ORI GIN AL PA PER

Preliminary Evidence for Iodate Reduction in BottomWaters of the Gulf of Mexico During an Hypoxic Event

Piers Chapman • Victor W. Truesdale

Received: 1 July 2010 / Accepted: 24 February 2011 / Published online: 12 March 2011� Springer Science+Business Media B.V. 2011

Abstract The distributions of iodate and total inorganic iodine concentrations in the

waters on the Texas–Louisiana shelf in April, June, and August 2004 are described.

Iodine–salinity graphs show three-end-member mixing involving onshore and offshore

surface waters and deep offshore water. The April survey showed simple mixing on the

surface, but in the later surveys, iodate concentrations were often much lower than pre-

dicted by the mixing curve while those for total inorganic iodine were higher. This

demonstrated both iodate reduction in the water and iodide addition, although individual

samples did not show equivalent speciation changes. Hydrographically, the system consists

of the estuaries of the Mississippi and Atchafalaya rivers as they spill onto the shelf. The

waters are stratified seasonally by a robust halocline, leading to hypoxia in the bottom

waters from the combined effect of restricted downward diffusion of oxygen and the

sinking of the luxuriant growth of phytoplankton induced by riverine nutrient supply. The

distributions of iodate and total inorganic iodine are, therefore, interpreted in terms of

water–sediment interaction as the shelf shoals to the north.

Keywords Gulf of Mexico � Texas � Anoxia � Hypoxia � Iodine � Total inorganic iodine �Iodate

1 Introduction

Dissolved iodine is typically present as iodide and iodate in oxic seawater at concentrations

of approximately 0.45 lM (Sugawara and Terada 1957; Elderfield and Truesdale, 1980;

Nakayama et al. 1989; McTaggart et al. 1994; Truesdale 1994a; Wong 1991; Campos et al.

1996; Truesdale et al. 2000). Consistent with the main circulation of the oceans and the

P. Chapman (&)Department of Oceanography, Texas A&M University, College Station, TX 77843, USAe-mail: piers.chapman@tamu.edu

V. W. TruesdaleSchool of Life Sciences, Oxford-Brookes University, Headington, Oxford OX3 0BP, UKe-mail: vtruesdale@brookes.ac.uk

123

Aquat Geochem (2011) 17:671–695DOI 10.1007/s10498-011-9123-6

prevailing redox conditions, in deeper waters (generally [200 m) dissolved iodine is

present mainly as iodate (Truesdale 1994a; Campos et al. 1999; Waite et al. 2006; Chance

et al. 2010); very small concentrations of iodide (*10–20 nM) are generated from

decomposing organic matter that sinks toward the ocean bottom (Tsunogai and Sase 1969;

Wong 1991; Truesdale 1994a). In contrast, in near-surface waters (0–200 m), either in

tropical or sub-tropical areas (Chapman 1983; Elderfield and Truesdale 1980; Jickells et al.

1988; Wong and Zhang 1992; Campos et al. 1996; Truesdale and Bailey 2002; Wong and

Zhang 2003; Tian et al. 1996), or across the temperate shelf (Truesdale 1994b; Truesdale

and Jones 2000; Truesdale and Upstill-Goddard 2003; Truesdale et al. 2003a), up to about

50% of the iodate can be converted to iodide. Temperate and polar waters which are well

offshore are subjected to vertical exchange during winter but, so far, it is only in polar

waters that a seasonal change in iodate and iodide concentrations has been observed;

Chance et al. (2010) report finding concentrations of iodide of up to 150 nM during

summer, which can be linked to biological activity. In anoxic waters such as the Black Sea

(Luther 1991; Luther and Campbell 1991; Truesdale et al. 2001b), some deep trenches

(Wong and Brewer 1977; Wong et al. 1985; Ullman et al. 1990), and parts of some

estuaries (e.g., Luther and Cole 1988; Abdel-Moati 1999; Truesdale et al. 2001a), only

iodide is present. There is also mounting evidence that iodate can be reduced (chemically)

in hypoxic waters (Stipanicev and Branica 1996; Zic and Branica 2006a; Zic et al. 2008,

2010). Dissolved organic-I has been observed in coastal waters and estuaries (Luther et al.

1991; Wong and Cheng 1998; Wong and Zhang 2001, 2003) but concentrations are small

offshore (Truesdale 1975). Particulate iodine concentrations in seawater are in the pM

range, with most being re-cycled in surface waters (Wong et al. 1976).

Although several authors have shown that surface concentrations of total iodine are

lower than those in the deep ocean, implying either surface removal of small amounts of

total iodine, particularly in warmer waters (Sugawara and Terada 1957; Tsunogai and

Henmi 1971; Truesdale 1994a; Truesdale et al. 2000), or addition in the deep ocean,

interconversion (reduction of iodate to iodide and vice versa) is the predominant process in

open-ocean waters. There have been several investigations of iodate reduction (e.g., Wong

et al. 2002), whereas little is known of iodide oxidation to iodate (Edwards and Truesdale

1997; Truesdale 2007; Zic et al. 2010). (To avoid confusion, the term ‘iodate reduction’ is

restricted here to only mean chemical reduction of iodate.) Given the iodine distribution

described above with continual generation of iodide in shallow waters and the high pro-

portion of iodate in the deeper waters, the question is whether iodine distributions in

surface waters should be modeled as the net effect of simultaneous but opposite processes

of oxidation and reduction (Campos et al. 1996), or just reduction in combination with

advective exchange of iodine species between shallow and deep reservoirs, with iodide

oxidation back to iodate mainly in the deep reservoir (Truesdale et al. 2001b).

Given its initial discovery in the ash of burnt seaweeds by Courtois in 1812 (Brasted

1954), iodine is regarded as a biophilic element, and several authors have investigated the

role of primary production in the uptake and interconversion of iodine species in the sea

(Sugawara and Terada 1967; Truesdale 1978a; Moisan et al. 1994; Wong et al. 2002). It is

now clear that algae and bacteria of various types are able to assimilate and interconvert

iodine species in laboratory cultures (Farrenkopf et al. 1997b; Waite and Truesdale 2003;

Chance et al. 2007). However, the mechanism by which this occurs is disputed, as indeed is

its direct relevance to natural waters. An early suggestion was that as the iodate ion has a

similar structure to nitrate, it would compete with nitrate in the process of primary pro-

duction, with the enzyme nitrate reductase affecting the reduction in both (Sugawara and

Terada 1967). Correlation of nutrient concentrations with those of iodate also seemed to

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support a direct link between the two within the oceans (Wong and Brewer 1974; Elder-

field and Truesdale 1980), although Truesdale (1994a) interpreted this quite differently.

The hydrographic relevance of such effects may also have been exaggerated since labo-

ratory cultures of marine phytoplankton are usually much more dense than natural blooms.

In any case, iodate reduction was not provoked in mesocosm studies of either Antarctic

waters (Truesdale et al. 2003b) or upwelling regions (Truesdale and Bailey 2002). As a

result, Truesdale and Bailey (2000) and Truesdale and Upstill-Goddard (2003) suggested

that iodate reduction may occur along with re-mineralization. In addition, abiotic pro-

cesses, either photochemical (Spokes and Liss 1996; Truesdale 2007) or sedimentological

(Truesdale and Upstill-Goddard 2003), or through the influence of humic substances

(Gilfedder et al. 2008), have been considered.

There remains a need for further investigation of iodine distributions in more ‘exotic’

environments where the marine iodine system is stressed into disclosing its secrets. Such

initial studies have been conducted in anoxic regions (the Black Sea, the Cariaco and Orca

Basins), as well as in reduced oxygen environments such as the Benguela upwelling zone

(Chapman 1983), Port Hacking (Smith et al. 1990), the oxygen minimum zone (OMZ) of

the Indian Ocean (Farrenkopf et al. 1997a, b), and the eastern tropical North Pacific (Rue

et al. 1997). Investigations have also been made in fjords, where deeper waters are

sometimes isolated for many months or even years (Edwards and Truesdale 1997;

Truesdale et al. 2001a). More recently, iodine behavior has been studied in anchialine

objects (caves and water bodies close to the sea, but with restricted connections to it); these

can isolate seawater for lengthy periods in either intermittent light (Stipanicev and Branica

1996; Zic and Branica 2006a, b; Zic et al. 2010) or permanent darkness (Zic et al. 2008)

and thus, help elucidate the role of primary productivity or continued re-mineralization

upon iodine biogeochemistry. The most surprising finding so far has been that of high-

iodate/low-iodide waters of comparable composition to those in the deep ocean, but at a

depth of about 10 m (Zic et al. 2008).

This paper describes the results of a scouting expedition to the near-shore waters of the

Texas–Louisiana shelf in the northern Gulf of Mexico where iodine interconversion was

anticipated because of previous experience in St Helena Bay, South Africa (Chapman

1983; Chapman and Shannon 1985; Truesdale and Bailey 2000), where hypoxia develops

rapidly after coastal upwelling. Redox conditions on the Texas–Louisiana shelf change

from full oxygenation in spring to hypoxia in bottom waters in summer as a consequence

of eutrophication caused by the combined effects of nutrients from the Mississippi river

and local stratification. It seemed that this location might present more gentle and enduring

conditions than those encountered previously. The main finding is of lowered iodate

concentrations in the hypoxic bottom waters during warmer periods (July and August),

which feed into an otherwise simple estuarine iodine distribution, particularly evident in

cooler months, e.g., April.

2 Hydrography of the Study Area

2.1 The Estuarine Character of the Shelf Waters

This study covers the eastern portion of the Texas–Louisiana shelf west of the Mississippi

delta between 89o and 92.5oW longitude and about 28.5�–29.5�N (Fig. 1). In this area,

depths range between 0 and 100 m, while surface salinity decreases from an offshore value

of about 35.5 to minima of\10 inshore because of the freshwater flux from the Mississippi

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and Atchafalaya rivers. The riverine buoyancy flux leads to a stratified two-layer system,

particularly in summer following the spring runoff maximum, with salinity being the main

determinant of the density structure. Below the pycnocline salinity (35.5–36.5) is similar to

that of offshore waters in the Gulf of Mexico at the same depth (Nowlin 1972).

2.2 Stratification and Redox Conditions

The combination of the large freshwater buoyancy flux, high suspended solids loading in

the low-salinity surface layer, and the high local productivity, fueled by high nitrogen

concentrations in the river water, leads to seasonal hypoxia and occasionally even anoxia

(Rabalais et al. 2007; Bianchi et al. 2010 and references therein). The extent of the hypoxic

region is controlled partly by topography (DiMarco et al. 2010), and while the hypoxia has

increased considerably since about 1975, the area affected varies greatly from year to year

and even from month to month, depending on river flow, riverine nitrate flux, and the local

wind field (Hetland and DiMarco 2008; Bianchi et al. 2010).

Strong stratification enhances bottom water hypoxia by hampering oxygen diffusion

from the surface into the bottom layer. Close to the river mouths, sinking and decay of

suspended particulates at the freshwater/seawater interface (Burton 1976; Morris et al.

1978) can cause hypoxia through chemical means alone (Morse and Rowe 1999; Morse

and Eldridge 2007). Away from the river mouths sinking of organic matter, either as dead

phytoplankton or zooplankton fecal pellets, enhances respiration in the lower layer and in

the sediments (Rabalais et al. 2007 and references therein). Both processes reduce oxygen

concentrations in the bottom water to less than 1.4 ml/l (2.0 mg/l, 62 lM), the commonly

accepted definition of hypoxia (Rabalais et al. 1999). Further away still, the stratification is

still enough to maintain hypoxia at the bottom even though surface nutrients and hence

productivity have been reduced to near zero (Rowe and Chapman 2002). Atmospheric

Fig. 1 Geographic distribution of stations with Atatachalya & Mississippi outlets. The larger dots denotethe three main groups of stations corresponding to regions of differential control of hypoxia (Rowe andChapman 2002). The smaller dots denote stations added along the 10-m and 20-m isobaths to increase arealcoverage

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frontal passages in fall and winter ensure re-oxygenation of the bottom layer, but fronts

(and hurricanes) can also break down the stratification for short periods during the summer

(Bianchi et al. 2010). Hypoxic conditions may be found at any time between February and

November depending on the wind regime and river flow and may last for several months,

although the local variability is such that a particular site can experience several oxic–

hypoxic cycles during a year (Hetland and DiMarco 2008; Bianchi et al. 2010), with the

thickness of the hypoxic layer varying from 1–2 m to about half the water column depth.

These periods of low bottom oxygen concentrations result in increased nutrient concen-

trations at the bottom in a similar fashion to that seen in upwelling regions (e.g., Chapman

and Shannon 1985).

2.3 The Flow Regime on the Texas–Louisiana Shelf

The general pattern of currents on the shelf has been discussed by several authors

(Cochrane and Kelly 1986; Cho et al. 1998; Nowlin et al. 1998, 2005) and is summarized

by Bianchi et al. (2010). Immediately west of the delta, in the Louisiana Bight, there is a

quasi-permanent anticyclonic gyre, formed by the rotation of the Mississippi River plume

(Ichiye 1960). During non-summer months, the general flow over the shelf is from the delta

to the west, driven by both the wind field and Coriolis forcing, but in summer the wind field

in the western Gulf reverses, leading to flow to the east. This tends to pool fresher water on

the shelf, strengthening the stratification. Both models and field investigations, however,

have shown that the short-term flow regime is anything but steady, with reversals on the

scale of days to weeks, while frontal systems and hurricanes can disrupt the stratification

and cause re-oxygenation of the bottom layer at irregular intervals (DiMarco et al. 1995;

Bianchi et al. 2010). In the study area, patterns of hypoxia are affected by the coastal

topography on scales of less than 50 km (DiMarco et al. 2010), with overall scales of

physical variability on the order of 15–20 km (Li et al. 1996). Biochemical variability

seems to vary on similar space scales (Bianchi et al. 2010); moored data on bottom oxygen

concentrations show that a particular site can change from oxic to hypoxic and back again

in the space of a few hours (Bianchi et al. 2010, their Fig. 3).

3 Materials and Methods

Data were collected during three cruises of the R.V. Gyre during 2004 from April 2–8

(M1), June 26–July 2 (M2), and August 18–25 (M3), respectively. Three grids of stations,

west of the Mississippi delta in the Louisiana Bight, south of Terrebonne Bay, and south of

Atchafalaya Bay (Fig. 1), were occupied, with some additional stations along both the

10- and 20-m isobaths connecting the grids. The grids correspond to the three regimes

hypothesized by Rowe and Chapman (2002) in which hypoxia is presumed to be controlled

by reactions in the sediment, biological production, and physical stratification,

respectively.

At all stations, a full-depth CTD cast was taken to within about 1–2 m of the bottom

using a SeaBird 911 CTD fitted with a SB43 oxygen probe. Discrete water samples were

obtained from multiple bottles on each cast and analyzed for salinity (Guildline sali-

nometer), dissolved oxygen (WOCE 1991), and nutrients (nitrate, nitrite, phosphate,

ammonia, urea, and silicate) using standard autoanalyzer methods (WOCE 1991; ammonia

based on Harwood and Kuhn 1970; urea based on Rahmatullah and Boyde 1980). Samples

closer to the bottom were taken using a ‘Pogo’ sampler, which consisted of four bottles

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mounted on a frame that tripped when a large plate suspended beneath made contact with

the bottom. We estimate that this sampled within 30–80 cm of the bottom. Full-depth

sampling for iodine was carried out at nine stations per cruise; other stations were only

sampled at the surface (*1.5 m) and bottom. Bucket surface samples were also taken for

nutrient and salinity measurements; these were not analyzed for iodine species.

Samples for iodine analysis were drawn, unfiltered, at the same time as nutrient samples

into glass screw-capped bottles and maintained in cool conditions (4�C), in the dark until

analysis within 1 year. Analysis for total inorganic iodine and iodate was by Ce(IV)/As(III)

catalytic spectrophotometry (Truesdale and Chapman 1976) and iodimetry (Truesdale

1978b), respectively, each on a Technicon Autoanalyzer II system. Taken in the same

order, the standard deviation for analytical control standards carried through each process

(usually about 10 replicates per day) is typically 0.006 and 0.013 lM. The accuracy of

each analysis is probably within 5%. All iodate analyses were corrected for background

absorption by running trays a second time but with the potassium iodide reagent spiked

with sodium thiosulphate to remove I3- ions. The catalytic method suffers from a small

systematic under-estimate as the salinity of samples diverges from that of the standards in

35 S artificial seawater (Truesdale and Chapman 1976); as will be explained, the error does

not compromise any of the deductions made here.

This study did not include any dissolved organic-I measurements. Given the size and

complexity of the Mississippi river drainage basin, such an investigation may prove

worthwhile in the future. For example, Schwehr (2004) found up to 0.2 lM organic iodine

in samples from both Galveston Bay and within a warm-core ring in the Gulf of Mexico,

suggesting an active transfer between organic and inorganic forms of the element.

4 Results

4.1 General Property Distributions

4.1.1 Temperature and Salinity

Although absolute values of temperature and salinity vary across the region during the

year, their overall distributions are generally very consistent from one month to the next,

so we use data from the June/July cruise to illustrate the typical patterns seen. Over the

Texas–Louisiana shelf, the depth and strength of the pycnocline are determined by

buoyancy forcing and salinity gradients rather than temperature, so we will not describe

temperature variability. The near-surface (3 m) and bottom salinity distributions across the

area for late June/early July 2004 are shown in Fig. 2. In both maps, there is a general trend

for salinity to increase offshore in line with the deepening isobaths. At the surface, the

observed salinity depended on the presence or absence of the low-salinity plumes from the

Mississippi and Atchafalaya rivers. The lowest salinity (\20) was seen close to Southwest

Pass in the Mississippi delta, while salinities of 24–26 were found at the inshore stations

off Atchafalaya Bay. However, at the outer edge of the survey area, salinities approached

34, which is typical for this part of the shelf (Nowlin et al. 1998). Note that bucket samples

showed salinities as low as 7–10 close to the rivers, showing the extreme changes in

salinity in the upper few meters. The distribution of salinity on the bottom showed a similar

gradient, but from 31–32 to 36–37 as the depth increased from 10 to over 40 m, again in

agreement with previous data. Pycnocline depth increased from about 5 m at the inshore

edge of the survey to about 10–15 m at the 30-m isobath.

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This onshore–offshore variability was repeated in the surveys for April and August

(data not shown). April salinities were lower along the inshore stations, with lowest values

of \20 seen (at 3 m depth) near the Mississippi delta and 22–23 off Atchafalaya Bay.

August salinities were higher, with lowest values about 24. Again, in both cruises, the

upper 1–2 m of the water column showed somewhat lower salinities, with minima around

15 in April (apart from one sample of 11) and 16 in August. The area of low surface

salinity varies throughout the year depending on river flow; 2004 was a relatively low-flow

year, based on data from the U.S Army Corps of Engineers gauging station on the Mis-

sissippi at Tarbert Landing (see http://www.mvn.usace.army.mil/cgi-bin/wcmanual.

pl?01100). Short-lived maximal flows of about 24,000–26,000 m3 s-1 were measured in

late February and mid-March, with a more extended maximum of the same order over the

period June 10–July 10. As it takes about 1 month for the water measured at Tarbert

Landing to reach the Louisiana Bight (Rabalais et al. 1999), the April and June cruises

probably coincided with low discharges of fresh water from both rivers, and even the

August cruise distribution showed only a relatively small area with low salinity (\20).

4.1.2 Nutrients and Oxygen

As would be expected, highest surface nutrient concentrations coincided with lowest

salinities in all three cruises (Fig. 3). Thus, for the April cruise (not shown), the highest

concentrations were found in the Mississippi Bight close to the delta and at the shallow

stations off Atchafalaya Bay, with surface values of nitrate and silicate of about 12 and

Fig. 2 Near-surface (*2 m) and bottom salinity in June/July 2004. Color scale is the same for both plots.M2 on this and subsequent plots refers to the cruise number within the program; the April and Augustcruises were numbered M1 and M3, respectively

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20 lM, respectively. By the June cruise, concentrations close to the delta had risen to 80

and 40 lM, respectively, with 108 and 57 lM in the bucket sample at the station nearest to

Southwest Pass, but in August (not shown), nitrate concentrations were\1 lM throughout

the region apart from over the Mississippi Canyon in deeper water; total inorganic nitrogen

(nitrate ? nitrite ? ammonia ? urea) was also less than 1 lM everywhere except in the

Mississippi Bight and isolated stations off Terrebonne Bay.

Bottom concentrations throughout the region were higher than near-surface concen-

trations on all three cruises, except for samples from very close to the delta, suggesting that

organic matter produced at the surface was decomposing below the pycnocline or being

released from the sediments. Concentrations changed from month to month; during April,

nitrate concentrations were \5 lM across the whole shelf. In June, however (Fig. 3),

concentrations were higher ([20 lM) at inshore stations near the Atchafalaya, decreasing

offshore to\1 lM at the bottom along the 20–30 m isobath. Other than in the Mississippi

Bight, where they remained similar to June concentrations, August data showed total

inorganic nitrogen levels at 5–10 lM near the bottom. This decrease was, perhaps, caused

by the passage of Hurricane Bonnie (see section 5.1). While reduced nitrogen components

(ammonia, urea, and nitrite) were measured on all samples, only nitrite was found con-

sistently at concentrations greater than 1 lM, and such concentrations were only observed

during the June cruise. Maximum concentrations observed here were up to 12 lM (Fig. 4),

suggesting considerable release from the sediment or the reduction in nitrate in the bottom

water. Regions with highest nitrite concentrations coincided generally with bottom iodate

concentrations \0.20 lM (Fig. 7).

Fig. 3 Near-surface (*2 m) and bottom nitrate concentrations (lM) in June/July 2004. Color scale is thesame for both plots. White stations near the delta in the near-surface plot were very high concentrations([40 lM) that are off-scale

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Dissolved oxygen concentrations, when combined with nitrate concentrations, clearly

showed the development of hypoxia over the course of the three cruises (Figs. 5, 6).

Figure 6 shows the oxygen/nitrate relationship for all samples collected during the cruises,

not just those where iodine was sampled. April surface samples were frequently super-

saturated (at the temperature and salinity found during the cruise saturation was about

5.1 ml l-1; 225 lM) and clearly showed elevated nitrate concentrations within the surface

plume, particularly close to the delta. Out of the surface plume, in well-oxygenated water,

nitrate concentrations were \2 lM, but below the pycnocline, oxygen concentrations

declined to below 4 ml l-1 (178 lM) and nitrate increased again, showing that decom-

position was active in the bottom layer even this early in the season.

In June/July, water temperatures had increased to about 26�C, for which saturation

occurs at about 4.3 ml l-1 (190 lM). The surface layer remained supersaturated even

though nitrate concentrations were still up to 100 lM in the surface plume, particularly

close to the delta. Below the pycnocline, however, a dramatic decrease in dissolved

oxygen was noted, with many samples being in the 0.25–0.5 ml l-1 (10–20 lM) range.

Nitrate concentrations in the bottom layer increased as the oxygen concentration

declined, but there was no simple relationship across the three regions sampled, and the

lowest oxygen concentrations showed a wide range in nitrate concentrations (0–20 lM).

Reduced nitrogen species (ammonia and nitrite) generally mirrored nitrate concentrations

near the surface, with high concentrations of both being found in the surface plume in

the Mississippi Bight. At the bottom, however, nitrite was found to make up about 50%

or more of the dissolved nitrogen (\12 lM) in low-oxygen water west of the Bight, as

stated earlier. Despite this, ammonia levels remained low (\0.5 lM) at almost all sta-

tions in these areas.

The situation was very similar in August, even though surface nitrate concentrations

were very low at this time at all but one station. The reduced nitrogen species were found

in lower concentrations than in June, and ammonia was the dominant species near the

bottom close to the delta, suggesting that nitrate reduction had continued in the water

column. West of 90�W, however, the situation was again similar to June, with generally

higher concentrations of nitrite apart from at two or three inshore stations, but the absolute

levels were less than half those found earlier, being in the range of 1–3 lM.

Fig. 4 Bottom nitrite concentrations (lM) in June/July 2004

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4.2 The Horizontal Distribution of Iodine on the Shelf

Horizontal distributions of total iodine and iodate for all cruises are exemplified by the

June samples (Fig. 7). Total iodine concentrations at the surface varied from\0.25 lM in

the Mississippi Bight to [0.35 lM along the outer edge of the survey area. Below the

pycnocline, total iodine concentrations were more consistent, almost all samples con-

taining 0.40–0.45 lM (Fig. 7d). Only seven bottom samples showed concentrations

[0.45 lM (Table 1), while a similar number of samples, all taken along the 10-m isobath

south of Atchafalaya Bay, was below 0.40 lM. During both April and August, the general

trend for total inorganic iodine of lower concentrations inshore and higher ones offshore

remained, with the surface total inorganic iodine distributions similar to that found in June.

Even so, in April and August, more bottom samples displayed low concentrations

(\0.40 lM).

Iodate concentrations (Fig. 7a, b) also showed the same trend of low inshore, higher

offshore concentrations in both surface and bottom samples, with the lowest concentrations

(\0.15 lM) being found in a band stretching along (and presumably inshore of) the 10-m

isobath. Surface concentrations increased offshore to a maximum of 0.27 lM over the

Fig. 5 Near-surface (*2 m) and bottom oxygen concentrations (ml/l) for June/July 2004. Color scale isthe same for both plots

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Mississippi Canyon, but bottom values were generally about 0.05 lM higher than surface

samples, except over the Canyon where the greater depth meant that concentrations were

up to 0.36 lM, in line with known values from other areas at similar depths (e.g., Wong,

1995).

4.3 The Vertical Distribution of Iodine Toward the Edge of the Shelf, at the Canyon

Station

It is useful to examine the vertical distribution of iodine concentrations and salinity at the

Canyon station (28.76oN, 89.59oW) before proceeding to the more variable conditions on

Fig. 6 Oxygen/nitrate relationships for the three cruises. Note the different scales for the nitrateconcentrations

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Fig. 7 Near-surface (*2 m) and bottom iodate (a, b) and total iodine (c, d) distributions (lM) in June/July2004. Note the different color scales for iodate and total iodine

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the shallower part of the shelf. This suggests what the conditions on the shelf would have

been had they been controlled by the basic offshore oceanic stratification and not modified

by sedimentary and estuarine effects. Indeed, the depth profiles for salinity and total

inorganic iodine concentration below about 20–25 m (Fig. 8a) are relatively constant.

Meanwhile, the iodate concentration identifies two regions: one at 25–65 m and another at

about 80 m, where total and iodate concentrations eventually converge. At depths shal-

lower than about 25 m, all three variables decrease markedly. Plots of total inorganic

iodine and iodate concentrations (Fig. 8b), rationalized to salinity 35 (RTI and RIO3,

respectively), confirm this interpretation for depths greater than 20–25 m. However, they

show clearly that a part of the variability in the 0–10 m depth range is not due merely to

dilution, but to non-conservative mixing.

4.4 The Vertical Distribution of Iodine at Stations on the Shelf

Figure 9 is a composite of all rationalized iodine data for 0–40 m, taken cruise by cruise.

(Before these graphs were constructed, the small mismatch between standards and sample

salinities in the total iodine data was removed.) The dashed lines at 0.30 and 0.40 lM RIO3

and RTI concentration, respectively, are drawn to facilitate comparison with Fig. 8. The

data for all the samples collected in April are only slightly more scattered than they were at

the Canyon station. In sharp contrast, the RIO3 data around 5–20 m for June and August

spread much more markedly. The lowest RIO3 concentrations in these later periods largely

corresponded with near-bottom samples (Fig. 9), with the lowest concentration observed at

*10 m. Meanwhile, RTI concentrations are spread more in June and August; slightly

greater decreases seem to appear in the near-surface waters while even a slight increase

occurs at depths of about 10–30 m, particularly in June.

Table 1 The occurrence ofunexpectedly high total inorganiciodine (TII) concentrationsduring the cruises

Station Lat (�N) Long (�W) Depth (m) TII (lM)

June cruise

12A 28.97 N 89.49 W 31 0.48

17A 28.76 N 89.59 W 85 0.46

4B 28.84 N 90.68 W 16 0.46

12B 28.87 N 90.39 W 20 0.47

16B 28.98 N 90.32 W 15 0.54*

BC3 28.61 N 91.00 W 19 0.47

CG4 29.45 N 92.87 W 2� 0.63*

12 0.87*

August cruise

3B 28.58 N 90.78 W 0� 0.58*

3C 28.92 N 92.45 W 28 0.51*

5C 29.01 N 92.17 W 19 0.47

6C 28.92 N 92.23 W 25 0.50*

9C 28.90 N 92.07 W 25 0.47

10C 28.80 N 92.13 W 30 0.52*

13C 28.94 N 91.87 W 20 0.46

14C 28.84 N 91.92 W 25 0.47

Aquat Geochem (2011) 17:671–695 683

123

4.5 Iodine Versus Salinity Plots

In Sect. 4.4, interpretation of the data sets was aided by graphs of rationalized concen-

trations versus depth. Here, plots of iodine concentration versus salinity are explored, to

separate the onshore results from the offshore ones.

4.5.1 The April Cruise

The iodine versus salinity plot for all samples from the April cruise (Fig. 10a) shows that

both total inorganic iodine and iodate concentrations increased almost conservatively with

salinity over the salinity range 17–36. The curve actually represents three-end-member

mixing (Truesdale et al. 2003a) since the samples with highest salinity represent deeper,

offshore water. A second, less obvious limb of the mixing diagram is, therefore, present

just above salinity 36. The surface to deep mixing limb is more evident as a change of

about 0.12 lM in iodate concentration just above salinity 36. Indeed, this latter change is

essentially the local expression of the latitudinal difference in iodate concentrations

between surface and deep waters of the Atlantic system (Truesdale et al. 2000). For the

April cruise, straight lines were fitted by eye to the surface dilution lines. Neither line

passed through the origin, the intercepts for iodate and total iodine being about -0.03 and

-0.07, respectively. This relationship agrees with the low RTI and RIO3 concentrations of

Fig. 8 The distribution with depth of salinity and inorganic iodine at the Canyon station during the Aprilcruise. Blue diamond salinity; green triangle and red square—iodate and total inorganic iodine,respectively; blue triangle and pink square—RIO3 and RTI, respectively. The dashed lines are added forthe sake of comparison

684 Aquat Geochem (2011) 17:671–695

123

Fig. 9 and confirms that the near-shore surface waters contain proportionately less iodine

than the other waters on the shelf.

4.5.2 The June Cruise

The iodine/salinity plots for the June and August cruises (Figs. 10b, c) show essentially the

same background pattern of three-end-member mixing established on the April cruise. (For

comparison purposes, the straight lines established for the April results were added.) Total

iodine concentrations were mostly higher than those of April, while those for iodate were

persistently less. Occasional total inorganic iodine concentrations were exceptionally high,

e.g., 0.60 lM. The lowest iodate concentration of 0.02 lM corresponded with a salinity of

about 31.5; these lower iodate and raised total iodine concentrations were a persistent

feature at salinities between about 18 and 36.

To differentiate shoaling effects from those arising purely from an estuarine decrease in

salinity, samples from the bottom 20% of the water column have been color-coded differently

from those in the upper 80%. This particular split was chosen because at most stations the

depth of the pycnocline is shallower than the lower 20% of the water column. All the so-called

‘bottom samples’ cluster toward the right in Fig. 10 because they represent deeper, higher-

salinity water which penetrated onto the shelf from offshore. Note that as a result of the

estuarine stratification over the shelf, the difference in salinity between surface and bottom

waters at inshore stations is generally larger than at offshore stations. Hence, on both the total

inorganic iodine and iodate graphs, points for surface and deep samples at a given station will

be much more separated at inshore stations than is the case at offshore stations.

4.5.3 The August Cruise

Comparison of the individual total inorganic iodine concentrations of the August cruise

(Fig. 10c) with the straight lines imported from the April results shows a somewhat similar

0

20

40

Dep

th / m

[I]R / µMAug June /

July

0 0.2 0.4 0.60 0.2 0.4 0.60 0.2 0.4 0.6

[I]R / µM

0

20

40

Dep

th / m

[I]R / µMApril

0

20

40

Dep

th / m

Fig. 9 Variation with depth of rationalized total iodine (RTI, blue diamond) and rationalized iodate (RIO3,magenta square) in the three cruises

Aquat Geochem (2011) 17:671–695 685

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distribution to that in June/July, with only slightly higher total inorganic iodine at salinities

above 30. Evidently, there was even a loss of total iodine in some 4–5 samples with salinity

near 29. Occasional, unusually high, total iodine concentrations were observed at salinities

of 28 and 30.5. Meanwhile, as with the June cruise, iodate concentrations were also lower

0

0.1

0.2

0.3

0.4

0.5

0.6

Salinity

[Io

din

e] /

µM

April

June/July

0

0.1

0.2

0.3

0.4

0.5

0.6

14 16 18 20 22 24 26 28 30 32 34 36 38

August

0

0.1

0.2

0.3

0.4

0.5

0.6

[Io

din

e] /

µM

Salinity14 16 18 20 22 24 26 28 30 32 34 36 38

Salinity14 16 18 20 22 24 26 28 30 32 34 36 38

[Io

din

e] /

µM

Fig. 10 Iodine versus salinity plots for all three cruises. Straight lines (black total iodine, red iodate) showthe relationships for April 2004. On all graphs, pale blue and red points refer to samples from the bottom20% of the water column; dark blue and magenta are from the upper 80%

686 Aquat Geochem (2011) 17:671–695

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than in April at all salinities, although not as much. This suggests a similar overall rate of

decrease in iodate concentration with salinity in the system overall; the August sampling

did not include the deeper, offshore waters of salinity 36.4.

4.6 The Occurrence of Individual High Iodine Concentrations

Some 16 samples showed higher than expected total inorganic iodine concentrations

([0.45 lM). As it has been convenient to ignore these elsewhere in the results, they are

presented separately here. In total, they amount to only some 3% of the total number of

samples taken. Nevertheless, they may point to an interesting aspect of iodine biogeo-

chemistry, especially as similar occurrences have been witnessed elsewhere before

(Chapman 1983; Truesdale and Bailey 2000; Truesdale et al. 2001a; 2003a), with iodide

release from the sediment postulated as the cause. Table 1 shows that none were recorded

in the April cruise while eight occurred in each of the two later cruises. This difference in

frequency suggests then that they relate to the development of hypoxia on the shelf. Out of

the 16 occurrences only seven, split equally across the June and August cruises, gave

concentrations greater than 0.50 lM (*; Table 1). The majority occurred at depth with only

two occurrences at the surface (�; Table 1). Interestingly, there seems to have been a shift

in the high numbers from region B (south of Terrebonne Bay) in June westward to region C

(south of Atchafalaya Bay) in August, although given the small number of samples the

significance of this is tentative. Note that station CG4, where high concentrations were

found in both surface and bottom samples, is considerably west of the remainder of the

samples and does not appear on the horizontal plots.

5 Discussion

5.1 The Findings Overall

Once the effect of estuarine dilution across the Louisiana–Texas shelf is allowed for, the

results reveal three major effects: a small gain of inorganic iodine to the bottom waters in

summer (Figs. 8, 9); a temporary reduction in iodate in hypoxic bottom waters in the

summer months (Figs. 9, 10); and a year-round loss of total inorganic iodine from the

surface waters particularly evident in Fig. 8.

The simplest explanation for the total inorganic iodine results seems to rely upon

transport of iodine from the surface waters to the bottom waters by the same process that

causes the hypoxia. Thus, iodine taken up by phytoplankton in the enhanced productivity

of the eutrophic estuarine waters of the Mississippi River system (Fig. 9) re-appears in the

hypoxic bottom waters as iodide as the sunken particulate organic matter decomposes on

the bottom (also in Fig. 9). This is supported by the fact that both the loss to algal uptake

and the regeneration on the bottom would both be of similar, but relatively small mag-

nitude. The above-mentioned iodine mechanism would be seasonal as it followed the

seasonal generation of the hypoxia. In winter, bottom waters would be mixed into the

surface waters with the hypoxia and the changed iodine distribution being eliminated.

The reduction in iodate in the hypoxic bottom waters appears as a marked change in the

iodate distributions of June and August, as compared with that of April (Fig. 9), in the

presence of relatively little change in concentrations of total inorganic iodine. Chapman

(1983) found a similar situation in the Benguela upwelling region. The simplest expla-

nation for this is that it occurred concomitantly with the decomposition of the organic

Aquat Geochem (2011) 17:671–695 687

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matter that produced the hypoxic condition. Such a mechanism has been hypothesized as

causing the general reduction in iodate across temperate shelves (Truesdale and Upstill-

Goddard 2003), and this discovery of the effect in the seasonal hypoxia on the Louisiana

shelf makes further investigation of it that much easier. Indeed, as the effect on the

Louisiana shelf is seasonal and much more intense than in temperate waters, it will be

much easier to mount rigorous investigation of the dynamics of the process. If this is so,

organic-I (dissolved and/or particulate) likely plays a major role in this estuarine system, as

indeed, seen in several inshore-water systems (Wong and Cheng 1998; Wong and Zhang

2001). Unfortunately, organic-I concentrations were not measured in this preliminary

scouting expedition.

There is always a chance that the system is more complicated than that described above,

with a possible involvement of anoxic sediment underneath the hypoxic bottom water.

Thus, Zhang and Whitfield (1986) showed that iodate is rapidly reduced to iodide by the

bisulfide ion. As a result, iodate reduction is automatically expected in anoxic systems such

as the Black Sea waters deeper than about 60 m (Luther and Campbell 1991), fjord deeps

(Emerson et al. 1979), various trenches, e.g., the Cariaco Trench (Wong and Brewer 1977),

salt basins such as the Orca Basin (Wong et al. 1985) and those within the Mediterranean

Sea (Ullman et al. 1990), and anoxic sediments (Price and Calvert 1978; Kennedy and

Elderfield 1987a, b; Elderfield et al. 1981; Upstill-Goddard and Elderfield 1988; Anschutz

et al. 2000). However, in iodate reduction, the presence of sulfide is not vital. Using natural

samples of water from the oxygen minimum zone of the Arabian Sea Farrenkopf et al.

(1997b) found they could reduce lM amounts of added iodate within a few hours. In such

systems, nitrate rather than sulfate is used as an electron acceptor after oxygen is used up,

and sulfide is not generated. Farrenkopf et al. (1997b) also showed that Shewanella pu-trefaciens grown in the laboratory onshore, either aerobically on agar plates or anaerobi-

cally in liquid medium with nitrate, also readily reduced 250 lM amounts of iodate.

This more complicated mechanism relies upon the commonly established fact that

iodide is released from sediment pore waters into overlying waters, particularly in anoxic

or hypoxic regions (e.g., Price and Calvert 1978; Chapman 1983; Farrenkopf and Luther

2002). The occurrence of this in summer, but not in April of the same year (Fig. 9), would

result from the winter re-oxygenation of the bottom waters and the sediment surface. It is

known that oxic sediments sorb iodine, so that the I/C molar ratio in surficial deposits

becomes an order of magnitude greater than those in either anoxic sediments or the original

seston from which they are derived (Bojanowski and Paslawska 1970; Price et al. 1970;

Price and Calvert 1978). Therefore, the switching of the redox condition of the sediment

could temporarily reduce, or even halt, the flux of iodine leaving the sediment. Ullman and

Aller (1980) showed that the flux need not stop completely as the sediment is oxygenated.

The sorption of iodine to oxic sediments is believed to occur upon manganese and iron

oxides, and as anoxia creeps upwards in the summer, desorption will occur. Although

hypoxia can be found on the Louisiana–Texas shelf at any time between February and

November (Rabalais et al. 2007), the area affected varies considerably both from year to

year and month to month. Hypoxia is most commonly seen between May and September.

Even then, however, the hypoxia in the bottom layer can be alleviated for short periods as

atmospheric frontal systems move through the area and cause re-oxygenation of the bottom

water. Presumably, these changes in bottom water redox levels cause minor changes in

iodine species concentrations, but very intensive sampling would be needed to show this.

The lack of additional changes between the June/July and August cruises is presumably

linked to bottom water re-oxygenation following the passage of tropical storm Bonnie

688 Aquat Geochem (2011) 17:671–695

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(August 11–13) and possibly hurricane Charley (August 13–14, but further east), as

mentioned earlier.

5.2 Corroborating Examples of Similar Observations

Given the above, our results are consistent with the findings by Luther and Cole (1988)

and Ullman et al. (1988) that total iodine concentrations in Chesapeake Bay and some

other North American estuaries plotted consistently higher than the line depicting con-

servative mixing between the fresh water end-member and a marine one immediately

outside the Bay; within the Bay, iodide was released from the sediments to augment the

water column concentration. Interestingly, Ullman et al. (1988) were cautious about these

findings because the differences were not large and the phenomenon had not been

recorded previously. Our results and these two papers suggest that the case for accepting

all the records is now much stronger. In contrast, conservative mixing has been reported

in the estuaries of the rivers Yarra (Smith and Butler 1979), St. Lawrence (Takayanagi

and Cossa 1985), Tamar (Upstill-Goddard and Elderfield 1988), Krka (Zic and Branica

2006b), and indeed sometimes even within Chesapeake Bay itself (Ullman et al. 1988).

Our observations are also consistent with an increase in total inorganic iodine concen-

tration in near-bottom samples at both a 20-m station in Port Hacking (Smith et al.

1990), in Rogoznica Lake (Zic and Branica 2006a), and within a sub-pycnocline, low-

oxygen layer over the shelf in the upwelling region north of Cape Town, South Africa

(Chapman 1983; Truesdale and Bailey 2000). There, once again, the iodide concentra-

tions were in excess of those generated by reduction in iodate. In quite, a different

context Farrenkopf and Luther (2002) applied a similar explanation to the highest total

iodine concentrations ever recorded in an open-ocean water column. These occurred in

the oxygen minimum zone of the Arabian Sea, and it was postulated that iodide had been

advected into the region over thousands of miles from the highly reducing sediments off

the west coast of India. The fact that detection of non-conservative behavior is highly

dependant upon specific local conditions is well illustrated by Upstill-Goddard and El-

derfield’s (1988) conservative mixing line for the Tamar estuary existing alongside a

measured flux of iodide from the sediment; clearly, the flux, although significant in itself,

was insufficient to affect the water column as a whole.

Meanwhile, there are several examples of both marked non-conservative and near-

conservative mixing of iodate in estuaries. The problem, of course, is that whatever

analytical method has been used for iodate, differentiating between conservative and non-

conservative mixing becomes difficult when the variability in iodate concentration

approaches that of the natural uncertainty of the method. This is automatically more acute

at low salinities where the concentrations are lower. Nevertheless, on a somewhat arbitrary

basis, it seems that conservative mixing behavior for iodate was observed in the

St. Lawrence by Takayanagi and Cossa (1985), and in the Tamar by Upstill-Goddard and

Elderfield (1988). In contrast, a complete absence of iodate was observed at salinities

below 20 by Luther and Cole (1988) in Chesapeake Bay, while Abdel-Moati (1999) reports

non-conservative behavior for iodate in the Nile. These marked losses of iodate seem to

involve shallower waters where the bottom sediment has more influence on the water

column. Meanwhile in the hypoxic but not anoxic, 14-m water column of Rogoznica Lake,

RIO3 concentration decreased with depth, once again suggesting an effect of the sediment

(Zic and Branica 2006a; Zic et al. 2010).

Aquat Geochem (2011) 17:671–695 689

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5.3 The Dissipation of High Iodide Concentrations from the Louisiana–Texas Shelf

If water on the Louisiana–Texas shelf was not exchanged with that offshore, iodide con-

centrations would increase continuously with time while iodate would be continuously

depleted, unless, of course, a compensatory in situ oxidation mechanism exists. As

explained in the Introduction, the question as to where iodide oxidation occurs is the major

mystery of marine iodine biogeochemistry. Even though there is generally thought to be

little exchange between the shelf and the open Gulf of Mexico (Cho et al. 1988), some

exchange must occur when tropical storms and hurricanes pass through the region. It is also

known that water is drawn off the shelf between contra-rotating warm-core and cold-core

rings (Biggs et al. 2005) and that water tends to move inshore and offshore on a diurnal

time cycle (Bianchi et al. 2010). It, therefore, seems likely that the localized anomalies in

total inorganic iodine and iodate concentrations observed here on the shelf dissipate into

the Gulf proper and, perhaps, via the Loop Current, eventually into the Gulf Stream

system. Given the resulting large dilution effect of this western boundary current system,

present analytical techniques are unable to follow such changes. Nonetheless, this does

illustrate the possibility that iodide generated upon the Louisiana–Texas shelf might

actually be re-oxidised to iodate after the Gulf Stream water has been convected into the

deep-water circulation of the N. Atlantic, around Iceland (Waite et al. 2006).

Although iodate is the thermodynamically predominant iodine species in oxic systems

(e.g., Liss et al. 1973), iodide is kinetically meta-stable. Our work clearly suggests that

iodate reduction can occur during hypoxia. Therefore, more severe and continuing hypoxia

on the Texas–Louisiana shelf will probably lead to increasing iodate reduction in this

region, as suggested for the Indian Ocean by Farrenkopf and Luther (2002), and as known

from other anoxic regions. However, whether this occurs in both the water column and the

sediments on open shelves, as has proved to be the case in anchialine caves (Zic et al.

2008), remains unknown. (In the latter case, under hypoxic conditions, iodate reduction

accompanies respiration at the pycnocline.) Further work on the Texas–Louisiana shelf

may be able to resolve this question of iodate reduction. This study has shown that for

future work, sampling should be done at the formation sites of hypoxic water. Comple-

mentary studies stand a good chance of deciding whether localized, short-term changes in

sediment redox potential can produce more general changes in iodine speciation over

temperate shelves, as well as whether the return reaction, iodide oxidation, is ubiquitous or

occurs only remotely in deep waters.

5.4 Reservations Concerning Filtration and Storage of Samples

The preliminary status of this study acknowledges unintended uncertainty arising from the

prolonged storage of samples prior to analysis. The samples were taken unfiltered in April,

June, and August 2004 and, for operational reasons, were all analyzed together after

storage for 12, 9, and 7 months, respectively, instead of after an intended 1 month of

storage. We reason below that this is unlikely to have influenced the results unduly.

With open-ocean surface water samples, there would be little doubt attached to their

storage for these periods as, over about four decades, samples taken for iodate or total

inorganic iodine analysis have generally stored well for many months under the regime

adopted here (Truesdale and Spencer 1974; Campos 1997). The early and severe problems

of storage (Truesdale 1968) were caused by the accumulation of nitrite, from nitrification,

which boosted the apparent iodate concentration enormously. The severity of this effect

may even have given rise to the false impression of a layer of enhanced iodate

690 Aquat Geochem (2011) 17:671–695

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concentration associated with the Equatorial Undercurrent in the Atlantic (Wong 1977).

Sulfamic acid suppresses the effect of nitrite on iodate analysis (Johannesson 1958;

Truesdale and Spencer 1974; Chapman and Liss 1977, Truesdale 1978a, b). More recently,

while iodate is generally thought to be more stable in storage than total iodine, storage

seems to have been a problem mainly in organic-rich samples (Campos 1997), especially

phytoplankton or bacterial cultures (Tsunogai and Sase 1969; Moisan et al. 1994; Bluhm

et al. 2010). Storage can also cause a disproportionate error in iodide determination with

deep waters, or even surface ones from polar regions, where the iodide concentrations are

so very low. However, that does not apply here as iodate and iodide concentrations are

comparable. Finally, the type of material used for the containers appears to be important.

Truesdale (1968) found that glass was much more effective than polythene for storing Irish

Sea water, and Campos’ (1997) experience with HDPE seems to confirm this, unless

samples are frozen. Filtration itself has not raised any problems, but the lack of it in

combination with storage has sometimes been problematic, especially when iodide con-

centrations are low.

The samples collected here were from relatively high salinity hypoxic shelf waters;

none was anoxic (Sect. 4.1), and dissolved oxygen concentrations would undoubtedly have

increased during transport and storage through equilibration with oxygen in the head-space

of the sample bottles. These hypoxic waters were generally of low turbidity, the highest

values being found in the thin, low salinity, surface plume. Therefore, there is no reason to

suspect that the low iodate concentrations were caused by sulfide reduction in iodate during

storage or of tri-iodide during analysis. Incidentally, no systematic connection should be

anticipated between high total inorganic iodine and low iodate concentrations as a result of

either analytical method used. Although both rely upon oxidizing agents, I3- and Ce(IV)

ions, respectively, only the iodate method uses direct oxidimetry. Correlation coefficients

for total iodine v iodate concentrations for the individual samples from the three cruises

were 0.917 (April, n = 74), -0.052 (June, n = 118), and 0.117 (August, n = 56),

respectively, showing that the reduced iodate concentrations found in June and August did

not correspond with increased total iodine concentrations in the same samples.

Notwithstanding the above, the best evidence that storage was not a problem is that the

set stored for the longest period showed the least random variation in either total inorganic

iodine or iodate iodine (Figs. 9a, 10a). It is the samples stored for only 9 and 7 months (the

June and August sets, respectively) that display the low iodate concentrations of particular

interest here. Moreover, there is very little to distinguish between the two sets (Fig. 9),

when a variable storage effect would be expected to increase the spread and variability of

the results the longer the samples were stored (i.e., June [ August). Nonetheless, we

accept that any problem over storage would have been more likely to occur with the

hypoxic bottom waters of June and August.

Finally, and somewhat ironically, even if most of the decrease in the iodate concen-

trations witnessed in this study had occurred during the storage period, the experiment

could still be taken as a crude incubation experiment. Although ill-defined kinetically, the

result would still be useful biogeochemically as indicating some natural rate of iodate

reduction on the shallow shelf of the Gulf of Mexico. This would still fulfill the original

intention behind this study, of scouting for a location where more detailed studies, e.g.,

time series, might be mounted later, rather than that of providing a definitive answer to

iodine biogeochemistry. Thus, any future investigation would be wise to include alterna-

tive analytical methodology to consider, e.g., organic iodine concentrations (Luther et al.

1991) or bacterial reduction (Farrenkopf et al. 1997b), to confirm the effects observed here.

Aquat Geochem (2011) 17:671–695 691

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Acknowledgments The data used in this study were collected as part of the National Oceanic andAtmospheric Administration-funded NGOMEX program Mechanisms Controlling Hypoxia. We are gratefulto Steven DiMarco (co-chief scientist with PC on these cruises) for the hydrographic data, and to the Captainand crew of the R.V. Gyre and our many students and colleagues who made up the cruise parties. Furtherinformation on the data sets may be obtained from the authors. We also thank our two reviewers for theircomments on an earlier version of this manuscript; these have helped improve the discussion. This isNGOMEX contribution #134.

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