Time-series observations during the low sub-surface oxygen events ...
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Marine Chemistry 97
Time-series observations during the low sub-surface oxygen
events in Narragansett Bay during summer 2001
Deanna L. Bergondo *, Dana R. Kester, Heather E. Stoffel, Wendy L. Woods
Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA
Received 8 December 2003; received in revised form 7 January 2005; accepted 22 January 2005
Available online 6 September 2005
Abstract
A series of automated water column time-series measurement systems has been established in Narragansett Bay, Rhode
Island. These systems measure near surface and near bottom temperature, salinity, oxygen, pH, chlorophyll and pressure at 15-
min intervals. The data obtained from two buoy sites during the period of July through September 2001 reveal the occurrence of
episodic surface phytoplankton blooms followed by subsurface hypoxic events, particularly in the upper portions of the estuary
including the area known as the Providence River. Three hypoxic events occurred at monthly intervals in July, August and
September. Their timing, and that of the phytoplankton blooms that preceded them, is linked to the periodic weak neap tidal
cycles that occur alternately with somewhat stronger neap tidal cycles. The connection between surface blooms/subsurface
hypoxia and tidal range is attributed to water column stratification and the ability of moderate changes in tidal amplitude to
reduce stratification through vertical mixing. During the summers of 2002 and 2003 we predicted and subsequently observed
hypoxic events in the Upper Bay. Based on these findings we can project into future years the times when summer blooms and
hypoxia are most likely to occur in the upper portions of Narragansett Bay. The observations from summer 2001 suggest that
the oxygen consumption and renewal in the subsurface waters is delicately balanced. Further increases in inputs of nutrients,
organic matter, or oxygen-consuming substances could shift this balance from hypoxic to anoxic with substantial impacts on
fish and other marine organisms.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Estuaries; Oxygen; Phytoplankton; Stratification; Tides; Time-series analysis
1. Introduction
Eutrophication and its associated hypoxic condi-
tions are increasing concerns in coastal waters. Nutri-
ents and organic matter enter coastal waters from
0304-4203/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.marchem.2005.01.006
* Corresponding author.
E-mail address: [email protected] (D.L. Bergondo).
sewage and wastewater treatment facility effluents.
Fertilizers used for agriculture and landscaping in
urban and suburban coastal watersheds, as well as
animal organic wastes, contribute non-point source
pollution, aiding in the eutrophication process. In-
creases in organic material loading of coastal waters
can lead to oxygen depletion, especially beneath the
pycnocline of a stratified water column. Nutrients
(2005) 90–103
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–103 91
entering coastal waters stimulate algal photosynthesis
and the production of organic matter. Upon reaching
waters beneath the pycnocline this organic matter
contributes to oxygen depletion.
Hypoxia refers to conditions where the dissolved
oxygen concentration decreases to the point where
organisms are adversely affected. Various investiga-
tors have chosen different oxygen concentrations,
usually either V0.06 mmol/L (2 mg/L) or V0.09mmol/L (3 mg/L), as the criterion for hypoxia. Miller
et al. (2002) reported the results of a number of
experiments on the lethality of low oxygen to various
marine organisms. While some species and life-stages
are more tolerant than others to low oxygen levels,
concentrations at 50% lethality (LC50) as high as 0.09
mmol/L were found in a number of the experiments.
In this paper we consider oxygen concentrations
V0.09 mmol/L (38.3% saturation when t=19 8C and
S =29) to be hypoxic and a cause for concern regard-
ing the quality of waters for marine organisms in
Narragansett Bay.
Diaz (2001) provided a worldwide summary of
coastal water hypoxia during the last half of the
20th century based on observations from more than
20 countries and about 10 states. In the USA, hypoxic
conditions have been studied extensively in the Che-
sapeake Bay, the Gulf of Mexico, western Long Island
Sound, and several other coastal systems. Taft et al.
(1980) summarized the occurrence of hypoxic and
anoxic bottom waters in a northern section of Chesa-
peake Bay from near Baltimore to the Patuxent River
from 1964 to 1977. Oxygen-depleted waters were
associated with thermohaline stratification created by
river input and summer seasonal warming. In the
northern Gulf of Mexico, hypoxia is related to exces-
sive nutrients discharged from the Mississippi River,
and water column stratification caused by the river’s
plume along the shelf (Rabalais and Turner, 2001;
Rabalais et al., 2001). In Long Island Sound (LIS),
Parker and O’Reilly (1991) summarized data from
1950 to 1990 showing east-to-west depletion of bot-
tom O2 and progressively more severe hypoxia from
1975 to 1989. Welsh and Eller (1991) further exam-
ined the processes associated with LIS hypoxia. They
found a continual decline in bottom oxygen from May
to September when the water column was thermally
stratified. Anderson and Taylor (2001) reported results
of weekly bottom water oxygen measurements during
the summers of 1992 and 1993 in which episodic
hypoxia, as opposed to persistent hypoxia, was
observed throughout the summer in LIS.
The uppermost reaches of Narragansett Bay,
including the Seekonk and Providence Rivers, located
in the major urban center of Rhode Island (Fig. 1),
become seasonally hypoxic. While geographically
referred to as briversQ, the Seekonk and Providence
are actually tidal estuaries. Poor water quality condi-
tions have been known to exist for quite some time in
the Seekonk and Providence Rivers (Doering et al.,
1988a,b; Granger, 1994; Kester et al., 1996). Below
the Providence River, conditions are considered par-
tially to well mixed (Kremer and Nixon, 1975; Pilson,
1985). This has led to a general expectation that most
other portions of Narragansett Bay are in good con-
dition for public utilization (e.g., swimming and fish-
ing) and habitat utilization by organisms vital to the
diverse ecology of the Bay. However, during a July
1998 training exercise, investigators with the Envir-
onmental Protection Agency (EPA) Environmental
Monitoring and Assessment Program (EMAP)
observed lower than expected subsurface oxygen con-
centrations (b0.06 mmol/L) in the region south of
Greenwich Bay and north of Wickford Harbor (Fig.
1). Fish kills and sporadic measurements of oxygen
produced additional evidence of low oxygen condi-
tions beyond the tidal river section of the estuary
(Deacutis, 1999). In 1999 the north and south Pru-
dence Island buoys were established by the Rhode
Island Department of Environmental Management
(DEM) and the Graduate School of Oceanography
(GSO) to conduct more comprehensive monitoring
of oxygen in the upper and mid Bay regions. In
2001 additional monitoring sites in the Providence
River were established. These new measurement tech-
nologies have been used to investigate conditions and
processes occurring in Narragansett Bay.
This paper presents observations and analyses from
the summer of 2001 showing hypoxic conditions not
only in the Providence River, but also in the main
parts of Narragansett Bay. Low oxygen events are
documented in July, August, and September 2001 at
two buoy sites located in the Providence River (Bul-
lock Reach, BR) and the Upper Bay (North Prudence,
NP) (Fig. 1). This paper examines the mechanisms
that cause low oxygen concentrations, and the factors
that restore oxygen concentrations to more moderate
Fig. 1. Map of Narragansett Bay showing the locations of the two moored time-series buoys: BR=Bullock Reach Buoy, NP=North Prudence
Buoy, X=meteorological data from T.F. Green Airport. The Blackstone River, not shown, flows into the Seekonk River.
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–10392
values. We also show an ability to predict when sur-
face water phytoplankton blooms and sub-surface
hypoxia are most likely to occur in future years.
2. Study site
Narragansett Bay is a partially mixed estuary, run-
ning north–south from Rhode Island Sound (Fig. 1). It
was formed by the drowning of three river valleys,
which today form the three main regions of the bay:
West Passage, East Passage, and Sakonnet River. The
mean depth of Narragansett Bay is approximately 8.3
m includingMt. Hope Bay (Pilson, 1985). Narragansett
Bay is a relatively saline estuary (i.e., mean salinity 29–
31), with freshwater inputs mainly localized in the
uppermost areas of the bay (Ely, 2002). Seventy-eight
percent of the total surface freshwater input to the bay
comes from five major rivers: Tauton, Blackstone,
Pawtuxet, Woonasquatucket, and Moshassuck (Des-
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–103 93
bonnet and Lee, 1991). The Tauton River empties into
Mount Hope Bay, while the remaining rivers empty
into the Providence River. Additional fresh water input
comes from smaller rivers and land runoff (5%), sew-
age treatment effluent carried by rivers (4%), and pre-
cipitation (13%) (Ries, 1990, as cited in Desbonnet and
Lee, 1991). Flushing time of the bay is primarily con-
trolled by fresh water input rates, and varies from 10 to
40 days with a mean time of 26 days (Pilson, 1985).
Circulation in the bay is primarily tidally controlled,
though non-tidal circulation induced by winds and
freshwater inputs can exert considerable influence on
shorter temporal and spatial scales (Hicks, 1959; Weis-
berg and Sturges, 1976).
3. Methods
The measurements in this study were made with
Yellow Springs Incorporated (YSI) sensors and
sondes. Instrumented buoys were moored at two
sites: Bullock Reach (BR) and North Prudence (NP)
(Fig. 1). Each buoy consisted of a 1.2 m diameter
foam discus with a water-tight chamber that housed
the data logger, controlling electronics, and 12-V
batteries. A tripod structure on the buoy supported
three solar panels that recharged the batteries. Two
sondes (YSI model 6820 at Bullock Reach and YSI
model 600XL at North Prudence) were suspended
from each buoy at depths 0.5 m below the sea surface
and 1.0 m above the seafloor. The average depths of
the bottom sondes were 8.8 m and 10.3 m for Bullock
Reach and North Prudence respectively. Every 15 min
the sondes measured temperature, conductivity (from
which salinity was computed), dissolved oxygen
(using a rapid pulsed oxygen sensor), pH, and pres-
sure (from which the depth of the sensor was deter-
mined). Chlorophyll fluorescence was measured at the
near-surface depth at both sites. Chlorophyll was
measured at the Bullock Reach site with a YSI chlor-
ophyll fluorescence probe and at the North Prudence
site with a Seapoint Systems fluorometer. Data were
transmitted from the Bullock Reach buoy by radio
every 15 min and from the North Prudence buoy by
cellular telephone modem every 8 h.
During summer months, when biofouling rates were
relatively rapid in Narragansett Bay, the buoys were
serviced at 2-week intervals by swapping the surface
and bottom sondes with clean and recently calibrated
sondes. The sonde temperature sensors were factory
calibrated and did not require further calibration prior
to deployment. The conductivity sensors were cali-
brated using a secondary standard 0.2 Am filtered
coastal seawater with a salinity range typical of Narra-
gansett Bay. The salinity of this secondary standard
was determined using a Guildline Autosal based on
IAPSO standard seawater. The dissolved oxygen sen-
sor was calibrated at the atmospheric partial pressure of
oxygen as recommended by YSI, Inc. In earlier works
with these oxygen sensors, they showed that calibra-
tions were within F0.5% of Winkler titration values
(Kester and Magnuson, 1994; Magnuson and Kester,
1995). The pressure sensors were calibrated at atmo-
spheric pressure with a barometer.
Each 2-week deployment began with recently cali-
brated sensors. At the end of each deployment, the
data were examined to determine if post-deployment
corrections were needed to account for sensor drift or
biofouling. Several independent measures were used
to determine whether corrections were needed. The
sondes removed from the field were re-calibrated in
the laboratory prior to removal of any biofouling.
These post-deployment calibrations provided one
measure of the combined effects of sensor drift and
biofouling. As a second measure the last reading of
the sonde removed from the field and the first reading
of the newly deployed (recently calibrated) sonde
were compared. During each servicing of a buoy, a
recently calibrated sonde was used to measure the
vertical profile of properties at the buoy site. These
profiles were used to obtain information on vertical
gradients at the times when sondes were swapped.
To remove high frequency noise from the data
analysis, the 15-min measurements were digitally fil-
tered to 6-h and hourly values using a 24- and 4-point
filters. Hourly and 6-h filters were applied to the 15-
min data in a manner that eliminated potential phase
shifts between the original data and the filtered data.
4. Results
Hourly values of surface to bottom density differ-
ences and surface and bottom dissolved oxygen at the
two buoy locations (July 1, 2001 to September 30,
2001) are shown in Fig. 2. The variations in surface to
0
2
4
6
8
10
7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30
Bullock Reach North Prudence
0.0
0.2
0.4
0.6
0.8
7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30
Oxy
gen
(mm
ol/L
) .
Surface Bottom Bullock Reach
0.0
0.2
0.4
0.6
0.8
7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30
Oxy
gen
(mm
ol/L
) . Surface BottomNorth Prudence
0.5
0.8
1.1
1.4
1.7
7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30
Tid
al R
ange
(m) .
-10
-5
0
5
10
7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30
N-S
Win
d (m
/s) From the south
From the north
-10
-5
0
5
10
7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30
E-W
Win
d (m
/s) From the west
From the east
0
10
20
30
40
7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30
a.
b.
c.
d.
e.
f.
g.
Riv
er F
low
(m3 /s
) (k
g/m
3 )
∇ σ T
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–10394
Table 1
Summary of 2001 oxygen depletion rates at Bullock Reach and North Prudence Buoy Sites
Start date End date Initial dissolved
oxygen (mmol/L)
Final dissolved
oxygen (mmol/L)
Oxygen depletion
rate (mmol/L/day)
Bullock Reach
7/13/01 5:17 7/19/01 12:47 0.126 0.061 �0.008
8/13/01 8:32 8/17/01 13:32 0.101 0.033 �0.018
8/27/01 21:02 8/29/01 16:02 0.071 0.047 �0.026
9/2/01 4:47 9/4/01 2:32 0.112 0.061 �0.025
9/9/01 9:47 9/10/01 21:32 0.151 0.040 �0.033
9/12/01 5:02 9/14/01 0:02 0.115 0.052 �0.029
Average �0.02
North Prudence
7/13/01 11:47 7/17/01 5:17 0.100 0.037 �0.012
8/13/01 3:17 8/16/01 5:47 0.106 0.042 �0.013
8/28/01 12:17 8/31/01 3:02 0.148 0.045 �0.026
9/1/01 22:32 9/3/01 15:32 0.203 0.160 �0.025
9/6/01 0:32 9/7/01 18:47 0.180 0.115 �0.031
9/11/01 11:02 9/13/01 6:17 0.154 0.093 �0.039
Average �0.024
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–103 95
bottom density difference at the buoy sites reveal
episodic stratification events. The density differences
were primarily driven by changes in salinity. The
surface to bottom density difference at the Bullock
Reach site was greater than at the North Prudence site.
While the amplitudes of events varied with location,
they were nearly always in phase (Fig. 2).
During the summer of 2001 three major episodes
of hypoxia were observed in subsurface waters of
Narragansett Bay (Fig. 2). The first of the low oxygen
events occurred between July 13 to 23 at both the
North Prudence and the Bullock Reach sites (Fig. 2).
During this period, the tidal range was less than 0.9 m,
the winds were below 7 m/s and variable, and the
Blackstone River peaked (11 July, Fig. 2). At the
North Prudence site the dissolved oxygen dropped
below 0.09 mmol/L on 13 July and, based on the
15-min data, reached a low of 0.036 mmol/L on 17
July. The dissolved oxygen at the Bullock Reach site
declined from 13 July to 19 July at a rate of �0.008
mmol/L/day (Table 1), becoming hypoxic on 17 July.
The dissolved oxygen reached a minimum value of
0.043 mmol/L on 22 July. At about this time, fish kills
Fig. 2. The hourly time-series and metrological data: a) surface to bottom
(black), b) dissolved oxygen concentration at Bullock Reach, c) dissolved
RI, e) north–south component of wind from T.F. Green Airport, f) east–we
flow. The diamonds indicate dates of RIDEM dissolved oxygen surveys.
were reported and low oxygen was observed in shal-
low water (~3–4 m) along the western shore of Green-
wich Bay (Deacutis, personal communication, 2001).
At the North Prudence site, the event ended with
bottom oxygen values increasing steadily (19 July–
23 July) from 0.063 mmol/L to 0.19 mmol/L with
salinity and temperature indicating full mixing of the
water column. At the Bullock Reach site, the event
ended with a series of very large tidally driven varia-
tions in oxygen between 21 July and 23 July. The end
of the July event coincided with increased tidal range
during the new moon of July 20. Surface oxygen
values were more variable at the Bullock Reach site,
ranging from 0.22 to 0.53 mmol/L, with the North
Prudence site ranging from 0.19 to 0.35 mmol/L.
The second low oxygen event occurred between 13
August and 1 September (Fig. 2). At the Bullock Reach
site, dissolved oxygen concentrations decreased from
0.13 mmol/L on 13 August to 0.033 mmol/L on 17
August (rate=�0.018 mmol/L/day (Table 1)). The
tidal range at this time was less than 0.9 m, winds
were variable, and the river flow peaked on 14 July.
Based on 15-min readings, concentrations reached a
density difference for North Prudence (grey) and Bullock Reach
oxygen concentration at North Prudence, d) tidal range at Newport,
st component of wind from T.F. Green Airport, g) Blackstone River
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–10396
minimum of 0.021 mmol/L on 18 August. Between 18
and 22 August, tidal range was above 0.9 m, and
dissolved oxygen concentrations ranged tidally
between 0.03 and 0.09 mmol/L. The tidal range
began to fall on 21 August. Dissolved oxygen concen-
tration decreased from 0.103 mmol/L on 22 August to
0.041 mmol/L on 27 August, recovered briefly on 27–
28 August, and decreased to less than 0.06 mmol/L late
on August 28. The dissolved oxygen remained low
until 1 September. At the North Prudence site, hypoxia
occurred between 13 and 17 August, reaching a mini-
mum value of 0.042mmol/L on 16 August. The bottom
oxygen recovered to 0.16 mmol/L by the new moon on
18 August and remained high until 25 August. A short
Fig. 3. The 6-h surface (solid grey) and bottom (solid black) oxygen values
the relationships between bottom water hypoxia and surface phytoplankto
2001. Shaded regions are periods of low tidal range.
hypoxic event at North Prudence was observed
between 29 and 31 August. Surface oxygen values
ranged from 0.14 to 0.44 mmol/L at Bullock Reach
and 0.19 to 0.47 mmol/L at North Prudence.
In September, hypoxia occurred intermittently
from 2 to 14 September at the Bullock Reach site
(Fig. 2). Concentrations reached lows of 0.061 mmol/
L on 4 September, 0.040 mmol/L on 10 September,
and 0.052 mmol/L on 14 September. Surface oxygen
values ranged from 0.16 to 0.42 mmol/L at Bullock
Reach. Bottom water oxygen also declined at the
North Prudence site during this time period, but did
not decline to levels below 0.09 mmol/L for more than
a tidal cycle.
from (a) Bullock Reach and (b) North Prudence buoy data showing
n blooms and tidal range (grey with circles) during the summer of
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–103 97
5. Discussion
5.1. Tidal range and hypoxia
There are several important factors that lead to the
monthly subsurface hypoxic conditions observed
during the summer of 2001. The most important
factors were availability of nutrients in the euphotic
zone to support phytoplankton blooms, meteorologi-
cal conditions that enhanced water column stratifica-
tion (freshwater input and surface heating), and
variations in tidal amplitude. Our time-series data
show that tidal range has a strong influence on the
oxygen content of waters in Narragansett Bay. Fig. 3
shows the tidal range (low–high height difference)
during the summer of 2001, based on tidal predic-
0.0
0.1
0.2
0.3
0.4
0.5
0.6
8/7 8/8 8/9 8/10 8/11 8/12 8/13 8
0.00
0.05
0.10
0.15
8/7 8/8 8/9 8/10 8/11 8/12 8/13 8
0
200
400
600
800
1000
8/7 8/8 8/9 8/10 8/11 8/12 8/13 8
.
a.
b.
c.
Time
Sola
r Rad
iatio
n (W
/m2 )
Chl
orop
hyll
a ( µ
mol
/L)
Oxy
gen
(mm
ol/L
)
Fig. 4. The 15-min time-series data from the Bullock Reach buoy for A
dissolved oxygen values (b) chlorophyll a (Amol/L, molecular weight=89
Laboratory in Newport, RI.
tions at Newport, Rhode Island (which is character-
istic of nearly all portions of Narragansett Bay). The
values shown are the absolute fall (or rise) which
occurs about every 6 h due to the semi-diurnal
nature of tides in the Bay. In 2001, the tidal range
was less than 0.9 m on 11–17 July, 28 July–2
August, 10–16 August, 26–31 August, 7–13 Septem-
ber, and 23–29 September.
The relationship between bottom water hypoxia
and tidal range during the summer of 2001 is illu-
strated in the 6-h oxygen values from the Bullock
Reach and North Prudence buoy data (Fig. 3). Dur-
ing periods of low tidal range (around the neap
tides), there is oxygen depletion in subsurface
waters. A decrease in bottom oxygen concentration
was observed during each of the six neap tidal
/14 8/15 8/16 8/17 8/18 8/19 8/20 8/21
Surface Bottom
/14 8/15 8/16 8/17 8/18 8/19 8/20 8/21
Chlorophyll a
/14 8/15 8/16 8/17 8/18 8/19 8/20 8/21
Solar Radiation
-2001
ugust 7–21, 2001 (a) surface (solid grey) and bottom (solid black)
3.49 g/mol) (c) incoming solar radiation data measured by Eppley
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–10398
events. However, the occurrence of hypoxia in only
the four lower tidal range events indicates that
hypoxia does not occur during all neap tides. During
periods of high tidal range (around spring tides),
bottom oxygen concentrations are restored. Subsur-
face oxygen values were restored during the 20
August spring tide period at the North Prudence
site but not at the Bullock Reach site. This is con-
sistent with our observations that stratification tends
to be stronger at Bullock Reach than at North Pru-
dence, thereby requiring more tidal mixing to restore
oxygen to the bottom water. Dissolved oxygen con-
centrations decreased at rates of �0.008 to �0.033
mmol/L/day at Bullock Reach and at rates of
�0.012 to �0.39 mmol/L/day at North Prudence
during the summer of 2001 (Table 1).
5.2. Tidal range and phytoplankton blooms
There is a strong relationship between light and
photosynthetically produced oxygen. The large in-
creases in chlorophyll a and photosynthetically pro-
duced oxygen observed during clear sunny days are
absent during cloudy conditions (Fig. 4). The major
controlling variable for surface oxygen values is solar
irradiation. Important secondary variables causing
lower surface oxygen concentrations at high tide and
higher levels at low tide, are the horizontal oxygen
gradient within the Providence River and the tidal
excursion. The influence of these factors is much
smaller that the influence of solar irradiation. A
power spectral density plot of the Bullock Reach sur-
face water oxygen concentration shows that the semi-
0
1000
2000
3000
4000
5000
6000
7000
8000
0.0 0.5 1.0Frequency (c
Pow
er S
pect
ral D
ensi
ty
Fig. 5. Power spectral density of surface di
diurnal tidal frequency (1.932 cycles/day) is of very
small consequence in the oxygen variations, com-
pared to the day–night events (1 cycle/day) and the
lower frequency events such as the components at 8–9
days and longer (Fig. 5). These lower frequency
changes in oxygen correspond to bloom conditions
during which surface oxygen concentrations reach 0.3
mmol/L and greater (Fig. 2).
The periods of high surface oxygen values,
which are indicative of phytoplankton blooms, are
sensitive to water column stratification (Fig. 2).
Blooms occur when stratification is enhanced
(such as during neap tides or increases in freshwater
input). Blooms terminate during periods of increased
vertical mixing caused by spring tides, surface cool-
ing, or storm-induced wind mixing. Fig. 3 shows
evidence that changes in tidal range also affect the
surface oxygen concentration. At the North Pru-
dence buoy, high levels of photosynthetically pro-
duced oxygen were observed when the tidal range
was less than 0.9 m on 11–17 July, 27 July–2
August, 10–16 August, 27 August–2 September,
11–16 September and 23–26 September. Photosyn-
thetic blooms tend to occur during periods of low
tidal range (i.e., neap tides) while, conversely,
blooms are disrupted or cannot be sustained during
high tidal ranges (spring tides).
5.3. Restoration of bottom oxygen
Restoration of oxygen to bottom waters can occur
by two processes: downward mixing of euphotic
water or lateral advection of oxygen-rich bottom
1.5 2.0 2.5ycles per day)
BR Surface DO
ssolved oxygen concentrations at BR.
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–103 99
water from the southern portion of the Bay. From 22
to 25 July, bottom oxygen concentrations increased
with each tidal cycle, in phase with tidal height (Fig.
2). The large tidal pulses imply a large lateral oxygen
gradient over the tidal excursion distance (about 3
km). The change in surface to bottom density shows
(Fig. 2) that large spikes in dissolved oxygen concen-
tration occur during times when the surface to bottom
density differences are smallest (i.e., when the water
column is vertically mixed). The bottom dissolved
oxygen concentrations during the well-mixed periods
reached values as high as 0.27 mmol/L (Fig. 2). Sur-
face dissolved oxygen concentration ranged from 0.16
to 0.43 mmol/L at the BR site, whereas surface dis-
solved oxygen concentrations further south at the NP
site barely reached over 0.22 mmol/L in the same
period (Fig. 2). Therefore, it appears that both lateral
advection and vertical mixing restore oxygen to the
bottom waters at BR.
5.4. Predictions for hypoxia
The observed sensitivity of surface phytoplankton
blooms and subsurface hypoxia to tidal range in the
upper and mid-portions of Narragansett Bay is a sig-
nificant finding. While a number of factors influence
the occurrence of blooms and bottom water oxygen
depletion, tidal mixing is especially important during
the summer months. Other factors required to support
a phytoplankton bloom include availability of nutri-
ents and sunlight. Factors important for subsurface
oxygen depletion include surface salinity decreases
due to rainfall and river flows, and surface warming
due to solar irradiance. Hypoxia is, conversely, alle-
viated or eliminated by rapid surface cooling and wind
mixing events.
Despite a variety of influences on oxygen levels
in Narragansett Bay, observation in the present study
show that bloom and hypoxic events can be pre-
dicted. Fig. 6 shows the predicted tidal ranges at
Newport for the months of June through September
for 2002, 2003, and 2004. Based on the results for
2001, the critical events occurred when the tidal
range was less than 0.9 m over a period of 5–7
days. In 2002 the first of such periods occurred
from 30 June to 7 July and the second such period
was from 29 July to 6 August. During both of these
periods hypoxic conditions were observed at the
Bullock Reach and North Prudence buoy sites (Ber-
gondo, 2004). During the early August event the
subsurface oxygen concentration at the North Pru-
dence site reached 0.021 mmol/L, lower than any
value seen in 2001. The hypoxia was eliminated by
6 August when north winds completely mixed most
of the Bay. The critical periods for low oxygen in
2003 were 20–26 June, 20–27 July and 18–25
August. Hypoxia was observed at the NP site during
these periods (Narragansett Bay Window Collabora-
tive, unpublished data). On 20 August, a very large
fish kill was reported in Greenwich Bay (RIDEM,
2003). Projecting ahead to the summer of 2004,
exceptionally weak tidal ranges during the periods
of 7–14 August and 5–11 September were expected
to enhance water column stratification to an even
greater extent than was the case in 2001, 2002 and
2003 (compare Figs. 3 and 6). The 2004 dissolved
oxygen data from the North Prudence site showed
hypoxic events occurring around 11 August and 9
September (Narragansett Bay Window Collaborative,
unpublished data).
5.5. Past dissolved oxygen surveys
Are summer hypoxic events a recent occurrence in
upper Narragansett Bay? Low bottom water dissolved
oxygen concentrations were reported in the Seekonk
and Providence Rivers during surveys in 1923, 1947,
1955, 1959, 1983, and 1987 (Desbonnet and Lee,
1991). Summertime surveys of the Seekonk and Pro-
vidence in 1947, 1955 and 1987 showed increases in
dissolved oxygen over time, and also with distance
from the head of the Seekonk River (Desbonnet and
Lee, 1991). Assuming that the onset of hypoxia in
Narragansett Bay is as closely linked to tidal amplitude
as was observed in the 2001 time-series data, it is
interesting to relate tidal amplitude to past surveys
south of Conimicut Point (Table 2). During the summer
of 1959, the weak neap tide occurred during 27 June–3
July and 27 July–1 August. Low oxygen conditions
were observed on the 28, 29, 30, and 31 July surveys. In
the summer of 1980, four surveys occurred during the
weak tidal cycle, 20 June, 18 July, 15 August and 12
September; however, low oxygen concentrations were
not observed. In the summer of 1983, Granger (1994)
found 0.031–0.02 mmol/L oxygen concentrations in
the northern half of the Providence River; 0.062–0.16
Fig. 6. Relative tidal range predictions at Newport for (a) 2002, (b) 2003, and (c) 2004.
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–103100
mmol/L in the southern portion of the Providence
River; and 0.13–0.19 mmol/L in the upper Bay near
the present NP buoy site. These observations were
based on oxygen profiles taken from a small boat
about twice per month. Of the four surveys between
mid-June and mid-August 1983, three occurred near
times of high tidal range (N1.6 m), and one occurred
just prior to weak neap tide period (b0.8 m). The four
SINBADD cruises took place 21–24 October 1985,
18–21 November 1985, 7–10 April 1986 and 19–22
May 1986, times when low oxygen conditions were not
likely to occur. Another relevant historical data were
reported by Doering et al. (1988a,b) for the six SPRAY
surveys over an annual cycle in 1987–88. They also did
not observe hypoxia to the extent seen in 2001. The
automated time-series measurements reported in this
study obviously provide a more comprehensive record
of hypoxic events than can be obtained with occasional
boat surveys. However, properly timed surveys, such
as those reported by Deacutis et al. (in press), are of
great value in determining the spatial pattern and extent
of hypoxic conditions. In the eleven surveys conducted
prior to 1999 during periods when hypoxia was likely
to occur, low oxygen concentrations were only
Table 2
Previous dissolved oxygen studies in Narragansett Bay
Year Date Dates of low
tidal range
Likelya Observed
1959 29 Jun–3 Julb 27 Jun–4 Jul Y N
1959 13 Jul–17 Julb N N
1959 27 Jul–31 Julb 25 Jul–2 Aug Y Y
1959 11 Aug–13 Augb N N
1972 20–21 Junc 17 Jun–26 Jun Y N
1972 26–27 Julc 16 Jul–24 Jul Y Y
1980 6–Jund N N
1980 20–Jund 17 Jun–26 Jun Y N
1980 7–Juld N N
1980 18–Juld 17 Jul–25 Jul Y N
1980 15–Augd 15 Aug–23 Aug N N
1980 12–Sepd 14 Sep–21 Sep N N
1982 16–Sepe N N
1982 30–Sepe 21 Sep–30 Sep Y N
1982 15–Octe N N
1982 23–Nove N N
1982 10–Dece N N
1983 10–Dece N N
1983 24–Maye N N
1983 16–June N N
1983 1–Jule 28 Jun–7 Jul Y N
1983 14–Jule N N
1983 11–Auge 16 Aug–22 Aug N N
1983 28 Sep–3 Octe 26 Sep–2 Oct Y N
1985 21–24 Octf N N
1985 18–21 Novf N N
1986 7–10 Aprf N N
1986 19–22 Mayf N N
1986 11–Octg N N
1986 15–Decg N N
1987 11–Marg N N
1987 22–Aprg N N
1987 27–Jung 28 Jun–7 Jul N N
1987 12–Augg N N
1987 19–20 Augg 15 Aug–19 Aug Y N
1989 7–Seph 4 Sep–12 Sep Y N
1999 27 Jul–30 Juli 19 Jul–27 July Y Y
a The likelihood of hypoxia occurring is determined based on a
survey being conducted during a summer month (June–September)
and when tidal range is less than 0.9 m (Y=Yes, N=No).b US Army Corp of Engineers (1960).c Olsen and Lee (1979).d Oviatt (1980).e Granger (1994).f Pilson and Hunt (1989)Hunt et al. (1987).g Doering et al. (1988ab).h Doering et al. (1990).i North Prudence Buoy, unpublished data.
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–103 101
observed twice. It is likely that if hypoxia in the upper
Bay had been as extensive and extreme as was
observed in 2001–2003, it would have been detected
and reported in studies of the Bay prior to the mid-
1990s.
6. Summary
The occurrence of neap tidal subsurface hypoxic
events only in summer is likely caused by several
factors. One factor is stronger stratification of the
water column during summer and weaker sustained
winds, typically out of the southwest, versus strong
north to northeast winds typical of other seasons
(Magnuson, 1997). These seasonal changes in wind
direction may also affect the estuarine circulation and
flushing rate of the bay (Kincaid, personal commu-
nication). Another factor is accelerated rates of
organic matter decomposition by bacteria under
warm conditions. Summer decomposition rates are
sufficient to cause hypoxia during weeklong periods
of reduced vertical mixing (Nixon et al., 1976). At the
North Prudence buoy site, the spring tide mixing
events in 2001 were sufficient to restore the subsur-
face oxygen to values of 0.13 mmol/L and greater.
During the July and September events at the Bullock
Reach buoy site, this was also the case; however,
during the August hypoxic event, the strong spring
tide did not restore the subsurface oxygen. This was
due to stronger density-driven water column stratifi-
cation in this area of the Bay. These results indicate
that there is a delicate balance between oxygen con-
sumption and renewal in the upper portions of Narra-
gansett Bay, and particularly in the Providence River.
Increases in water column nutrients, organic matter, or
other oxygen-consuming substances could shift this
delicate balance from its current state of intermittent
hypoxic to a state of prevalent anoxia, with large
effects on water quality and survivability of fish and
other marine organisms.
Acknowledgements
This paper is dedicated in memory of Dana R.
Kester. We would like to thank Donald Pryor for
compiling the information on past dissolved oxygen
surveys in Table 2. We are grateful to Donald Pryor,
Christopher Deacutis, Warren Prell, Mimi Fox, and
Candace Oviatt for their guidance and support in
D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–103102
preparing this paper, for their discussions on hypoxia
and their helpful suggestions for improving the manu-
script. The thoughtful comments of Bob Byrne and
two anonymous reviews substantially improved the
manuscript. Support for this work came from the
NOAA CMER and EPA EMPACT programs.
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