Journal of Paleolimnology21: 215–233, 1999. 215c 1999Kluwer Academic Publishers. Printed in the Netherlands.

A multi-proxy trophic state reconstruction for shallow Orange Lake, Florida,USA: possible influence of macrophytes on limnetic nutrient concentrations

Mark Brenner1, Thomas J. Whitmore1, Margaret A. Lasi2, Jaye E. Cable1* &Peter H. Cable3**1Department of Fisheries and Aquatic Sciences, University of Florida, 7922 NW 71st Street, Gainesville, FL32653, USA (E-mail: Brenner@nervm.nerdc.ufl.edu);2St. Johns River Water Management District, PO Box 1429,Palatka, FL 32178, USA;3Department of Oceanography, Florida State University, Tallahassee, FL 32306, USA.*Present address: Coastal Ecology Institute, Department of Oceanography and Coastal Sciences, Louisiana StateUniversity, South Stadium Road, Baton Rouge, LA 70803, USA; **Present address: 310 Nassau Drive, BatonRouge, LA 70815, USA

Received 4 September 1997; accepted 14 March 1998

Key words:diatoms, Florida, geochemistry,210Pb dating, macrophytes, nutrients, paleolimnology, sediments,shallow lakes

Abstract

We retrieved four sediment cores from shallow, eutrophic, macrophyte-dominated Orange Lake (A = 51.4 km2,zmax < 5 m, zmean < 2 m), north-central Florida, USA. The210Pb-dated profiles were used to evaluate spatial andtemporal patterns of bulk sediment and nutrient accumulation in the limnetic zone and to infer historical changesin lake trophic state. Bulk density, organic matter, total carbon, total nitrogen, total phosphorus and non-apatiteinorganic phosphorus (NAIP) concentrations displayed stratigraphic similarities among three of four cores, as didaccumulation rates of bulk sediment, organic matter and nutrients. Accumulation rates were slower at the fourthsite. Nutrients showed generally increasing rates of accumulation since the turn of the century. Percentages ofperiphytic diatom taxa increased progressively in the cores after� 1930. Diatom-inferred limnetic total P trendswere similar among profiles. Eutrophic conditions were inferred for the period prior to the turn of the century. Thelake was hypereutrophic in the early decades of the 1900s, but inferred limnetic total P values declined after� 1930.Declining inferred limnetic total P trends for the last 60–70 years were accompanied by concomitant increases inaccumulation rates of total P and NAIP on the lake bottom. Several lines of evidence suggest that after� 1930,phosphorus entering Orange Lake was increasingly utilized by submersed macrophytes. Paleolimnological recordsfrom Orange Lake highlight the importance of using multiple sediment variables to infer past trophic state andsuggest that aquatic macrophytes can play a role in regulating water-column nutrient concentrations in shallow,warm-temperate lakes.

Introduction

The state of Florida, in the southeastern USA, possess-es some 8000 waterbodies, making it one of the richestlake districts in the world (Brenner et al., 1990). Lakesin the state are increasingly subject to human impacts,including residential development, urbanization, roadand railroad construction, ranching, citrus agriculture,row crop production and phosphate mining. Much ofthe anthropogenic influence has occurred during the

20th century. Census information indicates the statewas home to about a half million inhabitants at the turnof the century (Marth and Marth, 1990). Today, thereare more than 14� 106 residents in Florida.

There has been increasing concern that rapid pop-ulation growth and associated human activities havehad negative impacts on Florida’s surface waters. Highnutrient concentrations in some lakes are blamed onriparian human disturbances.Nevertheless, insufficientlimnological records exist to establish baseline, predis-

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turbance water quality conditions that could be used todocument cultural eutrophication. Earliest water qual-ity data for Florida lakes were collected in the late1960s (Huber et al., 1982), by which time many water-bodies demonstrated nutrient-rich conditions. Eventoday, despite concerted sampling efforts by severalstate agencies and university programs, water qualityanalyses have been completed on only about 10% ofthe state’s lakes (Florida Lakewatch, 1996).

Because long-term limnological data are lack-ing for Florida lakes, paleolimnological methods areincreasingly relied upon to provide insights into histor-ical trophic state conditions.Over the past two decades,we have explored quantitative and qualitative methodsto infer historical trophic state in Florida’s shallow,warm temperate and subtropical lakes. We have testedseveral geochemical and biological sediment variablesas indicators of human disturbance or lake trophic sta-tus. They include concentrations, ratios and accumula-tion rates of nutrients and pigments (Binford & Bren-ner, 1986; Brenner & Binford, 1988), concentrations,accumulation rates and relative abundances of diatoms(Whitmore, 1989, 1991), carbon isotope ratios (Bren-ner et al., 1995), and recent increases in226Ra activity(Brenner et al., 1997). The most reliable trophic statereconstructions are based on multiple sediment proxies(e.g. Brenner et al., 1993, 1995, 1996).

Study site

Orange Lake is a relatively large (A = 51.4 km2), shal-low (zmax < 5 m, zmean < 2 m) waterbody locatedat 29�27’20” N, 82�10’20” W, in Alachua and Mar-ion counties, north-central Florida, USA (Figure 1).The lake lies in the Alachua Prairies subdivision ofthe Northern Peninsula Plains division of the OcalaUplift District (Brooks, 1981). The Orange Lake sub-basin covers some 356 km2and is dominated by ter-race deposits of surface sand over clayey sand andclay. Basal deposits consist of preglacial Pleistocenelimestone.

Orange Lake receives surface hydrologic input atnorthern and northeastern sites from the Newnans andLochloosa subbasins via the River Styx and CrossCreek canal, respectively (Figure 1). The River Styxoriginally drained swamps and sloughs north of OrangeLake. Additional water was added to the river in the1920s when flow from Prairie Creek to Paynes Prairie,which lies northwest of the lake, was diverted fromthe prairie by construction of the Camps Canal andlevee system. Hydrologic diversion was undertaken to

convert the wet prairie to rangeland. Some flow fromPrairie Creek to Paynes Prairie was restored in 1975,when the Camps Canal levee was breached. Waterleaves Orange Lake via Orange Creek near Highway301, at the southeastern edge of the lake (Figure 1).Outflow passes over a weir that was constructed in 1963and remained at 16.92 m NGVD until 1990, when itwas illegally filled and raised to 17.40 m NGVD. Thelake also loses water to evaporation and via downwardleakage into sinkholes located in the southwestern partof the basin.

Orange Lake displays short-term (5–10 year) water-level fluctuations. Stage data for the last 50 years showa maximum excursion of� 3 m, but sub-decadal lakestage changes of� 1 m have been recorded routine-ly. Maximum water levels were measured in the mid-1940s and mid-1960s, and minimum lake level wasrecorded during a dry period in the mid-1950s.Becausethe waterbody is shallow (Figure 1),stage changes alterlake morphometry and surface area is reduced substan-tially during periods of low water (Colle et al., 1987).

Between 1967 and 1981, water quality datawere collected in Orange Lake by several agencies.Data were compiled in the Florida Lakes Data Base(FLADAB) by Huber et al. (1982) and summarizedin a survey of Florida lake water quality and sedimentcharacteristics (Brenner & Binford,1988). Median val-ues for limnological variables in the FLADAB indicatethat in recent decades Orange Lake has been circum-neutral, fresh, and moderately eutrophic (pH = 7.2,conductivity = 67�S cm�1, alkalinity = 18 mg L�1,Secchi depth = 0.80 m, total nitrogen = 1110�g L�1,total phosphorus = 40�g L�1, and chlorophylla =17 �g L�1). Since 1993, the Florida Lakewatch Pro-gram has collected monthly water samples in OrangeLake. For the period 1993–1995,mean values for waterquality variables were consistent with earlier findings(Secchi depth = 1.25 m, total nitrogen = 1150�g L�1,total phosphorus = 30�g L�1, and chlorophylla =18�g L�1; Florida Lakewatch, 1995).

Orange Lake is used primarily for boating, fishingand duck hunting. Since the turn of the century, severalhuman activities in the watershed may have influencedlake water quality, including road and railroad build-ing, residential development and agriculture. The lakehas also been influenced by hydrologic manipulation,introduction of exotic plants (e.g.Hydrilla andEich-hornia) and weed control measures. Although recenthuman activities were assumed to have affected OrangeLake’s water quality, the assumption could not be veri-

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Figure 1. Bathymetric map of Orange Lake showing locations of overland inflows and outflows, and sites 1, 2, 3, and 4, where sediment-waterinterface cores were collected. Solid black lines are roads. Insets show the Orange Creek Basin watershed with locations of other lakes mentionedin the text, and the location of the Orange Creek Basin in North- central Florida.

fied using limnological information because long-termdata were lacking.

We studied short (� 1.3 m),210Pb-dated sedimentcores to infer past trophic state conditions in OrangeLake and evaluate recent changes in water quality. Themajor objectives of our investigation were four-fold.First, we studied cores from four sites in Orange Laketo evaluate spatial differences in sediment stratigraphyand accumulation.Second, at each site we used nutrientconcentrations and ratios in sediments to assess recentchanges in the ecosystem. Third, we estimated tempo-ral changes in the rates of nutrient sequestering on thelake bottom at roughly decadal intervals over the lastcentury to estimate historical shifts in nutrient load-ing. Fourth, we used sedimented diatom assemblagesto infer historical water quality (i.e. total limnetic Pconcentrations) trends in Orange Lake during the last� 100–150 years. Multiple sediment indicators of pastwater quality from four cores were used to evaluate thereplicability of trophic state reconstructions at widely-spaced sites in this shallow, macrophyte-dominated,wind-stressed lake.

Field methods

We identified potential coring stations at four widely-spaced sites in the limnetic zone of Orange Lake (Fig-ure 1). To prevent retrieval of truncated sections, wesought coring sites with at least 1 m of accumulatedsediment. At each station, we anchored and determinedlatitude and longitude with a global positioning system(GPS). Water depth was estimated by lowering a Sec-chi disk on a metered rope to the lake bottom. Next,we measured sediment thickness by forcing calibrat-ed metal rods through the soft, organic sediment lensuntil they contacted hard bottom and then recorded thedepth from the water surface to hard bottom. Thicknessof the soft sediment lens was calculated by subtractingthe water depth from the total depth from the watersurface to hard bottom. Once an appropriate stationwas identified, we moved to the other end of the boatand collected a core from an undisturbed site.

Sediment cores were taken at each of the four siteswith a sediment-water interface piston corer that pos-sesses a clear, polycarbonate core barrel (Fisher et al.,1992). Cores were collected on 3 and 16 April, 1996

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and ranged in length from 136–148 cm. All possessedfine-grained organic sediments overlying more consol-idated, brown, peaty deposits. Profiles were assignednumbers based on the date of retrieval and the site num-ber (day-month-year-site). Cores are referred to by sitenumber in the text (see Figure 1).

Retrieved cores were extruded in a vertical positionon the boat to prevent mixing of unconsolidated upper-most deposits. Profiles were sectioned at 4-cm intervalsand extruded sediments were placed in preweighed,labelled polyethylene containers for transport to thelaboratory. During the extrusion process, subsamples(1–2 cm3) were removed from the top of each discrete4-cm interval for diatom analyses. Sediment samplesfor diatom enumeration were placed in labelled 20-mLplastic scintillation vials and fixed with alcohol.

Laboratory methods

Sediment chronology

Each preweighed polyethylene container with its wetsediment sample was weighed on a FisherTM S-400electronic balance. Wet sample mass was calculated bysubtracting the container weight (tare) from the con-tainer plus wet sample weight. Next, open containerswere placed in a VirtisTM Unitrap II freeze drier for�1 week. Containers with the dry samples were removedand weighed again. Dry mass was calculated by sub-tracting the container weight from the container plusdry sample weight. Percent dry mass was computed asdry mass/wet mass� 100. Dry sediments were groundwith a mortar and pestle.

In order to210Pb date cores, gamma spectrometrywas used to measure radioisotope activities in sedi-ments (Appleby et al., 1986; Schelske et al., 1994).Activities in cores from sites 1, 2, and 3 were mea-sured using a Canberra well-type germanium detec-tor. The core from site 4 was counted on an EG&GOrtecTM GWL high purity germanium well detec-tor. Prior to radiometric analysis, dry sediment wasadded to preweighed sample tubes and sediment masswas determined by reweighing. Tubes were filled tothe same height with sediment to standardize samplegeometry. Samples were sealed in the tubes with epoxyglue and permitted to equilibrate for at least 2–3 weeksto allow 214Bi to come into equilibrium within situ226Ra. In Florida lakes, supported210Pb activities canbe both high and variable over the lengths of cores,and supported210Pb activities must be measured direct-

ly as opposed to estimated by the downcore, asymp-totic total 210Pb activity (Brenner et al., 1994, 1995,1997; Schelske et al., 1994). Supported210Pb activity,expressed here as226Ra activity, was estimated by aver-aging the activities of214Pb (295.1 keV and 351.9 keV)and214Bi (609.3 keV). Because226Ra varied over thedatable portion of the cores, unsupported210Pb activitywas calculated by subtracting proxy estimates of sup-ported210Pb from the total210Pb activities (46.5 keV)on a level-by-level basis.137Cs activity was measuredby the 661.7 keV photopeak in an effort to identify theperiod of maximum fallout from atmospheric nuclearweapons testing (Krishnaswami & Lal, 1978) and cor-roborate210Pb dates.

Sediment age/depth relations were calculated usingthe c.r.s. (constant rate of supply) model (Appleby &Oldfield, 1983), which is the model of choice whenchanges in sediment accumulation rate are suspect-ed (Oldfield & Appleby, 1984; Binford & Brenner,1986). Calculated sediment dates correspond to thebase of each 4-cm section, and ages are expressed rel-ative to the date when the core was collected, i.e. April1996. Counting errors were estimated by first-orderapproximation, assuming that gamma disintegrationsare described by a Poisson distribution (Knoll, 1989).Age errors, expressed as one standard deviation aboutthe age were propagated by incorporating error asso-ciated with total210Pb and226Ra counts, backgroundcounts, and detector efficiency (Schelske et al., 1994).

Bulk sediment accumulation rates (g cm�2 yr�1)were computed from output of the c.r.s. model (Apple-by & Oldfield, 1983) and represent the mass of sedi-ment deposited in each 4-cm interval (g cm�2) dividedby the time represented in the interval (yr). Accumula-tion rates (mg cm�2 yr�1) of organic matter, total car-bon, total nitrogen, total phosphorus and non-apatiteinorganic phosphorus (NAIP) were computed for eachstratigraphic interval by multiplying the bulk sedimentaccumulation rate (g cm�2 yr�1) by the correspondingconcentration (mg g�1) of each constituent in the bulksediment.

Physical and chemical variables

Organic matter content was estimated by weight loss-on-ignition (LOI) at 550� (Hakanson & Jansson,1983).Sediment bulk density (g dry cm�3 wet) was calculat-ed from proportion dry/wet mass and organic/inorganiccontent, using the formula of Binford (1990). Total car-bon and total nitrogen were determined with a Carlo-ErbaTM NA 1500 C/N/S analyzer. Total phosphorus

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Table 1. Orange Lake core designations, water depth, soft sediment thickness, retrievedcore lengths, and locations of core sites as determined by Global Positioning System(GPS)

Water Soft

depth sediment Core length Latitude Longitude

Core (cm) (cm) (cm) (N) (W)

3-IV-96-1 265 140 136 29�26’42.5” 82�10’21.7”

3-IV-96-2 290 125 148 29�27’40.5” 82�10’27.4”

16-IV-96-3 310 335 148 29�28’12.3” 82�11’15.4”

16-IV-96-4 260 140 136 29�29’30.2” 82�12’23.1”

was measured using a TechniconTM Autoanalyzer IIwith a single-channel colorimeter, following digestionwith H2SO4 and K2S2O8. NAIP was measured by auto-analyzer according to the methods of Schelske et al.(1986). Organic matter, C, N, P and NAIP content insediments are expressed as amount per unit dry mass.

Diatom analyses

Diatom assemblages were analyzed at 15 stratigraphiclevels in each core from stations 1, 2, and 3, and at eightdepths in the core from site 4. Samples were digest-ed in hydrogen peroxide and potassium dichromateaccording to Van der Werff (1955) and slides were pre-pared with HyraxTM mounting medium.Diatoms werecounted at 1500� magnification and identified usingstandard floras (e.g. Hustedt 1930–1966; Patrick &Reimer, 1966–1975). At least 500 valves were count-ed and identified at each level.

We inferred past limnetic total phosphorus valuesusing a transfer function that relates nutrient concen-trations in lake water to diatom assemblages in surfacesediments from 47 phosphorus-limited (N/P> 30) andnutrient-balanced (N/P = 10–30) Florida lakes (Huberet al., 1982; Brenner et al., 1993, 1995). Lakes in thecalibration data set represent a wide range of troph-ic state conditions, from ultraoligotrophic to hypereu-trophic. The transfer function is a simple linear regres-sion equation (r2 = 0.81, P< 0.0001) that relates theTROPH1 diatom index (Whitmore,1989) to the depen-dent, water-column variable, total P. It takes the form:Log10 total P = –1.795 + 0.973 (Log10TROPH1) (Bren-ner et al., 1996). The TROPH1 diatom index is basical-ly a ratio of percentages of diatoms found principallyin high trophic-state waters to those found principallyin low trophic-state waters. Unlike multiple regressionmodels based on taxa, the linear regression model usedto predict total P in this study is insensitive to pres-

ence/absence of particular species and is not subject tocovariant pH effects (Whitmore, 1989).

Results

Typical of many Florida lakes, Orange Lake displaysmodest total thickness of soft sediment on the basinbottom. At sites 1, 2, and 4, measured soft sedimentthickness ranged from 125–140 cm,and cores retrievedat the sites effectively sampled the entire soft sedimentlens (Table 1). Sediments may be focused to site 3,where about 335 cm of soft sediment had accumulated.

Cores from sites 1, 3, and 4 displayed general-ly higher bulk density (g dry/cc wet) with increasingdepth in the sediment (Figure 2), reflecting greatercompaction of unconsolidated organic deposits overtime. The core from site 2 showed a similar trend,but also possessed several deep stratigraphic levelswith high bulk density, indicating sandy horizons (Fig-ure 2a).

Topmost deposits in Orange Lake cores are organic-rich, with LOI > 70% (Figure 2b). Organic mattercontent generally declines below 50 cm depth in theprofiles. Variable LOI at depth is related to changingsand content. Total carbon and nitrogen concentrationsparallel stratigraphic changes in organic matter content(Figures 2c & 2d), suggesting most C and N is bound inorganic form. Molar C/N ratios generally increase withgreater depth in the profiles (Figure 2e). Bottommostdeposits have molar C/N ratios>15, but values declineto about 12 at 50 cm depth in all cores. Above 50 cm,C/N ratios are slightly higher (� 13), particularly incores from sites 1, 2, and 3. Topmost deposits in allfour cores have C/N ratios� 11.5.

Total phosphorus concentrations in surface sed-iments vary little among stations, from 1.47–1.53 mg g�1 (Figure 2f). Total P concentrations in

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Figure 2. Physical and chemical constituents versus depth and age in Orange Lake sediment-water interface cores. (a) Bulk density (g drycm�3 wet); (b) Organic matter (% LOI 550�C); (c) Total carbon; (d) Total nitrogen; (e) Atomic C/N ratio; (f) Total phosphorus; (g) Non-apatiteinorganic phosphorus (NAIP).210Pb dates at 8-cm intervals are presented to the right of each plot.

sediments generally decline with increasing depth inthe profiles. A single high value at 24–28 cm in thecore from site 2, stands out. Values typically rangefrom 0.3–0.6 mg g�1 below 70 cm depth (Figure 2f).Decline in total P concentration with increasing depthoccurs principally in the topmost 50 cm of the cores,corresponding to the last 40–100 years. NAIP con-centrations are also higher in uppermost deposits and

generally decline with increasing depth (Figure 2g).Total P and NAIP concentrations are highly correlatedin cores 1 through 4 (r = 0.96, 0.84, 0.92, and 0.86,respectively), and the high total P value at 24–28 cmin the core from site 2 is associated with a high NAIPconcentration.

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Figure 2. Continued.

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Sediment dating and material accumulation rates

Cores 1, 2 and 3 displayed similar radioisotope profiles(Figure 3a), with supported210Pb activity (i.e.226Ra)reached at 76–80 cm. Total residual unsupported210Pbactivities measured at core sites 1, 2, and 3 were 38.1,40.0 and 38.5 dpm cm�2, respectively, equivalent to210Pb fallout rates of 1.19, 1.25 and 1.20 dpm cm�2

yr�1. Supported210Pb activity was reached at 52 cmin core 4 (Figure 3a). Total residual unsupported210Pbactivity at site 4 was 18.1 dpm cm�2, reflecting a210Pbfallout rate of 0.56 dpm cm�2 yr�1.

Radium-226 activities (i.e. supported210Pb activi-ties) were< 5 dpm g�1 in all four cores (Figure 3a).226Ra activity, nevertheless, declined with increasingdepth, particularly in Orange Lake cores 1, 2, and 3.The pattern is typical of many Florida lake sedimentprofiles (Brenner et al., 1997) and necessitated subtrac-tion of supported210Pb activity from total210Pb activ-ity on a level-by-level basis, to estimate unsupport-ed activity. Cesium-137 activities in all Orange Lakecores were< 5.1 dpm g�1 and did not display a defin-itive ‘bomb’ peak (Figure 3a).137Cs activity at depthin the Orange Lake profiles suggests postdepositionaldownward mobility of the highly soluble radioisotope.137Cs fails to display a discrete peak in many Floridalake cores (Brenner et al., 1994; Schelske et al., 1994)because, unlike particle-reactive210Pb that binds tight-ly to deposited lake sediment,137Cs can be mobilizedand transported through the sediment column.

Oldest210Pb dates for Orange Lake cores 1 through4, respectively, are 1856 (76 cm), 1845 (80 cm), 1835(80 cm), and 1833 (52 cm). We restrict discussion ofaccumulation rate trends to the post-1900 period fortwo reasons. First, old, low-activity samples yield highcounting errors that translate into large error estimateson both dates (Figure 3b) and accumulation rates. Sec-ond, the c.r.s dating model yields systematically ‘too-old’ dates in the oldest datable portions of cores (Bin-ford, 1990). Errant dates affect calculated accumula-tion rates and interpretations of ecosystem changes.

Mean net linear sedimentation rates in Orange Lakewere high and similar at sites 1, 2, and 3 during the lastcentury (0.79, 0.72, and 0.73 cm yr�1, respectively).During the same period, net linear sediment accumula-tion at site 4 was 0.46 cm yr�1. Mean mass accumula-tion rates since the turn of the century were also similarat sites 1, 2, and 3 (32.5, 32.1, and 32.3 mg cm�2 yr�1,respectively). At site 4, mass accumulation over thepast century averaged 17.3 mg cm�2 yr�1.

Since the turn of the century, changes in mass sed-iment accumulation at the four Orange Lake sites dis-play similar trends (Figure 4a). All four sites showedgenerally increasing accumulation over the last� 100years. At sites 1, 2, and 4, the general increase inaccumulation was reversed briefly at times during thesecond half of the century. Site 3 displays more short-term variability in sediment accumulation than sites 1,2, or 4. Site 4 differs from the other sites in displayinggenerally lower rates of bulk sediment accumulation.

Changes in both bulk sediment accumulation rateand sediment composition affect computed constituentaccumulation rates. In the datable portions of the cores,organic matter concentrations varied at most abouttwo-fold (Figure 2b), while bulk sediment accumula-tion rates displayed much greater variation (Figure 4a).Because changes in bulk sediment accumulation ratewere the primary determinant of the trends in organicmatter accumulation, temporal trends in organic matteraccumulation appear similar to shifts in bulk sedimentaccumulation. Short-term variability in organic matteraccumulation was greatest at site 3, and, organic mat-ter accumulation was generally slower at site 4 than atsites 1, 2, and 3 (Figure 4b).

Because total C and N are bound principally in theorganic fraction of Orange Lake sediments, accumula-tion rate trends for carbon and nitrogen in all four coresare similar to those for organicmatter (Figures 4b, 4c &4d). In Orange Lake, phosphorus is unique among thenutrients with respect to accumulation rate trends.Bothtotal P and NAIP display generally increasing rates ofaccumulation through time, with maximum or near-maximum rates measured in modern deposits (Figures4e & 4f). High recent rates of phosphorus accumulationreflect the combined effects of high nutrient concen-trations in uppermost deposits and high rates of bulksediment accumulation during recent times. Total Pand NAIP accumulation has been slower at site 4 thanat sites 1, 2, and 3.

Diatoms

Fifteen species with varying trophic state and lifeformpreferences have dominated the diatom flora in OrangeLake for more than 150 years (Figure 5, Table 2). In thebottommost enumerated samples from sediment corestaken at sites 1, 2 and 3, planktonic taxa represent�

60% of the diatom assemblages, but increase to maxi-mum relative abundance between 60 and 80 cm depth,i.e. around the turn of the century. In the core from site4, planktonic forms are maximal in the bottommost

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Figure 3. (a) Radioisotope (210Pb,226Ra [i.e. supported210Pb], 137Cs) activities versus depth in Orange Lake sediment-water interface cores;(b) Age versus depth in Orange Lake sediment-water interface cores.

counted sample at 56 cm. The most abundant diatomfound in the Orange Lake cores was the planktonic,eutrophic indicatorAulacoseira ambigua(Figure 5).A.ambiguadominated at 64 cm depth in core 3 (� 1921),representing 86% of the assemblage (Figure 5). Therehas been a progressive decline in planktonic represen-tation and a relative increase in periphytic diatom taxasuch asStaurosirella pinnata, Staurosira construens,S. construensvar.venterand var.pumila, Aulacoseiradistans, andAchnanthes minutissimasince� 1930.

Diatom-based total limnetic P inferences show thatOrange Lake has been eutrophic over the time spanrepresented by the sediment cores (Figure 6). Deep-est samples with diatom counts are> 100 years oldand yield eutrophic inferences. Beginning more than100 years ago, water-column nutrient concentrationsgenerally increased through the beginning of the 20thcentury, a trend that persisted until about the 1930s(Figure 6). Much of the increase in inferred limnetictotal P was driven by the abundance ofA. ambigua(Fig-ure 5). Diatom assemblages deposited between about1930 and the present record a progressive decline in

limnetic total P (Figure 6). Cores from sites 1, 2, and3 registered a slight increase in inferred total P dur-ing the 1980s, but minimal, or near-minimal valueswere recorded in modern, surface deposits. Surfacedeposits from site 4 display slightly higher inferredP values than samples deposited during the previousseveral decades.

Diatom-based, mean limnetic total P inferencesfrom surface mud at sites 1–4 were 32, 46, 49, and83�g L�1, respectively. Modern inferences for cores1, 2, and 3 are slightly higher than the mean water-column value of 30�g L�1 reported for 1995 (Flori-da Lakewatch, 1995), and comparable to the medianwater-column P concentration (40�g L�1) reportedin the FLADAB for the 1960s and 1970s (Brenner &Binford, 1988). The diatom-inferred limnetic P valuefor surface sediments from site 4 exceeds recent mea-sures in the water column.The high inferred value fromsite 4 is attributable to large percentages of eutrophicand hypereutrophic indicatorsAulacoseira ambigua,Staurosira construens, andStaurosira construensvar.pumila in surficial deposits, and may be an artifact of

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Figure 4. Bulk sediment and constituent accumulation rates versus date in Orange Lake sediment-water interface cores. (a) Bulk sediment; (b)Organic matter; (c) Total carbon; (d) Total nitrogen; (e) Total phosphorus; (f) Non-anatite inorganic phosphorus (NAIP).

sorting. Nevertheless, diatom-inferred limnetic total Pconcentrations from surface deposits at sites 1–3 aresimilar to recent, direct measures of water-column totalP concentrations, and stratigraphic trends in inferredlimnetic total P are similar among sites. This suggeststhat the diatom-based model yields reasonably accuratereconstructions of changing limnetic nutrient contentin Orange Lake.

Discussion

The four sediment-water interface cores from OrangeLake were taken at sites> 1.5 km apart. Stratigraph-ic similarities suggest they are fairly representative ofsediment deposition in the limnetic zone. Bulk densi-ty (g dry cm�3 wet) generally increases with greaterdepths in the cores, reflecting sediment compaction.Orange Lake surface deposits are high in organic mat-

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Figure 4. Continued.

ter (> 70%) relative to most Florida lakes (mean =39.7%, range = 0.8-84.2%, n = 97; Brenner & Binford,1988). Orange Lake cores display a general decreasein organic matter content with greater depth in the pro-files. Total C and N concentrations parallel the organicmatter curves, suggesting that C and N are bound pri-marily in the organic sediment fraction.

Stratigraphic shifts in the C/N ratio of the sedi-ments probably reflect changes in the source of organic

matter. Uppermost sediments consist largely of algalremains and submersed macrophytes, while deepersediments contain a larger proportion of allochthonousor autochthonous plant material with substantial struc-tural tissue. The average composition of plankton hasan atomic C/N ratio of about 6.6 (Hakanson & Jans-son, 1983; Takamura & Iwakuma, 1991). Many higherplants contain support tissue in the form of celluloseand consequently have higher C/N ratios than algae

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Table 2. Diatom taxa that represent> 5% of the assemblage in at least one sample fromthe Orange Lake cores and their trophic state and lifeform ecological preferences (fromWhitmore, 1989)

Species Trophic state Lifeform preference

preference

Achnanthes lanceolata oligotrophic to mesotrophic periphytic

A. linearis oligotrophic periphytic

Aulacoseira ambigua eutrophic planktonic

A. distans mesotrophic-eutrophic periphytic

A. granulata mesotrophic-eutrophic planktonic

A. granulatavar.angustissima mesotrophic-eutrophic planktonic

A. italica eutrophic planktonic-periphytic

Cyclotella meneghiniana eutrophic planktonic-periphytic

C. stelligera oligotrophic-eutrophic planktonic-periphytic

Staurosira construens eutrophic planktonic-periphytic

S. construensvar.pumila hypereutrophic planktonic-periphytic

S. construensvar.venter mesotrophic-eutrophic planktonic-periphytic

Staurosirella pinnata mesotrophic-hypereutrophic periphytic

Synedra filiformisvar.exilis mesotrophic-eutrophic periphytic

S. rumpensvar familiaris oligotrophic-eutrophic periphytic

(Hakanson & Jansson, 1983; Abbasi et al., 1990; Zim-ba et al., 1993).

Sediments deposited before� 1900 contain a highproportion of plant remains with support tissue,as indi-cated by visual observation of large plant fragments inthe lower halves of the profiles. C/N ratios are> 15at some levels in the bottom halves of the cores andgenerally decline with decreasing depth in the profiles.Moving upward in the profiles from sites 1, 2, and 3,C/N ratios decline to� 12 at about 50 cm depth, cor-responding roughly to the middle of the 20th century.Thereafter, the C/N ratio rises slightly, to about 13.This increase in C/N ratio may reflect an episode ofemergent plant proliferation that was associated withthe 1955–1958 drought and consequent low lake stage.When the dry period ended and lake level rose, largeareas of emergent vegetation were inundated. The briefperiod of higher C/N ratios may reflect the decompo-sition of emergent plants at the core sites. Topmostsediments at all sites possess C/N ratios between 11and 12. These deposits consist largely of fine-grainedorganic matter, dominated by algae and perhaps theremains of submersed plants such asHydrilla andCer-atophyllum, that have little support tissue and low C/Nvalues (Best et al., 1990; Zimba et al. 1993).

Total P and NAIP concentrations increase withdecreasing depth in the profiles. Survey studies havedemonstrated a positive correlation between total Pcontent of surficial lake sediments and water-column P

concentration (Brenner & Binford, 1988) or phospho-rus loading (Sondergaard et al., 1996). Upcore increas-es in the total P concentrationof Orange Lake sedimentprofiles may reflect increasing P loading through time.Alternatively, higher total P concentrations in veryrecent deposits of some lakes have been attributed todiagenetic factors. Sondergaard et al. (1996) suggestthat higher total P in uppermost deposits is, in part,a consequence of high concentrations of temporarilystored, organic-bound P. They argue that, given time,these high total P concentrations will decline as nutri-ents are released to overlying waters. In their studyof Danish lakes, higher residual P (i.e. organic-boundP) persisted over the topmost 10–20 cm of the sed-iment profiles. If Orange Lake sediments are affect-ed to similar depths, then total P concentrations mea-sured in sediments deposited over the last decade maydecline somewhat with the passage of time. It is equal-ly plausible, however, that total P concentrations inthese deposits will increase with the passage of timebecause of preferential diagenetic loss of C and N. Inany event, even if most recent deposits are affectedby diagenetic factors, long-term trends in both total Pand NAIP concentration display unequivocal increasesthrough time.

226Ra activities in Orange Lake sediments (range= 1.02–4.69 dpm g�1) are low compared with val-ues recorded in some Florida lake cores (Brenner etal., 1994, 1995, 1997), suggesting that bedrock near

227

Figure 5. Relative abundance of diatom taxa versus depth in Orange Lake sediment-water interface cores.210Pb dates at 8-cm intervals arepresented to the right of each plot.

Orange Lake contains little radium. At sites 1, 2, and3, supported210Pb activity was reached at 76–80 cmdepth. Supported levels were reached at 52 cm depthin the core from site 4, indicating a slower sedimen-tation rate. All four cores represent at least 140 years(i.e. > 6 half-lives) of sediment accumulation, andbasal deposits from the sections are probably severalcenturies old.

Total residual unsupported210Pb activity differedlittle among stations 1, 2, and 3 (range = 38.1–40.0dpm cm�2) and yielded excess210Pb deposition ratesthat were quite similar (range = 1.19–1.25 dpm cm�2

yr�1). The values are comparable to both the meanvalue of 1.01 dpm cm�2 yr�1 reported for a suite ofFlorida lake sediment cores (Binford & Brenner, 1986)and a global estimate of210Pb flux to the land surface(0.99 dpm cm�2 yr�1) (Krishnaswami & Lal, 1978).The total residual unsupported210Pb value at site 4 waslower, yielding a210Pb fallout rate of 0.56 dpm cm�2

yr�1.Similar total residual, unsupported210Pb accumula-

tions at sites 1–3 suggested similar sedimentation ratesat the three locations. This was confirmed by nearlyidentical 20th century, mean linear sediment accumu-

228

Figure 6. Diatom-based historical limnetic total P inferences versus depth in Orange Lake sediment-water interface cores.210Pb dates at 8-cmintervals are presented to the right of each plot. Plots show mean (solid line) and 95% confidence intervals (dotted lines) for inferred values.

lation rates among sites (0.79, 0.72, and 0.73 cm yr�1,respectively) and mass sediment accumulation rates atthe three sites (32.5, 32.1, and 32.3 mg cm�2 yr�1).Bulk sediment accumulated rapidly in Orange Lake,and rates were consistent with values measured in oth-er hypereutrophic Florida lakes (Brenner et al., 1993,1995, 1996). At northernmost site 4 in Orange Lake,sediment accumulated more slowly during this centu-ry (0.46 cm yr�1, or 17.3 mg cm�2 yr�1), perhapsindicating continual resuspension of sediments in thenorthern part of the lake and redeposition at sites far-ther south. This pattern of sedimentation is consistentwith the north-to-south direction of hydrologic flow inthe basin.

Because sediment accumulated rapidly in OrangeLake, 4-cm sectioning of cores permitted decadal tosub-decadal dating resolution for material depositedsince the mid-1920s. The two southernmost cores (sites1 and 2) show similar patterns of bulk sediment, organ-ic matter, total C and total N accumulation. These vari-ables show general increases in accumulation duringthe first six decades of the century and display relative-ly high values in the 1960s and 1970s. Accumulationrates dropped somewhat in the 1980s and then roseagain to the present.

At core site 3, bulk sediment, organic matter, totalC, and total N display a general increasing trend inaccumulation that peaked in the 1970s and 1980s.Accumulation rates declined somewhat in recent years.

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Site 3 shows more short-term variation in materialaccumulation than sites 1 and 2. Since the turn ofthe century, bulk sediment, organic matter, total C,and total N accumulation rates at site 4 were generallylower than at other sites in the lake.

All four sites display generally increasing total Pand NAIP accumulation over time, with maximumor near-maximum rates of accumulation recorded inrecent years. High computed accumulation rates forthe last decade may, in part, be a diagenetic artifactif organic-bound P has had insufficient time to min-eralize (Sondergaard et al., 1996). This caveat doesnot, however, invalidate the general, long-term trendof increasing phosphorus accumulation through time.

Some site-to-site variation in accumulation ratemeasures may be an artifact of where, stratigraphi-cally (i.e. chronologically), cores were sectioned. Forinstance, if the lake experienced a brief period of highsediment accumulation, lasting< 10 years, the episodemay be largely included in a single 4-cm section fromone core, but may be partitioned between two sectionsin another profile. In the latter case, the impact of thebrief period of rapid sedimentation is ‘diluted’ by beingdivided between two contiguous sediment sections.

Temporal shifts in P accumulation in Orange Lakesuggest that the system received generally increasingP loads throughout the 20th century. Very recent highrates of P sequestering may be a diagenetic artifact.Greater rates of P sequestering since the turn of thecentury, however, probably reflect both higher P load-ing rates to the lake and greater nutrient trapping effi-ciency in an increasingly macrophyte-dominated lake.Greater rates of C and N burial over time may be aconsequence of increased productivity within the lake,greater delivery of allochthonous organic matter, andchanges in the type or preservation of organic matterbeing deposited. Rates of nutrient burial may be relat-ed to both natural (climatic/hydrologic) factors andanthropogenic impacts on the lake.

The Orange Lake diatom community has under-gone long-term changes with respect to lifeform pref-erence. In the oldest part of the records from sites 1,2, and 3, planktonic diatoms dominated, constitutingabout 60% of the sedimented assemblage. By� 1900,the proportion of planktonic taxa had increased to 70–90% of the assemblage. In all four cores, there is evi-dence for a general decline in the percentage of plank-tonic forms during this century, and the assemblagein modern sediments contains only 30–40% plankton-ic taxa. The shifts are due largely to changes in therelative abundance ofAulacoseira ambigua, a species

that is found most often in lakes where wind-generatedsummer turbulence extends to the lake bottom (Brad-bury, 1977). Factors that decrease turbulence, suchas persistent stratification, cyanobacterial presence, ormacrophyte abundance limitAulacoseirapopulationsby causing the heavy frustules to settle out of the watercolumn. Shifts in the relative representation of plank-tonic versus periphytic diatoms in the Orange Lakecores probably reflect changes in lakewater mixing.

Diatom samples collected at 116 cm in cores 1–3show that periphytic taxa were reasonably well rep-resented, constituting nearly 40% of the assemblage.Shortly after the turn of the century, the percentage ofperiphytic diatoms fell to between 10 and 30%, andopen-water conditions are indicated by the prevalenceof planktonicA. ambigua. At site 4, it appears thatA.ambiguaalready dominated the flora well before theturn of the century.

From about 1900–1930, Orange Lake was relative-ly free of macrophytes, and subject to considerablelimnetic water-column mixing. The decline in rela-tive abundance of planktonic taxa over the last� 70years was probably related to conditions that imped-ed wind mixing of the water. Native populations ofcoontail (Ceratophyllum demersum), southern naiad(Najas guadalupensis), and bladderwort (Utriculariaspp.) were found in the lake prior to the 1970s (Shire-man et al., 1983) and occupied as much as 50% ofthe lake area. More recently,Hydrilla infestation hasplayed a significant role in preventing diatom resus-pension, at times causing a loss of up to 95% of theopen-water (Colle et al., 1987) and providing habitatfor periphytic taxa.

Generally low C/N ratios in recent sediments, rel-ative to basal sections of the core, suggest that macro-phyte proliferation within this century was probablydominated by submersed taxa with low amounts ofsupport tissue and low C/N ratio. Prior to the 1970s,submersed taxa such as coontail (C/N ratio = 12.1;Best et al., 1990) and bladderwort (C/N ratio = 11.9;Abbasi et al., 1990) probably limited water mixing.Thereafter,Hydrilla became the principal taxon limit-ing water mixing. Using mean reported C and N valuesfor Hydrilla in Lake Okeechobee, Florida (Zimba et al.,1993), we calculated a molar C/N ratio for the macro-phyte of 11.1, very close to the molar C/N ratio ofsurface sediments in Orange Lake.

Diatom-based limnetic total P inferences in OrangeLake also suggest the waterbody has experienced long-term changes in water quality. Deepest diatom samplesin cores from sites 1, 2, and 3 yield eutrophic infer-

230

ences, but nutrient concentrations in the water col-umn increased even further, reaching maximum valuesaround 1900 (Figure 6). Much of this shift is account-ed for by the prevalence ofA. ambigua, a eutrophicindicator in Florida lakes (Whitmore, 1989). In all fourprofiles, the period of highA. ambiguaabundance com-menced prior to the turn of the century and persisteduntil about the 1930s (Figure 5), when diatoms indicatemean limnetic total P concentrations of� 100–150�gL�1 (Figure 6). The exceptionally high relative abun-dance ofA. ambiguaat 64 cm depth in core 3 (Figure 5)may be a consequence of differential frustule transportto the site, that may have caused an inaccurately highestimate of limnetic total P. Nevertheless, the assem-blage in this sample suggests open water with highlimnetic nutrient concentrations in the early 1920s.

Inferred water-column total P concentrations beganto decline after� 1930. By the 1960s, inferred limneticnutrient concentrations had fallen to levels compara-ble to those measured in the basin today. During the20th century, sedimented P accumulation in OrangeLake yields inferences about changing trends in laketrophic status that are opposite to trends inferred fromdiatom assemblages. That is, phosphorus accumula-tion rates on the lake bottom increased at the sametime that diatom-inferred limnetic total P concentra-tions declined.

We propose several explanations to account forlower inferred limnetic P concentrations in light ofhigher nutrient loading. First, total P and NAIP accu-mulation rates on the lake bottom may have been unre-lated to bioavailable phosphorus loading and in-lake Pconcentrations if a substantial fraction of the recently-sedimented P was delivered to the lake in a particulateform that was unavailable to primary producers. Sec-ond, limnetic total P trends in Orange Lake are inferredfrom diatom assemblages dominated by periphytic andbenthic taxa. In many areas of the world,such as North-ern Ireland (Anderson et al., 1993), Michigan, USA,(Fritz et al., 1993), and British Columbia, Canada(Hall & Smol, 1992), calibration data sets for infer-ring historical trophic state conditions include diatomassemblages that are dominated by planktonic forms.Considering that most of the lakes in these studies wererelatively deep (zmax> 10 m), prevalence of plankton-ic taxa and their potential to predict epilimnetic total Paccurately, is expected.

In a study of 31 shallow, artificial, enriched pondsin southeast England, Bennion (1995) concluded thatcaution should be exercised in generating diatom-basedtransfer functions to predict trophic state using assem-

blages dominated by non-planktonic taxa. In theseshallow waterbodies, factors other than epilimneticnutrient concentrations evidently influenced diatomdistribution. Nevertheless, benthic/periphytic diatomassemblages prove to be strong predictors of troph-ic status in Florida lakes. Florida lakes are generallyshallow relative to natural lakes in temperate regions(Brenner et al., 1990). Lakes in our training data set,for which morphometric data are available, general-ly have mean depths of only� 2.5 m, and they aredominated by benthic and epiphytic taxa, regardless oftrophic state. Whitmore (unpublished data) analyzeddiatoms in surface sediment samples from 57 Floridalakes that displayed a range of limnetic total P from 1–192 �g L�1. Mean benthic/periphytic representationwas 71.9% and only eight lakes had assemblages with> 50% planktonic taxa. The TROPH1 diatom index(Whitmore, 1989) is thus based on assemblages dom-inated by benthic and periphytic forms and is a strongpredictor of limnetic total P (r2 = 0.81).

In three of four cores from Orange Lake, lim-netic total P inferences based on surface sedimentdiatom assemblages are comparable to modern lim-netic total P values, suggesting that the diatom-basedmodel performs well in this waterbody. It is con-ceivable, however, that diatom assemblages in olderOrange Lake deposits overestimate limnetic total P.Sediments deposited around the turn of the century areoverwhelmingly dominated by the eutrophic indicatorA. ambigua, and the very high percentages are outsidethe range of percentages in the calibration data set.

It is probable that limnetic total P concentrationsin Orange Lake did in fact decrease over the last�

60 years, even as nutrient loading increased. Declinesin diatom-inferred limnetic total P over the last fewdecades may be related to increased nutrient uptake byan expanding macrophyte community. Florida lakesthat become densely inhabited by macrophytes dis-play shifts in water-column characteristics that includelower nutrient and Chla concentrations and greaterwater clarity (Canfield et al., 1983a, 1984; Canfield& Jones 1984). Conversely, lakes in which macro-phytes are removed by herbicides or grass carp showincreases in limnetic nutrient concentrations and Chla, as well as a decrease in water clarity (Gasaway &Drda, 1978; Canfield et al., 1983b).Hydrilla influenceon lakewater nutrient and chlorophylla concentrationsbecomes apparent when submersed macrophyte cov-erage exceeds 40% of the lake area (Canfield et al.,1983b; Canfield & Jones, 1984). Long-term data onareal coverage of macrophytes in Orange Lake are not

231

available. Nevertheless, we note that the high rela-tive abundance ofA. ambiguain the early decades ofthe century implies relatively open-water conditions.Macrophytes probably began to expand in Orange Lakeafter 1930. By 1976, 80% of the lake surface was cov-ered byHydrilla (Colle et al., 1987).

In Orange Lake, some of the phosphorus thatentered the lake was utilized or mechanically trappedby macrophytes and delivered to the lake sediment, butdid not contribute to limnetic total P or algal productionin the open water. We inferred a similar, recent declinein limnetic total P for Lake Francis, Highlands County,Florida that was attributable to increasing macrophyteabundance during the last few decades (Brenner et al.,1993). Diatoms in Orange Lake cores record a slight,brief increase in inferred limnetic total P during the1980s that may be related to macrophyte control mea-sures.

Factors that affect water-column mixing and sedi-ment resuspension have influenced limnological con-ditions in Orange Lake since the turn of the century.Many shallow Florida lakes are subject to resuspen-sion of sediments caused by wind mixing of the watercolumn (Carrick et al., 1993; Whitmore et al., 1996).Sediment resuspension, in turn, contributes to elevatedlimnetic nutrient concentrations (Maceina & Soballe,1990; Brenner et al., 1993). When macrophytes arepresent, they not only offer habitat for epiphytic diatomtaxa, but minimize turbulence in the water column andconsequent sediment resuspension by reducing watervelocity and wave action (Dieter, 1990). Physical mix-ing processes affect the diatom community in twoways. First, they determine whether planktonic or peri-phytic forms will dominate by creating conditions thatare advantageous for either continued suspension inthe water column or settling. Second, sediment resus-pension can increase nutrients in the water column, sothat periods of frequent turbulence are characterizedby eutrophic indicators, while quiescent periods favordiatoms typical of lower-nutrient waters. The reduc-tion in planktonic diatom taxa in Orange Lake since� 1930 is a consequence of greater submersed macro-phyte abundance.

Although our paleolimnological data cannot iden-tify the ultimate causes for environmental changes inthe lake, we note that shifts in the diatom communi-ty coincided with completion of several projects thataltered the hydrology of the waterbody (Lasi et al.,1996). Highway 301 was constructed across the outletfrom Orange Lake in 1926, thereby restricting out-flow. Hydrologic inflow was enhanced in 1927 when

Camps Canal was completed, allowing diverted waterfrom eutrophic Newnans Lake to discharge into OrangeLake. Since that time, other hydrologic modificationsincluded installation of a fixed-notch weir at the out-flow in 1963, restoration of some flow from PrairieCreek to Paynes Prairie in 1975, and attempts to restrictoutflow from the sinkholes in the southwestern part ofthe basin. Superimposed upon these human-mediatedhydrologic changes, there have been pronounced cli-matic changes and consequent lake-level fluctuations.

Other anthropogenic activities have also influencedOrange Lake.Hydrilla introduction and biomass accre-tion during the last� 25 years is probably responsiblefor both nutrient removal from the water column andaccelerated nutrient sequestering in sediments, as wellas a shift to dominance of periphytic diatom taxa. Peri-odic efforts to control nuisance weed growth throughmechanical harvesting and herbicide application haveprobably affected sedimentation and indirectly influ-enced the diatom community.

The four sediment cores from Orange Lake provedthat coherent,210Pb-datable records can be obtainedfrom shallow, wind-stressed basins in Florida (seealso Whitmore et al., 1996). Stratigraphic similari-ties among the four sections indicate that single coresfrom the limnetic zone provide representative trendsfor both nutrient sequestering on the lake bottom anddiatom-based historical trophic state inferences. Themulti-proxy approach to reconstructing past trophicconditions elucidated the potential role that submersedmacrophytes can play in affecting open-water nutrientconcentrations in shallow Florida lakes.

Acknowledgments

This study was supported, in part, by the St. JohnsRiver Water Management District. Interpretations andconclusions are not necessarily those of the support-ing agency. Claire Schelske provided logistical supportand commented on the manuscript. Jason Kahne assist-ed fieldwork. William Kenney and Jason Curtis helpedwith laboratory analyses and William Burnett facilitat-ed210Pb dating. Euan Reavie, Phyllis Hansen, WilliamKenney, Christine Taylor, Frederick Aldridge and ananonymous reviewer provided helpful comments. Thispaper is Journal Series Number R-05923 of the FloridaAgricultural Experiment Station.

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