Fall Turnover Research Articles

Photosynthesis, respiration, and stable isotopes of oxygen in a large oligotrophic lake (Lake Erie, USA–Canada)

Dissolved oxygen (DO) concentration, isotopic composition (18:16O), and respiratory fractionation (αr) were measured in the east basin of Lake Erie to determine the patterns of planktonic photosynthesis (P) and respiration (R) and to assess a dual isotope, steady-state model for P/R ratios. Values of αr varied significantly between months but averaged close to a widely accepted norm for planktonic respiration (0.982). Isotope dynamics in situ predicted that 33%–53% of hypolimnetic oxygen consumption was by the sediments, consistent with previous knowledge. Except during fall turnover, DO was generally above, and 18:16O below, atmospheric equilibrium in the surface mixed layer (SML), suggesting P/R is usually >1 and agreeing with independent estimates by incubation experiments. P/R ratios calculated from the oxygen isotope model were often inconsistent with the DO and 18:16O data, likely because of uncertainties in αr and nonsteady-state conditions in the SML. The autumnal transition to P/R < 1 was captured better in the DO and 18:16O data than in the incubation experiments. Despite problems with the steady-state model, the isotope data were able to reveal spatial and temporal variations in lake metabolism and indicated a strong autochthonous basis for community metabolism in the SML.

Organic matter mineralisation in the hypolimnion of an eutrophic Maar lake

Constraints on hypolimnetic methane production in productive lakes are of interest owing to the importance of methane emissions from lakes for the atmospheric methane burden. We studied carbon fluxes and terminal electron accepting processes in the hypolimnion of eutrophic Meerfelder Maar, Germany. Carbon fluxes from epilimnion and sediments were estimated using sediment traps, porewater analysis and incubation techniques. The contribution of various redox processes to overall dissolved inorganic carbon (DIC) production was quantified from a seasonal hypolimnetic mass balance. Carbon sedimentation rates were 22 mmol m−2 d−1 from the epilimnion and 20–34 mmol m−2 d−1 from the hypolimnion. Anaerobic respiration accounted for a DIC production of 21 mmol m−2 d−1, and was thus capable of using a substantial part of primary production. The diffusive C flux from the sediment was small in comparison, although potential respiration rates in incubation experiments reached 63 – 75 mmol m−2 d−1. Ebullition of methane occurred, but could not be quantified. Following depletion of dissolved oxygen (DO) and nitrate, iron and sulphate reduction and methanogenesis proceeded concurrently in the hypolimnion. Estimated hypolimnetic DIC production decreased in the order methanogenesis (2267 mmol m−2) > oxic respiration (880 mmol m−2) > sulphate reduction (575mmolm−2) > denitrification (159 mmol m−2) > acetogenesis (157 mmol m−2) > iron reduction (19 – 144 mmol m−2). Methane production thus dominated respiration and was not inhibited by sulphate and iron reduction. It also strongly accelerated with increased carbon sedimentation rates in September, which apparently eased limiting factors on the process. Hypolimnetic methane production is thus likely an important process in similar lakes, even in the presence of other electron acceptors, and will contribute to methane emissions during fall turnover.

Implications of redox processes for the rehabilitation of an urban lake, Onondaga Lake, New York

A synthesis of the impacts of domestic and industrial wastes on oxygen resources and redox conditions of polluted Onondaga Lake, Syracuse, N.Y., is presented based on a long-term (1978–2005) monitoring program. Insights from this retrospective analysis are used to evaluate management alternatives for the remediation of oxygen resources and redox conditions. Reduced byproducts of anaerobic metabolism accumulated annually in the hypolimnion, causing low dissolved oxygen (DO) concentrations in the upper waters during fall turnover. The high natural sulfate (SO4 2−) concentration of the lake promoted DO depletion and the production of methylmercury (CH3Hg+). Severe depletions of DO occurred annually in the upper waters during fall mixing, representing violations of water quality standards for extended intervals from 1978 to 1996. Depletions of DO during fall were less severe from 1997 to 2004. The improvement is reported to be in response to: (1) more routine occurrence of spring turnover following closure of an industry; (2) reduction in primary productivity; (3) return of large bodied Daphnia from closure of the industry; (4) satisfaction of historic debt from earlier higher primary production levels; and (5) year-round nitrification at a contributing domestic wastewater facility. Additional improvements in oxygen resources and decreases in SO4 2− reduction are anticipated based on mandated future upgrades of phosphorus and ammonia treatment at the wastewater facility. Prevailing DO conditions during fall turnover, particularly within the context of anticipated improvements from the mandated upgrades, indicate hypolimnetic aeration or oxygenation is not required to meet water quality goals. Amendments to the hypolimnetic pool of NO3 − are recommended instead of aeration or oxygenation to inhibit production of CH3Hg+ by SO4 2− reducing bacteria.

Demonstrating the Stratification of Dimictic and Meromictic Lakes in a Fish Tank

The demonstration described in this paper attempts to illustrate the main controls on the vertical circulation of lake water. This phenomenon integrates the effects of annual air temperature cycles, fetch, and possible groundwater sources to a lake. Because the nature of a lake's circulation pattern has an effect on the “quality” of the sediment record preserved, it is among the most important limnological processes for students to understand.This demonstration involves a fish tank, colored water, salty water, and artificial sun and wind. The water in the tank “evolves” through the seasons of a typical meromictic lake, beginning with Spring turnover and ending with Fall turnover. The demonstration requires several hours to complete, and students measure water temperature and conductivity through the model year. I have found this demonstration to be a helpful illustration of dimictic and meromictic lakes; it is especially helpful to illustrate the seasonal evolution of lake water, which is not apparent during winter field studies of local northeastern US lakes.

Temporal and Spatial Variations in Water Quality on New York South Shore Estuary Tributaries: Carmans, Patchogue, and Swan Rivers

The chemical and biological impacts of anthropogenic physical modifications (i.e., channelization, dredging, bulkhead, and jetty construction) to tributaries were assessed on New York’s Long Island South Shore Estuary. Water-quality data collected on Carmans, Patchogue, and Swan Rivers from 1997 to 2005 indicate no significant differences in nutrient levels, temperature, or pH among the rivers, but significant differences in light transmittance, dissolved oxygen (DO), salinity, and sediments were observed. Patchogue River (PR) and Swan River (SR) were significantly more saline than Carmans River (CR), PR and SR had less light transmittance than CR, and both exhibited severe warm season hypoxia. CR was rarely hypoxic and only at the lower layer of the deepest station in warm seasons. Deep stations on PR had hypoxic readings year round, but the shallower SR was well-oxygenated at all stations after the fall turnover. There were wide diel and seasonal variations in chlorophyll a on each river, and measurements were significantly higher at poorly flushed stations. In warm seasons, this often resulted in hyperventilation with supersaturated DO in the upper water column on sunny days, and suboxic conditions at nights and/or in deeper layers. PR sediments were anoxic, SR sediments ranged from normal to anoxic, and CR sediments were normal at all stations. Polyaromatic hydrocarbon concentrations in PR sediments were over three orders of magnitude higher than SR and CR sediments. Benthic invertebrate assessment of species richness, biotic index, and Ephemeroptera, Plecoptera and Trichoptera richness indicated that PR was severely impacted, SR ranged from slightly to severely impacted, and CR ranged from non-impacted to slightly impacted. Diversity and abundance of plankton were comparable on SR and CR, and were significantly higher than on PR. The data indicate that nutrients do not play a major role in hypoxia in these estuarine tributaries but that physical forces dominate. The narrow inlets, channelization, and abrupt changes in depth near the inlets of PR and SR foster hypoxic conditions by inducing salinity stratification that limits vertical mixing and by restricting horizontal water mass exchange with the bay. The study suggests that other tributaries with such physical modifications should be examined to assess the temporal and spatial extent of hypoxia.

Internal and external sources of THM precursors in a midwestern reservoir

Trihalomethanes (THMs) are contaminants of drinking water produced by the reaction between chlorine and natural organic matter. Determination of THM formation potential (THMFP) is a means of quantifying precursor abundance in waters from diverse sources. THMFP in river water entering and leaving Taylorsville Lake (Ky.) was measured to assess internal and external sources of THM precursors. THMFP in Taylorsville Lake was largely determined by watershed inputs. External inputs from tributary streams accounted for 80% of reservoir THMFP, and internal processes resulted in a net generation of 20%. Chlorophyll concentrations in the main tributary (Salt River) were comparable to those measured in Taylorsville Lake, suggesting that algal production in source waters may be important in regulating precursor supply to the reservoir. The highest THMFP was found in hypolimnetic samples, and peak export from the reservoir occurred during fall turnover, suggesting that decomposition of sedimenting organic matter both delayed and enhanced precursor release.

Dynamics of CFCs in northern temperate lakes and adjacent groundwater

Three dimictic lakes and one meromictic lake in and near the Trout Lake, Wisconsin, watershed were sampled to determine the variation of chlorofluorocarbon (CFC) concentrations within the lakes. The lakes were sampled during stratified conditions, during fall turnover, and during ice cover. The results demonstrate a considerable variation in CFC concentrations and corresponding atmospheric mixing ratios in the lakes sampled, both with depth and season within a given lake, and across different lakes. CFC profiles and observed degradation were not related to the groundwater inflow rate and hence are likely the result of in‐lake processes influenced by CFC degradation in the (lake) water column, CFC degradation in the lake‐bed sediments, and gas exchange rates and the duration of turnover (turnover efficiency).

Comparison of an Urban Lake Targeted for Rehabilitation and a Reference Lake Based on Robotic Monitoring

A reference lake, Otisco Lake, NY, was selected to evaluate rehabilitation initiatives to mitigate severe cultural eutrophication of Onondaga Lake, NY. Onondaga Lake was mesotrophic before European development. The reference lake selection was based on an analysis of paired monitoring datasets for temperature, fluorometric chlorophyll (Chlf/a) and dissolved oxygen (DO), collected daily by robotic profiling platforms for the spring to fall interval of three years. The various metrics of trophic state documented here for Otisco Lake represent reasonable informal interim goals for the rehabilitation of the cultural eutrophication of Onondaga Lake. The use of Otisco Lake as a reference site is supported by its similar stratification/mixing regime and mesotrophic state, in addition to its proximity (~25 km) and similar morphometry with Onondaga Lake. Strong contrasts in water quality manifestations of trophic state are depicted in Onondaga Lake, including higher Chlf/a (3.5-fold), prevalence of blooms, greater deviations of DO from saturation conditions, much lower minimum DO values at fall turnover in the upper layers and a higher volumetric hypolimnetic oxygen deficit (VHOD; 1.55-fold). Advantages of the fine vertical and temporal scale capabilities of the monitoring platforms are demonstrated in characterizing these and other limnological features. Continued robotic monitoring at Otisco Lake as a reference site and Onondaga Lake through the rehabilitation program will support ongoing comparisons to assess progress and will help engage stakeholders in the process.

Distribution of the thiols glutathione and 3‐mercaptopropionic acid in Connecticut lakes

The spatiotemporal chemical characteristics of the water column in Linsley Pond, a freshwater lake in Connecticut, were studied from just before its stratification in April through the fall turnover in December. Two low‐molecular‐weight thiols, glutathione and 3‐mercaptopropionic acid (MPA), were detected at nanomolar concentrations in the water column. GSH was detected (average concentrations were ca. 5 nmol L−1 in the particulate phase and ca. 3 nmol L−1 in the dissolved phase) in surface and near‐surface waters only and covaried with chlorophyll a. Both particulate and dissolved MPA were measured in the oxic and anoxic regions. In the metalimnion, MPA concentrations were greater than in the oxic water layers above. At times, the MPA concentrations in the metalimnion were as high as those found in the anoxic hypolimnion. The MPA present at different depths in the Linsley Pond water column is most likely produced through dissimilar mechanisms. Throughout the water column, MPA may be a product of the metabolic degradation of sulfur‐containing organic compounds; however, in the hypolimnetic waters, dissolved sulfide may play an important role in MPA formation through abiotic nucleophillic addition to unsaturated functionalities in dissolved organic matter. Parallel laboratory experiments were performed to assess the importance of MPA in copper speciation. The results confirmed that despite nanomolar levels in Linsley Pond, MPA does not play an important role in copper speciation. A survey of six additional lakes in Connecticut was made for detectable thiols. Only MPA was detected in four of these lakes, and its distribution was similar to that in Linsley Pond.

Assessment of Long-term Trends in the Oxygen Resources of a Recovering Urban Lake, Onondaga Lake, New York

Long-term trends in dissolved oxygen (DO) resources are documented for ionically enriched, hypereutrophic Onondaga Lake, New York, for 1978–2002. Assessments of oxygen resources are based on DO profiles of 1 m resolution conducted hourly to weekly during April-October. Closure of an industry that operated on the lake's shore until 1986 resulted in an abrupt decrease in primary production and organic carbon deposition and marked changes in hypolimnetic and epilimnetic DO regimes. The hypolimnetic DO response was gradual, reflecting the influence of sediment feedback. The onset of complete hypolimnetic anoxia, which occurred by early June through the 1980s, took place approximately one month later over 1997–2002. The anoxic factor decreased significantly over the study interval, from 97±5 d to 67±2 d. Epilimnetic DO concentrations responded rapidly to the abrupt change in lake metabolism. Before closure of the industry, 41% of the April to mid-September DO observations deviated from saturation concentrations by >30%. In the years immediately following industry closure, this degree of disequilibrium occurred in 23% of DO observations. Percent saturation values during fall turnover increased gradually during the study, from 49±2 to 65±2. DO concentrations in the lake's upper waters have been subject to substantial diel fluctuations (>4 mg/L), with daily minima and maxima generally occurring at approximately 7:00 and 16:00, respectively. Measurements made at 11:00 and 22:00 were most likely to be representative of daily average conditions. Diel variations and sampling frequency were demonstrated to be important issues for the evaluation of status with respect to water quality standards for DO. Levels of DO in the lake's upper waters have been positively correlated with chlorophyll a concentrations (r = 0.52). Despite marked improvements in DO resources, the lake continues to lack cool, well-oxygenated waters during summer necessary to support coldwater fish species.

Determination of mercury methylation potentials in the water column of lakes across Canada

A stable isotope technique was used to trace the formation of methylmercury in lake water incubation assays at in situ conditions in five lakes across Canada. Methylation activity was only detected in the anoxic hypolimnia of lakes. The stable isotope was methylated at varying rates between lakes and depths within lakes ranging from 0.56%/day to 14.8%/day. A peak in methylation potential was typically observed just below the oxycline, which decreased with increasing depth. The depth and rates of methylation potential changed seasonally with no methylation activity occurring after fall turnover. A decrease in the sulfate concentration was concomitant with the zone of mercury methylation potential indicating the likely involvement of sulfate reducing bacteria in the methylation process. A simple correlation test between DOC concentrations and methylation rates indicated a positive relationship ( r 2 = 0.62; p = 0.006; n = 27). The demethylation rate constant in the anoxic hypolimnia was less than 0.12 d − 1 .

SEDIMENT RESUSPENSION AND DRAWDOWN IN A WATER SUPPLY RESERVOIR1

ABSTRACT: The magnitudes and patterns of sediment resuspension are assessed in Cannonsville Reservoir, New York, to quantify and characterize this internal source of sediment. The assessment is based on analyses of sediment trap collections from 10 sites over the spring to fall interval of two years. Temporal and spatial patterns in sediment deposition are demonstrated to be driven by resuspension/redeposition processes. Sediment that had been resuspended and redeposited represented 80 to 96 percent, on average, of the depositing solids collected along the main axis of the lake. About 90 percent of the redeposited sediment was inorganic. Increased resuspension caused by drawdown of the reservoir surface and fall turnover resulted in 10 to 50‐fold increases in deposition rates compared to levels observed when the reservoir was full and strongly thermally stratified. Elevated levels of redeposition from resuspension in the reservoir have been driven by both higher water column concentrations of suspended solids and settling velocities. Recurring longitudinal and lateral gradients in resuspension are delineated, establishing that resuspended solids are transported from the riverine to the lacustrine zone and from near‐shore to pelagic areas. Resuspension is demonstrated to cause increases in inanimate particle (tripton) concentrations. Higher tripton levels have been observed in years with greater drawdown. Water quality impacts of the resuspension phenomenon are considered.

Seasonal variation of herbicide concentrations in prairie farm dugouts.

Prairie farm dugouts are frequently constructed for use as potable water sources. Consequently, cumulative pesticide inputs via atmospheric deposition and surface runoff may constitute a risk to human health. Since, relative to other pesticides, herbicides are used in greatest amount on the Canadian prairies, herbicide concentrations were intensively monitored in three dugouts over three growing seasons. Herbicides were detected in the water of all three dugouts each growing season which may reflect cumulative inputs from atmospheric and surface processes over the lifetimes of the dugouts, which varied from 11 to 22 yr. Detections, which were not continuous, tended to be seasonal in nature. During the 3-yr study, detections were most frequent during the spring application period and late fall following dugout turnover. Between these periods, herbicide concentrations generally decreased to below detection limits. The reappearance of herbicides in the dugout water during fall turnover and in concentrations generally greater than those present during the spring application period suggest that, under appropriate environmental conditions, the bottom sediments may act as a source of herbicides to the water column. In general, herbicide inputs due to deposition of application drift did not result in detectable concentrations of herbicides in the dugouts. In the only year that winter samples were monitored, herbicides were also detected during ice cover. On the basis of monthly sampling over each growing season, median concentrations of 9 of the 10 herbicides monitored were less than 0.05 microg L(-1). The exception, 2,4-D, which has been used extensively on the Canadian prairies for more than 50 yr and in greatest amounts, was the most frequently detected herbicide. In no case did herbicide concentrations exceed Canadian drinking water guidelines; however, on occasion maximum herbicide concentrations did exceed aquatic life and irrigation water guidelines.

Seasonal variability of nitrate supply and potential new production in the Gulf of Maine and Georges Bank regions

The mean annual cycle of new primary production is determined for each of five hydrographic provinces contained within the combined Gulf of Maine and Georges Bank region using potential new production (PNP) as a proxy for new primary production. PNP is computed using in situ data and defined as the difference between the time rate‐of‐change of vertically integrated nitrate (NO3) in the euphotic zone and vertical flux of NO3 into the euphotic zone assuming Fickian diffusion, after conversion of nitrogen to carbon using the Redfield ratio. Maximum recharge rate of NO3 in the euphotic zone occurs primarily during fall, between yeardays 300 and 330 (late October to late November), in agreement with the timing of fall turnover of the water column. Maximum utilization rate of NO3 in the euphotic zone occurs between yeardays 60 and 120 (March to April), in agreement with the timing of the spring bloom, with little phase difference between provinces. Peak‐to‐peak amplitude (maximum recharge rate to maximum utilization rate) is largest for Jordan Basin, Northeast Channel, and the western Gulf of Maine province. Wintertime NO3 recharge into the euphotic zone within the Gulf of Maine is largely the result of vertical NO3 flux, except for eastern Gulf of Maine waters where advective NO3 flux into surface waters is important. However, there still exists a significant deficit for fall NO3 recharge for eastern Gulf of Maine which cannot be accounted for by either vertical diffusive or horizontal advective NO3 fluxes. Annual totals of integrated total primary production (ITPP) and PNP summed over the full study region (including the southern New England shelf) are 44.33 tg C yr−1 and 9.96 tg C yr−1, respectively, for an annual mean potential f‐ratio of 0.22. Exclusion of the southern New England shelf increases the annual mean potential f‐ratio value for the remaining Gulf of Maine and Georges Bank region to 0.25. Inclusion of a time‐varying horizontal advective flux of NO3 into the eastern Gulf of Maine south of Cape Sable, Nova Scotia, increases the annual total of PNP for the full study region to 10.66 tg C yr−1, for an annual mean potential f‐ratio of 0.24. Annual mean potential f‐ratio for only the Gulf of Maine (excluding Georges Bank and southern New England shelf) using model‐derived Kz values and horizontal advective flux of NO3 into eastern Gulf of Maine waters is 0.17, above oligotrophic ocean values of ∼0.1, but well below the value of 0.40 used to calculate nitrification within the Gulf of Maine. Greater than one half (53.3%) of total regional PNP occurs within the hydrographic province (annual mean potential f‐ratio = 0.58) bounded by the tidal mixing front on Georges Bank but comprises less than one fifth (17.3%) of the region's total area. PNP indicates primary production on the crest of Georges Bank is largely recycled, with an annual mean potential f‐ratio of much less than 0.1, suggesting nitrogen limitation. However, error estimates for PNP and potential f‐ratio on the crest of Georges Bank are large due to the paucity of data collected in this shallow province.

A Limnological Survey of Third Sister Lake, Michigan with Historical Comparisons

ABSTRACT Third Sister Lake in southeastern Michigan has been a focus of ecological studies since 1904. In 1999, we surveyed several physical, chemical, and biological parameters of the lake and compiled data collected by University of Michigan students since 1975. Comparison of dissolved oxygen profiles from years 1927–29, 1939–42 to recent data (1992–99) indicate a decrease in maximum oxygenation of bottom waters at fall turnover (8 mg O2·L−1 maximum 1927–29, 1939–42 vs. 4 mg O2·L−1 1980, 1992–93) and more rapid depletion of oxygen in the hypolimnion following stratification. In early studies, oxygen persisted (> 3 mg L−1) at all depths from mid-November to May (1927–29, 1939–42). There has been no evidence of spring mixing in recent years and bottom waters have become anoxic by late January. The diversity and density of offshore benthic organisms has declined from at least 12 species and an average density of 167000·m−2 in 1927 to 4–5 species at 15144 · m−2 in 1999. Overall, benthic populations shifted from the deepest portions of the lake (16–18 m benthos) to shallower depths and only Chaoborus was found at bottom contours > 10 m in 1999. Mean chloride concentration [Cl·] increased nearly 13-fold from 19 mg L−1 to 241 mg L−1 between 1981 and 1988, with 260 mg L−1 recorded in 1999. Calculations of whole-lake stability incorporating increases in [Cl·] indicate that about 60% more energy would be required to mix the lake following ice-out in 1999 compared to 1981. During the winter-spring transition, net respiration dominated under ice (4.1 g C·m−2 d−1) and immediately afterice-out(0.32 g C·m−2 d−1)in 1999. The phytoplankton community was dominated by cyanobacteria (Oscillatoria) from February to April. Under ice cover, bacterial abundance increased with depth from an average of 4.17 × 105 cells ml−1 (1–5 m) to 18.9 × 105 cells ml−1 (15 m).

The Relationships between Chemical Measures and Potential Predictors of the Eutrophication Status of Inlets

Measurements of nutrients, dissolved oxygen and trace metals in bottom waters, taken just before the fall turnover, have been evaluated as indicators of eutrophication in inlets. Samples for these analyses were collected in 34 inlets in eastern Canada. The dominant factor (31% of the variance of the dataset) from a principal component analysis of the resulting data was clearly related to eutrophication. This factor included phosphate, ammonia, silicate, dissolved oxygen, iron and manganese, but not cadmium and zinc. It was used to rank inlets according to eutrophication. Comparisons of these rankings with measures of inlet shape revealed that several measures of the significance of sills were good predictors of the eutrophication status. Tidal prism flushing times, and other geometric measures, were poor predictors of eutrophication. Measures of anthropogenic inputs to the inlets were also poor predictors of the eutrophication status: apparently natural processes dominate anthropogenic inputs in these inlets.

Seasonal dynamics of carbon dioxide and methane in two clear-water lakes and two bog lakes in northern Wisconsin, U.S.A.

We investigated carbon dynamics in the water column and CO2 and CH4 exchange across the air-water interface in four closed-basin lakes in northern Wisconsin: two with clear, low dissolved organic carbon (DOC) waters and two bog lakes with stained, high-DOC waters. Clear-water lakes quickly became undersaturated following ice-out and remained undersaturated until fall turnover. Bog lakes were supersaturated in CO2 throughout the ice-free season, although surface CO2 concentrations dropped sharply following ice-out and increased again during autumn turnover. Differences in seasonal patterns of CO2 were attributed to morphometry and the timing and intensity of mixing events. Ice-free season fluxes of CO2 were 6.7 and 10.0 mol·m-2 in the bog lakes and 1.2 and 0.09 mol·m-2 in the clear-water lakes. Fluxes of CH4 were significant only immediately after ice-out and during autumn turnover and were <0.4 mol·m-2 in the bog lakes and <0.05 mol·m-2 in the clear-water lakes. Compared with changes in carbon pools in the lakes, our results indicate rapid carbon turnover rates in bog lakes, as opposed to clear-water lakes. We suggest that allochthonous inputs of CO2 may be responsible for this rapid turnover.

Effects of Hypolimnetic Oxygenation on the Food Resources and Feeding Ecology of Cisco in Amisk Lake, Alberta

Although the ability of hypolimnetic oxygenation to enhance coldwater fish populations depends, in part, on the availability of prey within the hypolimnion, studies that evaluate both fish diets and prey distributions during treatment are rare. This paper compares the diet of cisco Coregonus artedi the dominant coldwater fish in Amisk Lake, Alberta, Canada, with the composition and vertical distribution of prey during the summers of 1989 and 1992 when one basin of this two-basin eutrophic lake was oxygenated; the second basin served as a reference. During periods of thermal stratification, ciscoes in the reference basin were restricted by hypoxia to epilimnetic and metalimnetic waters whereas those in the treated basin distributed into the hypolimnion, where dissolved oxygen (DO) concentrations remained above 1.0 mg/L. Higher DO concentrations enabled ciscoes in the treated basin to feed throughout the water column and incorporate hypolimnetic and benthic prey species into their diets, whereas ciscoes in the reference basin ate primarily epilimnetic prey species. As a result, similarity in the composition of fish diet, which was high in early summer and during fall turnover, decreased through midsummer when differences in hypolimnetic DO concentrations between the two basins were most severe. Although foraging habitat for fish increased during oxygenation, whole-lake fish biomass also increased and resulted in a substantial reduction in per capita food levels; the amount of food in fish stomachs in 1989 was at least 30 times as high as in 1992. These results demonstrate that oxygenation can enhance availability of fish forage organisms in deep water. However, expanded foraging habitat may not always translate into increased food consumption if fish densities increase. Thus, the enhancement of coldwater fish populations during oxygenation will also depend on the nature of density-dependent interactions between fish and their prey.

Prediction of annual and seasonal phosphorus concentrations in stratified and polymictic lakes

Phosphorus (P) concentrations in lakes arc summarized and reported in different ways. Annual whole lake P (Pann), epilimnetic summer P (Pepi), and mixed water column P during fall turnover after summer stratification (Pfall) were investigated with data from up to 150 lakes. Differences between P averages are caused by temporal variation in thermal stratification and the size of internal P sources. To predict various P averages in specific lake types, I combined a P retention model that predicts annual retention due to sedimentation (Rpred) with external and internal load estimates (Lext, Lint) in a steady‐state, P mass balance approach.Retention of internal and external loads in stratified and polymictic lakes can be predicted by the same model, Rpred = 15/(18 + qs), where qs is annual water load (m yr−1). However, lakes with chemically enhanced P coprecipitation (e.g., with calcium) are an exception. P averages can be predicted in stratified lakes, as urn:x-wiley:19395590:media:lno19984371544:lno19984371544-math-0001although Pepi is overestimated in lakes with oxic hypolimnia. P concentrations can be approximately predicted in polymictic lakes as urn:x-wiley:19395590:media:lno19984371544:lno19984371544-math-0002

Seasonal variability in phosphorus speciation and deposition in a calcareous, eutrophic lake

Settling particulate matter was collected in sediment traps from the major depositional basin of Onondaga Lake, a calcareous, eutrophic lake in Syracuse, New York, U.S.A. Sediment traps were deployed at three depths from July to November and for a brief period under ice cover in February to investigate both vertical and seasonal variations in sedimentation. A sequential chemical extraction (fractionation) method was applied to the collected material to quantify the following phosphorus (P) fractions, or forms: loosely bound-P (sorbed-P, CaCO 3 associated-P, and Fe and Al bound-P), extractable biogenic-P (easily degradable organic-P and biogenic inorganic poly phosphates), calcium mineral-P (e.g., apatite), and refractory organic-P. Extractable biogenic-P and refractory organic-P were strongly correlated, suggesting a common, autochthonous origin in plankton. Calcium mineral-P appears to be of terrigenous origin. A dramatic increase in the loosely bound-P content of the particulate matter was observed during fall turnover when dissolved Fe 2+, which had accumulated in the anoxic hypolimnion, mixed with the surface waters and oxidized with attendant adsorption of P. We estimate that the labile-, or exchangeable-P (i.e. loosely bound- and extractable biogenic-P forms) content of the settling particulates constitutes ~ 50% of the total P influx to the sediments over the period of study. The labile-P flux over the July–November study period was 11 mg P m −2 day −1 and the labile-P flux estimated from sediment traps deployed under ice cover was 6 mg P m −2 day −1. From these values the annual average labile-P flux to the sediments was estimated to be 9 mg P m −2 day −1, only slightly less than the estimated annual average sediment P release rate of 10 mg P m −2 day −1. This indicates that the sediments are approximately at steady state with respect to current external P loadings to the lake.