Annual cycle of circulations in Lake Biwa, part 2: mechanisms

We investigate the mechanisms of circulations in Lake Biwa using model results that are driven by the momentum and heat fluxes evaluated from meteorological parameters observed around and on the lake. The kinetic energy budget shows that the wind stress over the lake, which has positive (negative) curl over the northern (southern) region of the lake throughout the year, generates the current system of multiple gyres in the stratified season (May–January), while the thermal forcing makes a minor contribution. This mechanism is quite different from the previous picture, in which the thermal forcing was dominant in forming the gyre system during the heating season. In the cooling season, the overturning of the lake occurs not in a simple one-dimensional (vertical) process, but in a time-dependent three-dimensional one. Cold water that is formed in the shallow eastern region descends the bottom slope to the deep lake in December to March while such a descent hardly occurs in the western region. A large shallow area (small heat capacity) as well as intense cooling in the eastern region effectively produces cold water. Wind again plays an important role by intensifying the cooling through its higher speed and by driving slope current. Further, the fluctuating wind direction causes significantly time-dependent behaviors of the slope current or bottom water formation.     

Spatial and temporal variations of the water–sediment thermal structure in shallow ice-covered Lake Vendyurskoe (Northwestern Russia)

The thermal structures in the vicinity of the ice–water and water–sediment boundaries of a shallow lake, L. Vendyurskoe (Northwestern Russia) during four winter seasons are described. The heat flux at the water-ice boundary was 0.1–0.2 W m⁻² during winter. The maximal heat flux at the water–sediment boundary was 4.5 W m⁻² at the beginning and 0.5 W m⁻² at the end of winter. The daily average value of the solar radiation penetrating into the water was 0.5 W m⁻² during main part of winter and 2–50 W m⁻² during April. During winter, temperature showed an oscillation in the vicinity of the sediment-water interface. Most periods corresponding to the main oscillation frequencies in the near-bottom water layer (0–0.4 m) and upper layer sediment (0–0.35 m), identified by FFT analysis, fall within the scale of synoptic variations (3–10 days), and in a number of cases were equal to 1 day. The theoretical periods of the first baroclinic seiche mode of Lake Vendyurskoe are 4.5–8.5 days that compares well with identified temperature oscillation periods. The comparison between the rate of heat content change in a water column and the difference of vertical heat fluxes from sediment to water and from water to ice show that the horizontal heat transport takes place in the lake during winter as a result of heat advection along the bottom.     

Winter anoxic layer in Lake Hibara

The concentration of dissolved oxygen in waters 0.5–0.6 m above the bottom of Lake Hibara, a dimictic lake, was zero in early spring of 1994 and 1997. The concentrations in early spring of 1992, 1993, 1995, 1996, and 1998 ranged from 3.75 to 10.1 mg l⁻¹. The depth profiles of water temperature suggest that water had not circulated prior to the sample collections of 1994 and 1997, but it had done so in the cases of the other years, suggesting that winter conditions were well preserved in the former years. On the other hand, the dissolved oxygen in the same strata decreased severely in summer. However, more or less titratable amounts of dissolved oxygen still remained (0.01–0.73 mg l⁻¹) at the final stages of summer stratification from 1992 to 1998. These facts show that a completely anoxic condition is not formed in this lake in summer but is sometimes formed in winter. It is interesting to note that in spite of unfavorable winter conditions for oxygen consumption, i.e., shorter duration and lower water temperature, oxygen is exhausted. These facts suggest that ventilation to the depths is much greater in summer than in winter. Received: March 5, 1999 / Accepted: October 18, 1999     

Vulnerability of a large monomictic lake (Lake Biwa) to warm winter event

Detailed cross-sectional temperature and oxygen distributions during winter were examined over 5 years (2003–2007) in large, warm, monomictic Lake Biwa. The distribution patterns showed year-to-year variation in the degree of spatial heterogeneity and indicated that both convective mixing and gravity currents contributed to the delivery of oxygen to the profundal zone. We encountered a warm winter in 2007, when deep-water oxygenation was delayed by more than a month relative to normal years. Data obtained during the period of oxygenation in 2007 suggest that the timing of complete oxygenation of the bottom water (or lower layers of the hypolimnion) was controlled by physical processes responsible for destruction of the weak thermal stratification near the bottom. This study revealed that Lake Biwa is sensitive to year-to-year variations in winter meteorological conditions. Specifically, the bottom environments are highly susceptible to a warm winter, especially when it follows a cold winter.     

Love Handles in Aquatic Ecosystems: The Role of Dissolved Organic Carbon Drawdown, Resuspended Sediments, and Terrigenous Inputs in the Carbon Balance of Lake Michigan

During the unstratified (winter) and stratified (summer) periods of 1999 and 2000, we examined carbon (C) dynamics in the upper water column of southern Lake Michigan. We found that (a) bacterial respiration (BR) and planktonic respiration (PR) were major sinks for C, (b) C flux through bacteria (CFTB) was diminished in winter because of reduced bacterial production (BP) and increased bacterial growth efficiency (BGE) at colder temperatures, and (c) PR exceeded primary production (PP) during the spring–summer transition. Drawdown of dissolved organic C (DOC), resuspended organic matter from the lake floor, and riverine organic matter likely provided organic C to compensate for this temporal deficit. DOC in the water column decreased between winter and summer (29–91 mg C m² d⁻¹) and accounted for 20%–53% of CFTB and 11%–33% of PR. Sediment resuspension events supported elevated winter heterotrophy in the years that they occurred with greatest intensities (1998 and 2000) and may be important to interannual variability in C dynamics. Further, riverine discharge, containing elevated DOC (5×) and dissolved P (10×) relative to lake water, peaked in the winter–spring season in southern Lake Michigan. Collectively, terrigenous inputs (river, stream, and groundwater discharges; storm water runoff; and atmospheric precipitation) may support approximately 10%–20% of annual in-lake heterotrophy as well as autotrophy. Terrestrial subsidies likely play a key role in the C balance of even very large lakes, representing a critical linkage between terrestrial and aquatic ecosystems. Received 11 June 2001; Accepted 14 December 2001.     

Thermal,chemical, and optical properties of Crater Lake,Oregon

Crater Lake covers the floor of the Mount Mazama caldera that formed 7700 years ago. The lake has a surface area of 53 km² and a maximum depth of 594 m. There is no outlet stream and surface inflow is limited to small streams and springs. Owing to its great volume and heat, the lake is not covered by snow and ice in winter unlike other lakes in the Cascade Range. The lake is isothermal in winter except for a slight increase in temperature in the deep lake from hyperadiabatic processes and inflow of hydrothermal fluids. During winter and spring the water column mixes to a depth of about 200–250 m from wind energy and convection. Circulation of the deep lake occurs periodically in winter and spring when cold, near-surface waters sink to the lake bottom; a process that results in the upwelling of nutrients, especially nitrate-N, into the upper strata of the lake. Thermal stratification occurs in late summer and fall. The maximum thickness of the epilimnion is about 20 m and the metalimnion extends to a depth of about 100 m. Thus, most of the lake volume is a cold hypolimnion. The year-round near-bottom temperature is about 3.5°C. Overall, hydrothermal fluids define and temporally maintain the basic water quality characteristics of the lake (e.g., pH, alkalinity and conductivity). Total phosphorus and orthophosphate-P concentrations are fairly uniform throughout the water column, where as total Kjeldahl-N and ammonia-N are highest in concentration in the upper lake. Concentrations of nitrate-N increase with depth below 200 m. No long-term changes in water quality have been detected. Secchi disk (20-cm) clarity varied seasonally and annually, but was typically highest in June and lowest in August. During the current study, August Secchi disk clarity readings averaged about 30 m. The maximum individual clarity reading was 41.5 m in June 1997. The lowest reading was 18.1 m in July 1995. From 1896 (white-dinner plate) to 2003, the average August Secchi disk reading was about 30 m. No long-term changes in the Secchi disk clarity were observed. Average turbidity of the water column (2–550 m) between June and September from 1991 to 2000 as measured by a transmissometer ranged between 88.8% and 90.7%. The depth of 1% of the incident solar radiation during thermal stratification varied annually between 80 m and 100 m. Both of these measurements provided additional evidence about the exceptional clarity of Crater Lake.     

Predictors of Seasonal Oxygen Levels in Small Florida Lakes: The Importance of Color

This study examines the relationship of profundal oxygen concentrations in 55 shallow Florida lakes to humic color, trophic state, and lake size during different seasons. The data set represented a broad range of color and trophic state. The percent saturation of dissolved oxygen remained relatively constant during the fall (mean 78.4%), winter (mean 81.3%), and spring (mean 82.5%), but declined markedly during summer (mean 65.2%). Chlorophyll a concentrations were highest during the winter (mean 2.52 mg m^–3) and lowest during the fall (mean 1.17 mg m^–3), while color peaked during the fall (mean 30.1 mg Pt l^–1) and was lowest during the summer (mean 12.7 mg Pt l^–1). The relative importance of lake size, chlorophyll a, and color in explaining variation in percent oxygen saturation was examined using multiple regression. Percent oxygen saturation was negatively correlated with color during the winter, spring, and summer, and positively correlated with lake size in the winter and spring. However, percent oxygen saturation showed no relationship with chlorophyll a during any season. These results suggest that colored Florida lakes are naturally oxygen depleted and that profundal oxygen values have little relationship to lake trophic state. This revised version was published online in July 2006 with corrections to the Cover Date.     

Do warmer winters change variability patterns of physical and chemical lake conditions in Sweden?

In this study, the effect of a warmer winter climate on variability patterns of physical and chemical lake conditions was examined by using monthly air temperature data from 72 meteorological Swedish sites, ice breakup data from 77 Swedish lakes and monthly data of 17 water chemical variables from 11 nutrient-poor Swedish reference lakes during 1988–2005. The results showed significantly increasing variations of lake ice breakup dates and nitrate concentrations over Sweden along with increasing winter air temperatures. Variability patterns of other water chemical variables were not affected by warmer winters. Nitrate concentrations increased their variability in spring and early summer not only between lakes but also within lakes, which was attributed to a climate-induced increase in spring nitrate concentrations in particular in southern Sweden, while summer nitrate concentrations remained rather constant and low all over Sweden (median 10 μg l⁻¹). Since nitrate concentrations play an important role for primary production, highly varying concentrations will be a challenge for biota to adapt.     

Some generalizations based on stratification and vertical mixing in meromictic Lake Shira,Russia, in the period 2002–2009

In a brackish, temperate, 24-m-deep Lake Shira, the profiles of salinity, temperature, oxygen and sulfide concentrations were measured on a seasonal basis from 2002 to 2009. The lake was shown to be meromictic with autumnal overturn restricted to mixolimnion. The depth of mixolimnion and position of oxic–anoxic interface varied annually. The spring mixing processes contribute to the formation of mixolimnion in autumn. The exceptionally windy spring of 2007 caused the deepening of mixolimnion in the winter of 2008. The winter position of oxic–anoxic interface was affected by the position of lower boundary of mixolimnion in all winters. The salinity in the winter mixolimnion increased compared with the autumn because of freezing out of salts from the upper water layers meters during ice formation and their dissolution in water below. The profiles of salinity and temperature were simulated by the mathematical 1-D model of temperature and salinity conditions taking into account ice formation. The simulated profiles generally coincided with the measured ones. The coincidence implies that simplified one-dimensional model can be applied to roughly describe salinity and density profiles and mixing behavior of Lake Shira.     

A five-year study of autotrophic winter picoplankton in Lake Balaton,Hungary

Picoeukaryotes dominate the phytoplankton of Lake Balaton—the largest shallow lake in Central Europe—in the winter period. We examined the annual dynamics of picoplankton abundance and composition in the lake in order to establish if the picoeukaryotes merely survive the harsher winter conditions or they are able to grow in the ice-covered lake when the entire phytoplankton is limited by low light and temperature. Lake Balaton has an annual temperature range of 1–29°C, and it is usually frozen between December and February for 30–60 days. In the spring-autumn period phycocyanin and phycoerythrin rich Cyanobacteria are the dominant picoplankters, and picoeukaryotes are negligible. Our five-year study shows the presence of three types of picophytoplankton assemblages in Lake Balaton: (1) Phycoerythrin-rich Cyanobacteria—the dominant summer picoplankters in the mesotrophic lake area; (2) Phycocyanin-rich Cyanobacteria—the most abundant summer picoplankters in the eutrophic lake area and; (3) Picoeukaryotes—the dominant winter picoplankters in the whole lake. The observed winter abundance of picoeukaryotes was high (up to 3 × 10⁵ cells ml⁻¹), their highest biomass (520 μg l⁻¹) exceeded the maximum summer biomass of picocyanobacteria (500 μg l⁻¹). Our results indicate that the winter predominance of picoeukaryotes is a regular phenomenon in Lake Balaton, irrespective of the absence or presence of the ice cover. Picoeukaryotes are able to grow at as low as 1–2°C water temperature, while the total phytoplankton biomass show the lowest annual values in the winter period. In agreement with earlier findings, the contribution of picocyanobacteria to the total phytoplankton biomass in Lake Balaton is inversely related to the total phytoplankton biomass, whereas no such relationship was observable in the case of picoeukaryotes.     

Evidence for predatory control of the invasive round goby

We coupled bioenergetics modeling with bottom trawl survey results to evaluate the capacity of piscivorous fish in eastern Lake Erie to exert predatory control of the invading population of round goby Neogobius melanostomus. In the offshore (>20 m deep) waters of eastern Lake Erie, burbot Lota lota is a native top predator, feeding on a suite of prey fishes. The round goby invaded eastern Lake Erie during the late 1990s, and round goby population size increased dramatically during 1999–2004. According to annual bottom trawl survey results, round goby abundance in offshore waters peaked in 2004, but then declined during 2004–2008. Coincidentally, round goby became an important component of burbot diet beginning in 2003. Using bottom trawling and gill netting, we estimated adult burbot abundance and age structure in eastern Lake Erie during 2007. Diet composition and energy density of eastern Lake Erie burbot were also determined during 2007. This information, along with estimates of burbot growth, burbot mortality, burbot water temperature regime, and energy densities of prey fish from the literature, were incorporated into a bioenergetics model application to estimate annual consumption of round goby by the adult burbot population. Results indicated that the adult burbot population in eastern Lake Erie annually consumed 1,361 metric tons of round goby. Based on the results of bottom trawling, we estimated the biomass of yearling and older round goby in offshore waters eastern Lake Erie during 2007–2008 to be 2,232 metric tons. Thus, the adult burbot population was feeding on round goby at an annual rate equal to 61% of the estimated round goby standing stock. We concluded that the burbot population had high potential to exert predatory control on round goby in offshore waters of eastern Lake Erie.     

Adélie penguin survival: age structure,temporal variability and environmental influences

The driving factors of survival, a key demographic process, have been particularly challenging to study, especially for winter migratory species such as the Adélie penguin (Pygoscelis adeliae). While winter environmental conditions clearly influence Antarctic seabird survival, it has been unclear to which environmental features they are most likely to respond. Here, we examine the influence of environmental fluctuations, broad climatic conditions and the success of the breeding season prior to winter on annual survival of an Adélie penguin population using mark–recapture models based on penguin tag and resight data over a 16-year period. This analysis required an extension to the basic Cormack–Jolly–Seber model by incorporating age structure in recapture and survival sub-models. By including model covariates, we show that survival of older penguins is primarily related to the amount and concentration of ice present in their winter foraging grounds. In contrast, fledgling and yearling survival depended on other factors in addition to the physical marine environment and outcomes of the previous breeding season, but we were unable to determine what these were. The relationship between sea-ice and survival differed with penguin age: extensive ice during the return journey to breeding colonies was detrimental to survival for the younger penguins, whereas either too little or too much ice (between 15 and 80% cover) in the winter foraging grounds was detrimental for adults. Our results demonstrate that predictions of Adélie penguin survival can be improved by taking into account penguin age, prior breeding conditions and environmental features.     

A photographic method for detailing the morphology of the floor of a dynamic crater lake: the El Chichón case (Chiapas,Mexico)

The active volcano El Chichón (Chiapas, Mexico) hosts a shallow acidic crater lake. During the period 2001–2007, 26 photographs of the crater lake were taken from the same spot at the eastern crater rim, ~160 m above the crater floor. The size of the lake was extremely variable. Using a GPS track from around the lake shore as a reference, 26 digitized lake outlines were corrected simultaneously for the perspective angle. The corrected lake outlines were superposed, leading to a “morphological map” of a large section of the lake bottom. This map provides insight into the erosive–sedimentary regime of the lake floor. The inner section of the lake is more stable due to the precipitation of sealing clays. This is probably one of the reasons why the El Chichón crater lake has never disappeared during the past 28 years. The sealing clays at the lake bottom can be considered the superficial analog of impermeable clay caps at the depths of hydrothermal systems. The photographic procedure presented here may be useful for other limnological and (volcanic) lake studies aimed at describing lake morphology, and for eventually deducing the surface area and volume of the lake.     

Some features of the thermal and dissolved oxygen structure in boreal,shallow ice-covered Lake Vendyurskoe,Russia

The 5-year-long (2001–2005) studies of the winter thermal structure and the dissolved oxygen (DO) dynamics in Lake Vendyurskoe, Russia, a typical boreal shallow mesotrophic lake of glacial origin, revealed still poorly studied features of lake-wide dynamics, such as net lateral heat flux towards deeper parts of a lake and development of the anaerobic zone over the deepest points of the lake basin. We estimated magnitude of the heat transport along the bottom slope based on scaling analysis. The seasonal changes in DO concentration appear to be controlled mostly by biochemical consumption. We identify four factors controlling the extent of anoxic zones in shallow ice-covered lakes: (1) the amount of organic matter stored in the bottom layers, including the sediments surface during the autumnal bloom; (2) the length of the ice-covered period; (3) heat content of bottom sediments; and (4) the initial water temperatures at the time of the ice cover formation.     

Occurrence of a thermocline in two lakes of the semi-arid region in western Rajasthan,India

Water quality samples were obtained monthly or bimonthly 17 times from May 1974 to May 1975 at three stations in Delaware Bay. In addition, two 12-hour cruises were also conducted at one station in February and April 1975. Surface and bottom water samples were taken. Measurements and analyses included temperature, salinity, dissolved oxygen, silicate, nitrate and nitrite, orthophosphate, ammonia, chlorophylls a, b, and c, phaeopigments, and carotenoids. The annual pattern of temperature was typical of an estuary in the mid-Atlantic Bight. Salinity and dissolved oxygen ranged from 22.9 to 29.7‰ and from 4.53 to 8.53 ml/l, respectively. Nutrient and pigment values showed seasonal peaks. Silicate (30.3 μg-at/1) and orthosphate (1.59 μg-at/1) were highest in September. Highest concentrations of ammonia were commonly measured in July (6.80 μg-at/1) and September (5.13 μg-at/1), and peak concentrations of nitrate and nitrite were recorded in January (24.27 μg-at/l), February (18.2 μg-at/1), and May (16.37 μg-at/1). Peak concentrations of chlorophyll a were measured in August (17.2 μg-at/1), October (15.70 μg-at/1), and March (15.33 μg-at/1). In general, the annual pattern for chlorophylls b and c were similar to chlorophyll a. Comparison with other estuaries and bays (Narrangansett Bay, Long Island Sound, Raritan Bay, and Chesapeake Bay) indicated that concentrations of nutrients and pigments in Delaware Bay were generally similar in magnitude and seasonality, These are the first set of seasonal water quality data for lower Delaware Bay.     

Composition and variations in the occurrence of dissolved free simple organic compounds of an unproductive lake ecosystem in northern Sweden

Low molecular weight organic carbon compounds are potentially important carbon and energy substrates to heterotrophic production in the aquatic environment. We studied the occurrence of dissolved free amino acids (AA), monosaccharides (CHO), and carboxylic acids (CA) in the subarctic Lake Diktar-Erik. The lake is unproductive with slightly humic water, and receives water via one major inlet stream draining a birch forest environment. The concentration of dissolved organic carbon (DOC) in the inlet stream was strongly correlated with the discharge. This relationship changed from season to season, indicating changes in the sources of the DOC entering the stream. AA and CHO each accounted for an average of less than 0.5% of the DOC. After high discharge events during the ice-free period, AA and CHO occurred in especially high concentrations. CA occurred in higher concentrations during the ice-free period, when it generally accounted for 20–30% of the DOC pool. The CA content relative to the total DOC pool was strongly inversely correlated with overall DOC concentration, and at low DOC levels the relative content of CA was high and vice versa. This followed a seasonal trend, with CA accounting for a smaller proportion of the DOC in winter and a larger part in spring/early summer. A conservative estimate suggested that the studied simple organic carbon compounds potentially could cover 30% of the bacterial gross production in the lake and therefore potentially also was an important source of CO₂ that occur in supersaturated concentrations in the lake.     

Sediment phosphorus cycling in a large shallow lake: spatio-temporal variation in phosphorus pools and release

Sediment and water column phosphorus fractions were recorded monthly for one year (April 2004–April 2005) in a shallow lake recovering from nutrient pollution (Loch Leven, Scotland). Equilibrium phosphate concentration (EPC0) and gross sediment phosphorus (P) release rates were estimated from laboratory experiments. Pore water and organic P pools were lowest during warm water periods whereas bottom water P was lowest during cold water periods. Reductant-soluble, organic, metal oxide-adsorbed, residual and sediment total phosphorus pools all varied significantly with overlying water depth. Short-term, high magnitude, redox initiated P release events occurred in late summer and winter as a result of anoxic sediment conditions. Lower magnitude long-term release conditions were maintained for most of the year, most likely as a result of organic P cycling and maintenance of high concentration gradients between the pore and bottom water P pools. Estimates of summer P uptake/release rates, across an intact sediment-water interface, suggested that maximum gross internal release was ~12 mg SRP m⁻² lake surface area d⁻¹ with EPC0 values ranging between 180 and 270 μg P L⁻¹. This study highlights the biological mediation of internal loading in shallow eutrophic lakes, and in particular, the role of sediment algae in decreasing, and sediment bacteria in enhancing, sediment P release.     

A one-dimensional model of vertical stratification of Lake Shira focussed on winter conditions and ice cover

In meromictic lakes such as Lake Shira, horizontal inhomogeneity is small in comparison with vertical gradients. To determine the vertical distribution of temperature, salinity, and density of water in a deep zone of a Lake Shira, or other saline lakes, a one-dimensional (in vertical direction) mathematical model is presented. A special feature of this model is that it takes into account the process of ice formation. The model of ice formation is based on the one-phase Stefan problem with the linear temperature distribution in the solid phase. A convective mixed layer is formed under an ice cover due to salt extraction in the ice formation process. To obtain analytical solutions for the vertical distribution of temperature, salinity, and density of water, we use a scheme of vertical structure in the form of several layers. In spring, the ice melts as top and bottom. These processes are taken into account in the model. The calculated profiles of salinity and temperature of Shira Lake are in good agreement with field measurement data for each season. Additionally, we focussed on the redox zone, which is the zone in which the aerobic layers of a water column meet the anaerobic ones. Hyperactivity of plankton communities is observed in this zone in lakes with hydrogen sulphide monimolimnion, and Lake Shira is among them. The location of the redox zone in the lake, which is estimated from field measurements, coincides with a sharp increase in density (the pycnocline) during autumn and winter. During spring and summer, the redox zone is deeper than the pycnocline. The location of pycnocline calculated with the hydro physical model is in good agreement with field measurement data.     

Physical background of the development of oxygen depletion in ice-covered lakes

The effect of the heat interaction between a water column and sediments on the formation, development, and duration of existence of anaerobic zones in ice-covered lakes is estimated based on observational data from five frozen lakes located in northwestern Russia and North America. A simple one-dimensional model that describes the formation and development of the dissolved oxygen deficit in shallow ice-covered lakes is suggested. The model reproduces the main features of dissolved oxygen dynamics during the ice-covered period; that is, the vertical structure, the thickness, and the rate of increase of the anaerobic zone in bottom layers. The model was verified against observational data. The results from the verification show that the model adequately describes the dissolved oxygen dynamics in winter. The consumption rates of DO by bacterial plankton and by bottom sediments, which depend on the heat transfer through the water–sediment interface, are calculated. The results obtained allow the appearance of potentially dangerous anaerobic zones in shallow lakes and in separate lake areas, which result from thermal regime changes, to be predicted. Priority programme of the German Research Foundation—contribution 10.     

Food selection in <Emphasis Type="Italic">Eucypris virens</Emphasis> (Crustacea: Ostracoda) under experimental conditions

Identification of sources of nutrient pollution is a first step towards remediation of eutrophication in aquatic ecosystems. The stable isotope nitrogen-15 (¹⁵N) is a natural indicator of nitrogen (N) source and biogeochemistry. We sampled Lake Taihu, a hyper-eutrophic lake in eastern China, and major inflow rivers during winter and spring of 2004 to determine concentration and δ¹⁵N of nitrate (NO ₃ ⁻ ). Nitrate concentrations in rivers and the lake were higher, in most cases, in spring than in winter. δ¹⁵N of NO ₃ ⁻ was not correlated with NO ₃ ⁻ concentration, indicating that concentrations alone are insufficient to describe N sources. Results show that riverine N inputs in winter are influenced by discharge of human sewage into rivers and the lake. In spring, however, wastewater inputs to the lake appear to be balanced by fertilizers, atmospheric, and/or N₂ fixation sources. Rain NO ₃ ⁻ concentrations were seasonally high and isotopically enriched compared to potential sources, indicating that rain may be a significant or even dominant source of N to the lake during the rainy season. δ¹⁵N values show that urbanized areas of the lake have more sewage-derived N than those areas dominated by agriculture, aquaculture, or industry. This observation has important implications for human health, since Lake Taihu is a source of drinking and irrigation water as well as fish for human consumption.