The identification of nutrient limitations on eutrophication in Dianchi Lake,China

Due to rapid urbanisation and economic development, Dianchi Lake has been eutrophic since the 1980s. Control of nutrient loading is regarded as the primary restoration method. Two independent approaches: analysis of 13 years of Dianchi Lake monitoring data using Support Vector Machines (SVM) and a nutrient‐enrichment experiment, identified phosphorus and nitrogen as simultaneously limiting. Total phosphorus (TP) has a clear, stable correlation with chlorophyll‐a (Chl‐a) concentrations in Dianchi Lake. Compared to total nitrogen (TN), TP shows a stronger correlation when Chl‐a concentration is below 0.051 mg/L and an almost equal correlation when it is above this level. An excessive phosphorus to nitrogen ratio may stimulate compensatory fixation of atmospheric N₂ by cyanobacteria; therefore, only reducing the external nitrogen input may not necessarily result in lower total nitrogen and chlorophyll concentrations. We recommend that reducing the lake TP load to the lowest economically‐feasible level is the most cost‐effective restoration measure.     

Controlling harmful cyanobacterial blooms in a hyper-eutrophic lake (Lake Taihu, China): the need for a dual nutrient (N & P) management strategy

Harmful cyanobacterial blooms, reflecting advanced eutrophication, are spreading globally and threaten the sustainability of freshwater ecosystems. Increasingly, non-nitrogen (N₂)-fixing cyanobacteria (e.g., Microcystis) dominate such blooms, indicating that both excessive nitrogen (N) and phosphorus (P) loads may be responsible for their proliferation. Traditionally, watershed nutrient management efforts to control these blooms have focused on reducing P inputs. However, N loading has increased dramatically in many watersheds, promoting blooms of non-N₂ fixers, and altering lake nutrient budgets and cycling characteristics. We examined this proliferating water quality problem in Lake Taihu, China’s 3rd largest freshwater lake. This shallow, hyper-eutrophic lake has changed from bloom-free to bloom-plagued conditions over the past 3 decades. Toxic Microcystis spp. blooms threaten the use of the lake for drinking water, fisheries and recreational purposes. Nutrient addition bioassays indicated that the lake shifts from P limitation in winter-spring to N limitation in cyanobacteria-dominated summer and fall months. Combined N and P additions led to maximum stimulation of growth. Despite summer N limitation and P availability, non-N₂ fixing blooms prevailed. Nitrogen cycling studies, combined with N input estimates, indicate that Microcystis thrives on both newly supplied and previously-loaded N sources to maintain its dominance. Denitrification did not relieve the lake of excessive N inputs. Results point to the need to reduce both N and P inputs for long-term eutrophication and cyanobacterial bloom control in this hyper-eutrophic system.     

The use of algal bioassays to predict the short- and long-term changes in algal standing crop which result from altered phosphorus and nitrogen loadings

Bioassays were performed on filtered water from Jackson Lake, Wyoming, using the Algal Assay Procedure-Bottle Test with Selenastrum capricornutum as the assay organism. It is hypothesized that (i) short- and long-term additions of phosphorus (P) and nitrogen (N) to waters should be evaluated separately, and (ii) only bioassay treatments of lake water plus phosphorus plus nitrogen (LW + P + N) should be used to predict the effect of long-term phosphorus additions on algal standing crop. Equations are developed directly from the corresponding bioassay responses to predict short-term increases in the lake's algal standing crop. Accepting the hypothesis, an equation describing the response of S. capricornutum to LW + P + N treatments is derived to predict the long-term effects of phosphorus additions on algal standing crop. This equation is then combined with another derived to describe the steady state concentration of phosphorus in lake water. The resultant equation predicts the percent change in algal standing crop as a function of any change in the lake's phosphorus loading. In most cases, use of LW + P rather than LW + P + N treatments to predict changes in algal standing crop will lead to quite different interpretation of bioassay data, and probably to different management decisions.     

Enriching effects of Urban runoff on the productivity of a mesotrophic lake

Sewage was diverted from Lake Sammamish in September, 1968, and since then there have been no significant responses by trophic indicators which indicate the lake is beginning to recover. To explain this delay in response, extensive urban development in the lake's watershed was considered as a factor which might be acting to inhibit the lake's recovery. In vitro uni-algal experiments with water from 13 streams which drain urban and undeveloped areas showed that about half of the streams stimulated algal growth significantly, but these streams constitute less than 14 per cent of the lake's total water income. Of these six streams only two drain urban areas and account for less than 2 per cent of the water income. In situ experiments showed that no streams were significantly stimulatory to natural populations of phytoplankton. Other in situ studies showed that one urban stream and two from undeveloped areas caused substantial increases in periphyton growth after a 12-day period, although these increases were not statistically significant. Additional in situ nutrient limitation experiments in August of 1970 and 1971 revealed that P and N were limiting only in combination. Results from these studies do not support the contention that urban runoff is seriously enriching the limnetic region of Lake Sammamish.     

Internal phosphorus loading in a shallow eutrophic lake

Internal loading of phosphorus has been implicated as a major eutrophicating factor in Long Lake, Washington (Kitsap County). As a result of such loading, summer total phosphorus concentrations approach or exceed 100 μgP l⁻¹. Most of the summer loading of phosphorus is thought to have been released directly from the rich, flocculent sediment in the mid and northern part of the lake as a result of high pH (up to 10) related to phytoplankton photosynthesis. The lake also supports a dense submersed macrophyte crop (areal weighted mean dry weight of about 220 g m⁻²) composed primarily of Elodea densa. Although excretion of phosphorus from healthy E. densa was found to be minimal, the potential contribution of P indirectly from sediment via macrophyte uptake and subsequent decomposition in the winter was on the order of 200–400 kg yr⁻¹ or about 25–50% of the external loading. Nevertheless, loading of phosphorus during predrawdown summers is thought to have originated largely as a direct release from sediment due to high pH. Estimates of sediment phosphorus release determined from laboratory experiments, mass balance and core analyses ranged from 2.2 to 5.6 mg m⁻² day⁻¹.As a component of the restoration program, the lake (mean depth of 2 m) was drawn down nearly 2 m for 4 months during the summer and fall of 1979. The lake's trophic status was expected to improve due to sediment consolidation and/or macrophyte reduction. The drawdown resulted in an 84% reduction in macrophyte biomass in 1980 but minimal sediment consolidation (0.1 m). Winter decomposition of the much smaller macrophyte crop, apparently provided insufficient phosphorus in the spring to stimulate phytoplankton and to enrich midlake sediments, which has probably occurred in previous years. Low water column pH during the postdrawndown summer of 1980 resulted from relatively low rates of plankton photosynthesis. During the summer of 1980 internal loading of phosphorus was reduced and total phosphorus remained below 50 μg l⁻¹.     

Snow and ice in the phosphorus budget of a lake in South Central Ontario

Roles of snow and various forms of ice in the phosphorus cycle of a lake are discussed. It is pointed out that precipitation falling directly onto a lake ice sheet during the winter has a disproportionately large impact on the lake system in the spring. Phosphorus derived from snow incorporated into the ice sheet must be included if accurate estimates of spring phosphorus loading are to be made. Data from Coon Lake, Ontario, Canada are used to illustrate points made.     

Comparison of seasonal and diurnal patterns of some physico‐chemical parameters of the open and closed parts of Loktak Lake,Manipur, India

A comparative study of the periodic and aperiodic variations of some physico‐chemical parameters of the open and closed parts of the Loktak Lake, Manipur, India, was carried out for a period of three years from 1976–78 to investigate the effects, if any, of human activities on these variations. The closed part of the lake, used for controlled fishing was well protected, whereas the open part was subjected to considerable human activities. The variation in the physico chemical parameters in the open lake were different from those of the closed lake in that (a) there was distinct seasonal variation in DO which was not evident in the closed lake and (b) there was direct relationship between diurnal variation of CO₂ and pH instead of an indirect relationship in the closed lake. The variation in pH, electrical conductivity, bicarbonate, free carbon dioxide and chloride between 1976–78 had a similar pattern in the two parts of the lake.     

Decision support for the selection of an appropriate in-lake measure to influence the phosphorus retention in sediments

Many in-lake measures which aimed to influence the phosphorus retention in lake sediments have failed to improve the trophic state of the lakes. The present paper introduces a systematic approach to select an appropriate in-lake measure. Before selecting an in-lake measure the goal of the measure should be defined, the problems of the lake must be identified, and the probability of success must be estimated. The proposed decision support consists of two parts. Part A, pre-selection, excludes inappropriate measures. It uses six criteria, which are mainly based on a simple mass-balance model, and the targets of restoration. The criteria describe the magnitude of the external versus the internal phosphorus loading, the dynamics of the internal load, and the lake morphometry. Each measure is weighted differently with respect to importance and suitability by specific quantified limits. Part B, selection, uses qualitative criteria, which are specific to the measure in question. Checking these criteria will help to select a measure with a low risk of failure. The suggested decision support is illustrated in flow charts and exemplified by Lake Arendsee in Germany.     

Groundwater seepage as a nutrient source to a drainage lake; Lake Mendota, Wisconsin

The flux of groundwater through shallow-water sediments into Lake Mendota was calculated from hydrologic studies and was measured directly with seepage meters at 106 sites around the lake. Groundwater accounted for a substantial amount (around 30%) of the water budget. Pore water in seepage zones was collected for chemical analysis by two methods: (1) dialysis samplers; and (2) direct gentle suction. This pore water, which was assumed to represent seepage inflow, was considerably lower in nitrogen and phosphorus than surface inflow but was higher in phosphorus and lower in nitrogen than well water, indicating that well chemistry does not provide a good indication of the composition of groundwater entering lakes. Calculations indicated that seepage accounted for 12% of the total phosphorus loading to Lake Mendota and 2% of the total nitrogen budget. These results are interpreted in terms of the annual nutrient loading estimates that have been done on Lake Mendota involving only surface water measurements.     

A simple method to estimate lake phosphorus concentrations resulting from natural, background, loadings

The relation of mean total phosphorus concentrations to morphoedaphic index (calculated as the ratio between the mean depth and the alkalinity or the conductivity) has been examined for a large, diverse, widely distributed group of lakes.A highly significant correlation was found in lakes practically not affected by anthropogenic input of phosphorus. As a consequence the phosphorus concentration in a lake resulting from natural, background, loading could be calculated on the basis of the morphoedaphic index. The evaluation of natural phosphorus concentration allows the prediction of the maximum extent to which actual phosphorus concentrations could be reduced in culturally eutrophied lakes by eliminating the anthropogenic inputs and to calculate “permissible” loadings necessary to achieve natural trophic conditions that will not necessarily correspond to oligotrophy.     

Groundwater seepage nutrient loading in a Florida Lake

Twenty-five seepage meters were positioned in East Lake Tohopekaliga, Florida, to determine groundwater seepage contributions of water and nutrients to the lake in 1983. Seepage was found to be an important source of water to the lake, contributing 14.3% of the water sources, and rates decreased significantly (P < 0.01) with distance from shore. A comparison of the piezometer and seepage meter techniques for measuring nutrient loading to the lake indicates the direct seepage meter technique overestimated nutrient inputs due to the enclosure to the sediments, possibly resulting in anaerobic conditions and increased release rates of ammonium nitrogen and phosphate. These results suggest that past studies employing this technique may be in error. Nutrient loading, calculated from piezometer nutrient data and seepage meter flow data, show that the groundwater nutrient loading in the lake was significant, contributing 8.7 and 17.6% of the total phosphorus and total nitrogen inputs to the lake, respectively.     

Nitrate suppresses internal phosphorus loading in an eutrophic lake

The presence of nitrate in the hypolimnion of the eutrophic, dimictic Upper Mystic Lake has been previously shown to suppress the release of arsenic from lake sediments during seasonal anoxia, in large part by oxidizing iron (II) and producing iron oxyhydroxides that sorb inorganic arsenic. Because of the importance of internal phosphorus loading in the phosphorus budget of many eutrophic lakes, the chemical similarities between phosphate and arsenate, and the need to account for internal phosphorus loading as part of many lake restoration strategies, we carried out measurements to determine if the presence of nitrate also suppressed the release of phosphorus from the sediments of this lake during anoxia. Observations showed that this was the case. Arsenic, phosphorus, and iron (II) concentrations were strongly correlated in the water column, as expected, and the depths below which phosphorus and iron concentrations increased relative to epilimnetic values was predicted by the depth at which nitrate concentration approached zero. The results suggest that knowledge of a lake’s nitrogen budget may be a useful tool in the design of lake remediation efforts, even though phosphorus is typically the limiting nutrient.     

Bioassays and the algal populations of Hastings Lake, Alberta, Canada

During summer 1978 an analysis was made of the biomass development and algal species composition of Hastings Lake, a shallow eutrophic lake near Edmonton, Alberta, Canada. The algal growth potential of Hastings Lake water during the period of investigation was of particular interest. Bioassays were carried out using untreated water, water inoculated with Anabaena flos-aquae (competition experiments) and filtered or autoclaved water.The results of bioassays with untreated Hastings Lake water were well correlated with the biomass development in the lake. Bioassays with unialgal cultures in filtered or autoclaved water were carried out to obtain information concerning the nutrient situation in the lake. Iron and/or phosphorus appear to be important limiting factors during the summer period in Hastings Lake.     

A preliminary modeling analysis of water quality in Lake Okeechobee, Florida: Diagnostic and sensitivity analyses

A deterministic mass balance model for nutrient and phytoplankton dynamics was previously developed and calibrated to a comprehensive set of field data for Lake Okeechobee. In the present study, diagnostic and sensitivity analyses were conducted with the calibrated model to better understand factors controlling phytoplankton and load-response dynamics in the lake. Phytoplankton growth rate limitation due to underwater light attenuation appears to be substantially greater than growth rate limitation due to non-optimal phosphorus concentrations. Phytoplankton biomass appears strongly controlled by the supply rate of dissolved available phosphorus to the water column. The dynamics of total phosphorus and chlorophyll a concentrations in the lake are strongly influenced by sediment-water phosphorus fluxes. There is a wide range of uncertainty in responses of total phosphorus and cholorophyll a concentrations to changes in tributary phosphorus loadings. Much of this uncertainty is due to a lack of quantitative understanding of sediment responses to changes in tributary loadings. Other important factors are inter-annual variability in hydrometeorological conditions and the potential influence of wind-induced resuspension of particulate phosphorus. Responses of total phosphorus and chlorophyll a concentrations for a given change in tributary loading depend not only on the magnitide of the loading change, but also on the time frame after the loading change due to a lag in sediment response. Load-response predictions for Lake Okeechobee must take into account changes in available phosphorus loadings to the water column, and must be premised on assumptions for changes in internal phosphorus loadings from the sediments. Results from this preliminary modeling analysis are provisional in that they do not include potential nitrogen limitation, potential interactions between phosphorus and nitrogen, or phytoplankton responses to potential nitrogen fixation.     

On the relationship between lake trophic level and lake sediments

The purpose of this work has been to study the relationship between lake type, as expressed by various trophic characteristics, and sediment type, as expressed by determinations of simple chemical data on nitrogen, phosphorus, carbon and loss on ignition. The study is based on two sets of data. The first one emanates from 71 lakes for which information is available from published sources on trophic level and N-content and organic content (loss on ignition) from surficial sediments. More detailed information has been obtained from 12 Swedish lakes/basins on the following water indicators of trophic level: transparency, chlorophyll-a, total-N, total-P, organic-N, inorganic-N. The sediment data from these 12 lakes include: mean values and characteristic values for organic content (loss on ignition, IG) C-, N- and P-contents from surficial sediments. A BPN-value (bioproduction number) is defined by the slope coefficient of the regression line between N-content and IG-content of surficial sediments (0–1 cm). It has been shown that the BPN-value provides most accurate information about lake trophic level on the scale from oligotrophy to eutrophy, provided that more than 50% of the lake area has lower loss on ignition than 20% (per dry substance). The BPN-value cannot be used as an indicator of trophic level if the IG-content is predominantly higher than 30% in a lake. In the latter case the ratio IG/N can be used as a means to differentiate lake humic level. The BPN-value, which is determined from sediment samples providing an even spread throughout the lake surface, yields lake typical information, whereas C/N ratios yield site specific information. A key diagrammatic interpretation between lake trophic type and lake sediments has been presented.     

A preliminary modeling analysis of water quality in Lake Okeechobee, Florida: Calibration results

To gain understanding of nutrient and phytoplankton dynamics in Lake Okeechobee, Florida, we developed and applied a deterministic, mass balance, water quality model at the whole-lake spatial scale. The model was calibrated to a comprehensive set of field data for 1985–1986, and then used to simulate the period 1973–1992. The model represented the mean behavior of in-lake total phosphorus, dissolved available phosphorus, total nitrogen and chlorophyll a concentrations reasonably well during the calibration period. The model did not represent dissolved available nitrogen concentrations very well, nor did it capture much of the observed temporal variability during the calibration period. The model results identified important information needs to improve our understanding of the nitrogen cycle including, sediment-water nitrogen fluxes, denitrification and nitrogen fixation. Results from the 1973–1992 simulation indicated that model assumptions and/or calibration parameters were not uniformly applicable over this period. Total phosphorus concentration results from this model were compared with results from two site-specific, empirical loading models for the lake. None of these models represented annual average concentrations uniformly well over the entire 20-year period, and none captured much of the observed inter-annual variability. External total phosphorus loadings and lake hydrology are not sufficient to fully describe total phosphorus dynamics in Lake Okeechobee. Other important factors are diffusive sediment-water fluxes, wind-induced sediment resuspension, and the spatial heterogeneity in the lake.     

Predicting the long-term variations in stream and lake inorganic aluminium concentrations for acidic and acid sensitive catchments

A modification to the one-box version of the MAGIC model of stream and lake acidification is given which presents a new hypothesis for describing inorganic aluminium controls in streams draining acidic catchments. This modification entails the removal of any assumption of Al(OH)₃ solubility control in the stream. Success of this modification is demonstrated for data collected from a regional survey of Welsh streams and lakes. The modified model is used with data from the acidic and acid impacted Dargall Lane catchment of the Galloway region of South West Scotland to provide an example of the long-term changes in stream acidity and inorganic aluminium concentration.     

Calcium carbonate deposition in Ca2+ polluted Onondaga Lake,New York,U.S.A.

Seasonal and long-term trends in the rate of calcium carbonate (CaCO₃) deposition are documented for Ca²⁺ polluted Onondaga Lake, New York. These observations are based on particulate inorganic carbon and particulate calcium analyses of sediment trap collections, made weekly over the May-September interval for 10 years. Continuous deposition of CaCO₃ was evident for the monitoring period of all 10 years. The average rate of CaCO₃ deposition for the deployment interval decreased from about 0.196 to 0.082 mol·m⁻²·d⁻¹ following the closure of the industrial source of Ca²⁺ pollution. This decline was generally consistent with the reductions in Ca²⁺ loading and lake Ca²⁺ concentrations over the same period. The prevailing rate of CaCO₃ deposition in Onondaga Lake remains substantially higher than the rates documented for three other hardwater lakes in the literature. Calcium carbonate is the primary component and principal regulator of the dynamics of total solids deposition in the lake.     

A comparison of rain-related phosphorus and nitrogen loading from urban, wetland, and agricultural sources

Comprehensive watershed studies have been conducted for two lakes located in the Lake Michigan drainage system. Studies were conducted from March through October of 1979. During that interval, large differences in storm-related nutrient loading were measured from urban, wetland, and agricultural sources. Eliminating runoff due to melt of the snow pack, it was found that rain-related discharge from the urban area studied was 0.578 kg total-P and 3.688 kg total-N ha⁻¹ of watershed. Rain induced runoff from marshes in the same drainage basin transported 0.023 kg total-P and 0.585 kg total-N ha⁻¹ of catchment. Rainfall of approximately the same amount caused runoff from agricultural land of 0.180 kg total-P and 5.965 kg total-N ha⁻¹. Algae of both lakes were phosphorus limited; nitrogen was present in excess. Using constants from Nichols-Dillon relationships in the literature regarding phosphorus, phytoplankton biomass, and secchi disc transparencies, the urban input of phosphorus ha⁻¹ of drainage was sufficient to bring 0.96 ha-m of lake water to undesirable algal bloom status. Similarly, march input ha⁻¹ would bring an estimated 0.04 ha-m into bloom. By the same calculation, storm-related agricultural runoff would result in 0.30 ha-m of lake water becoming undesirably rich in algae. Knowing the number of hectares in these types of catchment and the volume available in a particular lake for phytoplankton production, decisions regarding cost-effective treatment of rain-related discharge can be made.     

The spatial distribution and emission of nitrous oxide (N2O) in a large eutrophic lake in eastern China: Anthropogenic effects

The emission of N₂O from China is globally significant, but relatively few direct observations have been made in many of the fresh water environments most likely to be important sites of N₂O production. In this paper, N₂O saturations were examined in the ecologically heterogeneous, eutrophied, Lake Taihu, as well as in surrounding rivers in eastern China. The emissions of N₂O were estimated and compared with those from other landscapes within the Lake Taihu drainage basin. We found that anthropogenically-enhanced inorganic N inputs act as a limited primary control on the spatial distribution of N₂O saturations in heavily eutrophied parts of the lake only and that overall, lake N₂O production and emission are not raised as significantly as expected due to high N inputs. In comparison, the heavily eutrophied river network is an important fraction of the local N₂O budget, and when considered together with emissions of N₂O from the lake, constitute a major (10-50% depending on season) fraction of total N₂O emissions from the Lake Taihu drainage basin.