Total inputs of phosphorus and nitrogen by wet deposition into Lake Taihu, China

Lake Taihu suffers from eutrophication caused by riverine nutrient inputs and air deposition. To characterize wet deposition of phosphorus (P) and nitrogen (N) to the lake, precipitation collection and measurements of total phosphorus (TP) and total nitrogen (TN) and other components at five cities around Lake Taihu were made from July 2002 to June 2003. TP and TN concentrations and deposition rates exhibited strong spatial variation in the whole catchment. An inverse correlation between station-averaged TP and TN concentrations and precipitation amount was found. Maximal TP concentration in rainfall was found in Suzhou, and maximal TN in Wuxi. However, highest wet deposition rates of TP and TN were found in Suzhou, which suggests that atmospheric nutrients are mostly from the east and northwest area of Lake Taihu. Mean TP and TN deposition rates were 0.03 and 2.0 t km⁻² year⁻¹ respectively in Lake Taihu, which are greater than reported values in other areas by comparision. Total N and P contributed to the lake by wet deposition were 75 and 4720 t per year, respectively, which represent about 7.3% and 16.5% of total annual N and P inputs via inflow rivers. Wet deposition, especially N, could have significant effects on eutrophication in the lake, which shows that air deposition should be taken into account while reducing the external nutrients in the lake.     

Nuisance cyanobacteria in an urbanized impoundment: interacting internal phosphorus loading, nitrogen metabolism, and polymixis

Severe nuisance blooms of cyanobacteria, mainly Aphanizomenon and Microcystis, historically have plagued polymictic Ford Lake, one of the most productive warm-water sport fishing lakes in Michigan, U.S.A. Biomass development in the lake is known to be limited by phosphorus. Alternative theories ascribed nuisance conditions either to allochthonous inputs or to internal loading of phosphorus from anoxic sediments. From 2003 to 2009, there was a strong linear relationship between allochthonous total phosphorus income and phosphorus retained within the basin. The relationship had a negative intercept, implying negative retention, or positive export, of phosphorus from the lake from May to September. Mass balance calculations at lake inlet and outlet were consistent with rates of sedimentary phosphorus accumulation measured from sediment cores. Release rates of soluble reactive phosphorus from anoxic sediments were half that of allochthonous inputs. However, severe declines in nitrogen to phosphorus ratio developed in the anoxic, nitrate-poor hypolimnion because accumulation of ammonium was only tenfold that of phosphate. The result was a steep decline in ratio of total nitrogen to total phosphorus during July and August throughout the lake after episodic mixing events, followed by and coinciding with development of heterocystous Aphanizomenon populations. Lake sediment composition determined by X-ray fluorescence in addition to results of sediment core experiments indicates that phosphorus release is governed by an iron trap mechanism such that phosphate and iron are released only when both oxygen and nitrate are depleted.     

Substance budgets of an upland catchment: the significance of atmospheric phosphorus inputs

1. Precipitation inputs and outflow stream outputs are presented for 1993 in an upland lake and catchment system on the Antrim Plateau, Northern Ireland. 2. Phosphorus, potassium, chloride and possibly sulphate behaved conservatively inputs were approximately balanced by outputs. Combined nitrogen outputs were very much less than inputs, whereas there was a net export of calcium, magnesium, sodium and silica from the catchment. 3. Precipitation phosphorus inputs (22 kg P km^?2yr^?1) are compared with literature data and are shown to be near the median value. 4. The phosphorus budget is discussed in relation to the fact that there are few oligotrophic lakes in Northern Ireland. It is suggested this is due to the rainfall inputs, low phosphorus retention by the catchment and rapid flushing rates in the lakes.     

A Long Term Study of the Phosphorus Cycle in Lough Neagh,Northern Ireland

Eleven years of weekly observations on the phosphorus cycle in Lough Neagh are analysed. The data comprised inputs, outputs and lake concentrations of the soluble and particulate phosphorus fractions. It is shown that the particulate phosphorus input quickly sediments to the bottom and the particulate phosphorus in the lake is largely made up of phytoplankton. The average phosphorus retention in the lake was 34 %, similar to the particulate phosphorus input. Climate has a big influence on the phosphorus input and sediment release, both of which varied two-fold. A simulation model suggested that suppression of sediment P release would reduce lake winter phosphorus concentrations by an average of 29 %.     

Evaluation of recent limnological changes at Lake Apopka

Recent changes in submersed macrophytes and water quality variables have been offered as the strongest evidence that the current restoration program at Lake Apopka will be effective (Lowe et al., 2000); however, the new beds of submersed plants in Lake Apopka are found only on hard substrates on the fringes of the lake within 40 m of shore and are protected from waves by cattails (Typha spp.). They occupy only 0.02% of the lake area, and there is no indication that they can colonize the flocculent sediments that make up 90% of the lake area. There is no correlation between annual inputs of phosphorus and total phosphorus concentrations in the lake, and patterns of change in chlorophyll and other water quality variables do not follow changes in phosphorus loads. Rather than reflecting decreases in phosphorus loading, the recent changes could be related to the harvest of benthivorous fish or are just the normal fluctuations found in lakes that have not been perturbed. Regardless of the reason the macrophytes were lost in the 1940s, the new analyses confirm our previous findings that the high turbidities in Lake Apopka are due to the resuspension of sediments, and that the fluid mud cannot support the colonization of submersed aquatic macrophytes. Even without the fluid mud, the target phosphorus concentration of 55 mg m^–3 is too high to bring about the restoration of the former macrophyte beds in the lake.     

Whitemere, a lake that defies some conventions about nutrients

1. Whitemere, a kettle-hole lake in north-west England (z_m,14 m, area 22.5 ha) has extremely high maximal concentrations (around 1 mg L⁻¹) of total phosphorus (TP) and soluble reactive phosphorus (SRP), and comparatively low maximal concentrations of dissolved inorganic nitrogen (<0.5 mg N L⁻¹). Bioassays indicate that its phytoplankton is nitrogen limited, and it has surface blue-green algal blooms in summer. Palaeoecological investigations have shown that high populations of cyanophytes occurred more than 6000 years ago, even before human settlement of the catchment.
2. The reasons for this combination of features are not immediately clear. This paper attempts to distinguish between two hypotheses: that the high phosphorus concentrations arise from input of phosphate-rich groundwater; and that the high phosphorus concentrations arise from concentration and recycling mechanisms within the lake.
3. The lake is entirely fed by ground water, direct rainfall and surface sheet flow. The concentrations of phosphorus in the ground water are much lower than in the lake water. The lake is stratified and has high phosphorus concentrations in the anaerobic hypolimnion in summer as a result of substantial release from the sediment. Vertical turbulence in the relatively weakly stratified water column may transfer substantial amounts to the epilimnion.There is also substantial phosphorus release from the sediments under the aerobic epilimnion.
4. There is little outflow of water and phosphorus. Most of the phosphorus is returned to the sediment under winter isothermal conditions, to be released again the following summer. The lake has probably maintained these mechanisms for millennia. Recent eutrophication is likely to have been driven by nitrogen inputs from greater intensification of agriculture in the catchment.     

Transport of groundwater-borne phosphorus to Lake Bysjön,South Sweden

Lake Bysjön is a hypertrophic seepage lake, with groundwater as a main external source of phosphorus. Twelve groundwater samples from the vicinity of the lake were high in phosphate (0.4 to 11 mg l^–1, mean value 2.57 mg l^–1 PO₄-P), both within the riparian zone and in two shallow wells located upstreams the lake in the nearby village. Phosphorus sorption capacity of four sand samples measured with the Langmuir isotherm method was low (7.3 to 121,1 mg kg^–1 PO₄-P), with the lowest values found within the riparian zone. It is suggested that the phosphorus originates from garden fertilizers and other human sources, and that the low absorption capacity of the soils is caused by the leaching of calcium from the watershed, a process which started some 3000 years ago. Riparian zone itself has almost no retention capacity, and processes within it (e.g., redox-related) have only secondary importance for the transport of phosphorus to the lake.     

Limnology of naturally acidic,oligotrophic dune lakes in subtropical Australia,including chlorophyll — phosphorus relationships

This study provides the most comprehensive physico-chemical and phytoplankton data yet available for Australian dune lakes, which are among the world's most naturally acidic and oligotrophic freshwaters. Seasonal and spatial variations were examined in Blue Lagoon and Lake Freshwater, two ‘water-table window’ lakes in south-east Queensland. Like other dune lakes, they are acidic (minimum pH 4.20 and 4.55, respectively), polymictic water bodies with low concentrations of marine-derived major ions and almost undetectable levels of trace metals. While linmologically similar in winter, during spring-summer Lake Freshwater has significantly higher levels of chlorophyll-a, total phosphorus (TP) and turbidity than Blue Lagoon and other dune lakes, indicating seasonal mesotrophy. The key nutrient is TP, which has recently increased to a maximum of 17 μ· l⁻¹, due either to inputs from recreational sources, or to the death and decomposition of littoral vegetation resulting from falling water levels over the last decade. Inorganic nitrogen, though present only in small amounts, does not appear to limit the eutrophication process because of a shift in phytoplankton dominance from the usual desmids and dinoflagellates to N₂-fixing blue-green algae. A chlorophyll - TP linear regression derived for dune lakes indicates that at TP < 20 μg · l⁻¹ chlorophyll ‘yield’ is higher than in other lake types represented by regressions from the literature. This may be due to a more efficient utilization of the limited available phosphorus by dune lake algae which have adapted to the naturally oligotrophic environment. The implications of these findings for lake management are discussed.     

Input-output budgets at Langtjern,a small acidified lake in southern Norway

Precipitation and streamwater volume and chemical composition have been measured since 1974 at Langtjern, a small, acid (pH 4.6–4.8) lake on granitic-gneissic bedrock in coniferous forest located ca. 100 km north of Oslo, Norway. The area receives acid precipitation (weighted average pH 4.28). The 7-year input-output budgets for major ions at two terrestrial subcatchments indicate that for Na, K, SO₄ and Cl outputs approximately equal inputs, for H⁺, NH₄ and NO₃ outputs are much less than inputs, and for Ca, Mg and Al outputs greatly exceed inputs. The sulfate budgets (which include estimated dry deposit) indicate that the terrestrial catchment retains about 20% of the incoming sulfate, perhaps due to absorption in the soil, plant uptake, reduction and storage in peaty areas or reduction and release of H₂S to the atmosphere. The budgets for Langtjern lake itself indicate that for most components output equals inputs to within 10%, i.e. these compounds simply pass through the lake. For H⁺, and possibly NH₄ and NO₃, inputs exceed outputs. Because gaseous phases are not measured the N budgets are uncertain. A mechanism that leads to ‘retention’ of both H⁺ and SO₄ is sulfate reduction and incorporation of sulfides in the lake sediments. Such has been documented in the experimentally-acidified Lake 223, Experimental Lakes Area, Ontario, Canada. Although there is no evidence suggesting the development of anoxic bottom waters at Langtjern, such reduction might occur at the water-sediment interface and in the sediments. The budgets for the pollutant components H⁺ and SO₄ at Langtjern differ substantially from those at the relatively unaffected Lake 239, in the Experimental Lakes Area.     

Some Bag Experiments on Tibbs Run Lake,W.V., A Eutrophic Acidified Lake

Tibbs Run Lake was sampled from December 1979 to December 1980, monthly during the winter and biweekly during the spring, summer, and autumn. Primary production was measured from March 1980 to February 1981, either monthly or biweekly, as indicated above. The mean annual hydrogen ion concentration was equivalent to pH 4.33. This pH was 46% lower (based on H⁺ concentrations) than the 1977–1978 mean. The low pH of the lake is due to inputs of acid precipitation and the low buffering capacity of the watershed. Predictions of further reductions in the lake pH, based on mean annual hydrogen ion concentrations and hydrogen ion retention coefficients (RH) from Shellito (1979) and this study, indicate that the lake pH should reach an equilibrium with the precipitation pH between 1982 and 1985, assuming the precipitation pH continues to average 4.10. The stratified period mean chlorophyll a and total phosphorus concentrations were 22.1 mg/m³ and 19.3 μg/1, respectively. Total phosphorus loading to the lake was 0.233 gP/m²/yr. To examine the effects of neutralization and fertilization with phosphorus an in situ bag enclosure experiment, lasting 16 days, was performed during the summer of 1980. In the bags, phosphorus (20 μg/1 on days 0,4,8,12 as KH₂PO₄) was added alone and in combination with a base addition (IN KOH). The base additions raised the pH to approximately 7.8. Other treatments included base addition alone and a control. The maximum chlorophyll a concentration was found in the phosphorus treatment (∼30 mg/m³) on day 6. The phosphorus plus base treatment exhibited a 10 day lag before reaching a maximum chlorophyll a concentration (∼27 mg/m³) on day 16. The highest production rate was found in the phosphorus plus base treatment on day 12 (∼27 mgC/m³/hr.). A similar bag experiment, lasting 46 days, was performed during the late summer and early autumn of 1980. The treatments were the same as in the first experiment, however, the nitrate-nitrogen depletion in the lake and treatments necessitated the addition of nitrogen (2 mg/1 as NaNO₃) to one of each replicate treatment on day 5. On day 34, the maximum chlorophyll a concentrations were found in the phosphorus plus base nitrogen treatment (∼120 mg/m³) and the phosphorus plus nitrogen treatment (∼38.8 mg/m³). The control plus nitrogen treatment was submerged and possibly contaminated on day 5. All treatments not receiving nitrogen, except for the phosphorus plus base treatment, decreased in the chlorophyll a concentration throughout the experiment. The phosphorus plus base treatment peaked in the chlorophyll a concentration on day 45 (∼28 mg/m³) despite non-detectable levels of nitrate-nitrogen. The maximum primary production rate was found in the phosphorus plus base plus nitrogen treatment (∼32 mgC/m³/hr.) on day 11.     

Phosphorus inputs to Lake Naivasha,Kenya, from its catchment and the trophic state of the lake

The main river supplying Lake Naivasha, Kenya, the Malewa, drains a catchment given over to largely subsistence cultivation and animal husbandry. The lake itself is the focus for an intensive horticultural industry based upon irrigation from the lake. The Malewa, however, is relatively independent of the impact of industry, and so its contribution to eutrophication of the lake was evaluated. Two periods of study, a very wet-dry and a `normal' wet-dry season showed that the river contribution of phosphorus led to a total phosphorus loading of 1.4 g m<sup>–2</sup> lake surface ann<sup>–1</sup> in the very wet period compared to 0.2 in the `normal'. Chlorophyll `a' in the open water of the lake was significantly related to soluble reactive phosphorus. The lake is now eutrophic by normal limnological criteria.     

The Proportion of Empneuston and Total Atmospheric Inputs of Carbon,Nitrogen and Phosphorus in the Nutrient Budget of a Small Mesotrophic Lake (Piburger See,Austria)

Litter and wet traps were employed to determine the inputs of coarse organic matter (empneuston), total organic carbon, total nitrogen and phosphorus through dry fallout and precipitation to a soft-water, mesotrophic lake. The dispersal of airborne material over the lake surface was investigated, and a method for the calculation of total input was developed. The component of the particulate matter larger than 1 mm contained the largest proportion of organic carbon, while the dissolved and the fine particle (smaller than 1 mm) fractions made greater contributions to the inputs of nitrogen and phosphorus compounds. The importance of the large particle fraction of the airborne organic matter in the lake's nutrient budget during the autumnal fall of litter was confirmed. However, the maximum total airborne input seems to occur during spring and summer. The input of phosphorus compounds through the atmosphere to small forest lakes is large compared to that from other sources. Airborne material accounts for 39% of the total phosphorus loading.     

The effects of land use upon water chemistry,particularly nutrient enrichment,in shallow lowland lakes: comparative studies of three lochs in Scotland

Three shallow, lowland lochs (lakes) in the Tayside region of Scotland, experiencing the same climatic regime, were found to be dimictic lakes showing similar clinograde oxygen distributions in summer. Land use differences in their catchments were shown to result in estimated total nutrient surface loadings from 0.3 to 32 g m^–2 a^–1 phosphorus and from 4 to 240 g m^–2 a^–1 nitrogen. The major ions in the lochs were calcium and carbonate, but with elevated sulphate levels in all three lochs and an increase in sodium, chloride and sulphate in Forfar loch, which was affected by sewage effluent. Conductivity and total alkalinity showed marked increases with greater intensity of land use, from 64 to 439 µS cm^–1 and 0.5–3 meq l^–1 Maximum winter loch concentrations of soluble reactive phosphorus ranged from 60 to just under 5 000 mg m^–3 and of inorganic nitrogen from 500 to 10500 mg m^–3. Maximum chlorophyll a ranged from 20 to 250 mg m^–3 and comparisons indicated that above winter levels of 5000 mg m^–3 N and 500 mg m^–3 P, the nutrient-chlorophyll relationships did not hold. Predictions of nutrient input, from land use categories and soil losses of N and P derived from other north temperate areas, were shown to be comparable with inputs calculated from loch measurements. Models predicting loch concentrations of phosphorus from inputs were comparable with measured concentrations, but predictions of chlorophyll and transparency became less accurate with higher nutrient levels. The lochs were mesotrophic (the Lowes), eutrophic (Balgavies) and hypertrophic (Forfar) under the several classification systems used. The implications of their nutrient status for lake management are discussed and the value of studying this unique lake series in a similar physical environment but with considerable chemical differences is considered.     

城市化背景下白洋淀入湖营养盐负荷模拟研究

过量的营养盐输入导致白洋淀富营养化程度较高。雄安新区建设伴随着快速城镇化进程,将可能进一步增加入湖营养盐负荷。为摸清新区建设背景下白洋淀入湖营养盐负荷的变化趋势及削减量,基于入湖营养盐负荷模型计算了历史不同时期（1995-2015年）白洋淀入湖营养盐盐负荷,预测了土地利用、农业管理和污水处理系统变化下入湖营养盐负荷的变化趋势。结果表明,白洋淀历史时期入湖总氮和总磷年平均负荷分别为2018 t和313 t,主要来源分别是耕地和畜禽粪便排放。就土地利用变化而言,以2010年为基准,2050年白洋淀入湖总氮和总磷负荷在建设用地快速增长（RAP）情景下增长率最高,分别增长了56%和60%,主要原因在于城镇人口增加,从而增加了畜禽需求量和生活污水排放。就农业管理而言,削减化肥使用有利于降低入湖总氮负荷,控制畜禽粪便排放更有利于入湖总磷负荷的降低。2050年,生活污水将成为白洋淀入湖营养盐的重要来源,因此,在制定未来社会经济发展路径时,应重点提高农村地区生活污水收集率和处理效率。PCLake模型对不同入湖负荷下湖泊营养状态演变的模拟发现,削减入湖总磷负荷是白洋淀水质恢复的关键。白洋淀水质达到III类和IV类标准时,入湖总磷负荷阈值分别为2.1和2.6 mg P m^-2 d^-1。在建设用地快速增长情景下,若使白洋淀水质达到III类标准,入湖总磷负荷应减少131 t。     

Nitrogen and phosphorus budgets for the sub-tropical Richmond River catchment, Australia

Nitrogen and phosphorus budgets were developed forfour sub-catchments in the Richmond River catchmentfor two study years. The catchment is used for avariety of farming pursuits including dairying, beef,cropping, fruit, nuts, forestry, and sugar cane. Eachsub-catchment varies in hydrology, the proportion ofeach land use, and the population density whichenabled a unique opportunity to study fluxes andstorage associated with a variety of environmentalfactors. Total loadings entering each sub-catchmentvaried from 12 to 57 kg ha^–1yr^–1 fornitrogen and 0.25 to 6.6 kg ha^–1yr^–1 forphosphorus with little inter-annual variation.Averaged across the whole catchment, nitrogen fixation(47%) dominated the inputs; fertiliser (26%) andrainfall (21%) made up the next largest inputs.Fertiliser inputs dominated the phosphorus budget(65.5%); rainfall and manures making up 13% and 12%respectively. Produce dominated the outputs of bothnitrogen and phosphorus from the four sub-catchmentsbeing greater than the riverine export. The deliveryof nitrogen to catchment streams ranged from <1 to24% of the total inputs and the delivery of phosphorus to catchment streams ranged from <1 to 39%. Storage of phosphorus in catchment soils varied between –0.32 and 4.46 kg ha^–1yr^–1. Whendenitrification and volatilisation were estimated using data from other studies, storage of nitrogen ranged from 1 to 24 kg ha^–1yr^–1. Despite theepisodic nature of runoff in the sub-tropical RichmondRiver catchment, the magnitude of nutrient fluxes andstorage appear similar to other catchments of theworld which have mixed land use and relatively lowcatchment nutrient loadings.     

The atmospheric deposition of phosphorus in Lake Victoria (East Africa)

Wet and dry atmospheric fluxes of total phosphorus (TP) and soluble reactive phosphorus (SRP) measured at four sites over a 12-month period were used to estimate lake-wide atmospheric phosphorus (P) deposition to Lake Victoria, East Africa. Atmospheric samples were collected in plastic buckets with top diameter of 25.5 cm by 30 cm deep. The highest P loading rates of 2.7 (TP) and 0.8 (SRP) kg ha^–2 year^–1 were measured at Mwanza compared to less than 1.9 (TP) and 0.65 (SRP) kg ha^–2 year^–1 measured in other three sites. By applying these loading rates to the lake surface, it was estimated that 13.5 ktons (13.5 × 10³ kg) of TP were deposited annually into the lake from the atmosphere. Thirty-two percent of the total was found to be in the SRP form. Dryfall, a component ignored in previous studies exceeded wet deposition by contributing 75% of the total P input. However, materials deposited by dryfall made a lesser contribution to soluble form of phosphorus, as SRP concentrations in the wet samples were 2–3 times higher than SRP concentrations in dry samples. The annual fluxes of phosphorus measured on the south and western shores of Lake Victoria (1.8–2.7 kg ha^–2 year^–1) are near the upper range of similar fluxes measured in the tropics. In comparison with the existing estimates of municipal and runoff P inputs from other studies, it is estimated that atmospheric deposition represent 55% of the total phosphorus input to the Lake Victoria. The four sampling sites were fairly clustered and wet and dry P deposition data were collected from shore/land stations and applied to open lake areas to estimate lake-wide P deposition. In this regard, the estimates determined here should be viewed as a first order approximation of actual P load deposited into the lake.     

A review and reassessment of lake phosphorus retention and the nutrient loading concept

1. We conducted a statistical reassessment of data previously reported in the lake total phosphorus (TP) input/output literature (n = 305) to determine which lake characteristics are most strongly associated with lake phosphorus concentration and retention. We tested five different hypotheses for predicting lake TP concentrations and phosphorus retention. 2. The Vollenweider phosphorus mass loading model can be expressed as: TP_out = TP_in/(1 + στ_w), where TP_in is the flow‐weighted input TP concentration, τ_w is the lake hydraulic retention time and σ is a first‐order rate constant for phosphorus loss. 3. The inflow‐weighted TP input concentration is a moderately strong predictor (r² = 0.71) of lake phosphorus concentrations when using log–log transformed data. Lake TP retention is negatively correlated with lake hydraulic retention time (r² = 0.35). 4. Of the approaches tested, the best fit to observed data was obtained by estimating σ as an inverse function of the lake's hydraulic retention time. Although this mass balance approach explained 84% of the variability in log–log transformed data, the prediction error for individual lakes was quite high. 5. Estimating σ as the ratio of a putative particle settling velocity to the mean lake depth yielded poorer predictions of lake TP (r² = 0.77) than the approach described above, and in fact did not improve model performance compared with simply assuming that σ is a constant for all lakes. 6. Our results also demonstrate that changing the flow‐weighted input concentration should always have a directly proportionate impact on lake phosphorus concentrations, provided the type of phosphorus loaded (e.g. dissolved or particulate) does not vary.     

Water and Nutrient Mass Balance of the Partly Meromictic Temperate Lake Verevi

Mass balances of total nitrogen and total phosphorus were calculated for Lake Verevi (area 0.126 km², maximum depth 11 m, mean depth 3.6 m), a sharply stratified small lake located in South Estonia within the borders of the town Elva. The lake has up to 10 small inflows but only three of them are nearly permanent. Accidental overflows from near-by oxidation ponds during high floods have been the major source of the nutrient load of the lake in the past. L. Verevi receives a significant part of its inflow from groundwater, which is difficult to measure. In dry years the outflow is temporary. During summer the lake is sharply thermally and chemically stratified. The spring turnover is often incomplete even in homothermal conditions, thus giving the lake some meromictic features. The influx of nitrogen exceeded the outflux at any supposed proportion (20%, 50%, 80%) of surface runoff. The lake retained 45–90% of the nitrogen influx by sedimentation and/or by denitrification. The largest nitrogen losses with loss rates more than 10 kg N d⁻¹ occurred in May and June. The calculated phosphorus retention rate became strongly negative during mixing periods. From June to November, phosphorus release from the sediment exceeded sedimentation by 205 kg in 1991 and by 79 kg in 1993. Earlier stagnation and absence of a full spring turnover in the 2000 has slowed down the recovery of the lake because less phosphorus is flushed out. However, the stronger stratification and significantly smaller phosphorus content in the epilimnion limits biological activity and as a result improves the water quality of the surface layer.     

Response of a shallow boreal lake to recent nutrient enrichment – implications for diatom-based phosphorus reconstructions

Water quality of the shallow, mesotrophic, and macrophyte-dominated Lake Kaljasjärvi has been monitored at three to four year intervals since 1978. During the monitoring period, surface-water total phosphorus (TP) concentrations have typically varied between 20 and 25 g P l^–1. However, elevated total phosphorus concentrations were measured in 1987, 1991, and 1999. Diatom-based reconstruction of the historical lake-water TP concentrations was therefore employed to study the recent development of the lake. However, the diatom-TP model did not predict the high measured phosphorus concentrations despite the changes observable in diatom assemblages. In addition, the ratio of sedimentary diatom remains to chrysophycean stomatocysts declined towards the top of the sediment core, indicating decreasing trophy rather than eutrophication. Analysis of sedimentary pigments and phosphorus fractions, used to examine further the changes, also produced results that contradicted the simple eutrophication hypothesis. In particular, the proportion of chlorophyll derivatives instead of carotenoids increased and there was a rise in the concentration of refractory instead of NaOH-extracted phosphorus. These features appear to be related to the extensive littoral areas of the lake since enhanced littoral production can explain both the observed changes in sediment chemistry and the low diatom-inferred TP (DI-TP). Littoral primary producers are suggested to have benefited from the increased phosphorus inputs to the lake, transferring some of the phosphorus to the detrital pool and contributing to the increased pigment concentrations of sedimentary organic matter. High proportions of non-planktonic diatoms in the samples lower DI-TP because periphytic taxa are assigned low TP optima in the inference models used. Abundant aquatic macrophytes may also have made the lake resistant to eutrophication by assimilating nutrients, providing refuge for zooplankton, and having an allelopathic effect on phytoplankton. Since 1980, however, the sedimentary diatom assemblages also indicate increasingly eutrophic conditions. Additional loading from numerous cottages during the last 20 years seems to have caused observable changes in the phytoplankton communities.     

CARBON,NITROGEN, AND PHOSPHORUS AND THE EUTROPHICATION OF FRESHWATER LAKES1

The question of nutrients responsible for eutrophication of freshwater lakes is reviewed, and recent additions to the literature on nutrient limitation are discussed. The paper by Lange is criticized on several grounds, including the facts that utilization of HCO₃^? by phytoplankton and the invasion of lake waters by atmospheric CO₂ are ignored as sources of photosynthetic carbon. The phosphorus and nitrogen concentrations used in Lange's experiments are far higher than values published by others for Lakes Erie and Ontario. Preliminary results of fertilizing a small oligotrophic lake with nitrogen and phosphorus are described. The standing crop of phytoplankton increased by 30–50 ×, while the P:N:C ratio in seston did not change from ratios found in unfertilized lakes. Other experiments done in water columns isolated with polyethylene film showed that addition of carbon did not increase the phytoplankton standing crop. Since the fertilized lake was initially lower in total CO₂ than any other recorded in the literature, it is concluded that carbon is unlikely to limit the standing crop of phytoplankton in almost any situation. Measurements of invasion of atmospheric gases to the fertilized lake by the Rn²²² technique were compared with phytoplankton production measurements, revealing that atmospheric invasion of CO₂ is sufficient to support the high phytoplankton standing crop in the epilimnion of the lake. Possible errors in interpretation of culture and bottle-bioassay experiments with respect to eutrophication are discussed.