Development and practical application of limiting values for the phosphate concentration in surface waters in the Netherlands

Summary To counteract eutrophication it is important to possess sufficient information (1) on the relationship between the content of nutrients and the algal biomass, and (2) on the cycling of nutrients in lakes.A comparative study of a large number of surface waters in the Netherlands has demonstrated that it is possible to derive a relationship between the nitrogen and phosphorus concentrations and the upper limit of the chlorophyll concentration, averaged over the summer season.For the authority in charge of water quality it is essential to know how far the phosphorus loading must be reduced to attain the desired phosphate concentration. The results of an extensive study of the nutrient budgets of Lake Wolderwijd-Nuldernauw over the period 1976–1979 demonstrate that in certain cases a relative high phosphate concentration can occur (approx. 0.30 mg/l) in spite of a low external phosphorus loading (approx. 0.7 g P/m² year). Such high concentrations can be explained by the continuous presence of a bloom of blue-green algae and the release of phosphate from the aquatic sediments during the summer. In such lakes it is necessary to take additional measures, such as flushing with water poor in algae and phosphates, or, where necessary, the removal of aquatic sediment rich in phosphate.     

Long-term changes in the phytoplankton of large lakes in response to changes in nutrient loading

In Sweden the recovery process of the four largest lakes (<500 km²) has been followed since the mid-1970's when the phosphorus loading was reduced by approximately 50%. A ten-year investigation period before that acts as a basis of comparison. The response of the dominating species in the phytoplankton communities in the eutrophic Lake Mälaren and the oligotrophic Lake Vättern is discussed also in relation to a pristine state of the lakes. The long-term investigations with monthly sampling make a natural phytoplankton biomass variation range possible to establish - a range that is mainly due to effects of weather fluctuations. In order to get a tool for water management where disturbing and highly disturbing total phytoplankton volumes can be anticipated from a certain phosphorus concentration, a model is suggested based on experiences from more than 50 Swedish lakes investigated during a series of years.     

Lake restoration by reducing external phosphorus loading: the influence of sediment phosphorus release

SUMMARY. 1. This review considers the factors which determine the recovery of eutrophic lakes following a reduction in the external phosphorus loading.
2 The mean phosphorus content of a lake should decrease roughly in proportion to the reduction in phosphorus input. Where the lake phosphorus concentration does not decrease as predicted, then the release of phosphorus from the sediment is implicated.
3. The current understanding of the processes by which sediment phosphorus is mobilized and transported into the photic zone is described. The extent to which phosphorus release can maintain lake phosphorus concentrations following the reduction in external loading is influenced by: lake morphometry, flushing rate, sediment type, trophic state and history of enrichment.
4. A reduction in the phytoplankton biomass of a lake is dependent upon the size of the decrease in lake phosphorus concentration and the degree to which phosphorus limits primary production. The importance of phosphorus in limiting phytoplankton production tends to decrease with increasing lake trophic status.
5. Improvements in the condition of highly eutrophic lakes require very large reductions in external phosphorus loading, whereas in mildly enriched lakes moderate changes in the supply of phosphorus have noticeable effects on phytoplankton biomass.     

Influence of internal loading on phosphorus concentration in shallow lakes before and after reduction of the external loading

An analysis of data from 49 shallow lakes showed, that the parameters of empirical models between phosphorus loading and concentration in the lake (e.g. Vollenweider type of relations) differ significantly for lakes without or with a reduced external loading. For lakes without a reduction of the external loading the summer phosphorus concentration is determined by the external phosphorus loading and the hydraulic loading. For these lakes the classical models suffice; deviations between calculations and measurements are partly due to errors made in the determination of the loading.In contrast, for lakes where the external loading was reduced, the measured internal loading explains most of the variation in the summer lake concentration. The external loading is of minor importance and the classical models cannot be applied. The internal loading measured before reduction of the external loading is not useful in predicting the concentration afterwards. Instead of the internal loading, the sediment composition can be used. The advantage of using sediment composition is that these variables are easier to determine and vary less in time. The most promising variable is the ratio between total P and total Fe in the sediment.Abbreviations: Q_s hydraulic loading (m y^-1) - hydraulic retention time (y) - L_ext external phosphorus loading (gP m^-2 y^-1) - L_int internal phosphorus loading (gP m^-2 y^-1) - P_lake phosphorus concentration in the lake (gP m^-3) - P_inlet phosphorus concentration in the inlet water (gP m^-3) - P_sed phosphorus content on the sediment (gP kg^-1 d.w.) - Fe_sed iron content of the sediment (gFe kg^-1 d.w.) - Y dependent variable multiple regression calculations - X1, X2 independent variables multiple regressions calculations - a, a1 constants - a2, b constants     

Lake restoration with and without dredging of phosphorus-enriched upper sediment layers

Human activity has been the cause of continuing decline of water quality in most Dutch lakes. Development of lake restoration programmes must take into account the lake functions. Major reduction of the nutrient and pollutant loading is the primary step in lake restoration. Still, the recovery of eutrophic lakes is retarded frequently because of internal phosphorus loading by the lakes' sediments. Sediment dredging, as an additional tool for water quality management to accelerate accomplishing the desired water quality, is studied. In this paper we evaluate the preliminary results of eight lake restoration projects in the Netherlands. The lakes are compared in order to estimate the magnitude of the internal phosphorus loading. Dredging as an additional measure was carried out twice in the peatlake Geerplas. In the Nieuwkoop Lakes only the external phosphorus loading was substantially reduced from 0.9 to 0.2 g P m^–2 y^–1. Provisional results of these two shallow peatlake restoration projects focussed on eutrophication abatement with and without dredging, are presented. Both show a decrease in phosphorus concentration in the lakes. The necessity to dredge the lakes is discussed.     

Biological indicators track differential responses of pelagic and littoral areas to nutrient load reductions in German lakes

External nutrient loading was reduced over the past decades as a measure for improving the water quality of eutrophic lakes in western Europe, and has since been accelerated by the adoption of the European Water Framework Directive (WFD) in 2000 (EC, 2000). A variety of eutrophication-related metrics have indicated that the response of biological communities to this decreased nutrient loading has been diverse. Phytoplankton, a major component of the pelagic community, often responded rapidly, whereas a significant delay was observed for submerged macrophytes colonizing littoral areas. In this study we tested whether assessment methods developed for phytoplankton and macrophytes in lakes during Germany's implementation of the WFD reflect this differential response. An assessment of 263 German lakes confirmed that a lower ecological state was recorded when based on the biological quality element (BQE) for macrophytes than the BQE for phytoplankton during the investigated period (2003–2012). On average, lakes had a moderate ecological status for both phytoplankton and macrophyte BQEs, but differences of up to three classes were observed in single cases. Long-term data were available for five lowland lakes subject to strong reductions in phosphorus loading. Their phytoplankton-based assessments indicated a constant improvement of the ecological status in parallel to decreasing water phosphorus concentrations. In contrast, macrophyte-based assessments indicated a 10–20 year delay in their ecological recovery following nutrient load reduction. This delay was confirmed by detailed data on the temporal development of macrophyte species diversity and maximum colonization depths of two lakes after nutrient load reduction. We conclude that the available WFD assessment methods for phytoplankton and macrophyte BQEs are suitable to track the differential response of pelagic and littoral areas to nutrient load reductions in German lakes.     

Seasonal response of nutrients to reduced phosphorus loading in 12 Danish lakes

1. Concentrations of phosphorus, nitrogen and silica and alkalinity were monitored in eight shallow and four deep Danish lakes for 13 years following a phosphorus loading reduction. The aim was to elucidate the seasonal changes in nutrient concentrations during recovery. Samples were taken biweekly during summer and monthly during winter. 2. Overall, the most substantive changes in lake water concentrations were seen in the early phase of recovery. However, phosphorus continued to decline during summer as long as 10 years after the loading reduction, indicating a significant, albeit slow, decline in internal loading. 3. Shallow and deep lakes responded differently to reduced loading. In shallow lakes the internal phosphorus release declined significantly in spring, early summer and autumn, and only non‐significantly so in July and August. In contrast, in deep lakes the largest reduction occurred from May to August. This difference may reflect the much stronger benthic pelagic‐coupling and the lack of stratification in shallow lakes. 4. Nitrogen only showed minor changes during the recovery period, while alkalinity increased in late summer, probably conditioned by the reduced primary production, as also indicated by the lower pH. Silica tended to decline in winter and spring during the study period, probably reflecting a reduced release of silica from the sediment because of enhanced uptake by benthic diatoms following the improved water transparency. 5. These results clearly indicate that internal loading of phosphorus can delay lake recovery for many years after phosphorus loading reduction, and that lake morphometry (i.e. deep versus shallow basins) influences the patterns of change in nutrient concentrations on both a seasonal and interannual basis.     

中国典型湖泊富营养化现状与区域性差异分析

为全面科学地综合评估全国富营养化现状, 以全国五大湖区22个典型湖泊为研究对象, 科学评估了其富营养化状态, 分析了全国湖泊富营养化状态的区域性差异, 并探讨了富营养化状态与总磷的定量关系。结果表明, 2010—2011年, 59.1%的调研湖泊处于不同程度的富营养化状态, 其中云贵湖区的富营养化程度最为严重, 蒙新湖区的富营养化呈两极分化状态, 东北山地-平原湖区与东部湖区的湖泊基本均处于中营养-轻度富营养之间, 青藏高原湖区的富营养化程度最低。通过分析日照数、无霜期、气温、水深、海拔、降雨与湖泊营养状态的关系, 揭示了湖泊所处的地理位置是影响湖泊富营养化的基本因素, 具有区域性的分布规律。综合分析结果表明, 全国范围内湖泊中叶绿素a与总磷浓度存在显著相关性, 其中东部平原湖区、东北平原-山地湖区、青藏高原湖区和云贵高原湖区的叶绿素a与总磷浓度符合三次曲线方程, 蒙新湖区的叶绿素a与总磷浓度符合S型曲线方程; 东部平原湖区、东北平原-山地湖区、青藏高原湖区叶绿素浓度随着总磷浓度的增加, 首先出现1个极小值点, 然后出现1个极大值点, 其中3个湖区极小值点对应的总磷浓度分别为: 0.054、0.089和0.072 mg/L, 可为我国对应湖区的湖泊富营养化控制指标提供借鉴。     

Drought-induced changes in nutrient concentrations and retention in two shallow Mediterranean lakes subjected to different degrees of management

While extensive knowledge exists on the relationship between nutrient loading and nutrient concentrations in lakes in the cold temperate region, few studies have been conducted in warm lakes, not least in warm arid lakes. This is unfortunate as a larger proportion of the world’s lakes will be situated in arid climates in the future due to climate change and a larger proportion will suffer from a higher frequency of intensive drought. We conducted a comprehensive 11–13 year mass balance study in two interconnected shallow Mediterranean lakes in Turkey, covering a period with substantial changes in climate conditions. The upstream lake was only affected by natural changes in nutrient loading, while the downstream lake was additionally influenced by sewage diversion and restoration by fish removal. Contrasting to experience from north temperate lakes we found an increase in in-lake concentrations of total phosphorus and inorganic nitrogen (ammonia as well as nitrate) in dry years despite lower external nutrient loading, and submerged macrophytes did not increase the nitrogen retention capacity of the lakes. In contrast, fish removal modulated the nitrogen concentration as in north temperate lakes, but the effect was not long-lasting. Our results suggest that climate warming reduces the nutrient retention capacity of shallow lakes in the Mediterranean and exacerbates eutrophication. Lower thresholds of nutrient loading for shifting turbid shallow lakes to a clear water state are therefore to be expected in arid zones in a future warmer climate, with important management implications.     

Fish manipulation as a lake restoration tool in shallow,eutrophic temperate lakes 1: cross-analysis of three Danish case-studies

The use of fish manipulation as a tool for lake restoration in eutrophic lakes has been investigated since 1986 in three shallow, eutrophic Danish lakes. The lakes differ with respect to nutrient loading and nutrient levels (130–1000 μg P l⁻¹, 1–6 mg N l⁻¹). A 50% removal of planktivorous fish in the less eutrophic cyanobacteria-diatom dominated Lake V?ng caused marked changes in lower trophic levels, phosphorus concentration and transparency. Only minor changes occurred after a 78% removal of planktivorous fish in eutrophic cyanobacteria dominated Frederiksborg Castle Lake. In the hypertrophic, green algae dominated Lake S?byg?rd a low recruitment of all fish species and a 16% removal of fish biomass created substantial changes in trophic structure, but no decrease in phosphorus concentration. The different response pattern is interpreted as (1) a difference in density and persistence of bloomforming cyanobacteria caused by between-lake variations in nutrient levels and probably also mixing- and flushing rates, (2) a difference in specific loss rates through sedimentation of the algal community prevaling after the fish manipulation, (3) a decreased impact of planktivorous fish with increasing mean depth and (4) a lake specific difference in ability to create a self-increasing reduction in the phosphorus level in the lake water. This in turn seems related to the phosphorus loading.     

Lake responses to reduced nutrient loading – an analysis of contemporary long-term data from 35 case studies

1. This synthesis examines 35 long‐term (5–35 years, mean: 16 years) lake re‐oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L^?1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north‐temperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in‐lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in‐lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100–150 μg L^?1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re‐oligotrophication.     

Predicting concentration of total phosphorus and chlorophyll a in a lake with short hydraulic residence time

The relationship between total phosphorus and chlorophyll a concentration was determined for Skinner Lake, Indiana over an annual cycle in 1978–79. Total nitrogen:total phosphorus ratios in the epilimnion ranged from 19 to 220 suggesting a phosphorus-dependent algal yield in the epilimnion. Approximately 90% of annual TP loading reached the lake via streamflow, and 93% of this entered during snowmelt and spring-overturn periods. At that time incoming water flushed the lake 2.4 times. Atmospheric loading accounted for 1.4% of annual TP load. Internal hypolimnetic TP loading occurred during summer stratification. Mean [chl a] for the ice-free period was 15.15 mg m^–3, within the range expected for eutrophic lakes.The 1978–79 data were used in conjuction with the Vollenweider & Kerekes (1980) model to produce a model specific for the Skinner Lake system. The model predicted mean epilimnetic total phosphorus and chlorophyll a concentrations from mean total phosphorus concentration in inlet streams and from lake water residence time during the period of spring overturn and summer stratification. The Skinner-specific model was tested in 1982 and it closely predicted observed mean epilimnetic [TP] and [chl a] during the ice-free period. This study shows that variability in lake models which average data over an annual period can be reduced by considering lake-specific seasonal variation in hydrology and external TP loading.     

The occurrence of internal phosphorus loading in two small,eutrophic, glacial lakes in Northeastern Ohio

Internal loading of phosphorus for the summers of 1972–1974 in the eutrophic Twin Lakes, Ohio, USA was calculated from nutrient budgets, and was found to account for 65–100% of the increase in phosphorus content of the lakes during this period. Recovery of lakes of this type after nutrient diversion may be delayed by internal loading and chemical inactivation of phosphorus may be needed. A discussion of sources of this internal loading is presented.     

Environmental factors influencing chlorophyll v. nutrient relationships in lakes

SUMMARY. 1. A model relating log chlorophyll a concentration to log epilimnetic total phosphorus (TP) concentration was re-examined based on: (a) comparative and temporal studies of four stratifying Wisconsin and other highly eutrophic temperate lakes; (b) comparative summer lake surveys from Iowa and Alberta.
2. Although P-limited, deeper lakes with long hydraulic residence times and low external and internal nutrient loading in summer had summer chlorophyll a yields below model predictions based on spring and summer epilimnetic TP concentrations.
3. For lakes with summer epilimnetic TP between 30 and 80 mg m⁻³, chlorophyll a concentrations exceeded model predictions based on summer TP. This relationship held even for Lake Delavan, Wisconsin, where the ratio of available N to P was unfavourably low during spring turnover, and where the trans-thermocline N:P flux ratio was sub-optimal for algal needs in early summer.
4. With increasing summer TP concentrations and/or increasing epilimnetic circulation depth (>5m), chlorophyll a concentrations fell below model predictions—independent of the potential for N-limitation. This plateauing in chlorophyll a response occurred at lower epilimnetic TP content (−2) in lakes with elevated non-algal light extinction coefficients. Using Tailing's algorithm for the'column compensation point' (algal photosynthesis = algal respiration over diel cycle), light limitation best explains this fall-off in chlorophyll a yield.
5. The failure of the Dillon & Rigler (1974) spring TP v . summer chlorophyll a model for these Wisconsin lakes is unrelated to N-limitation. Instead, it reflects internal adjustment in take TP in response to stratification and seasonal external P loading.     

Iron:phosphorus ratio in surface sediment as an indicator of phosphate release from aerobic sediments in shallow lakes

Analysis of Danish lakes showed that both mean winter and mean summer concentrations of lake water total phosphorus in the trophogenic zone correlated negatively with the total iron to total phosphorus ratio (Fe:P) in surface sediments. No correlation was found between the water total phosphorus concentration and either the sediment phosphorus concentration alone or with sediment calcium concentration. The increase in total phosphorus from winter to summer, which is partly a function of net internal P-loading, was lowest in lakes with high Fe:P ratios in the surface sediment.A study of aerobic sediments from fifteen lakes, selected as representative of Danish lakes with respect to the sediment Fe and phosphorus content, showed that the release of soluble reactive phosphorus was negatively correlated with the surface sediment Fe:P ratio. Analysis of phosphate adsorption properties of surface sediment from 12 lakes revealed that the capability of aerobic sediments to buffer phosphate concentration correlated with the Fe:P ratio while the maximum adsorption capacity correlated with total iron. Thus, the Fe:P ratio may provide a measure of free sorption sites for orthophosphate ions on iron hydroxyoxide surfaces.The results indicate that provided the Fe:P ratio is above 15 (by weight) it may be possible to control internal P-loading by keeping the surface sediment oxidized. Since the Fe:P ratio is easy to measure, it may be a useful tool in the management of shallow lakes.     

Cyanobacteria and cyanotoxins: the influence of nitrogen versus phosphorus

The importance of nitrogen (N) versus phosphorus (P) in explaining total cyanobacterial biovolume, the biovolume of specific cyanobacterial taxa, and the incidence of cyanotoxins was determined for 102 north German lakes, using methods to separate the effects of joint variation in N and P concentration from those of differential variation in N versus P. While the positive relationship between total cyanobacteria biovolume and P concentration disappeared at high P concentrations, cyanobacteria biovolume increased continually with N concentration, indicating potential N limitation in highly P enriched lakes. The biovolumes of all cyanobacterial taxa were higher in lakes with above average joint NP concentrations, although the relative biovolumes of some Nostocales were higher in less enriched lakes. Taxa were found to have diverse responses to differential N versus P concentration, and the differences between taxa were not consistent with the hypothesis that potentially N(2)-fixing Nostocales taxa would be favoured in low N relative to P conditions. In particular Aphanizomenon gracile and the subtropical invasive species Cylindrospermopsis raciborskii often reached their highest biovolumes in lakes with high nitrogen relative to phosphorus concentration. Concentrations of all cyanotoxin groups increased with increasing TP and TN, congruent with the biovolumes of their likely producers. Microcystin concentration was strongly correlated with the biovolume of Planktothrix agardhii but concentrations of anatoxin, cylindrospermopsin and paralytic shellfish poison were not strongly related to any individual taxa. Cyanobacteria should not be treated as a single group when considering the potential effects of changes in nutrient loading on phytoplankton community structure and neither should the N(2)-fixing Nostocales. This is of particular importance when considering the occurrence of cyanotoxins, as the two most abundant potentially toxin producing Nostocales in our study were found in lakes with high N relative to P enrichment.     

Nutrient and Temperature Limitation of Bacterioplankton Growth in Temperate Lakes

Limitation of bacterioplankton production by nutrients and temperature was investigated in eight temperate lakes in summer. Six of the lakes were resampled in autumn. The lakes differ in nutrient content, water color, and concentration of dissolved organic carbon. Nutrients (phosphorus, nitrogen, and organic carbon) were added alone and in all possible combinations to filtered lake water inoculated with bacteria from the lake. After incubation for 36–40 h at in situ temperatures (ranging from 7 to 20°C), the response in bacterioplankton production was determined. The effect of increased temperature on bacterioplankton growth was also tested. Bacterioplankton production was often limited by phosphorus alone, organic carbon alone, or the two in combination. Phosphorus limitation of bacterioplankton production was more common in the summer, whereas limitation by organic carbon was more frequently observed in the autumn. There was a close balance between limitation by phosphorus and organic carbon in the epilimnion in the summer. In the hypolimnion in the summer, bacterioplankton growth was primarily phosphorus-limited. The effect of phosphorus additions decreased with increasing phosphorus concentrations in the lakes. However, there were no correlations between the effect of added organic carbon and water color, dissolved organic carbon concentration, or phosphorus concentration. When temperature was low (in the hypolimnion in the summer, and throughout the water column in the autumn) temperature also limited bacterioplankton production. Thus, temperature and inorganic nutrients or organic compounds can limit bacterioplankton growth both alone and simultaneously. However, at low temperatures, temperature is the most important factor influencing bacterioplankton growth.     

The spatio-temporal variations of sedimentary phosphorus in Taihu Lake and the implications for internal loading change and recent eutrophication

Internal phosphorus loading is particularly concerned for the shallow lakes due to the frequent sediment disturbance, which may play a vital role in changing nutrient level in overlying water. A historical perspective on internal phosphorus loading may contribute to understanding its contribution to recent eutrophication. In this work, a study on the changes in internal phosphorus loading and release potential in Taihu Lake, a shallow eutrophic lake in China, was performed based on the analysis of spatio-temporal variations of sedimentary total phosphorus and three operationally defined fractions (NaOH-P, HCl-P and OP). The influencing factors for changing internal loading were discussed. The results showed that internal phosphorus loading was elevated compared to pre-eutrophication periods and the increase has occurred since approximately the late 1970s to early 1980s. Changes in internal phosphorus loading were primarily attributed to the NaOH-P and OP fractions, relating to anthropogenic inputs and enhanced productivity, respectively. The internal phosphorus release potential may be enhanced by up to 22% currently relative to the pre-eutrophication period; however, it should play a secondary role to external input in enhancing nutrient levels and sustaining the eutrophication in Taihu Lake.     

Predictability and possible mechanisms of plankton response to reduction of planktivorous fish

The predictability of plankton response to reductions of planktivorous fish was investigated by comparing the plankton community in three biomanipulated lakes and ten unmanipulated lakes differing in intensity of fish predation. Data collected on total phosphorus, phytoplankton and zooplankton biomass and share of cyanobacteria and large grazers, as well as specific growth rate of phytoplankton, were further used to test some of the proposed underlying response-mechanisms. In the biomanipulated lakes the algal biomass and share of cyanobacteria decreased, specific growth rate of phytoplankton increased, and zooplankton biomass and share of large grazers increased or remained unchanged. This pattern was largely reflected in the differences in food-chain structure between the unmanipulated lakes with highversus those with low fish predation. The qualitative response to planktivorous fish reduction thus seems largely predictable. The biomanipulated lakes differed, however, in magnitude of response: the smallest hypertrophic, rotenone-treated lake (Helgetjern) showed the most dramatic response, whereas the large, deep mesotrophic lake (Gjersjøen), which was stocked with piscivorous fish, showed more moderate response, probably approaching a new steady state. These differences in response magnitude may be related to different perturbation intensity (rotenone-treatmentversus stocking with piscivores), food-chain complexity and trophic state. Both decreased phosphorus concentration and increased zooplankton grazing are probably important mechanisms underlying plankton response to biomanipulation in many lakes. The results provide tentative support to the hypothesis that under conditions of phosphorus limitation, increased zooplankton grazing can decrease algal biomassvia two separate mechanisms: reduction of the phosphorus pool in the phytoplankton, and reduction of the internal C:P-ratio in the phytoplankton cells.

     

Sediment resuspension: rescue or downfall of a thermally stratified eutrophic lake?

The aim of this study was to estimate the effect of sediment resuspension, a common phenomenon in many lakes, on the phosphorus budget of a eutrophicated lake. We used two different approaches, mass balance calculations and spatially comprehensive resuspension measurements, to determine the level of phosphorus loading from which rehabilitation action is started in a dimictic north temperate lake. The effect of resuspension was assumed to be significant, since it often is a governing process for cycling of material in lakes. Internal loading was multifold to that of external loading as determined by the budget calculation. Spatially comprehensive sedimentation and resuspension measurements were necessary, since deep site versus spatially comprehensive measurements had a marked difference in their results. Resuspension of P slightly exceeded the internal loading assessed by budget calculations and thereby proved its significance as a governing in-lake process that influences P cycling strongly. The shallow areas were of importance, since most of the total P load originated from there. The fate of P after resuspension depends on the retention capacity of resuspended particles in addition to prevailing biological and physico-chemical conditions. Therefore, sediment resuspension can either strengthen or diminish internal nutrient load and the processes of the shallow zones are of importance.