Epilimnetic Total Phosphorus Concentrations Research Articles

Global warming affects nutrient upwelling in deep lakes

Measures to reduce lake phosphorus concentrations have been encouragingly successful in many parts of the world. After significant eutrophication in the twentieth century, nutrient concentrations have declined in many natural settings. In addition to these direct anthropogenic impacts, however, climate change is also altering various processes in lakes. Its effects on lacustrine nutrient budgets remain poorly understood. Here we investigate the total phosphorus (TP) concentrations in the epilimnion of the meromictic Lake Zug under present and future climatic conditions. Results are compared with those of other deep lakes. Data showed that TP transported from the hypolimnion by convective winter mixing was the most important source of TP for the epilimnion, reaching values more than ten times higher than the external input from the catchment. We found a logarithmic relationship between winter mixing depth (WMD) and epilimnetic TP content in spring. Warming climate affects WMD mainly due to its dependence on autumn stratification. Model simulations predict a reduction of average WMD from 78 (current) to 65 m in 2085 assuming IPCC scenario A2. Other scenarios show similar but smaller changes in the future. In scenario A2, climate change is predicted to reduce epilimnetic TP concentrations by up to 24% during warm winters and may consequently introduce significant year-to-year variability in primary productivity.

A first chironomid-based summer temperature reconstruction (13–5 ka BP) around 49°N in inland Europe compared with local lake development

Abstract: Temperature reconstructions for the end of the Pleistocene and the first half of the Holocene based on biotic proxies are rare for inland Europe around 49°N. We analysed a 7 m long sequence of lake deposits in the Vihorlat Mts in eastern Slovakia (820 m a.s.l.). Chironomid head capsules were used to reconstruct mean July temperature (TJuly), other proxies (diatoms, green algae, pollen, geochemistry) were used to reconstruct local environmental changes that might have affected the climate reconstruction, such as epilimnetic total phosphorus concentrations (TP), lake level changes and development of surrounding vegetation. During the Younger Dryas (YD), temperature fluctuated between 7 and 11 °C, with distinct, decadal to centennial scale variations, that agree with other palaeoclimate records in Europe such as δ18O content in stalagmites or Greenland ice cores. The results indicate that the site was somewhat colder than expected from the general south-to-north YD temperature gradient within Europe, possibly because of north-facing exposition. The warmer phases of the YD were characterised by low water level or even complete desiccation of the lake (12,200–12,400 cal yr BP). At the Late-Glacial/Holocene transition TJuly steeply increased from from 11 to 15.5 °C (11,700–11,400 cal yr BP) – the highest TJuly for entire sequence. This rapid climate change was reflected by all proxies as a compositional change and increasing species diversity. The open woodlands of Pinus, Betula, Larix and Picea were replaced by broad-leaved temperate forests dominated by Betula, later by Ulmus and finally by Corylus (ca 9700 cal yr BP). At the same time, input of eroded coarse-grained material into the lake decreased and organic matter (LOI) and biogenic silica increased. The Early-Holocene climate was rather stable till 8700 cal yr BP, with temporary decrease in TJuly around 11,200 cal yr BP. The lake was productive with a well-developed littoral, as indicated by both diatoms and chironomids. A distinct decline of TJuly to 10 °C between 8700 and 8000 cal yr BP was associated with decreasing chironomid diversity and increasing climate moistening indicated by pollen. Tychoplanktonic and phosphorus-demanding diatoms increased which might be explained by hydrological and land-cover changes. Later, a gradual warming started after 7000 cal yr BP and representation of macrophytes, periphytic diatoms and littoral chironomids increased. Our results suggest that the Holocene thermal maximum was taking place unusually early in the Holocene at our study site, but its timing might be affected by topography and mesoclimate. We further demonstrated that temperature changes had coincided with variations in local hydrology.

Landscape factors influencing lake phosphorus concentrations across Minnesota

Total phosphorus (TP) concentrations are known to be a significant factor influencing fish populations in Minnesota lakes. Consequently, a primary focus of the Minnesota Department of Natural Resources to address fish habitat in lakes across the state has been to determine relationships between TP concentrations and watershed conditions in Minnesota lakes. Because phosphorus concentrations in Minnesota lakes vary widely corresponding to differences in geomorphology, nutrient criteria were established by the Minnesota Pollution Control Agency for specific ecoregions. To refine these relationships in Minnesota lakes managed for fishing, we gathered mean summer epilimnetic TP concentrations on 1330 natural lakes to identify where agricultural and urban development have elevated phosphorus levels. Random forest, regression tree, and generalized additive models were used to model spatial variation in lake phosphorus concentrations across Minnesota. Key landscape variables known to influence TP concentrations in lakes, including lake depth and watershed size, were used as explanatory variables in these models, along with agricultural and urban development quantified for lake watersheds from the National Land Cover Dataset. These models explained up to 60% of the variation in TP in lakes across the state and showed a critical benchmark of anthropogenic land use disturbance at 40%, that once exceeded could significantly alter TP levels and consequently fish populations. This information should be useful for fish managers to prioritize conservation efforts and to set appropriate fish population goals.

Submerged Macrophyte Responses to Reduced Phosphorus Concentrations in Two Peri‐Urban Lakes

Eutrophication of two urban temperate dimictic lakes in Berlin (Germany), smaller Schlachtensee (0.4 km2) and larger Lake Tegel (3 km2), caused total phosphorus (TP) concentrations up to 800 μg/L and a complete loss of their diverse submerged vegetation in the 1960s due to poor light conditions. Phosphorus stripping of their inflow began in the 1980s and caused a pronounced decline of their epilimnetic TP concentrations, eventually leading to reduced phytoplankton biomass and turbidity. Despite increased light availability, recovery of abundance as well as species diversity of submerged macrophytes was delayed by more than a decade, especially in the smaller lake. Slow oxidization of sapropelic sediment unsuitable for macrophyte growth, periphyton shading, herbivory, and/or lack of a viable seed bank were potential hampering factors. The present submerged vegetation, however, may already support mechanisms positively influencing water transparency such as providing habitat to enhance the ratio of piscivorous to planktivorous fish. Characeae meadows, typical for both lakes during their former mesotrophic state, so far only reoccurred in smaller Schlachtensee. Neither species composition nor abundance reversed back to the macrophyte community present in the nineteenth century. Although TP concentrations may decline further and some rare species have been detected, reassembly of this plant community will most probably not occur because many submerged macrophyte species have become rare throughout northwest Europe.

A diatom-based inference model for autumn epilimnetic total phosphorus concentration and its application to a presently eutrophic boreal lake

A diatom transfer function to infer epilimnetic total phosphorus (TP) concentration was derived using surface sediment diatom data from 68 medium-sized (10–1000 ha) lakes in Southern Finland. Publicly available monitoring records were used in lake selection to avoid gradients caused by pH and humic substances. Constrained and partially constrained ordinations indicated that TP was an important variable influencing diatom assemblages. A long floristic gradient in relation to TP was also apparent and therefore an inference model was developed for TP using unimodal-based regression and calibration methods. The final model included 61 lakes with epilimnetic TP concentrations between 3 and 89 μg P l–1, measured during the autumnal circulation period. It has a jackknifed-estimated root mean squared error of prediction of 0.16 log μg P l–1, a maximum bias of 0.28 log μg P l–1, and an r2jack of 0.76.

Monitoring lake recovery from point‐source eutrophication: the use of diatom‐inferred epilimnetic total phosphorus and sediment chemistry

SUMMARY1. Diatom and geochemical responses to reduced nutrient loading were followed in a small, monomictic eutrophic lake in Northern Ireland by use of short sediment cores taken c. 15 years after redirection of creamery waste away from the lake.2. Epilimnetic total phosphorus (TP) concentrations (fig TP 1‐−1) were estimated for the period 1850–1990 using weighted averaging regression and calibration. Background TP levels, inferred using the diatom model, were c. 35μg TP 1‐−1 and increased to >140μg TP 1‐−1 in the late 1960s to early 1970s. Total P concentrations dropped to 80 μg TP 1‐−1 within 5 years of waste diversion (c. 1978–79), but varied between 1980 and 1990 (range 70–140 μg TP 1‐−1), perhaps due to internal loading, occasional continued disposal from the creamery and natural variations in stream P load.3. Diatom‐inferred TP concentrations were compared with monitored data where available, and the diatom model tended to overestimate TP concentrations by about 25 fig TP 1‐−1. Possible reasons for this are discussed (errors in the diatom model, stratigraphic variability, variability in the monitoring data).4. Post‐1980 geochemistry profiles (concentrations and accumulation rates) indicated some changes when compared with sediments deposited before 1980, perhaps reflecting the redirection of the creamery waste and reduced productivity of the lake (e.g. reduced calcium deposition). Phosphorus concentrations in the sediments changed very little over the last 150 years and, while sedimentary TP fluxes (g cm−2 yr−1) increase steadily up‐core, they do not record the effluent redirection in the mid‐1970s. There is, however, some indication of a slight lowering of P retention in the most recent sediments (1985–90).5. The general implications of such an approach to monitoring (i.e. the use of short cores) are discussed and the value of diatom‐inferred TP assessed. Diatom models offer the possibility of determining background TP concentrations and indicate that, despite the redirection of the creamery waste over 15 years ago, the pre‐creamery epilimnetic TP concentrations have not yet been reached.

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 (<c. 400 mg m−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.

Hypolimnetic Withdrawal as Lake Restoration Technique

Hypolimnetic withdrawal is a lake restoration technique based on the damming of surface outflow and the discharge of hypolimnetic water. In 17 lakes, hypolimnetic withdrawal has been used for 1–10 yrs. In general, anoxia and total phosphorus (TP) concentrations of epiand hypolimnetic water decreased after the commencement of withdrawal. The decrease of epilimnetic TP concentration correlates to TP export via hypolimnetic withdrawal and becomes stronger with years of operation. While it is advisable to maximize TP export to improve lake conditions, two constraints have to be observed: (1) the lake level should remain constant; and (2) the thermostructure must not be disturbed. Maximum TP export can be obtained by positioning the withdrawal pipe deep in the hypolimnion and operating the system when hypolimnetic TP concentrations are high. Under these circumstances treatment of the withdrawn water might become necessary to avoid fertilization of downstream waters.

PREDICTING THE SUMMER PEAK BIOMASS OF FOUR SPECIES OF BLUE‐GREEN ALGAE (CYANOPHYTA/CYANOBACTERIA) IN SWEDISH LAKES1

ABSTRACT: Stepwise multiple regression analysis was used to develop models predicting the summer peak biomass of Aphanizomenon flos‐aquae, Anabaena flos‐aquae, Oscillatoria agardhii, and Microcystis aeruginosa in four Swedish lakes. These analyses suggest that while epilimnetic total phosphorus concentration is the principal predictor of their peak biomass, other factors such as station mean depth, water temperature, total nitrogen, and total CO2 concentration are also important.