To identify the seasonal pattern of nitrogen (N) and phosphorus (P) limitation of phytoplankton in four different lakes, biweekly experiments were conducted from the end of March to September 2011. Lake water samples were enriched with N, P or both nutrients and incubated under two different light intensities. Chlorophyll a fluorescence (Chla) was measured and a model selection procedure was used to assign bioassay outcomes to different limitation categories. N and P were both limiting at some point. For the shallow lakes there was a trend from P limitation in spring to N or light limitation later in the year, while the deep lake remained predominantly P limited. To determine the ability of in-lake N:P ratios to predict the relative strength of N vs. P limitation, three separate regression models were fit with the log-transformed ratio of Chla of the P and N treatments (Response ratio = RR) as the response variable and those of ambient total phosphorus:total nitrogen (TN:TP), dissolved inorganic nitrogen:soluble reactive phosphorus (DIN:SRP), TN:SRP and DIN:TP mass ratios as predictors. All four N:P ratios had significant positive relationships with RR, such that high N:P ratios were associated with P limitation and low N:P ratios with N limitation. The TN:TP and DIN:TP ratios performed better than the DIN:SRP and TN:SRP in terms of misclassification rate and the DIN:TP ratio had the highest R2 value. Nitrogen limitation was predictable, frequent and persistent, suggesting that nitrogen reduction could play a role in water quality management. However, there is still uncertainty about the efficacy of N restriction to control populations of N2 fixing cyanobacteria.

ABSTRACT:The frequent occurrence of the cyanobacterial toxin cylindrospermopsin (CYN) in the (sub)-tropics has been largely associated with cyanobacteria of the order Nostocales of tropical origin, in particular Cylindrospermopsis raciborskii. C. raciborskii is currently observed to spread northwards into temperate climatic zones. In addition, further cyanobacteria of the order Nostocales typically inhabiting water bodies in temperate regions are being identified as CYN-producers. Therefore, data on the distribution of CYN in temperate regions are necessary for a first assessment of potential risks due to CYN in water used for drinking and recreation. A total of 127 lakes situated in the north-eastern part of Germany were investigated in 2004 for the presence of the toxin CYN and the phytoplankton composition. The toxin could be detected in half of the lakes (n ¼ 63) and in half of 165 samples (n ¼ 88). Concentrations reached up to 73.2 g CYN/g DW. CYN thus proved more widely distributed than previously demonstrated. The analyses of phytoplankton data suggest Aphanizomenon sp. and Anabaena sp. as important CYN producers in Germany, and confirm recent findings of Aphanizomenon flos-aquae as CYN-producing species frequently inhabiting water bodies in temperate climatic regions. The data shown here suggest that CYN may be an important cyanobacterial toxin in German water bodies and that further data are needed to assess this. # 2007 Wiley Periodicals, Inc. Environ Toxicol 22: 26-32, 2007.

Purpose In lake restoration, the redox sensitivity of iron (Fe)-phosphorus (P) compounds has been regarded as detrimental for a sustainable increase in sedimentary P retention since developing low redox potentials release Fe-bound P. Thus, Fe salts alone have rarely been used successfully to inactivate sediment P, and there are no studies on the long-term effects of in-lake Fe applications on P retention. Here, we analyzed for how long, and how efficiently, a single and continuous Fe application can affect the P budget of lakes. Materials and methods Two aerated lakes in Berlin, Germany were compared: Lake Tegel (TEG) experienced a continuous Fe supply via its tributaries, while Lake Groß-Glienicke (GGS) was treated once with Fe in the winter of 1992/1993. By controlling the operation of aerators, their effectiveness on P exchange at the sediment-water interface (non-aerated vs. aerated) was directly compared in spring and autumn between 2008 and 2010. The amount of P controllable by aeration (P control ) was experimentally determined by non-aerated vs. aerated sediment cores (at 10 and 16°C). Core stratigraphy of Fe was observed by high-resolution μX-ray fluorescence analysis. Results and discussion In TEG, the mobility of Fe was limited due to its sulfidic fixation, and thus Fe only accumulated slightly at the sediment surface (Fe/P ratio, ∼3). P control corresponded to only 4 % of the P content of the lake and 18 % of P loading. Hence, aeration only slightly influenced trophy-relevant epilimnetic P. In GGS, the single Fe application still ensures a high P binding ability of sediment since Fe relocated towards the surface (Fe/P ratio, ∼7). P control corresponded to 38 % of the P content of the lake and 74 % of annual P loading. Thus, the P release is not relevant for the P supply to the epilimnion since with the lake's overturn P is co-precipitated by the hypolimnetically accumulated Fe. Conclusions When external P loading is sufficiently reduced, as in GGS, amendments to Fe precipitants can increase sediment P retention in a redox-dependent manner over the long term. Thus, the redox-dependent mobility of Fe should no longer be regarded as a disadvantage of Fecontaining precipitants. To compensate for co-precipitation and complexation of Fe with sediment organic matter, a high Fe dosage (≥200 gm −2 ) is needed.

Eutrophication of two urban temperate dimictic lakes in Berlin (Germany), smaller Schlachtensee (0.4 km 2 ) and larger Lake Tegel (3 km 2 ), 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.

Main Berlin waterways are lowland rivers Spree, Dahme, and Havel, which are connected to some shallow lakes. High nutrient concentrations support heavy blooms of cyanobacteria which ( ) ( ) produce toxic substances ''cyanotoxins'' . Concentrations of microcystins MC , anatoxin-a, chloro-[ phyll-a, and cyanobacterial biomass were analyzed in surface water. Two methods enzyme linked ( ) ( ) ] immunosorbert assay ELISA and high-pressure liquid chromatography HPLC were used for MC analysis. Different preparation techniques were carried out determining dissolved and cell-bound microcystins. Furthermore, the health implication for people using lakes for swimming and recreation was assessed as a basis of a proposal to health and water authorities for further regulations.( ) ( ) Concentrations of total cell-bound and dissolved microcystin with ELISA varied between 0.14 and 119 ( ) ( ) g / L; 95 of 128 samples showed values above 1 g / L nearly 74% , 49 above 10 g / L nearly 38% , ( ) and 2 above 100 g / L nearly 2% . With wide variability, concentrations of microcystins measured with ( ) HPLC were 65% median of values obtained using the immunologic method. Compared to total ( ) microcystin concentrations dissolved and cell-bound toxin proportion of dissolved toxin varied from 0.3 ( ) to 12% n = 46; mean: 4% . Using animal toxicity data for risk assessment we assume that for children a single intake of 25 g MC-LR has no adverse effects. Under conservative uptake assumption for contaminated water a concentration of 100 g MC / L in surface water is safe. On this basis, regulations ( ) as provisional orientation are proposed to health and water authorities. Levels of anatoxin-a measured ( ) in 14 surface samples of two Berlin lakes Unterhavel and Muggelsee were under detection limit of 0.01 g/L. ᮊ