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Lake Mustijärv was reconstructed by sediment removal following an almost complete siltation. Here we evaluate challenges and opportunities for the management of the lake. We focused on the stream discharging to the lake (i.e., external loading), sediment retention in accumulation basins (i.e., internal processes) as well as the ecosystem-level response to stressors based on biological variables (phyto- and zooplankton, macrophytes, fishes and zoobenthos). Urban and agricultural inputs elevated ammonium and total phosphorus concentrations in the lake, challenging lake reconstruction efforts. Sediment transport highly increased the risk of faster filling of the lake, associated with upstream streambed excavation. Sediments trapped at the accumulation basins release nutrients that enhance eutrophication. We, however, observed a rapid recovery in fish, macrophytes, and zoobenthos, despite the significant disturbances. Lake Mustijärv is in eutrophic condition, reflected by phytoplankton (Pseudanabaena, Closteriopsis dominance) and zooplankton (Keratella, Polyarthra dominance) composition. To improve lake water quality will require controlling external nutrient inputs, underlining the importance of better coordinated activities between the local (lake restoration) and regional (catchment use) scales. [ABSTRACT FROM AUTHOR]

The amplitude of natural fluctuation between annual averages of the water level (WL) of Lake Peipsi (3555 km2) is 1.5 m. A study aimed to examine the impact of WL fluctuations on phytoplankton, macrophytes, and their epiphyton was performed annually at littoral stations during 2005'2015. Also the characteristics of pelagic water were collated with the WL. Changes in littoral and pelagial phytoplankton were similar, with the exclusion of massive wind-caused accumulations of cyanobacteria in the littoral. At the lowest WL a significant increase occurred in (a) the biomass of phytoplankton and the share of phytoplankton-derived organic carbon in water and (b) the species richness and biomass of macrophytes, including submerged plants and macroalgae. The abundance of epiphytes did not reveal a clear relation with the WL. The ratios of biomasses in the years with the lowest and the highest average WL were 2.2 for Potamogeton spp. and 2.6 for phytoplankton. The assessment of ecological status at the minimum and the maximum WL differs at least by one quality class. Decisions about ecological status based on phytoplankton and large filamentous green algae at low water may be contrary to decisions based on macrophytes: high biomasses of phytoplankton and macroalgae indicate hypertrophic status, but species-rich macrovegetation and high biomasses of charophytes and elodeids are considered to be characteristic of meso- to eutrophic water bodies [ABSTRACT FROM AUTHOR]

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Hydrochemical and phytoplankton data from L. Peipsi (3555 km², mean depth 7.1 m) for July-September 1997.2011 (two lake basins) and for August 2003.2011 (three lake basins) were analysed. Our aim was to explain the impact of nutrient content and weather factors on the composition and species dominance of cyanobacteria. The share of cyanobacteria was on average 64% of the total biomass, maximum values amounted to 93% and 38 g m-3 in the areas of open water. Close to the lake shores these values reached 99% and 100 g m-3 in some cases. The most prevalent taxa affecting cyanobacterial biomass were Gloeotrichia echinulata in the littoral areas and Microcystis species in the open water. Principal component analysis placed all dominant genera (Aphanizomenon, Anabaena, Gloeotrichia, and Microcystis) separately from each other. Stepwise multiple analysis showed G. echinulata to be fairly independent of nutrients and related to the days with water temperature over 22 °C. The biomass of the genus Microcystis was found to have evidently the strongest positive connections with phosphorus and also with iron, as well as with all potentially toxic (vacuolated) forms of cyanobacteria. The other group of cyanobacteria, mainly small-celled colonial forms (Aphanocapsa, Aphanothece, Cyanodictyon, etc), constituted on average up to a fourth of the cyanobacterial biomass; its biomass showed a reasonably positive correlation with nitrogen and a negative correlation with water level. In summer, the succession of cyanobacterial genera in the lake started with Anabaena, then Gloeotrichia appeared (in the larger and deeper moderately eutrophic northern part of the lake), followed by Microcystis and Aphanizomenon. [ABSTRACT FROM AUTHOR]

The material of the present study was collected from the pelagial of L. Peipsi (Estonia/Russia) mainly during the vegetation periods of 1997.2008. The zooplankton : phytoplankton biomass ratio (BZp/BPhyt) was followed in different lake parts: the northern eutrophic L. Peipsi s.s. and the southern hypertrophic L. Pihkva (together with L. Lämmijärv connecting the other parts). During 1997.2008 the average values of BZp/BPhyt for the growing season were lower than 0.5 (0.27 and 0.16 for L. Peipsi s.s. and L. Lämmijärv, respectively). The average monthly values of BZp/BPhyt fluctuated in a wide range: from 0.06 to 0.62 for L. Peipsi s.s. and from 0.06 to 0.29 for L. Lämmijärv in May-October. For L. Pihkva, this value was 0.06 in August. Maximum values (up to 2) occurred for L. Peipsi s.s. in June and quite a high value (up to 1) was also found in May. In parallel with the increasing trophic state of the lake, the mean values of BZp/BPhyt for the growing season decreased from 0.34 in 1997 to 0.18 in 2008 in L. Peipsi s.s., and from 0.24 to 0.10 in L. Lämmijärv. The widest range of fluctuations in BZp/BPhyt was found in May and August. For May this is explicable with differences in the melting time of the ice cover and therefore, in the timing of the spring peak of phytoplankton in different years. In August the absolute values of phytoplankton (water bloom caused by cyanobacteria) varied by years and depended mainly on weather conditions. The ratio demonstrated lowest variability in autumn months. This ratio revealed significant differences in the content of nutrients and characteristic features in plankton between the lake parts with different levels of trophy (P-level < 0.0001). We consider mean values of the ratio BZp/BPhyt in a growing season to be quite a reliable indicator for assessing the quality of the ecosystem and water of a large lake. [ABSTRACT FROM AUTHOR]

In the 1990s a sharp decrease in nitrogen loading occurred in Estonian rivers, bringing about a reduction of the nitrogen-to-phosphorus ratio (N:P ratio) in the large shallow lakes, Peipsi (3,555 km2, mean depth 7.1 m) and Võrtsjärv (270 km2, 2.8 m). The average mass ratio of total nitrogen (TN) and total phosphorus (TP) in Võrtsjärv (45) was about twice as high as that in Peipsi (22). In Peipsi, the N2-fixing Gloeotrichia echinulata, Aphanizomenon flos-aquae and Anabaena species prevailed in the summer phytoplankton, while in Võrtsjärv the dominant cyanobacteria were Limnothrix planktonica, L. redekei and Planktolyngbya limnetica, which cannot fix N2; the main N2-fixing taxa Aphanizomenon skujae and Anabaena sp. seldom gained dominance. In May–October the critical TN:TP mass ratio, below which N2-fixing cyanobacteria (Nfix) achieved high biomasses, was ∼40 in Võrtsjärv and ∼30 in Peipsi. The percentages of both total cyanobacteria (CY) and Nfix (CY% and Nfix%) in Peipsi achieved their maximum values at an N:P mass ratio at or below 20 for both TN:TP and Nmin:SRP. In Võrtsjärv, the TN:TP supporting a high Nfix% was between 30 and 40 and the Nmin:SRP supporting this high percentage was in the same range as that in Peipsi (<20), though the maximum Nfix% values in Võrtsjärv (69%) were much lower than in Peipsi (96%). The Nmin:SRP ratio explained 77% of the variability in Nfix% in May–October. The temperature dependence of Nfix% approximated to the maximum function type, with an upper limiting value at a certain water temperature, and this was most distinct in May–October. The critical TN:TP ratios obtained from our study (roughly 30 for Peipsi and 40 for Võrtsjärv) are much higher than the Redfield N:P mass ratio routinely considered (7). Our results represent valuable guidelines for creating effective management strategies for large shallow lakes. They provide a basis for stressing the urgent need to decrease phosphorus loading and to keep the in-lake P concentration low, and not to implement nitrogen reduction measures without a simultaneous decrease of phosphorus concentration. [ABSTRACT FROM AUTHOR]