Atmospheric stilling offsets the benefits from reduced nutrient loading in a large shallow lake

As part of a global phenomenon, a 30% decrease in average wind speed since 1996 in southern Estonia together with more frequent easterly winds resulted in 47% decrease in bottom shear stress in the large (270 km2), shallow (mean depth 2.8 m), and eutrophic Lake Võrtsjärv. Following a peak in eutrophication pressure in the 1970s–80s, the concentrations of total nutrients were declining. Nonmetric Multidimensional Scaling (NMDS) ordination of a 54-year phytoplankton community composition time-series (1964–2017) revealed three distinct periods with breaking points coinciding with changes in wind and/or water level. Contrary to expectations, we detected no decrease in optically active substances that could be related to wind stilling, whereas phytoplankton biomass showed an increasing trend despite reduced nutrient levels. Here we show how opening of the “light niche,” caused by declining amount of suspended sediments, was capitalized and filled by the light-limited phytoplankton community. We suggest that wind stilling is another global factor, complementary to climate warming that counteracts eutrophication mitigation in lakes and may provide a challenge to assessment of the lake ecological status.Main financial support for EMU: European Union’s Horizon 2020 research and innovation programme Under the Marie Skłodowska-Curie Action, Innovative Training Networks, European Joint Doctorates.Project name, acronym and grant number: Management of climatic extreme events in lakes and reservoirs for the protection of ecosystem services, MANTEL, grant agreement No 722518.Publication date and, if applicable, length of embargo period: Published as Early View on 07.10.2019, no embargo period.Main financial support for EMU: European Union’s Horizon 2020 research and innovation programme Under the Marie Skłodowska-Curie Action, Innovative Training Networks, European Joint Doctorate

Lake catchment characteristics and external P load — cultivated area/lake area ratio as a tool for evaluating the risk of eutrophication from land use information

The effects of land use on external P load (LP), critical P load (LC) and on LP/LC of 27 lakes in Finland were examined with regression analyses. Catchment area to lake area ratio (AC/AL), cultivated catchment area to lake area ratio (AF/AL) and the percentage of cultivated land in the catchment (F%) were used as independent variables. We studied whether these parameters can be used to estimate the risk of exceeding the critical P load (LP/LC > 1). All three parameters had a positive effect on LP and LP/LC. Compared with AC/AL and F%, AF/AL had a higher coefficient of determination in predicting LP and LP/LC. This was because AC/AL does not include land use variations in catchments and F% does not include information on catchment size, while AF/AL includes information on both. Critical P load was exceeded when AF/AL > 1.63. AF/AL can be a useful and simple tool for evaluating the risk of lake eutrophication

Macrophyte-induced thermal stratification in a shallow urban lake promotes conditions suitable for nitrogen-fixing cyanobacteria

© 2017, Springer International Publishing AG. Densely vegetated shallow lakes often experience low water column nutrient levels and reduced phytoplankton growth, but in some cases a high phytoplankton biomass can co-exist with subme rged macrophytes. The conditions that favour phytoplankton blooms within areas colonized by submerged macrophytes remain largely unexplored. We investigated changes in water quality variables and phytoplankton community composition data in relation to macrophyte-induced thermal stratification in a shallow urban lake. The results indicate that submerged macrophytes may favour internal loadings of phosphorus and low DIN:TP ratios, in periods when macrophyte-induced thermal stratification of the water column is common. Blooms of nitrogen-fixing cyanobacteria occurred under the strongly stratified conditions triggered by the high macrophyte biomass and elevated canopy, even though nitrogen limitation was apparent during the whole growing season. These findings suggest that submerged macrophytes can promote blooms of nitrogen-fixing cyanobacteria once they are sufficiently tall and dense to induce stable water column conditions