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Lake eutrophication is a pervasive problem globally, particularly serious in agricultural and densely populated areas. Whenever nutrients nitrogen and phosphorus do not limit phytoplankton growth directly, high growth rates will rapidly lead to biomass increases causing self-shading and light-limitation, and eventually CO 2 depletion. The paradigm of phytoplankton limitation by nutrients and light is so pervasively established, that the lack of nutrient limitation is ordinarily interpreted as sufficient evidence for the condition of light limitation, without considering the possibility of limitation by inorganic carbon. Here, we firstly evaluated how frequently CO 2 undersaturation occurs in a set of eutrophic lakes in the Pampa plains. Our results confirm that conditions of CO 2 undersaturation develop much more frequently (yearly 34%, summer 44%) in these agriculturally impacted lakes than in deep, temperate lakes in forested watersheds. Secondly, we used Generalized Additive Models to fit trends in CO 2 concentration considering three drivers: total incident irradiance, chlorophyll a concentration, and lake depth; in eight multi-year datasets from eutrophic lakes from Europe, North and South America, Asia and New Zealand. CO 2 depletion was more often observed at high irradiance levels, and shallow water. CO 2 depletion also occurred at high chlorophyll concentration. Finally, we identified occurrences of light- and carbon-limitation at the whole-lake scale. The different responses of chlorophyll a and CO 2 allowed us to develop criteria for detecting conditions of CO 2 limitation. For the first time, we provided whole-lake evidence of carbon limitation of phytoplankton biomass. CO 2 increases and eutrophication represent two major and converging environmental problems that have additive and contrasting effects, promoting phytoplankton, and also leading to carbon depletion. Their interactions deserve further exploration and imaginative approaches to deal with their effects., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

This paper investigates, using mainly data of 19 85-2003, the influence of water temperature and water level on nutrients and on most important phytoplankton groups in Lake Peipsi. The water environment characteristics (water level at sampling time, in spring, in autumn, in the whole sampling year, and in the previous year, and the corresponding water temperatures) were considered as potential factors determining nutrient and phytoplankton fluctuations in the lake. To overcome problems with statistical correlation between environmental characteristics, these were linearly combined into a single Water Condition Factor (WCF). This factor was calculated with the Canonical Analysis individually for each nutrient and phytoplankton variable and used then to study how and to which extent different environmental conditions determine the corresponding water variable. It appeared that in the WCF of nutrients the most important role was played by water level characteristics while in the case of phytoplankton biomasses (particularly cyanobacteria) also the water temperature parameters had a significant effect. The water temperature had some effect also on the concentration of dissolved inorganic phosphate and dissolved inorganic nitrogen. In the shallower parts of the lake, lakes Lämmijärv and Pihkva (Pskov), the correlations between WCF and the water variables were higher than in the deepest part of the lake, L. Peipsi s.s. [ABSTRACT FROM AUTHOR]

In the 1990s, as a consequence of a decline in agricultural production in the watershed and a decrease in the amount of waste water discharged into rivers, the nutrient load carried from the catchment area into large, shallow, eutrophic Lake Peipsi (area 3,555 km[sup 2]) decreased. The aim of the present study was to analyze the in-lake response of key physical and chemical variables and the biota to large-scale changes in the nutrient load. Yearly changes in water transparency, nutrient elements of surface water, chlorophyll a content, as well as in phyto- and zooplankton biomass were studied during the growth season of 1992-2000. A clear decline in total nitrogen, ammonium ion, and orthophosphate ions was revealed in the northern part of the lake over the studied years. Beginning from 1996-1997, a decreasing tendency was revealed for the nitrate ion and total phosphorus. A significant decline of the total nitrogen:total phosphorus ratio was also observed from 1992-2000. However, the biomass of phytoplankton (particularly cyanobacteria) and chlorophyll a concentration did not follow the dynamics of nutrients, but displayed an increasing trend. Concentrations of nutrients in the lake during the last decade were not so low as to limit phytoplankton growth directly. In Lake Peipsi, strong and long-lasting algal blooms were observed in recent years, despite a definite decline in the nutrient content of surface water. Weather conditions appear to be very important factors in causing algal blooms in Lake Peipsi. [ABSTRACT FROM AUTHOR]