Food Web Responses to Artificial Mixing in a Small Boreal Lake

Abstract

Abstract:In order to simulate food web responses of small boreal lakes to changes in thermal stratification due to global warming, a 4 year whole-lake manipulation experiment was performed. Within that time, period lake mixing was intensified artificially during two successive summers. Complementary data from a nearby lake of similar size and basic water chemistry were used as a reference. Phytoplankton biomass and chlorophyll a did not respond to the greater mixing depth but an increase was observed in the proportional abundance of diatoms, and the proportional abundance of cryptophytes also increased immediately after the onset of mixing. Obligate anoxic green sulphur bacteria vanished at the onset of mixing but gradually recovered after re-establishment of hypolimnetic anoxic conditions. No major effect on crustacean zooplankton was found, but their diversity increased in the metalimnion. During the mixing, the density of rotifers declined but protozoan density increased in the hypolimnion. Littoral benthic invertebrate density increased during the mixing due to Ephemeroptera, Asellus aquaticus and Chironomidae, whereas the density of Chaoborus larvae declined during mixing and lower densities were still recorded one year after the treatment. No structural changes in fish community were found although gillnet catches increased after the onset of the study. The early growth of perch (Perca fluviatilis) increased compared to the years before the mixing and in comparison to the reference lake, suggesting improved food availability in the experimental lake. Although several food web responses to the greater mixing depth were found, their persistence and ecological significance were strongly dependent on the extent of the disturbance. To better understand the impacts of wind stress on small lakes, long term whole-lake experiments are needed.

Ecological Responses of Lakes to Climate Change

“…To evaluate how altered thermal properties of lakes might affect the structure and function of their food webs, studies have compared lakes at different latitudes, studied lakes for a long period of time as they have warmed, and in this special issue Arvola et al [14] present the results from an experiment where the mixing regime was artificially altered over a 4-year period. Two small lakes (4.7 and 4.1 ha) in Finland were selected for the study.…”

Polyhumous Dystrophic Pit Lakes: Hydrographic and Hydrochemical Characteristics on the Example of Reservoirs in the Włoszczowska Basin, Central Poland

“…These reservoirs also have a thick layer of organic sediments. Lakes of this type occur mainly in the areas of north-eastern Europe within the limits of the last glaciation [24]. They are scarce across the remaining part of the continent.…”

High Mercury Concentrations of European Perch (Perca fluviatilis) in Boreal Headwater Lakes with Variable History of Acidification and Recovery

“…One possible explanation for increased perch THg might have been enhanced bioavailability of mercury through increased methylation by sulphate-reducing bacteria (SRB) in anoxic conditions (Eckley et al, 2005;Ullrich et al, 2001). The importance of oxygen stratification to MeHg dynamics and perch mercury concentrations was shown in a lake-scale thermocline manipulation experiment during 2004(Arvola et al, 2017Forsius et al, 2010) where deepening of the thermocline and consequent decrease in the area of oxycline of a small lake resulted in a decrease of epilimnetic MeHg concentration in the water (Verta et al, 2010), followed by a decrease of THg in small perch (Rask et al, 2010). However, as two of the four lakes with highest perch mercury increase between the 1980s and 2010s (Orajärvi and Vähä Valkjärvi) are shallow (maximum depth 4.5 and 3 m, respectively) and lack an anoxic hypolimnion, (1983-1994 vs. 2013-2014), lake and perch length (covariate) and their interactions on perch THg-concentration…”