Responses of zooplankton to long-term environmental changes in a small boreal lake

Peer reviewe

Food Web Responses to Artificial Mixing in a Small Boreal Lake

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.Peer reviewe

Valkea-Kotisen planktonyhteisö ja sen muutokset

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Responses of zooplankton to long-term environmental changes in a small boreal lake

Zooplankton dynamics were examined in a small boreal lake over a 20-year period and interpreted in relation to climate change, brownification and recovery from acidification. Significant changes were recorded in the abundance of dominating crustacean species but not of rotifer species. According to redundancy analysis (RDA), the long-term pattern in crustacean zooplankton was mainly associated with abiotic factors like water colour, alkalinity and total phosphorus. Primary production of phytoplankton was the most important biological parameter whereas planktivorous perch and Chaoborus larvae had a marginal contribution. Biological factors were relatively more important for rotifers than for crustaceans, primary production being the most powerful explanatory parameter, followed by alkalinity, total phosphorus and colour. The changes in the zooplankton community were mainly related to increased organic carbon load and recovery from acidification. Within the food web, bottom-up regulation seemed to exceed the importance of top-down control.</p

Rehevöityneiden järvien hoitokalastuksen vaikutukset Vuosiraportti 1998

Toim. Mikko Olin ja Jukka Ruuhijärv