AB 14:105-120 (2012)  -  DOI: https://doi.org/10.3354/ab00379

Response of a microbial food web to prolonged seasonal hypoxia in a boreal lake

Grażyna Bręk-Laitinen*, Jessica López Bellido, Anne Ojala

Department of Environmental Sciences, University of Helsinki, 15140 Lahti, Finland

ABSTRACT: The clearwater boreal Lake Vesijärvi experiences annual periods of hypolimnetic hypoxia with unknown consequences for the microbial food web. We quantified the abundance of heterotrophic bacteria, autotrophic picoplankton (APP), nanoflagellates (NF) and ciliates in Lake Vesijärvi throughout the water column and vegetation season and determined how these varied with changes in environmental conditions, including dissolved oxygen concentrations. The abundance and diversity of ciliates increased significantly throughout the water column with the development of thermal stratification and the onset of hypoxia leading to distinct ciliate communities in the oxic epilimnion and hypoxic hypolimnion. Bacterivorous taxa dominated in the hypoxic zone, while in the epilimnion, bacterivorous, algivorous and predatory taxa feeding on other protozoa were present. Seasonal changes in oxygen and temperature explained nearly 80% of variance in ciliate community composition. Changes in total ciliate numbers correlated with seasonal dynamics of bacteria, but not with the dynamics of NF. In the epilimnion, NF were abundant in early spring and their numbers were positively correlated to numbers of prostomatids and haptorids, but not to APP or bacteria. In the hypolimnion, a significant correlation was found between NF and bacteria, but not NF and ciliates. Significant positive correlations between ciliates and bacteria suggest that concentrations of food resources are important in determining the abundance of ciliates in the plankton. Moreover, we recorded close coupling between seasonal dynamics of bacteria and productivity of phytoplankton, including APP.

KEY WORDS: Autotrophic picoplankton · Bacteria · Nanoflagellates · Ciliates · Anoxia · Hypoxia

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Cite this article as: Bręk-Laitinen G, López Bellido J, Ojala A (2012) Response of a microbial food web to prolonged seasonal hypoxia in a boreal lake. Aquat Biol 14:105-120. https://doi.org/10.3354/ab00379

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