A mesocosm experiment testing the combined effects on plankton community of a terrestrial organic matter runoff and a warming in small freshwater reservoirs

Small freshwater reservoirs are ecosystems of global importance both for
the great biodiversity they shelter, the ecosystem services they provide
and the freshwater resources they constitute. An increase in temperatures
and precipitation intensity due to climate change is expected by the end
of the century. These changes should result in an increase in interactions
between lakes and watersheds, leading in particular to increased inputs of
terrestrial organic matter (tDOM) likely to impact the functioning of
small lake ecosystems. In this context, a mesocosm experiment was carried
out to study the combined effect of two disturbances - input of tDOM and
temperature rise - on pelagic communities. The experiments were conducted
within the National Experimental Platform for Aquatic Ecology (PLANAQUA)
at CEREEP-Ecotron Ile-de-France located in Saint-Pierre-lès-Nemours
(France). The experiment was carried out in twelve 1m3 mesocosms placed
outside during 6 weeks, from June 8 to July 19, 2021. The mesocosms are
circular tanks with walls allowing the circulation of a heat transfer
fluid whose temperature is controlled by thermo-refrigerator pumps. The
mesocosms were randomly filled with unfiltered water taken from small
oligotrophic tanks located on the site and containing plankton and
microorganisms. Four juvenile fish roach (Rutilus rutilus) were introduced
at the start of the experiment in each mesocosm. Three replicates of two
crossed treatments were carried out in these mesocosms. These treatments
included a temperature differential of 6°C (18°C and 24°C) maintained
throughout the experiment and, for the addition of tDOM, either no
addition or an initial increase in tDOM concentration of 65 µM dissolved
organic carbon originating from natural forest soil, representing a total
enrichment in organic carbon of +25%. The mesocosms were manually stirred
every day before sampling and measurements with different probes (see
below for details).