Phytoplankton cell size is a key trait subject to a ubiquitous trade-off between resistance to grazers and competitive ability. As a result, cell size holds the potential to explain and predict phytoplankton dynamics in response to storm-induced terrestrial runoffs in lakes under realistic environmental conditions. This study investigates whether cell size can explain phytoplankton community structure following transient changes in light and nutrient conditions in a clearwater lake. An in-lake enclosure experiment was conducted using a natural community, simulating different intensities of nutrient-replete coloured dissolved organic matter (cDOM) runoff. Over six weeks, phytoplankton species composition and biomass were monitored along gradients of light, phosphate and mesozooplankton biomass. Findings indicate that cell size explains phytoplankton community structure during transient reduction in light availability. Initially, the size distribution was unimodal with small-celled species dominating up to 60% of the community biomass. Phosphate addition and mesozooplankton biomass had no effect on phytoplankton size distribution. However, the relative biomass of large-celled species significantly increased as light availability decreased in enclosures with added cDOM. Our results suggest a distinct competitive advantage of larger cells over small-celled species under reduced light availability, significantly altering the phytoplankton size distribution following the mimicked storm event.

Primary Presenter: Alexis Guislain, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) (alexis.guislain@igb-berlin.de)

Authors:

Alexis Guislain, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB)  (alexis.guislain@igb-berlin.de)

Jens Nejstgaard, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB)  (jens.nejstgaard@igb-berlin.de)

Jan Köhler, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB)  (jan.koehler@igb-berlin.de)

Anne Lyche Solheim, Norwegian Institute for Water Research (NIVA)  (anne.lyche.solheim@niva.no)

Ute Mischke, Bavarian Environment Agency, Ecology of lakes and rivers  (ute.mischke@lfu.bayern.de)

Birger Skjelbred, Norwegian Institute for Water Research (NIVA)  (birger.skjelbred@niva.no)

Hans-Peter Grossart, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB)  (hanspeter.grossart@igb-berlin.de)

Mark Gessner, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB)  (mark.gessner@igb-berlin.de)

Stella Berger, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB)  (stella.berger@igb-berlin.de)