Winter temperature and shifts in phytoplankton assemblages in a small Chara-lake

• We show effects of contrasting weather conditions on phytoplankton dynamics in a Chara-lake.
• Abundance and biomass were compared in a Chara-lake after warm and cold winters.
• Phytoplankton was far less abundant after warm winters.
• Biomass of filamentous Cyanoprokaryota was higher after warm winters.
• Abundant charophytes may modify the impact of climate warming on shallow lakes.

The structure of phytoplankton was analyzed along four successive growing seasons in relation to environmental conditions following contrasting winters in a shallow, mesotrophic Chara-lake (Lake Jasne, western Poland). The 2008 and 2009 growing seasons followed mild winters, whilst these of 2010 and 2011, had severe winters. After mild winters, despite higher availability of nutrients in the lake water, phytoplankton was less numerous than after severe winters, as reflected in the total abundance and the abundance and biomass of Chrysophyceae. Moreover, water clarity was higher after mild than after severe winters (5.2 m versus 3.8 m, mean Secchi depths). Contrary to abundance, total phytoplankton biomass showed no differences between the studied periods. However, a greater biomass of Cyanoprokaryota, particularly Dolichospermum lemmermannii (Richter) Wacklin, Hoffmann et Komárek and Aphanothece clathrata W. et G.S. West., was observed after mild winters, possibly in response to greater nutrient availability. A. clathrata also had higher abundance after mild winters. Nutrient budget and solute content as well as color, clarity, and, to a lesser extent, temperature of water were explanatory variables. We postulate that extensive charophyte meadows may be a crucial factor to control the phytoplankton dynamics in Chara-lakes. Overwintering charophytes may reduce phytoplankton densities by modifying phytoplankton responses to nutrient concentrations, which are higher due to shorter and warmer winters induced by climate change.