Abstract
Reduction in body size has been proposed as a universal response of organisms, both to warming and to decreased salinity. However, it is still controversial if size reduction is caused by temperature or salinity on their own, or if other factors interfere as well. We used natural benthic diatom communities to explore how “body size” (cells and colonies) and motility change along temperature (2–26°C) and salinity (0.5–7.8) gradients in the brackish Baltic Sea. Fourth-corner analysis confirmed that small cell and colony sizes were associated with high temperature in summer. Average community cell volume decreased linearly with 2.2% per °C. However, cells were larger with artificial warming when nutrient concentrations were high in the cold season. Average community cell volume increased by 5.2% per °C of artificial warming from 0 to 8.5°C and simultaneously there was a selection for motility, which probably helped to optimize growth rates by trade-offs between nutrient supply and irradiation. Along the Baltic Sea salinity gradient cell size decreased with decreasing salinity, apparently mediated by nutrient stoichiometry. Altogether, our results suggest that climate change in this century may polarize seasonality by creating two new niches, with elevated temperature at high nutrient concentrations in the cold season (increasing cell size) and elevated temperature at low nutrient concentrations in the warm season (decreasing cell size). Higher temperature in summer and lower salinity by increased land-runoff are expected to decrease the average cell size of primary producers, which is likely to affect the transfer of energy to higher trophic levels.
Highlights
Climate change over the century is projected to alter the environmental conditions for primary producers
Temporal autocorrelation was expected to occur because the samples were taken at regular intervals during the season and all sites were strongly influenced by seasonal variation in temperature, solar irradiation and inorganic nutrient concentrations
Cell size: a combined result of temperature and nutrients The linear cell-size decrease of 2.2% per uC we recorded in the natural diatom communities is similar to the linear cell-size decrease of 2.5% per uC found in a meta-analysis of single-species cultures of protists [52]
Summary
Climate change over the century is projected to alter the environmental conditions for primary producers. The Intergovernmental Panel on Climate Change (IPCC) predicts a global temperature increase of 2–4.5uC over the coming 100 years [1]. Regional climate scenarios for the brackish Baltic Sea area predict, besides an increase in the sea surface temperature, a decrease in salinity with increased land runoff due to precipitation [2,3]. Signs of an on-going climate change have already been observed in the temperature record. Historical data on the maximum summer surface water temperature in the Baltic Sea show an increase of ,1.3uC between 1861–1900 and 1985–2005 [4]. Decadal variations in the mean surface salinity in the Baltic Sea have been recorded, no overall long-term trend was detected in the 20th century [5]
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