Abstract
Shrinking of body size has been proposed as one of the universal responses of organisms to global climate warming. Using phytoplankton as an experimental model system has supported the negative effect of warming on body-size, but it remains controversial whether the size reduction under increasing temperatures is a direct temperature effect or an indirect effect mediated over changes in size selective grazing or enhanced nutrient limitation which should favor smaller cell-sizes. Here we present an experiment with a factorial combination of temperature and nutrient stress which shows that most of the temperature effects on phytoplankton cell size are mediated via nutrient stress. This was found both for community mean cell size and for the cell sizes of most species analyzed. At the highest level of nutrient stress, community mean cell size decreased by 46% per °C, while it decreased only by 4.7% at the lowest level of nutrient stress. Individual species showed qualitatively the same trend, but shrinkage per °C was smaller. Overall, our results support the hypothesis that temperature effects on cell size are to a great extent mediated by nutrient limitation. This effect is expected to be exacerbated under field conditions, where higher temperatures of the surface waters reduce the vertical nutrient transport.
Highlights
Shrinking of body size has been proposed as one of the universal responses of organisms to global climate warming [1,2] and related to classic biogeographic rules [3,4] and to the temperature-size rule (TSR) [5]
Interest in the temperature response to size has been revived by Global Change research and by the ‘‘metabolic theory of ecology’’ [6,7] and phytoplankton has become one of the model systems to study the size effect of warming
Enhanced size-selective grazing under warmer conditions [8,9,11,12,13,14] has been suggested as proximate cause, but it is general knowledge in biological oceanography that small phytoplankton tend to dominate in warm, nutrient poor waters while large ones tend to dominate in cold, nutrient rich waters [15,16,17,18]
Summary
Shrinking of body size has been proposed as one of the universal responses of organisms to global climate warming [1,2] and related to classic biogeographic rules [3,4] and to the temperature-size rule (TSR) [5]. Interest in the temperature response to size has been revived by Global Change research and by the ‘‘metabolic theory of ecology’’ [6,7] and phytoplankton has become one of the model systems to study the size effect of warming. While most phytoplankton studies support the general trend [8,9,10], the mechanism remain still unresolved. Warming of the surface waters intensifies vertical density stratification and, thereby, reduces vertical nutrient transport through the thermocline into the well illuminated surface zone
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