Scaling of growth rate and mortality with size and its consequence on size spectra of natural microphytoplankton assemblages in the East China Sea

Abstract

Allometric scaling of body size versus growth rate and mortality has been suggested to be a universal macroecological pattern, as described by the metabolic theory of ecology (MTE). However, whether such scaling generally holds in natural assemblages remains debated. Here, we test the hypothesis that the size-specific growth rate and grazing mortality scale with the body size with an exponent of −1/4 after temperature correction, as MTE predicts. To do so, we couple a dilution experiment with the FlowCAM imaging system to obtain size-specific growth rates and grazing mortality of natural microphytoplankton assemblages in the East China Sea. This novel approach allows us to achieve highly resolved size-specific measurements that would be very difficult to obtain in traditional size-fractionated measurements using filters. Our results do not support the MTE prediction. On average, the size-specific growth rates and grazing mortality scale almost isometrically with body size (with scaling exponent ∼0.1). However, this finding contains high uncertainty, as the size-scaling exponent varies substantially among assemblages. The fact that size-scaling exponent varies among assemblages prompts us to further investigate how the variation of size-specific growth rate and grazing mortality can interact to determine the microphytoplankton size structure, described by normalized biomass size spectrum (NBSS), among assemblages. We test whether the variation of microphytoplankton NBSS slopes is determined by (1) differential grazing mortality of small versus large individuals, (2) differential growth rate of small versus large individuals, or (3) combinations of these scenarios. Our results indicate that the ratio of the grazing mortality of the large size category to that of the small size category best explains the variation of NBSS slopes across environments, suggesting that higher grazing mortality of large microphytoplankton may release the small phytoplankton from grazing, which in turn leads to a steeper NBSS slope. This study contributes to understanding the relative importance of bottom-up versus top-down control in shaping microphytoplankton size structure

Cell Size as a Key Determinant of Phytoplankton Metabolism and Community Structure

“…First, it is now clear that heterotrophic protists (in particular dinoflagellates) are important consumers of large cells such as diatoms (Calbet 2008, Sherr & Sherr 2009. Second, although some studies have found that the smallest phytoplankton (picocyanobacteria and picoeukaryotes) do suffer higher grazing losses than larger cells (Latasa et al 1997, Landry et al 2000, Strom et al 2007, others have reported similar grazing pressure on phytoplankton groups of markedly different mean cell size (Latasa et al 2005, Gutiérrez-Rodríguez et al 2011, Teixeira et al 2011, Chang et al 2013. In addition, global analyses of grazing rate by microzooplankton, which collectively consume most of the phytoplankton daily production, indicate that losses to predation are of the same magnitude (approximately 60-70% of daily primary production) in regions that are known to have widely contrasting phytoplankton size structure (Calbet & Landry 2004, Schmoker et al 2013.…”

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing – an overview and synthesis

“…The decoupling between growth and mortality may be the result of non-steady-state conditions due to physical forcing and food web complexity, which makes it difficult to predict the grazing mortality at global scales. Chang et al (2013) assumed that allometric scaling of phytoplankton cell size of natural assemblage to growth and mortality could be described by the metabolic theory of ecology (MTE). While their experiments in the ECS did not support the MTE, they suggested that the higher grazing impacts of large phytoplankton cells release the grazing mortality of small phytoplankton cells.…”

“…Metazoans connect grazing and microbial food webs through their feeding on phytoplankton and protozoans (e.g., Kobari et al, 2003), suggesting that metazoan growth is an integration of production at lower trophic levels. Similarly to Chang et al (2013), tested the MTE for a copepod community in the ECS. The results from their experiments generally agreed with the MTE because growth rates of the copepod community showed positive correlations with ambient temperature and negative to their body size.…”

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing – an overview and synthesis

“…Metazoans connect grazing and microbial food webs through their feeding on phytoplankton and protozoans (e.g., Kobari et al, 2003), suggesting that metazoan growth is an integration of production at lower trophic levels. Similarly to Chang et al (2013), tested the MTE for a copepod community in the ECS. The results from their experiments generally agreed with the MTE because growth rates of the copepod com-munity showed positive correlations with ambient temperature and negative to their body size.…”