The effects of changing climate on microzooplankton grazing and community structure: drivers, predictions and knowledge gaps

Abstract

Microzooplankton dominate trophic interactions and biogeochemical processes at the base of pelagic marine food webs and so their responses to a changing ocean environment have potentially large implications for ocean ecosystem functioning. This diverse array of mostly protistan species constitutes an important source of phytoplankton and bacterial mortality, and contributes significantly to the food available to higher trophic levels by packaging minute prey into larger particle sizes that can be consumed by metazooplankton. Microzooplankton are pivotal species in oceanic food webs and nutrient remineralization and so it is essential that we understand the effects that changing climate may have on the biomass, species composition and trophic activities of these assemblages. Yet, our present understanding of this topic is derived from experimental studies of relatively few species subjected to specific environmental variables (e.g. changes in temperature, CO2, pH) in isolated culture. Most experiments and models employed to predict the effects of climate change have focussed on primary productivity and phytoplankton community structure, with less attention paid to microbial heterotrophy. Here we outline some of the major direct and indirect changes in environmental variables that are anticipated to accompany global climate change, and our present state of knowledge regarding their potential impacts on natural microzooplankton assemblages. We highlight a few specific areas for studies to address glaring omissions in our knowledge regarding how global change influences microzooplankton abundances and activities, and hypothesize that their ecological and biogeochemical roles may become even more prominent due to expected future shifts in marine chemistry and climate.