Life in a warming ocean: thermal thresholds and metabolic balance of arctic zooplankton

The magnitude and characteristics of the response of Arctic marine ecosystems to the challenges resulting from climate change are not known. Among the drivers of change, temperature plays a fundamental role, influencing biological processes from individual organisms to whole ecosystems, and sets the thresholds for species performance, abundance and distribution, and is responsible for massive shifts in ecosystem structure and function. The metabolic theory of ecology is generally invoked to ascertain the effects of global temperature changes on shifts in ecosystems, from individual size and species composition to global trophic status. However, as generally occurs with most scaling laws, there is a lively debate about its usefulness to predict something more than gross tendencies. In general, to explain variability is much more interesting than to predict average values. The successful performance of species and the trophic status of ecosystems are controlled by the balance between energy gains and losses. The temperature-induced mismatch between the positive and negative terms of the metabolic balance appears to depend on precise characteristics of their respective thermal windows, hardly identifiable by the averaging predictions made by the metabolic theory. As a case study, we discuss the response to temperature changes of the balance between ingestion and respiration rates of the copepod Calanus glacialis, a fundamental component of Arctic pelagic food webs. We suggest using the response of the metabolic balance (at the organismal, community or ecosystem level) to temperature changes to identify thermal thresholds leading to tipping points and nonlinear ecosystem shifts.