Turning up the Heat – Implications of Rising Temperatures for Arctic Zooplankton

The Arctic Ocean and adjacent ice-covered seas are the marine areas most rapidly affected by global warming. The air temperature is warming two to four times faster in the Arctic than the global average, with dramatic consequences for the ecosystem. Polar zooplankton species have to cope with this rise in temperature, whilst simultaneously facing increasing competition by boreal-Atlantic sister species advected into the Arctic Ocean via a stronger Atlantic inflow. To assess the sensitivity of Arctic and Atlantic zooplankton to rising temperatures, respiration rates were measured at ambient temperatures from 0 to 10ºC during three expeditions with RV Polarstern to the Arctic Fram Strait. Dominant Arctic species such as the copepods Calanus hyperboreus, Calanus glacialis, and Paraeuchaeta glacialis, as well as the amphipod Themisto libellula showed rather stable respiration rates despite increasing temperatures, indicating low Q10 ratios. In contrast, boreal-Atlantic representatives, i.e. Calanus finmarchicus, Paraeuchaeta norvegica, Themisto abyssorum, and the euphausiid Thysanoessa longicaudata, responded to higher temperatures by increasing their respiration rates. This unexpected result implies that Arctic species seem to be tolerant to warming but may be outcompeted by their Atlantic congeners, given their performance at higher temperatures. Changes in zooplankton community composition and biodiversity will have major consequences for Arctic ecosystems since polar species tend to be larger in size and have a higher lipid content than their southern congeners, providing a more nutritious food source for higher trophic levels.