Structure, inter-annual variability, and long-term change in zooplankton communities of the Chukchi Sea

Abstract

The Chukchi Sea is a complex transition zone between the Pacific and Arctic Oceans that has been experiencing dramatic change in recent decades due to shifting sea ice cover and increasing temperatures. We examine summer mesozooplankton communities of the Chukchi Sea in Alaskan and Russian waters during summers 2004, 2009, 2010 and 2012 within the scope of the RUSALCA (Russian-American Long Term Census of the Arctic) program. Community structure was highly variable between the study years, but was overall tightly correlated to water mass properties, with bottom temperature being the most significant factor influencing communities. Zooplankton biomass was dominated by the large copepod Calanus glacialis, while abundance was dominated by small shelf species of copepods, such as Pseudocalanus spp. and Oithona similis. The “cold" summers of 2009-2012 had nearly twice the biomass and abundance of zooplankton compared to the oceanographically “warm" summer of 2004. We discuss the implications of the inter-annual variability of planktonic communities within the Chukchi Sea, and the possible effects of longer-term climate change. We then look at distribution and population structure of an ecologically important species complex within the zooplankton, Pseudocalanus spp, and evaluate the implications of a warming climate for this group of copepods. While numerically dominating the communities, Pseudocalanus spp. has been historically understudied at the species level due to very subtle morphological differences between the species. Our approach used a combination of microscopic identification as well as a novel species-specific PCR identification method to discriminate between the four species found in the Chukchi Sea. Our results suggest that shifting oceanographic patterns and climate warming will have unequal impact on this group of organisms, arising from species-specific life histories and tolerance to environmental conditions. These recent observations on zooplankton are then placed into a historical context through comparison to data collected throughout the past half-century (1946-2012). Despite significant challenges associated with the highly variable spatial coverage and methodology of the available datasets, significant trends were detected. In addition to high levels of inter-annual variability, we demonstrate significant increases in zooplankton biomass and abundance in recent years compared to historical studies, as well as shifting distribution ranges for several key species. This signal was most pronounced within the copepods, particularly Calanus glacialis, which appears to be indirectly benefiting from warming of the region. While summer zooplankton communities of the Chukchi Sea have been primarily Bering-Pacific in character for as long as our records exist, continuing warming and ice loss are increasing the influence of Bering-Pacific fauna within the Chukchi region.