Joint effects of sea ice melt, freshwater discharge and tidal currents on zooplankton abundance in the Sea of Okhotsk: 2004 and 2013

Abstract

The foremost flooding event of the past century happened in 2013 on the Amur River, which flows into the Sea of Okhotsk. Concurrently, the winter of 2012–2013 was a year of heavy sea ice and delayed onset of melting in this region. To examine the joint effects of these major, regional-scale hydrological freshening events on oceanographic processes, we compared physical (CTD, tides and currents) and biological (zooplankton) data measured in 2004 and 2013 in Academy Bay, Sea of Okhotsk. Our results indicate that the difference in sea water temperature between the two years played a primary role in shaping zooplankton variability. Data collected in 2013 showed that water temperature was colder and that the upper layer was substantially fresher (∼4–5 psμ) than in 2004. This decrease in water temperature and salinity reduction was accompanied by a significant decrease in the abundance of some key zooplankton species including Calanus glacialis, Pseudocalanus spp. and Sagitta elegans and a corresponding increase in the abundance of Limacina helicina. Delayed melting of sea ice in 2013 potentially triggered a mismatch in pelagic production that may have impacted the recruitment of calanoid copepods. Variation in tidal and subtidal advection of cold and highly saline Okhotsk Sea shelf water appears to be an important process influencing regional variation in zooplankton abundance. The occurrence of such hydrological events has the potential to trigger cascading effects through the food web.