Effect of ice algal community on the increase of chlorophyll a concentration during spring in coastal water of the Sea of Okhotsk

A seasonal study of size fractionated chlorophyll α concentration was conducted weekly in Monbetsu Harbor from October 1996 to November 1997 to investigate the annually persistent occurrence of the spring peak of the chlorophyll α concentration in the >10μm size fraction immediately after the retreat of sea ice, as described by K. Hamasaki et al. (Plankton Biol. Ecol., 45,151,1998). Species composition of natural phytoplankton assemblages was also investigated to study whether phytoplankton or ice algae were responsible for the spring peak in the coastal water. The spring peak occurred immediately after the retreat of sea ice but timing of the occurrence was different between the stations occupied in the present study. The spatial heterogenity in occurrence of the spring peak seemed to be related to the sea ice distribution between the stations. New sea ice provided only a small supply of ice algae due to the relatively short growth period inside of the harbor. Large ice floes provided for a large supply of ice algae due to the long growth period outside of the harbor. The magnitude of the spring peak was related to sea ice growth. However, those ice algae seemed to sink to the bottom with little contribution to phytoplankton assemblage in the harbor, while ice algae contributed significantly to the spring peak outside of the harbor. Species composition revealed relatively fast response of phytoplankton to the environmental change after the disappearance of sea ice. Surface assemblages of phytoplankton including ice algae seemed to respond fully to the regional optical condition by changing in the species composition

オホーツク海沿岸におけるClione limacina およびLimacina helicinaの出現時期

第6回極域科学シンポジウム[OB] 極域生物圏11月16日（月）　国立極地研究所１階交流アトリウ

Seasonal variability of zooplankton size spectra at Mombetsu Harbour in the southern Okhotsk Sea during 2011: An analysis using an optical plankton counter

To evaluate the temporal changes in zooplankton size spectra, optical plankton counter (OPC) measurements were made of high-frequency time-series zooplankton samples collected at approximately 3.5-day intervals in Mombetsu Harbour, which is located in the southern Okhotsk Sea, from January to December 2011. Based on biomasses of 47 equivalent spherical diameter (ESD) size classes binned at 0.1 mm intervals across 0.35-5 mm, the Bray-Curtis similarity index separated the zooplankton community into six groups (A-F). The occurrence of each group was separated seasonally. Thus, groups A and B were observed during the ice-covered season and summer season, respectively. During March and June, groups C-F were observed. Their occurrence varied in the short term in relation to the exchange of water masses. Groups A and C, which were observed from January to April, showed flatter normalized biomass size spectra (NBSS) slopes ( -0.85 to -1.1), which indicate low productivity. In contrast, the other groups showed steeper slopes (-1.31 to -1.52) from May to December, with high productivity. Throughout the year, the frequency of highly productive groups occurred at a high level (95.2%). Although the seasonal variability in zooplankton size and productivity in Mombetsu Harbour was mainly governed by water mass exchanges, the productivity was continuously high throughout nearly all of the one-year study period. (C) 2018 Published by Elsevier B.V