Seasonal and inter-annual changes in mesozooplankton community at Mombetsu Harbor, southern Okhotsk Sea during 1997 to 2012

To evaluate seasonal and inter-annual changes in zooplankton community, we analyzed mesozooplankton samples collected by vertical tows of 335 μm NORPAC net from 0-8 m depths at Mombetsu Harbor, southern Okhotsk Sea with ca. 1.2 day interval during April 1997 to December 2012. Through 16 years studied, integrated mean temperature and salinity ranged -1.4-19.8 ˚C and 31.5-33.6, respectively. Chl. a exceeded 1 mg m^-3 throughout the year, and formed two peaks in March (ice-edge bloom) and May (open water bloom). 183 zooplankton species were identified from the samples. Zooplankton abundance had minimum (1,198 ind. m^-3) in January and maximum (7,926 ind. m^-3) in April. Cluster analysis (Bray-Curtis method) based on abundance data classified mesozooplankton community into five groups (A-E). Occurrence of each group well corresponded with season. Thus, group A, B, C, D and E was mainly observed during January-March, April-May, June-July, August-October and November-December, respectively. Dominant species was varied with group. Group A, D, E was dominated by copepods Pseudocalanus spp. (A, E) and Paracalanus parvus s. l. (D). While euphausiid eggs and cladocerans (Evadne, Podon and Peniria spp.) were the dominant taxa/species for groups B and C, respectively. As inter-annual changes, following four years were marked. Thus, 2000 and 2005: zooplankton community characterized by cold-water small copepods (group A) extended long period. 2002: Cladoceran dominated warm-water community (group C) started faster and remained long period. 2006: The period of cold-water group B (characterized with euphausiid eggs) was long, while the following warm-water community (group C) was very short. The 16-years zooplankton data set in Mombetsu may be valuable to compare with those collected in the other geographical region. Through these comparisons, recent trends in coastal ecosystem may become clear.