Physical and biological factors influencing the seasonal variation in distribution of zooplankton across the shelf at Nordvestbanken, northern Norway, 1994

This paper presents data on species composition, abundance, horizontal and vertical distributions of zooplankton from selected sites across Nordvestba nken, northern Norway, sampled monthly from March to September 1994. The sa mpling covered three stations: A (closest to shore), C and D (close to the shelf break). There was a distinct succession in species composition over t he season, with a shift from a community almost entirely dominated by Calan us finmarchicus in May, towards a larger contribution of smaller, neritic s pecies in August and September Numerically C. finmarchus made up about 70 % of the total zooplankton stock at stations D and C in May (about 90 % if n auplii, which mostly belonged to this species, are included), whereas the p roportion of this species was about 50 % at station A this month. The popul ation development of the most common species agreed with earlier findings i n the area. A CCA analysis was performed including a total of 18 copepod sp ecies and 8 environmental variables. The pattern found demonstrated an insh ore-offshore community gradient correlated to temperature, salinity and dis tance to shore. The most abundant copepod species at Nordvestbanken constit uted a group which showed little or no correlation with either of the envir onmental variables included in the CCA analysis. These species are probably present in the area throughout the season, and are breeding successfully o n the shelf, Temora longicornis, Acartia longiremis, and Paracalanus parvus represent the more neritic, shallow water, autumn species, and their prese nce is probably related to the offshore extension of the coastal water duri ng summer. A third group composed of deep-water species such as Heterorhabd us norvegicus, Chiridius armatus, Scoleciticella minor and Pleuromamma robu sta could also be distinguished, most likely introduced to the outer shelf area by events of intrusion of deeper slope water.