Seasonal population dynamics and production of Microsetella norvegica, a widely distributed but little-studied marine planktonic harpacticoid copepod

Abstract

Microsetella norvegica is a widely distributed marine planktonic harpacticoid copepod, which is poorly known from the biological point of view. We investigated the seasonal population dynamics and production of M. norvegica in the central part of the Inland Sea of Japan. It occurred throughout the year, whilst its reproduction was confined to the warm season between May and November. The proportion of ovigerous females, which carry a single egg sac, was low (mean: 23.1%) in August and September, and high (53.6%) in October. Their brood size attained a maximum (mean: 15.8 eggs per sac) in July and August and declined gradually to 6.2 eggs in November. Duration time from egg laying to moulting to adulthood was temperature-dependent; it was 31.9 and 14.3 days at 20 and 27°C, respectively, under excess food conditions in the laboratory. An enormously high population abundance (7.32 × 104 individuals m-3), which accounted for 86.5% of the total copepods, and biomass (69.6 mg C m-3) gave an annual maximum production rate of 4.90 mg C m-3 day-1) in October. Nauplii and copepodites disappeared in December, and the overwintering population was represented by adults, mainly large females. Associations of M. norvegica with marine snow aggregates, which have often been found in oligotrophic waters, were not observed in the foodrich environment of the Inland Sea of Japan.