Biomass and Size Composition of the Comb Jelly Mnemiopsis sp. in the North-western Black Sea During Spring 1997 and Summer 1995

Abstract

This study assessed the abundance, biomass and size structure of the ctenophore Mnemiopsis sp. in the north-western (NW) Black Sea during the summer of 1995 and the spring of 1997, i.e. several years after this species became the dominant zooplankter in this area. Biomass was measured directly as wet weight and then converted to carbon units. The total population biomass was low in spring (114 mg C m −2) and peaked in July (630 mg C m −2). Spawning increased enormously when the water temperature exceeded 22 °C and was primarily restricted to coastal stations. Larval biomass was highest in the plume of the river Danube. The body length (L, in cm) and volume (V, in ml) of a large number of animals were measured in each season and related to each other according to V=a L b. The parameters of the equation changed significantly from spring (V=0·244 L 2·25) to summer (V=0·575 L 1·88). With all data pooled, the equation was V=0·303 L 2·268), which is similar to earlier results from other areas. Length to volume relations have been used by previous authors to calculate Mnemiopsis biomass in the Black Sea. The use of various allometric equations published in the literature resulted in estimates of the population biomass of Mnemiopsis differing by a factor of two. For a given size class, the biomass of Mnemiopsis individuals increased from spring through to late summer. The population biomass of Mnemiopsis was similar to estimates obtained from the same area in the early 1990s when comparable methods were used. Current estimates of the total biomass of Mnemiopsis are, however, subject to serious biases originating from the use of different sampling devices and methods of calculation.