Vertical distribution, grazing and egg production of calanoid copepods during winter-spring in Gullmarsfjorden

Abstract

The vertical distribution of copepods in relation to their potential food was examined in Gullmarsfjorden, Sweden (58°15.6′ N, 11°27.2′ E). Plankton distributions were determined from bottle samples at 5 m intervals on four occasions; 30 January, 28 February, 7 March and 10 April 1996. Potential food was crudely divided into chlorophyll containing protists (generally phytoplankton) and ciliates. Weak correlations between copepods and phytoplankton were found in January and April, while there were no correlations during the diatom spring bloom (February-March) when phytoplankton were superfluous throughout the water column. Copepods and ciliates were never correlated. Ingestion as determined from gut fluorescence and egg production analyses suggested a higher degree of herbivory during the spring bloom than before and after. There was potential for copepod predation control of ciliate biomass throughout the study. Due to low copepod abundance in January through March estimated grazing pressure was not sufficient to control phytoplankton biomass until after the diatom spring bloom. Egg production rates were constant for Centropages hamatus (~ 20 eggs female⁻¹ day⁻¹) on all dates but more variable (1–26 eggs female⁻¹ day⁻¹) for the other species. Temora longicornis and Acartia sp. both revealed their lowest fecundity during the bloom.