Food web interactions in a Calanus finmarchicus dominated pelagic ecosystem - A mesocosm study

Abstract

The significance of nauplii versus copepodite stage V of Calanus finmarchicus grazing and their effects on the structure of the food web were investigated during two sampling periods of 7-8 days in March and April in experimental mesocosms held in a Norwegian fjord over a 2 month period. The mesocosms were manipulated by the addition of two different levels of inorganic nutrients (control versus enriched). During the 'naupliar' period in March, the phytoplankton was characterized by a diatom bloom while during the 'copepodite' period in April, it was in a post-bloom phase characterized by small-celled species, mainly Phaeocystis pouchetii. Phytoplankton, bacterial and protozooan biomass and production rates were measured in addition to copepod biomass. Copepod grazing was estimated by three different methods: (i) gut fluorescence; (ii) chlorophyll clearance from the water; and (iii) growth method measured as body carbon increase. The two latter methods gave similar results for nauplii, but all three gave different results for the copepodites. Independent somatic growth, based on changes in abundance and individual carbon content, and grazing estimates revealed an overall growth efficiency of 0.66 ± 0.20 (mean ± S.E.) for copepodites. Empirical carbon flow models were constructed, which indicated that the nauplii could not control either phytoplankton or protozoan growth in either the control or in the enriched system. Ignoring recycling and sedimentation, the fate of the primary production for the nauplii-dominated community was to be grazed by a diverse and abundant protozooplankton community. In the copepodite-dominated community, the copepods grazed >100% of the daily primary production, and also grazed heavily on a protozooplankton community of low biomass and diversity and presumably on detritus. The fate of the primary production in the two different copepod scenarios followed predicted routes for 'low meso-zooplankton' and for 'high meso-zooplankton' biomass systems, as suggested by Wassmann (Wassmann, 1998).