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

The significance of nauplii versus copepodite stage V of Calanus finmarchic us grazing and their effects on the structure of the food web were investig ated during two sampling periods of 7-8 days in March and April in experime ntal mesocosms held in a Norwegian fjord over a 2 month period. The mesocos ms were manipulated by the addition of two different levels of inorganic nu trients (control versus enriched). During the 'naupliar' period in March, t he phytoplankton was characterized by a diatom bloom while during the 'cope podite' period in April, it was in a post:bloom phase characterized by smal l-celled species, mainly Phaeocystis pouchetii. Phytoplankton, bacterial an d protozooan biomass and production rates were measured in addition to cope pod biomass. Copepod grazing was estimated by three different methods: (i) gut fluorescence; (ii) chlorophyll clearance from the water; and (iii) grow th method measured as body carbon increase. The two latter methods gave sim ilar results for nauplii, but all three gave different results for the cope podites. Independent somatic growth, based on changes in abundance and indi vidual carbon content, and grazing estimates revealed an overall growth eff iciency of 0.66 +/- 0.20 (mean +/- S.E.) for copepodites. Empirical carbon how models were constructed, which indicated that the nauplii could not con trol either phytoplankton or protozoan growth in either the control or in t he enriched system. Ignoring recycling and sedimentation, the fate of the p rimary production for the nauplii-dominated community was to be grazed by a diverse and abundant protozooplankton community. In the copepodite-dominat ed community, the copepods grazed >100% of the daily primary production, an d also grazed heavily on a protozooplankton community of low biomass and di versify and presumably on detritus. The fate of the primary production in t he two different copepod scenarios followed predicted routes for 'low meso- zooplankton' and for 'high meso-zooplankton' biomass systems, as suggested by Wassmann (Wassmann, 1998).