Food web interactions in a Calanus finmarchicus dominated pelagic ecosystem--a mesocosm study |
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Authors: | Hansen, B.W. Hygum, B.H. Brozek, M. Jensen, F. Rey, C. |
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Affiliation: | Roskilde University, Department of Life Sciences and Chemistry, PO Box 260, DK-4000, Denmark and 1 Université P. et M. Curie (Paris IV), C.N.R.S.- I.N.S.U., URA 2077, Station Zoologique, PB 28, F-06230 Villefranche-sur-Mer, France |
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Abstract: | The significance of nauplii versus copepodite stage V of Calanusfinmarchicus grazing and their effects on the structure of thefood web were investigated during two sampling periods of 78days in March and April in experimental mesocosms held in aNorwegian fjord over a 2 month period. The mesocosms were manipulatedby the addition of two different levels of inorganic nutrients(control versus enriched). During the naupliarperiod in March, the phytoplankton was characterized by a diatombloom 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 protozooanbiomass and production rates were measured in addition to copepodbiomass. 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. Thetwo latter methods gave similar results for nauplii, but allthree gave different results for the copepodites. Independentsomatic growth, based on changes in abundance and individualcarbon content, and grazing estimates revealed an overall growthefficiency of 0.66 ± 0.20 (mean ± S.E.) for copepodites.Empirical carbon flow models were constructed, which indicatedthat the nauplii could not control either phytoplankton or protozoangrowth in either the control or in the enriched system. Ignoringrecycling and sedimentation, the fate of the primary productionfor the nauplii-dominated community was to be grazed by a diverseand abundant protozooplankton community. In the copepodite-dominatedcommunity, the copepods grazed >100% of the daily primaryproduction, and also grazed heavily on a protozooplankton communityof low biomass and diversity and presumably on detritus. Thefate of the primary production in the two different copepodscenarios followed predicted routes for low meso-zooplanktonand for high meso-zooplankton biomass systems,as suggested by Wassmann (Wassmann, 1998). |
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