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AB 2:93-103 (2008)  -  DOI:

High reproduction, but low biomass: mortality estimates of the copepod Acartia tonsa in a hyper-eutrophic estuary

Peter Tiselius1,*, C. Marc Andersen Borg2, Benni Winding Hansen3, Per Juel Hansen4, Torkel Gissel Nielsen5, Bent Vismann4

1Department of Marine Ecology, Kristineberg Marine Research Station, Göteborg University, 450 34 Fiskebäckskil, Sweden
2Department of Systems Ecology, Stockholm University, 106 91 Stockholm, Sweden
3Department of Environment, Social and Spatial Change, Roskilde University, PO Box 260, 4000 Roskilde, Denmark
4University of Copenhagen, Marine Biological Laboratory, Strandpromenaden 5, 3000 Helsingør, Denmark
5National Environmental Research Institute, Aarhus University, Department of Marine Ecology, Frederiksborgvej 399, 4000 Roskilde, Denmark

ABSTRACT: Production, abundance and mortality of the copepod Acartia tonsa were studied for a period of 9 d in a hyper-eutrophic estuary, Mariager Fjord, Denmark. The estuary is characterised by oxygen-depleted and often sulphidic bottom water, and a relatively sparse mesozooplankton community with low species diversity. During the study, an intense phytoplankton bloom consisting mainly of the diatom Skeletonema costatum developed with chlorophyll a concentrations reaching 46 µg l–1. Egg production rate (EPR) in A. tonsa ranged from 30 to 65 eggs female–1 d–1, and egg hatching success was >90%, yet the abundance of copepods remained low (1 to 3 nauplii l–1, 0.3 to 1.5 copepodites l–1). Calculated daily copepod mortality ranged from 18% for nauplii, 16% for Copepodite Stage C1, up to 70% for C2 and C3, then declining for older stages. The vertical distribution of copepodites in relation to the depth range of mussel beds suggests strong predation by suspension-feeding Mytilus edulis at depths of 5 to 10 m. Moreover, anoxia and the presence of sulphide in deep water, which prevented hatching of A. tonsa eggs and decreased the survivorship of older life stages sinking below 20 m depth, further contributed to the low copepod abundance.

KEY WORDS: Acartia tonsa · Anoxia · Egg production · Mortality · Mytilus edulis

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Cite this article as: Tiselius P, Borg CMA, Hansen BW, Hansen PJ, Nielsen TG, Vismann B (2008) High reproduction, but low biomass: mortality estimates of the copepod Acartia tonsa in a hyper-eutrophic estuary. Aquat Biol 2:93-103.

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