MEPS 178:109-119 (1999)  -  doi:10.3354/meps178109

ABSTRACT: A 16 d mesocosm experiment was performed during summer 1995 with 6 m³ bags filled with natural fjord water and plankton in a Danish embayment, Knebel Vig. Two control mesocosms simulated in situ conditions and a further 2 were enriched with inorganic nutrients to stimulate the primary producers. Zooplankton food availability in terms of 2 to 20 µm particles in the control and the enriched mesocosms revealed a mean ± SD of 582 ± 109 and 1191 ± 505 µg C l^-1, respectively. The major increase of particulate carbon in the enriched mesocosms was due to a bloom of the cryptophyte Hemiselmis virescens. Cohorts of the polychaete larvae Polydora spp., Spio/Microspio spp. and Nereis (Neanthes) succinea were identified. The natural instantaneous mortality rate of the larvae revealed a mean of 0.16 and 0.37 d^-1 in the control and enriched mesocosms, respectively. Mean growth in terms of setiger increase, body length increase and carbon content increase showed a fast growth phase followed by a stationary phase for Polydora spp. and Spio/Microspio spp. During the fast growth phase Polydora spp. had a specific growth rate of 0.21 to 0.31 d^-1 in both the control and the enriched mesocosms. A statistically non-significant higher pragmatic mean specific growth rate was found when enriched phytoplankton was offered Spio/Microspio spp. (both controls 0.20 d^-1, enriched 0.25 and 0.28 d^-1). N. (Neanthes) succinea grew more continuously throughout the study and in 1 of the enriched mesocosms with a significantly higher mean value (0.15 and 0.16 d^-1 in controls and 0.19 and 0.31 d^-1 in enriched mesocosms). This indicates functionally food limited growth for polychaete larvae in this summer situation despite high particulate carbon content in the controls.

KEY WORDS: Planktotrophic polychaete larvae · Polydora spp. · Spio/Microspio spp. · Nereis (Neanthes) succinea · Mortality · Growth · Food limitation · Mesocosm