The fate of discarded appendicularian houses: degradation by the copepod, Microsetella norvegica , and other agents

Despite the potential importance of zooplankton in degradation of marine snow, the association of colonising zooplankton with discarded appendicularian houses has not been investigated in northern areas. We sampled the vertical distributions of appendicularians, houses and potential zooplankton colonisers at two stations in the central North Sea during late summer. In addition, grazing experiments were performed with the copepod Microsetella norvegica, which was assumed to be the main contributor to house degradation. The results were used in (i) inverse modelling, to estimate the factors which were significant in shaping the vertical distribution of houses and (ii) calculations to estimate potential house degradation rates. M. Norvegica was able to feed on appendicularian houses, with feeding rates up to 0.42 g C (g C)^-1 day^-1 (0.14 μg C ind.^-1 day^-1). The model results suggested that the vertical distribution of houses was shaped by sinking of houses, bacterial degradation and feeding of M. norvegica and invertebrate larvae. The estimated community degradation rate by M. norvegica was low, whereas invertebrate larvae had degradation rates close to bacterial degradation. We conclude that at the typical concentrations of M. norvegica in the North Sea (≤10⁴ ind. m^-2), its role in marine snow degradation is likely to be small. Degradation by other zooplankton groups, such as invertebrate larvae, can, however, be substantial.