Ecological relevance of salps and doliolids in the northern Benguela Upwelling System

Thaliacea (Tunicata) are fragile pelagic organisms, switching between sexual and asexual reproduction during their life cycle, occasionally occurring in massive blooms. During these blooms they graze large amounts of phytoplankton and microzooplankton, subsequently providing an enhanced carbon flux into deeper layers by sinking of faeces and dead and moribund animals. Seasonal variations and spatial distributions off the northern Namibian coast were analysed for December 2009, September 2010 and February 2011. Three Doliolida and four Salpida taxa were detected with a decreasing diversity towards the coast. Generally, Thaliacea preferred the upper 200 m of the water column but were also found in the mesopelagic zone. The abundances accounted for <1% of the mesozooplankton in most samples, but blooms with up to 63% were detected at some stations during moderate upwelling conditions. Intense upwelling seems to hamper the development of Thaliacea. During bloom periods, the relative abundance of asexually bred blasto-or gonozooids was highest. First estimates for a salp bloom off Walvis Bay (similar to 11.000 ind. m(-2), 0-200 m) revealed that 25-100% of the daily primary production was consumed during the bloom. Stable isotope analyses suggest a higher trophic level for Thaliacea than for herbivorous copepods.