Meroplankton distribution and its relationship to coastal mesoscale hydrological structure in the northern Bay of Biscay (NE Atlantic)

19 The relationship between meroplankton distribution and spatio-temporal variability of coastal 20 mesoscale hydrological structure was investigated in the northern Bay of Biscay, North-East 21 Atlantic. For the three coastal polychaetes studied, i.e., Pectinaria koreni, Owenia fusiformis, 22 Sabellaria alveolata, the highest larval abundances were sampled in low salinity, low density and 23 high temperature river plume waters. For two species (P. koreni and O. fusiformis), maximal 24 abundances were observed in the surface and thermocline layers due to ontogenic migrations. 25 Variance partitioning based on multiple regression and redundancy analyses were used to assess the 26 relative roles played by the hydrological environment alone, the geographical space alone, and their 27 interactions, i.e. the spatial structure of the hydrological environment. These analyses demonstrate 28 the key role played by hydrological spatial structure in the distribution of larval abundances, 29 indicating that larvae mainly act as passive particles. The hydrological environment alone was 30 insignificant, whereas geographical space alone explained a significant part of the variability in 31 meroplankton distribution, probably in conjunction with ecological processes. For species whose 32 benthic populations are spatially structured, the distribution and the size of adult populations and 33 the timing of spawning events can significantly affect larval distribution and dispersal. 34