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