Influence of spring phytodetritus sedimentation on intertidal macrozoobenthos in the eastern English Channel

The North Sea shallow waters of the eastern English Channel are recurrently affected by the high algal spring bloom of the Prymnesiophyte Phaeocystis spp.When the bloom has run its course, foam accumulations resulting from the release of mucilaginous polysaccharides can be observed both on the sea surface and on beaches.Four beach sites, 2 that were affected and 2 that were not, were sampled from April to December 2001 and from April to August 2001 respectively.The physical and chemical conditions of the sediment, as well as the macrofauna associated with it, were assessed in the Canche Bay before, during and after the phytodetritus sedimentation period.Phaeocystis spp.-derived material significantly changed the grain-size distribution in accumulation areas as shown by the resulting fine-particle input (> 60%) observed.Due to the rapid decomposition of decaying colonies, organic content increased in the sediments towards the end of the bloom, marked by a single peak on sands or strong, persistent enrichment on muds.As a result of this increase in organic matter, oxygen flux increased over time.In the low-energy silty environment, severe anoxic conditions rapidly occurred whereas on areas subject to tidal or wind-induced turbulence, superficial residual crusts resulting from drying foam prevented oxygen diffusion into sandy sediment.At the impacted sites, all macrobenthic taxa were affected, and both the number of species and their densities were simultaneously reduced.Sandy assemblages showed a better recovery, starting in July, due to more suitable environmental conditions, while on muddy assemblages full recovery had still not occurred by the end of the year.Spring recruit mortality was partly responsible for the lack of faunal recovery, which was mainly dependant on the migration of adults from the surrounding area.