Field investigations of pumping activity of the facultatively filter-feeding polychaete Nereis diversicolor using an improved infrared phototransducer system

Fleld data were obtained that document the extent to which the facultatively filter-feeding polychacte Nerels dlverslcolor undertakes suspension feeding at different times of the year in 2 D a n ~ s h shallo\v water areas, eutrophicated (Odense Fjord) and unpolluted (Fyns Hoved). A modified phototransducer system was used to make continuous long-tern~ recordings of the characteristic undulating body movements ( p u m p ~ n g activity) indicative of filter feedlng. F~l ter feeding was 'triggered' by the presence of phytoplankton in the water, and the worm was fllter feeding 50 to 100% of the time monitored during the productive summer period. In early spnng and autumn filter feeding occurred for about 5 to 20% of total time. At Fyns Hoved in September, when l ~ g h t was beginning to be a limiting factor for primary production, the chlorophyll a (chl a) concentration fluctuated around the 'trigger level' of 1 to 3 pg chl a I-', and filter feeding only took place during the afternoon when primary production had elevated the phytoplankton biomass above the trigger level. The grazing impact of N. diversicolor (potentially 5.6 m3 m-' d ' in May, representing a volume 11 to 55 times greater than the overlying water column in Odense Fjord) was probably restricted on days with low wind-caused mixing of the water column by a thin layer of refiltered water overlying the sediment. lncreased pumping activity with rising temperature led to a d o u b h g of the population filtration potential from March to June in Odense Fjord even though the population biomass was concomitantly reduced by 3 C % . No temperature adaptation of pumping activity was found in N. diversicolor. This study confirms that Al. diversicolor utilizes a number of feeding methods, depending on season, phytoplankton biomass and local conditions.