Changes in morphometric characteristics of nematode communities during a spring phytoplankton bloom deposition

Nematode body size was investigated in terms of body length, width and length/width (L/W) ratios, before, during and shortly after a spring phytoplankton bloom deposition in a station in the southern North Sea (20 m depth). Sediments consisted of medium sand (median grain size: 333 µm) and were devoid of mud. Redox values in the upper 6 cm of the sediment were positive (>100 mV) throughout the sampling period. During the peak of the spring phytoplankton bloom in May 1999, several small-sized species (adult length < 700 µm) emerged. Most prominent was the appearance of a 'stout' nematode assemblage characterised by low L/W ratios. Most of these small nematode species were virtually absent before the peak blooming, and they decreased in abundance shortly after deposition of phytoplankton to the seafloor. This indicates the opportunistic behaviour of these nematodes, which is consistent with their small length, enabling them to reach adulthood rapidly. The net rate of increase of the stout nematodes during the bloom was estimated as 6.4% d-1. This is much larger than the estimated net rate of 1.5% d-1 for the total nematode community. The species composition of the stout nematode assemblage differed from similar stout assemblages described for continental slope and deep-sea areas. In the southern North Sea, Epsilonematidae were dominant while members of the Desmoscolecidae were prominent in offshore deeper areas. Possibly these differences reflect the relatively strong hydrodynamic forces at the North Sea site. In contrast with their short temporal appearance at our coastal North Sea site, stout nematodes seem to be a consistent member of deep-sea nematode communities. We hypothesise that this is caused by the quality of organic matter reaching the seafloor, together with differences in sedimentology and temperature, influencing the duration of the presence of suitable food items for these nematodes. [KEYWORDS: Benthic-pelagic coupling · Size · Shape]