Chaetognaths and ctenophores in the holoplankton of the Bristol Channel

The geographical distributions, seasonal variations in numerical abundance and biomass (mg C m^-3) of the predators of the holoplankton of the Bristol Channel, between November 1973 and February 1975, are described. The predator numbers and biomass were dominated by the chaetognath Sagitta elegans Verrill. This species represented 96% of the holoplankton carnivore biomass in the outer, seaward region of the Channel and 60% in the inner region; the remainder being ctenophores. The maximum numerical abundance of S. elegans occurred in September at 129 individuals m^-3 (18 mg C m^-3). Juveniles (<5 mm) reached maximum numbers of 55 individuals m^-3 during June, August and September, demonstrating the reproductive activity of the population. The peak numbers were probably the result of the development of two major generations over the 90 d period from mid-June to mid-September. The tentaculate ctenophores were represented by Pleurobrachia pileus (O. F. Müller). The highest abundance was 81 individuals m^-3 (3.0 mg C m^-3) at a single site in July in the South Central Channel. However, June was the only month when the ctenophores dominated the carnivore biomass in all regions of the Channel; thereafter, S. elegans was more abundant. Reproduction of the ctenophore occurred from April to September, with juveniles reaching maximum abundance in June at 12 individuals m^-3. The estimated food demand of the population in May for the outer region of the Channel was approximately 31% of the daily production of copepods. When the population reached its peak abundance in June, the estimated food requirement outstripped the daily production of copepods and a decline in both the prey and predator standing stocks was observed. Similar estimations were derived for the inner region of the Channel. S. elegans increased from a standing stock of 0.038 mg C m^-3 in March to 6.35 mg C m^-3 in September. Estimates of the copepod production compared with the derived demand of the chaetognath population showed that the decline in the copepods in the late summer was the result of feeding by this predator. The holoplankton carnivore population was approximately 66% of the copepod standing stock for the 10 mo period November 1973 to September 1974 in the outer region of the Channel and 45% of that in the inner region. The carnivores formed the greater part of the total holoplankton biomass from September through the winter months to February, suggesting a predator-dominated community.