Utility and limits of biomass spectra: the nekton community sampled with the RMT 25 in the Scotia Sea during austral summer

An RMT 25 opening/closing net was used to sample the nekton community at 2 stations in the ice free zone (IFZ) of the Scotia Sea (Stn 1 approximately 10 nautical miles south of the Antarctic Polar Front; Stn 2 on the edge of the South Georgia Shelf Break to the northwest of the island). Oblique hauls sampled 200 m depth layers to 1000 m during both day and night. Total and individual volumes of each species in each 200 m layer were measured by displacement. The data were used to generate biomass and numerical spectra for day and night at each station for the whole water column to 1000 m. At both stations the relationship between log(10) biomass density (BIA) and log(10) individual body mass (M) were strongly positive. Slopes of the biomass spectra were not significantly different among the day and night stations and an overall regression showed that biomass density scaled as M(0.61), Analysis of biomass spectra revealed that although the species composition and biomass density varied between the 2 stations, energy turnover in the nekton community in the 2 areas was similarly dominated by animals of larger size. Considering energy turnover in terms of taxonomic groups revealed that Stn 1 turnover was dominated by tunicates (salps) followed by fish and cnidarians and at Stn 2 turnover was dominated by crustaceans followed similarly by fish and cnidarians. Use of biomass spectra in this case study was shown to enhance insight into the comparative function of 2 pelagic systems obtained using a conventional taxonomic approach. The analysis of biomass spectra in the absence of taxonomic data would have had limited value as it would not have emphasised the major difference between the 2 stations: the domination by tunicates, an energetic dead end, at Stn 1 and crustaceans, which are available to predators, at Stn 2.

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