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

Abstract

An RMT 25 opening/closing net was used to sample the nekton comrnuruty at 2 stations m the ice free zone (IFZ) of the Scotia Sea (Stn 1 approximately 10 naut~cal m l e s south of the Antarchc 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 dunng both day and night. Total and ~ndividual volumes of each species in each 200 m layer were measured by displacement The data were used to generate biomass and numencal spectra for day and night at each statlon for the whole water column to 1000 m At both stat~ons the relationship between loglo b~omass density (B/A) and loglo individual body mass (M) were strongly pos~tive. Slopes of the biomass spectra were not significantly d~fferent among the day and n ~g h t stations and an overall regression showed that b~omass dens~ty scaled as MO6' Analysis of biomass spectra revealed that although the species composlbon and biomass density vaned between the 2 stations, energy turnover in the nekton community in the 2 areas was similarly dominated by animals of larger slze Considenng energy turnover m terms of taxonomic groups revealed that Stn 1 turnover was dormnated by tunicates (salps) followed by fish and cnidanans and at Stn 2 turnover was domnated by crustaceans followed smlarly by fish and cn~danans Use of biomass spectra in this case study was shown to enhance Insight into the comparabve funct~on of 2 pelaglc systems obtained using a conventional taxonomc approach The analys~s of b~omass spectra In the absence of taxonom~c data would have had h i t e d value as it would not have emphasised the major Mference between the 2 stations the domination by tunicates, an energebc dead end, at Stn 1 and crustaceans, w h ~c h are available to predators, at Stn 2