Metazoan meiofauna of the deep Arabian Sea: standing stocks, size spectra and regional variability in relation to monsoon induced enhanced sedimentation regimes of particulate organic matter

Effects of monsoon-induced enhanced depositional regimes of particulate org anic carbon (POC) on regional variability and distribution patterns and siz e spectra of metazoan meiofauna, particularly of nematodes, were investigat ed at five sites 3158-4414 m deep in the Arabian Sea. The sampling sites we re subjected to different flux rates of POC. Total meiofaunal abundance ran ged from 109 to 320 ind./10 cm(2). Nematodes were the numerically most abun dant taxon, with a relative abundance of 82.5-88.7%, followed by copepods a nd ostracods. Mean individual nematode biomass ranged from 0.0272 to 0.1033 mu gC, and Mean nematode population biomass varied between 0.0026 and 0.01 33 mgC/10 cm(2). Mean nematode lengths ranged from 614.2 to 832.6 mu m. The length distributions of nematodes at the different sites were typically sk ewed with the distributions extending into the longer size classes. At the sites with higher POC deposition rates, nematodes displayed deeper distribu tions in the sediment column (47.4-58.5% of nematodes in the top 1 cm layer of the sediment) in contrast to very shallow distributions at a site of lo w POC flux (75.1% of nematodes in the top 1 cm of the sediment). Regional v ariability of nematode biomass, size and vertical distribution was related to monsoon-driven gradients of POC- and chlorophyll a (chl. a) flux rates a nd bacterial biomass i.e. bioavailable organic carbon. This was in contrast to nematode abundance which did not correlate significantly with any of th ese environmental parameters. The differential pattern between biomass and abundance, distribution might be related to POC-dependent alterations in th e species composition of the nematode assemblages at the different sites. T he hypothesis of increased meiobenthic stocks due to monsoon-induced enhanc ed sedimentation could not be confirmed compared to data from other less pr oductive oceanic regions. Nematode abundance and biomass in the Arabian Sea were similar to values obtained from the abyssal temperate NE-Atlantic. (C ) 2000 Elsevier Science Ltd. All rights reserved.