Description: As part of the large-scale, interdisciplinary deep-sea study “BIGSET”, the relationship between the monsoon-induced regional and temporal variability of POC deposition and the small-sized benthic community was investigated at several sites 2316–4420 m deep in the Arabian Sea during four cruises between 1995 and 1998. Vertical and horizontal distribution patterns of chloroplastic pigments (a measure of phytodetritus deposition), readily soluble protein and activity, and biomass parameters of the small-sized benthic community (Electron Transport System Activity (ETSA); bacterial ectoenzymatic activity (FDA turnover) and DNA concentrations) were measured concurrently with the vertical fluxes of POC and chloroplastic pigments. Sediment chlorophyll a (chl. a) profiles were used to calculate chl. a flux rates and to estimate POC flux across the sediment water interface using two different transport reaction models. These estimates were compared with corresponding flux rates determined in sediment traps. Regional variability of primary productivity and POC deposition at the deep-sea floor creates a trophic gradient in the Arabian Basin from the NW to the SE, which is primarily related to the activity of monsoon winds and processes associated with the topography of the Arabian Basin and the vicinity of land masses. Inventories of sediment chloroplastic pigments closely corresponded to this trophic gradient. For ETSA, FDA and DNA, however, no clear coupling was found, although stations WAST (western Arabian Sea) and NAST (northern Arabian Sea) were characterised by high concentrations and activities. These parameters exhibited high spatial and temporal variability, making it impossible to recognise clear mechanisms controlling temporal and spatial community patterns of the small-sized benthic biota. Nevertheless, the entire Arabian Basin was recognised as being affected by monsoonal activity. Comparison of two different transport reaction models indicates that labile chl. a buried in deeper sediment layers may escape rapid degradation in Arabian deep-sea sediments.

Description: Effects of monsoon-induced enhanced depositional regimes of particulate organic carbon (POC) on regional variability and distribution patterns and size spectra of metazoan meiofauna, particularly of nematodes, were investigated at five sites 3158–4414 m deep in the Arabian Sea. The sampling sites were subjected to different flux rates of POC. Total meiofaunal abundance ranged from 109 to 320 ind./10 cm2. Nematodes were the numerically most abundant taxon, with a relative abundance of 82.5–88.7%, followed by copepods and ostracods. Mean individual nematode biomass ranged from 0.0272 to 0.1033 μgC, and Mean nematode population biomass varied between 0.0026 and 0.0133 mgC/10 cm2. Mean nematode lengths ranged from 614.2 to 832.6 μm. The length distributions of nematodes at the different sites were typically skewed with the distributions extending into the longer size classes. At the sites with higher POC deposition rates, nematodes displayed deeper distributions 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 low POC flux (75.1% of nematodes in the top 1 cm of the sediment). Regional variability of nematode biomass, size and vertical distribution was related to monsoon-driven gradients of POC- and chlorophyll a (chl. a) flux rates and bacterial biomass i.e. bioavailable organic carbon. This was in contrast to nematode abundance which did not correlate significantly with any of these environmental parameters. The differential pattern between biomass and abundance, distribution might be related to POC-dependent alterations in the species composition of the nematode assemblages at the different sites. The hypothesis of increased meiobenthic stocks due to monsoon-induced enhanced sedimentation could not be confirmed compared to data from other less productive oceanic regions. Nematode abundance and biomass in the Arabian Sea were similar to values obtained from the abyssal temperate NE-Atlantic.

Description: Large amounts of the greenhouse gas methane are released from the seabed to the water column where it may be consumed by aerobic methanotrophic bacteria. This microbial filter is consequently the last marine sink for methane before its liberation to the atmosphere. The size and activity of methanotrophic communities, which determine the capacity of the water column methane filter, are thought to be mainly controlled by nutrient and redox dynamics, but little is known about the effects of ocean currents. Here, we report measurements of methanotrophic activity and biomass (CARD-FISH) at methane seeps west of Svalbard, and related them to physical water mass properties (CTD) and modelled current dynamics. We show that cold bottom water containing a large number of aerobic methanotrophs was rapidly displaced by warmer water with a considerably smaller methanotrophic community. This water mass exchange, caused by short-term variations of the West Spitsbergen Current, constitutes a rapid oceanographic switch severely reducing methanotrophic activity in the water column. Strong and fluctuating currents are widespread oceanographic features common at many methane seep systems and are thus likely to globally affect methane oxidation in the ocean water column.