Across-slope relations between the biological populations, the euphotic zone and the oxygen minimum layer off the coast of Oman during the southwest monsoon (August, 1994)

An area some 120 x 70 km off the eastern coast of Oman (containing the UK J GOFS Arabesque station) was intensively studied over a 17 day period in Aug ust 1994, with the objective of determining the relationships between the b iological populations, the oxygen minimum layer and the dynamic hydrography of the euphotic zone. The outer margin of the area was delimited by a rect angle of 15 full depth CTD casts and the hydrography was further defined by two Seasoar surveys within the box. Midwater trawls were used to sample th e populations at three stations, oceanic, slope and shelf edge respectively . Day and night samples of macroplankton and micronekton were taken at each station to determine the extent of diel vertical migration and the effect of the hypoxic region on these migrations. Concurrent ADCP data were used t o follow the migrations and spatial changes in real time. Despite the limit ed area studied the patterns of upwelling and their temporal and spatial ch anges were complex. Coastal upwelling was observed directly only at the sou thwestern edge of the area during both Seasoar surveys. Persian Gulf Water was a consistent but spatially discrete feature of the region at depths of 200-300 m. Arabian Sea Surface Water was present at the eastern margin of t he first survey. Between these two water masses was a large area with small horizontal gradients and variable silicate and chlorophyll levels. Satelli te data suggest that this water may have been advected as a filament from a more northerly coastal source. Very marked changes took place in the hydro graphy and in the phytoplankton composition and abundance at the reference station at 19 degrees N 59 degrees E over the 16 day period between visits. The highest biomass of plankton and micronekton (expressed as wet volume o r as carbon) occurred in the upper 100 m, closely correlated with the relat ively high oxygen levels at these depths. Gelatinous animals predominated i n these layers, with additional swarms of swimming crabs. Quite large popul ations of myctophid and photichthyid fishes and of decapod crustaceans were present below the oxycline by day. Most of these migrated into the surface layers at night, leaving minimal biomass behind, with the result that the ADCP backscatter data from beneath the oxycline at night were often below i nstrument resolution. Daytime ADCP data, on the other hand, showed multiple fine layering, some of which correlated with salinity differences. At the base of the oxygen minimum layer there was a large increase in biomass, mar king the presence of a more typical bathypelagic fauna. (C) 1998 Elsevier S cience Ltd. All rights reserved.