Transfer of organic matter in the deep Arabian Sea zooplankton community: insights from delta N-15 analysis

Zooplankton samples were obtained in the central Arabian Sea using a 1 m(2) -MOCNESS with 333 mu m mesh aperture to investigate the nitrogen stable iso topic composition of different mesozooplankton size classes (< 0.5, 0.5-1, 1-2, 2-5 mm) from the surface down to 3900 m depth in intervals of less tha n or equal to 500 m during April 1997 and February 1998. Samples from the u pper 500 m indicated temporal differences between two years, with the lowes t value (4.5 parts per thousand,) for the 1-2 mm size class between 250 and 500 m depth in April 1997 and the highest value (13.5 parts per thousand,) for the 2-5 mm size class in the same depths range in February 1998, The u pper bathypelagic zone (1000-2500 m) showed an increase in delta(15)N with increasing depth for all size classes. In April 1997, the size-dependent di stribution showed higher values in the larger size classes, indicating high er trophic levels as compared to smaller size classes. Such a size-dependen t increase was not evident in February 1998. Below 2500 m, the delta(15)N v alues were more or less stable with increasing depth, or even decreased as exemplified by the smallest size class( < 0.5 mm). The size-dependent distr ibution for both investigated periods showed increasing delta(15)N values w ith increasing size. These results give an insight in the trophic structure of the zooplankton community in the deep Arabian Sea. Differences between size classes were less than one trophic level in the upper bathypelagic zon e (1000-2500 m) and one to two trophic levels in the lower bathypelagic zon e ( > 2500 m). The amount of diet needed by the different mesozooplankton s ize classes to build up the measured biomass is estimated for the deep bath ypelagic zone. (C) 2000 Elsevier Science Ltd. All rights reserved.