Transfer of organic matter in the deep Arabian Sea zooplankton community: insights from δ15N analysis

Zooplankton samples were obtained in the central Arabian Sea using a 1 m2-MOCNESS with 333 μm mesh aperture to investigate the nitrogen stable isotopic 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 ⩽500 m during April 1997 and February 1998. Samples from the upper 500 m indicated temporal differences between two years, with the lowest value (4.5‰) for the 1–2 mm size class between 250 and 500 m depth in April 1997 and the highest value (13.5‰) for the 2–5 mm size class in the same depths range in February 1998. The upper bathypelagic zone (1000–2500 m) showed an increase in δ15N with increasing depth for all size classes. In April 1997, the size-dependent distribution showed higher values in the larger size classes, indicating higher trophic levels as compared to smaller size classes. Such a size-dependent increase was not evident in February 1998. Below 2500 m, the δ15N values were more or less stable with increasing depth, or even decreased as exemplified by the smallest size class (<0.5 mm). The size-dependent distribution for both investigated periods showed increasing δ15N values with 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 zone (1000–2500 m) and one to two trophic levels in the lower bathypelagic zone (>2500 m). The amount of diet needed by the different mesozooplankton size classes to build up the measured biomass is estimated for the deep bathypelagic zone.