Note
Bacterial and zooplankton distribution in deep waters of the Arabian Sea

https://doi.org/10.1016/j.dsr.2005.06.012Get rights and content

Abstract

The distribution of free-living bacteria and mesozooplankton was investigated at two deep sites (∼4000 m) in the western (WAST) and central (CAST) Arabian Sea in February 1998 during the late NE monsoon. In the epipelagic zone (0–150 m), biomass expressed as carbon was lower in bacteria (WAST: 3.31–0.93 mg C m−3, CAST: 2.00–1.32 mg C m−3) as compared to mesozooplankton (WAST: 11.39–3.32 mg C m−3, CAST: 9.83–1.84 mg C m−3). In the core of the oxygen minimum zone (150–400 m), mesozooplankton biomass abundance (WAST: 0.26 mg C m−3, CAST: 0.08 mg C m−3) decreased sharply whereas bacterial biomass increased from minima of 0.28 mg C m−3 and 1.32 mg C m−3 to 0.75 mg C m−3 and 12.42 mg C m−3 at WAST and CAST, respectively. At greater depths (>400 m), mesozooplankton biomass was elevated in oxygenated waters, but decreased more distinctly from 0.53 mg C m−3 at WAST and 0.42 mg C m−3 at CAST to 0.02 mg C m−3 (WAST and CAST at 50 m above bottom) than bacteria, which varied between 1.18–1.56 mg C m−3 at WAST and 0.53–1.13 mg C m−3 at CAST below 700 m. This results in a high relative abundance of bacteria in the bathypelagic zone (>1000 m). A comparison of bacterial cell numbers compiled from different sources and seas shows that, in general, the decrease in abundance with depth is low below 600–800 m. Irrespective of the ocean region, the abundance ranged within one order of magnitude, with lowest values at oligotrophic sites and highest values at eutrophic sites.

Introduction

Carbon dioxide is transformed into organic carbon via photosynthetic activity by phytoplankton in the euphotic zone of the oceans. Phytoplankton is consumed by herbivorous organisms that are fed upon by omnivorous and carnivorous animals. Ungrazed cells, viral lysis, transparent exopolymer particles (TEP), carcasses and fecal material produce particulate organic carbon (POC). This material is eaten by detritivorous zooplankton, is remineralized by bacterial activity, or sinks to greater depths. POC is partly transformed into dissolved organic carbon (DOC) by metabolic activity and fuels the microbial loop (see Williams, 1990; Ducklow, 2000).

Information on the vertical distribution of free-living bacteria and their contribution to carbon flux to bathypelagic depths is sparse (see Nagata et al., 2000; Hansell and Ducklow, 2003), as are papers that deal with comparisons of bacterial distributions and other planktonic groups to describe interactions of the biota in the ocean's interior. Patterson et al. (1993) and Tanaka and Rassoulzadegan (2002) investigated the distribution of bacteria and protozoa down to bathypelagic depths of the NE Atlantic and the NW Mediterranean, respectively, and Raghukumar et al. (2001) did so in the Arabian Sea. Yamaguchi et al. (2002) investigated concomitantly the deep distribution of bacterio-, phyto-, protozoo- and metazooplankton. Their study was done at sites of different productivity in the Pacific Ocean. Knowledge about the abundance and vertical distribution of functional groups within the plankton community will give insights into their contribution to carbon utilization in the sea.

Our paper will describe the vertical distribution of free-living bacteria and zooplankton at two deep sites in the Arabian Sea. We will show that bacterial biomass is higher than mesozooplankton biomass in the deep-sea and that the vertical distribution of bacteria and mesozooplankton is influenced by a very pronounced zone of permanent oxygen deficiency between the lower boundary of the euphotic zone and more than 1000 m depth that extends over large areas of the Arabian Sea (Sewell and Fage, 1948; Olson et al., 1993).

Section snippets

Material and methods

Water bottle and net samples were taken at two sites in the Arabian Sea (Fig. 1) to determine the abundance and distribution of bacteria and mesozooplankton during Sonne cruise 129 (Pfannkuche and Utecht, 1999): at the Western and Central Arabian Sea Sediment Trap stations WAST (16°15′N, 60°20′E, water depth 4050 m) and CAST (14°30′N, 64°30′E, water depth 3950 m). Samples were taken at the end of the NE monsoon, when enhanced production in the northern part of the basin occurs (Sarangi et al.,

Abiotic parameters

Temperature, salinity and oxygen content (Fig. 2) were highest in the upper 50 m of the water column at WAST (T=25.9 °C, S=36.3–36.4, O2=170 μmol l−1) and CAST (T=26.8–26.9 °C, S=36.4–36.5, O2=153–163 μmol O2 l−1). Between 50 and 200 m all parameters decreased sharply. Below 170 m at WAST and 130 m at CAST, a marked oxygen minimum zone (OMZ) extended to 930 m at WAST and 850 m at CAST, with concentrations of less than 6 μmol O2 l−1. At greater depths, the increase of oxygen to 153 μmol O2 l−1 at 4000 m at WAST was

Discussion

Information about concomitantly taken samples of bacteria and zooplankton is sparse for the deep-sea (see also Yamaguchi et al., 2002) and, hence, the knowledge about the importance of these biotic groups in the deep-sea ecosystem is limited. This research note contributes to the knowledge about the distribution and ecological role of these groups in the deep ocean's interior. We will outline possible interactions of zooplankton and bacteria with particulate and dissolved organic material

Acknowledgements

This study is part of the German JGOFS Indian Ocean study funded by Grants of the German Ministry of Education and Research (FKZ's 03F0137A, 03F0183A, 03F0241A, TP 14) to H. Weikert. Our thanks goes as well to the ship staff and our colleagues on board of the R.V. Sonne cruise So129 for their skilful help and assistance. Thanks are due to three anonymous reviewers for helpful comments on the manuscript. Chlorophyll data were generously provided by the SeaWiFs project, NASA/Goddard Space and

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