Particulate Carbohydrate and Proteins in the Bay of Bengal

Particulate carbohydrates (PCHO) and proteins (PP) of the offshore waters (depth >200 m) were measured within the upper 150 m water column along the six transects covering 36 stations perpendicular to the coast in the Bay of Bengal. Concentrations of PCHO and PP ranged from 31 to 158 μg l−1and from 0•56 to 2•45 μg atomic (at) N l−1at the surface, and decreased to 7–58 μg l−1and 0•43–1•62 μg at N l−1at 150 m depth, respectively. In surface waters, the concentrations of these particulates showed high nearshore values with a pronounced seaward decrease, whereas subsurface concentrations did not reveal any particular trend. Regressions indicated that the detrital component may be in the form of terrigenous organic matter, and decomposition of plankton constituted a major fraction of particulate organic matter (POM). Subsurface chlorophyll maxima (SCM) were a consistent feature which occurred between 50 and 80 m. Chlorophylla(Chl a) content in the SCM was twice the observed surface values. A maximum Chl a/PP-N ratio of 0•48±0•07 ;μgChl;μg at N−1was recorded in the SCM, indicating that phytoplankton comprise a major proportion of biomass in the SCM. Furthermore, comparison of Chl a/PP-N ratios in the 150 m water column with those observed from other regions (oligotrophic and eutrophic), in addition to phytoplankton cultures, suggests that the Bay of Bengal appears to be oligotrophic rather than eutrophic, with a much higher contribution of non-plant material to the total biomass. Phytoplankton accounted for between 2•4 and 23% of the total carbohydrates, while its contribution to the protein pool ranged from 2 to 20% reflecting a proportion of phytoplankton in the seston. Ratios of PP/PCHO as a means of estimating plankton nutritional status indicated a severe lack of nitrogen or phosphorus deficiency. However, this ratio may not serve as a true indicator of nutrient deficiency in oligotrophic areas, where non-phytal biomass controls the distribution of particulate concentrations