Sedimentation of particulate material in stratified and nonstratified water columns in the Bombay High area of the Arabian Sea

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Abstract
Sedimentation of particulate material at 22, 42 and 62 m was recorded at a station in the Bombay High area of the Arabian Sea from September 1985 to March 1986. Diatom numbers and physical structure, especially thermal stratification, played an important role in controlling both magnitude of total flux and nature of sedimented material. Changes in suspended diatom numbers were, with some exceptions, reflected in the sedimented material caught by the traps. From September to March, mean sedimentation of total particulate material (TPM) and particulate inorganic material (PIM) increased with depth. Up to December, particulate organic carbon (POC), particulate organic nitrogen (PON), particulate biogenic silica (PSi), chlorophyll a equivalents (Chl a equiv.), total particulate carbohydrates (TCHO) and the constituent fractions dilute alkali-soluble (AL-CHO) and residual carbohydrates (RS-CHO) increased and dilute acid-soluble carbohydrates (AC-CHO) decreased with depth. From January to March, however, mean sedimentation rates of POC, PON, PSi and Chl a equiv. generally decreased with depth, whereas sedimentation fluxes of TCHO, AC-CHO, AL-CHO and RS-CHO decreased between 22 and 42 m and increased again at 62 m. Low POC/Chl a equiv. and POC/PSi ratios indicate that fairly fresh biogenic material settled during January to March when the water column was more or less homogeneous, as compared to September to December when the water column was thermally stratified. Sedimentation of organic carbon was correlated with sedimentation of biogenic silica, especially during January to March. There were positive and highly significant relationships between the flux of TCHO and those of TPM, POC, Chl a equiv. and PSi. From the latter 2 relationships, was deduced that diatoms were the major source of TCHO at this station. High sedimentation of TPM was associated with high flux of organic carbon and TCHO. This may have a stimulatory effect on benthic organisms and may also enhance the chances of burial of organic matter in sediments.

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