Organic production in a Tropical estuary

Abstract

In a highly turbid and polluted estuary the C¹⁴ assimilation is nearer to net production and the diurnal rhythm in photosynthesis is associated with the increase and decrease in daily illumination. There is no decline in the rate of photosynthesis at peak illumination but the difference between forenoon and afternoon production is significant. Photosynthesis measured in relation to illuminations gave different light saturation optima on different days probably due to highly variable nature of phytoplankton population. Estimates of gross and net primary production were made for the thin euphotic zone, the latter after computing the respiratory losses occurring during day and night which were large and inconsistent. Seasonal changes in the production rate were not well marked and showed only 3–4 fold increase in certain months. For most of the year, primary production seemed non-existent at depths greater than about 4 metres. None of the factors such as temperature and nutrients seems to be limiting in the estuary. Monthly variations in total solar radiation are not sufficiently large to affect seasonal changes in production, but the light penetration is greatly reduced which limits column production. Seasonal variations in salinity, primarily induced by the monsoon cycle, initiate a succession of brief pulses of bloom. The stability of the euphotic zone is important for maintaining plant population within the narrow zone of illumination and for favouring production. The range in carbon assimilation to chlorophyll ratio was large which signifies that photosynthesis and chlorophyll maxima are non-synchronous. From the assimilation ratio determined experimentally the estimates of production were made from radiation and chlorophyll data. The interrelationship between carbon assimilation, chlorophyll and detrital material has been discussed and the annual estimates of gross and net production have been given. The efficiency of the environment to convert solar energy into chemical energy was about 0·4%. A general lack of zooplankton grazers in the estuary leaves behind a considerable surplus of unconsumed basic food, much of which seems to be lost by sinking below the narrow euphotic zone.