Benthic Exchange of Biogenic Elements in the Estuarine and Nearshore Waters of Western India

Abstract

In order to get a quantitative understanding of coupling between benthic regeneration and fluxes with pelagic nutrient cycles and primary productivity in shallow environments of eastern Arabian sea, biogeochemical studies were carried out at two chosen sites i.e. Mandovi estuary (15.502oN, 73.825oE) and a coastal station (15.509oN, 73.65 oE) ~15 Km off Goa coast during 2004 to 2006. Monthly variation of relevant physico-chemical and biological parameters were also recorded at these sites. Benthic flux measurements were undertaken in premonsoon and postmonsoon period following standard methods of in-situ and laboratory incubation of sediment. Phosphorus dynamics in relation to its exchange across sediment-water interface were specially studied by a series of laboratory based incubation experiments mimicking the natural physical condition of the estuary. Nutrient enrichment experiments were carried out at these sites to understand the coupling between benthic inputs and ecosystem productivity. The rate of benthic respiration and nutrient exchange exhibited pronounced seasonality in the estuary. The estuarine sediment behaved as net source of DIN especially in the form NH4+ (70-100%) in the dry season (February-May) but became a net sink for combined N in the monsoon season implying active benthic denitrification. Nutrient fluxes peaked in April coinciding with high pelagic production but attained minimal in August reflecting the variation in temperature and availability of labile organic matter. Benthic supply of DIN, DIP and SiO4 4- met 49, 25 and 55% of algal N, P and Si demand respectively in premonsoon season. Shelf sediments were also found to be a significant source of nutrients to the pelagic algal community. The estuarine sediment showed high adsorbing capacity for DIP presumably due to presence of ferric oxide minerals which was further confirmed by exponential PO4 3- release from sediment under anoxic condition. An equilibrium concentration of 0.7- 0.9?M was maintained in the bottom water owing to PO4 3- buffering capacity of the sediment. Salinity of the overlying water was found to exert significant control on benthic P release agreeing with the results of in-situ measurements in different seasons. Nutrient uptake in the estuarine water was rapid after a time lag of ~24hours where as coastal phytoplankton took longer time (~48h) to respond to the nutrified ambience though followed by rapid uptake of nutrients which indicates their physiological adaptation to an environment which is nutrified in a typical natural process. Contrary to classical algal N preference, both estuarine and coastal phytoplankton preferred NO3 - rather than NH4 + even in the availability of the latter at a substantial concentration (~4?M) indicating species specific N preference. Simultaneous rapid uptake of NO3 -, PO4 3- and SiO4 4- by phytoplankton revealed the tight coupling between benthic fluxes and algal uptake of nutrients and a significant contribution of benthic fluxes to pelagic primary production.