Benthic fluxes in a tropical Estuary and their role in the ecosystem

Abstract

In-situ measurements of benthic fluxes of oxygen and nutrients were made in the subtidal region of the Mandovi estuary during premonsoon and monsoon seasons to understand the role of sediment-water exchange processes in the estuarine ecosystem. The Mandovi estuary is a shallow, highly dynamic, macrotidal estuary which experiences marine condition in the premonsoon season and nearly fresh water condition in the monsoon season. The benthic flux of nutrients exhibited strong seasonality, being higher in the premonsoon compared to the monsoon season which explains the higher ecosystem productivity in the dry season in spite of negligible riverine nutrient input. NH₄⁺ was the major form of released N comprising 70-100% of DIN flux. The benthic respiration rate varied from -98.91 to -35.13 mmol m^-2 d^-1, NH₄⁺ flux from 5.15 to 0.836 mmol m^-2 d^-1, NO₃^- + NO₂^- from 0.06 to -1.06 mmol m^-2 d^-1, DIP from 0.12 to 0.23 mmol m^-2 d^-1 and SiO₄^4- from 5.78 to 0.41 mmol m^-2 d^-1 between premonsoon to monsoon period. The estuarine sediment acted as a net source of DIN in the premonsoon season, but changed to a net sink in the monsoon season. Variation in salinity seemed to control NH₄⁺ flux considerably. Macrofaunal activities, especially bioturbation, enhanced the fluxes 2-25 times. The estuarine sediment was observed to be a huge reservoir of NH₄⁺, PO₄^3- and SiO₄^4- and acted as a net sink of combined N because of the high rate of benthic denitrification as it could remove 22% of riverine DIN influx thereby protecting the eco system from eutrophication and consequent degradation. The estuarine sediment was responsible for ~30-50% of the total community respiration in the estuary. The benthic supply of DIN, PO₄^3- and SiO₄^4- can potentially meet 49%, 25% and 55% of algal N, P and Si demand, respectively, in the estuary. Based on these observations we hypothesize that it is mainly benthic NH₄⁺ efflux that sustains high estuarine productivity in the NO₃^- depleted dry season.