Driving Forces behind Organic Matter and Nutrient Dynamics in a Mangrove Forest Ecosystem in South India

Organic matter is primary carbon source and energy for micro and macro fauna inhabiting in the mangroves, either derived from autochthonous or allochthonous sources. Species composition and sorption capacity of the sediment may alter the organic matter distribution in the mangrove forests. Salinity may also have some impact over the organic matter composition. To understand the organic matter dynamics in the mangrove forest, we conducted a survey on the Pichavaram mangroves lying on the southeast coast of India. We have observed a wide variation in the concentrations of nutrients in the mangrove water in time and space scale. An annual concentration of DOC is 4.19 mg L-1, POC 2.97 mg L-1, DON 3.23 mg L-1 and DOP 0.35 mg L-1. Nutrient dynamics in the mangrove water is significantly influenced by seasonal effects and internal microbial mechanisms. Among the plant species Rhizophora mucronata, R. apiculata and Avicennia marina are the dominant and contributing around 43.41%, 39.55% and 18.13% litter per year respectively. Benthic microbial mineralization of organic matter and total carbon oxidation were studied and found that in Avicennia zone was higher than Rhizophora zone. This was further supported by sulfate reduction. The relationship between salinity and nutrients were analyzed by plot of nutrients versus salinity for different seasons. It has been observed that the net gain of DIN (1.14% m-1) was more than (0.84% m-1), which suggests that conversion of inorganic to organic form was minimal in premonsoon. And the same trend was also observed in monsoon. But the reverse trend was observed in post monsoon (1.5% m-1) and summer (0.65% m-1). Mean C:N ratio increased from 13.6 to 30.2 reflecting the relatively losses of N from the mangrove ecosystem. This is supported by N:Si ratio reduced from 2.49 to 0.33, this may be attributed to relative contribution of diatoms production. Interchanges between nutrient pools were apparent throughout the system. Sediment, and even surface-water, concentration of nutrients seemed to respond to plant root zone oxidation and uptake and release of nutrients. Pore water biogeochemical processes were linked to surface-water nutrient dynamics as well. Based on these findings, tidal effects, pore water concentrations, seasonal variation and microbial transformations were identified as driving forces behind coastal outwelling of nutrients and organic matter from mangroves.