A synoptic monthly survey has been conducted on the Pichavaram mangroves of the south east India to understand the nutrient chemistry of the Pichavaram mangroves from July 2003 to June 2004. During monsoon, river inputs of nitrogen and silicate were significantly higher than phosphate concentration across the mangrove ecosystem. On the contrary, during non-monsoonal periods, oceanic influence was greater, leading to depleting nutrient concentrations. Correlation analysis explains that salinity had a good correlation with nitrate (R^2 = 0.69) and silicate (R^2 = 85) and moderate correlation found between salinity and phosphate (R^2 = 0.48) inferring that P is limiting for primary productivity indicating salinity control (scavenging) over phosphate distribution. Molar Si:N ratio was nearer to the Readfield ratio, which is probably due to more diatom productivity among the phytoplankton pool. Higher concentration of ammonia would also contribute to increase consumption ratio of Si/N amounts which may be leads to higher planktonic production.
Significant nutrient concentrations were observed in the sedimentary environment of the mangroves. Grain size controls the Carbon distribution in the sediments. The vertical distribution of the nutrients in the core sediments has been controlled by the physical forces, pH, temperature and redox potential. Total P ranges from 357 – 437 micro g g^1 and organic phosphate (OP) accounts for 37.52% of the total P pool. Higher concentration of inorganic phosphate (IP) was observed in the estuarine zones of the mangrove and OP was observed in the interior mangrove zone. IP was generally higher than OP in the core sediments. In surfacial sediments oxide bound-P was the dominant fraction than the other fractions where as in core sediments the authigenic / detrital fractions are dominant with increasing depth and was site specific.
Rates of the total carbon oxidation (TCOX) were measured and higher in the undisturbed A. marina forest (199 mol C m^-2 y^-1) than in the two disturbed stands (43 and 79 mol C m^-2 y^-1); rates of total carbon oxidation in the R. apiculata forest averaged 75 mol C m^-2 y^-1. Sulphate reduction (21 – 319 m mol S m^-2 d^-1) was the major decomposition pathway of TCOX, except at the most disturbed forest. Rates of sulphate reduction at all sites peaked in subsurface soils to a depth of ≈ 1m, leading to little carbon burial (3-5% of total C input). Significant Fe-Mn reduction was also observed in the mangrove sediments. Rates of the microbial activity was rapid than the other mangrove strands, reflecting high rates of the primary production and organic matter retention and internal cycling in the mangrove ecosystem.
Trophic dynamics has been studied by the stable isotopes (δ13C and δ15N). Significant variation in the stable carbon (δ13C) in the nutrient sources has been observed. However, δ15N signatures were not significantly different among the different potential nutrient sources. High organic matter with low C:N ratios with relatively negatively fractionated isotopes indicate that mangrove derived organic matter was the prime diet for the invertebrate communities in the mangrove ecosystem.
It has been calculated that net removal of DIN and DIP ranges from -0.15 to -0.09 mol m^-2 y^-1 and -0.95 to -0.04 mol m^-2 y^-1 respectively. The agricultural water canal system (Khan Shabib canal) supply considerable amount of nutrients to the Pichavaram mangrove The magnitude of net primary production (p – r) is + 956629 mol C d^-1, indicating that the mangrove is in autotrophic state.