THE INFLUENCE OF FOREST TYPE ON MICROBIAL-NUTRIENT RELATIONSHIPS IN TROPICAL MANGROVE SEDIMENTS

Microbial productivity, nutrient chemistry and rates of nutrient regeneration were examined in muds of different mangrove forests within the Fly Delta, Papua New Guinea, to assess the effect of forest type on microbial and nutrient processes, and their interactions. Three major forest types were examined: Rhizophora-Bruguiera, Nypa and Avicennia-Sonneratia forests. For most variables, variations within a forest type were as great as, or greater than, differences between forest types. A high-intertidal Nypa site was most different in edaphic characteristics compared to five low-intertidal stations (two stations in each forest type) suggesting that differences among forest types in earlier studies were mainly a function of tidal elevation rather than species-specific ability of mangroves to influence redox and nutrient status. Dissolved inorganic nutrients were dominated by high (approximately 200-500 muM) concentrations of silicates, but porewater phosphate levels were usually below detection limits (less-than-or-equal-to 0.02 muM). Measured rates of nutrient regeneration were either slow into the sediment, or undetectable, despite a high concentration gradient for some solutes such as silicate. Rates of bacterial DNA and protein synthesis, and patterns of benthic primary production, indicate uptake of nutrients at the sediment-water interface by epibenthic microalgae and sequestering of porewater solutes by very active, subsurface bacterial communities. Rapid growth of these bacteria may be partially maintained by the decomposition and release of nutrients of mangrove roots and rhizomes, as suggested by the dominance of silicate in the porewater. Correlation analysis supports the notion of nutrient (mainly P) limitation of bacteria and microalgae in mangrove muds. It appears that a close microbe-nutrient-plant connection serves as a mechanism to conserve scarce nutrients necessary for the existence of these tropical tidal forests.