Carbon, nitrogen and phosphorus dynamics in creek water of a southeast Asian mangrove forest

Abstract

Water column dynamics of carbon, nitrogen and phosphorus in the main creek of the Bangrong mangrove forest, Phuket Island, Thailand, were examined during the dry season. Water sampled from the upper and lower reaches of the creek throughout entire neap and spring tide periods was incubated under saturated irradiation and in the dark. The activity of microbial primary producers and heterotrophs were evaluated from changes in O2, TCO2, DOC, DIN, DON and PO43- concentrations. Gross primary production was moderate (1.6-2.4 μM C h-1) with no pronounced spatial and temporal variations. A large fraction of the assimilated inorganic carbon and nitrogen was released in the form of DOC (50-90%) and DON (50-60%) indicating that primary producers were under stress or nutrient depleted. PO43- and occasionally DIN appeared to be the limiting nutrients. The pelagic heterotrophic community was supported by DOC (50-100%) and DON (40-90%) excreted by primary producers when exposed to light. However, rapid light attenuation in the turbid creek water rendered the entire water column strongly heterotrophic at all times (1.1-2.8 μM C h-1). The microbial heterotrophs are therefore largely supported by particulate and dissolved substrates derived from tidal resuspension, mangrove root exudates and leachates from fallen leaves on the forest floor. The relatively high concentrations of metabolites (e.g. CO2, NH4+) in creek water were primarily supplied by microbial mineralization. Water seeping from creek banks, which was only enriched in TCO2 (2 times) and PO43- (20-100 times) compared with creek water, is not considered an important source of solutes in the waterways of the Bangrong mangrove forest. Although the results obtained here are only strictly representative for the dry season, water column processes in the wet season are not expected to differ much due to the limited seasonal variations with respect to physical, chemical and biological parameters in the Bangrong area.