The importance of mineralization based on sulfate reduction for nutrient regeneration in tropical seagrass sediments

Nutrient dynamics (nitrogen and phosphate), sediment organic matter mineralization (sulfate reduction rates) and stable carbon isotope composition in two tropical seagrass sediments were studied at a intertidal sandflat of Ban Pak Klok, Thailand. The seagrass beds were composed of meadows of Cymodocea rotundata (Ehrenb. and Hempr.) Aschers and Thalassia hemprichii (Ehrenb.) Aschers. The nutrient concentrations in porewaters were low (< 1 muM NO2- + NO3- 30-70 muM NH4+ and 2-5 muM PO43-) in the low-organic sediments (<0.6% DW POC and <0.03% DW PON). Sulfate reduction rates were significantly higher in the C. rotundata sediments (6.6 mmol m(-2) per day), whereas there was no difference between rates in the T. hemprichii and bare sediments (2.1 and 2.2 mmol m(-2) per day). The depth distribution of sulfate reduction rates in C. rotundata sediments was positively correlated with the below-ground biomass suggesting that the enhanced anaerobic activity was due to supply of microbial substrates from the seagrasses. The stable isotopic composition of the sediment bacteria resembled the seagrasses suggesting that the bacteria used organic matter derived from the seagrasses. A two-compartment experiment showed that both seagrasses released organic compounds from the roots (7.0 and 5.4% of photosynthetic rate for C. rotundata and T. hemprichii, respectively), and that the loss was sufficient to support the measured sulfate reduction rates. The contribution of sulfate reduction to nutrient availability was, however, low supporting only 6-22% of the nutrient demand, except for PO43- in C. rotundata sediments where 81% of the demand was covered by sulfate reduction. The relatively high nutrient content of the seagrasses (1.5-2.0% DW N and 0.18-0.20% DW P) suggested that their growth was not limited by nutrients. The low contribution of sulfate reduction to nutrient availability thus indicates that other mineralization processes or uptake of nutrients from the water column are important for plant growth. (C) 2001 Elsevier Science B.V All rights reserved.