Differences in Nutrient Concentrations and Resources Between Seagrass Communities on Carbonate and Terrigenous Sediments in South Sulawesi, Indonesia

Water column, sediment and plant parameters were studied in six tropical seagrass beds in South Sulawesi, Indonesia, to evaluate the relation between seagrass bed nutrient concentrations and sediment type. Coastal seagrass beds on terrigenous sediments had considerably higher biomass of phytoplankton, epiphytic algae and macroalgae, if compared to seagrass beds growing on carbonate sediments in oligotrophic reef flat environments. The size of leaves of seagrass plants of the same species was considerably larger at terrigenous sites than at carbonate sites. Seagrasses in carbonate-rich environments had invested considerably more in below-ground biomass relative to above-ground biomass than those growing on terrigenous sediments. Elementary composition of plant material indicated a richer nutrient supply of both N and P at the terrigenous sites (C:N:P = 340:19:1) than at carbonate sites (C:N:P = 565:18:1). Concentrations of dissolved reactive phosphate, ammonium, and nitrate + nitrite were low (<2 μm) in the water column at all sites, often below detectable limits, but considerably higher in sediment porewaters. Porewater phosphate concentrations (3–13 μm) were comparable between the two sediment types, but exchangeable phospborus contents were 2 to 5 times higher in carbonate sediments (18.6–23.6 mg P₂O₅˙100 g^–1 versus 4.4–10.9 mg P₂O₅˙100 g^–1 in terrigenous sediments). Porewater phosphate concentration decreased with increasing sediment depth in carbonate sediments. The relatively coarse composition of these sands might have limited the adsorption of phosphate onto the carbonate mineral surfaces. Carbonate sediments were extremely low in organic matter, compared to terrigenous sediments. Both biogeochemical properties of the sediment type and the degree of influence from terrigenous run-off were found to be important factors affecting the availability of nutrients to seagrass growth and determining the response in morphology, biomass and chemical composition of the seagrass material.