The dynamics of benthic nutrient pools and fluxes in tropical mangrove forests
Abstract:Variations in benthic nutrient pools and rates of dissolved nutrient exchange between the forest floor and tidal waters were examined over a 5-yr period in mixed Rhizophora forests lining Coral Creek on Hinchinbrook Island in Queensland, Australia. Seasonal and spatial changes in redox status, porewater and solid-phase nutrients, and in exchange rates were not consistent and did not correlate with temperature. Below-ground roots, on average, accounted for ≈79%, 37% and 26% of bulk sediment TOC, total N and total P pools, respectively. Porewater nutrient concentrations were dominated by Si(OH)4+ and DON with consistently low levels of NO2− + NO3−. At most sampling periods, porewater NH4+ and PO43− concentrations were higher in creek bank sediments than in mangrove sediments indicating uptake by trees. These sediments have low adsorption capacity (K = 0.17–0.47) for NH4+, but a moderate capacity (K = 0.8–4.8) for PO43− adsorption. Most measured benthic fluxes of dissolved nitrogen and phosphorus showed uptake by sediments, prop roots and timber lying on the forest floor. Relative (per ha) estimates indicate that low-intertidal Rhizophora forests import ≈2220 mmol N ha−1d−1 and ≈496 mmol P ha−1d−1, with sediments accounting for nearly all uptake while Si is exported (≈2475 mmol ha−1d−1. Mid-intertidal forests import ≈1385, 93 and 4720 mmol ha−1d−1 of N, P and Si, respectively; sediments, prop roots and timber respectively account for 36%, 62% and 2% of the N import. Mid-intertidal sediments account for all net P uptake, but prop roots and sediments account for 60% and 40% of total Si uptake. On an absolute basis, low-intertidal forests (78 ha total area) in Coral Creek import ≈881 kgN yr−1, 436 kgP yr−1 and export 1963 kgSi yr−1, and the mid-intertidal forests (338 ha total area) import ≈2392 kgN yr−1, 356 kgP yr−1 and 16300 kgSi yr−1. The sum of these estimates equates to ≈95% of the net annual import of total dissolved N and ≈66% of the net annual import of total dissolved P into the Coral Creek tidal basin from adjacent coastal waters. By difference, ≈14337 kgSi yr−1 is imported into the system. This indicates that mangrove forests are a very efficient sink of dissolved nitrogen, phosphorus and silicon in this tidally-driven coastal ecosystem. This import may be driven by the consistently high rates of microbial and plant growth and productivity within the forests.
Document Type: Research Article
Publication date: January 1, 1996
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