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 NO 2 - + NO 3 - . At most sampling periods, porewater NH 4 + and PO 4 3- 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 NH 4 + , but a moderate capacity (K = 0.8-4.8) for PO 4 3- 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 -1 d -1 and ∼496 mmol P ha -1 d -1 , with sediments accounting for nearly all uptake while Si is exported (∼2475 mmol ha -1 d -1 . Mid-intertidal forests import ∼1385, 93 and 4720 mmol ha -1 d -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.