Deposition, mineralization, and storage of carbon and nitrogen in sediments of the far northern and northern Great Barrier Reef shelf

The flow of carbon and nitrogen in sediments of the far northern and northern sections of the Great Barrier Reef continental shelf was examined. Most of the organic carbon (81–94%) and total nitrogen (74–92%) depositing to the seabed was mineralized, with burial of carbon (6–19%) and nitrogen (8–20%) being proportionally less on this tropical shelf compared with other non-deltaic shelves. Differences in carbon and nitrogen mineralization among stations related best to water depth and proximity to river basins, with rates of mineralization based on net ∑CO₂ production ranging from 17 to 39 ( mean=23) mmol C m⁻² d⁻¹. The overall ratio of O₂:CO₂ flux was 1.3, close to the Redfield ratio, implying that most organic matter mineralized was algal. Sulfate reduction was estimated to account for ≈30% (range: 6–62%), and denitrification for ≈5% (range: 2–13%), of total C mineralization; there was no measurable CH₄ production. Discrepancies between ∑CO₂ production across the sediment–water interface and sediment incubations suggest that as much as 5 mmol m⁻² d⁻¹ (≈25% of ∑CO₂ flux) was involved in carbonate mineral formation. Most microbial activity was in the upper 20 cm of sediment. Rates of net NH₄⁺ production ranged from 1.6 to 2.7 mmol N m⁻² d⁻¹, with highly variable N₂ fixation rates contributing little to total N input. Ammonification and nitrification rates were sufficient to support rapid rates of denitrification (range: 0.1–12.4 mmol N m⁻² d⁻¹). On average, nearly 50% of total N input to the shelf sediment was denitrified. The average rates of sedimentation, mineralization, and burial of C and N were greater in the northern section of the shelf than in the far northern section, presumably due to higher rainfall and river discharge, as plankton production was similar between regions. The relative proportion of plankton primary production remineralized at the seafloor was in the range of 30–50% which is at the high end of the range found on other shelves. The highly reactive nature of these sediments is attributed to the deposition of high-quality organic material as well as to the shallowness of the shelf, warm temperatures year-round, and a variety of physical disturbances (cyclones, trawling) fostering physicochemical conditions favorable for maintaining rapid rates of microbial metabolism. The rapid and highly efficient recycling of nutrients on the inner and middle shelf may help to explain why the coral reefs on the outer shelf have remained unscathed from increased sediment delivery since European settlement.