The flow of carbon and nitrogen in sediments of the far northern and northern sections of the Great Barrier Reef (GBR) 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 Total CO2 production ranging from 17 to 39 (mean=23) mmol C/m2/d. 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 approx 30% (range: 6-62%), and denitrification for approx 5% (range: 2-13%), of total C mineralization; there was no measurable methane production. Discrepancies between Total CO2 production across the sediment-water interface and sediment incubations suggest that as much as 5 mmol/m2/d (approx 25% of Total CO2 flux) was involved in carbonate mineral formation. Most microbial activity was in the upper 20cm of sediment. Rates of net ammonium production ranged from 1.6 to 2.7 mmol N/m2/d 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/m2/d). 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.
This research was undertaken to document benthic metabolic rates and pathways of carbon and nitrogen cycling at inner and middle shelf sites proximal to impacted catchments on the northern GBR and to less heavily impacted catchments on the far northern GBR shelf. Some first-order calculations are also made to understand the benthic contribution to nutrient dynamics on these sections of the shelf and how (or if) they differ to earlier studies in the central section of the GBR shelf.
This is a companion paper to:Alongi, D.M., Pfitzner, J., Trott, L.A. (2006). Deposition and cycling of carbon and nitrogen in carbonate mud of the lagoons of Arlington and Sudbury Reefs, Great Barrier Reef. Coral Reefs, 25, 123-143. This study focused on the lagoon sediments from two northern GBR reefs.
