Regional-scale nitrogen and phosphorus budgets for the northern (14°S) and central (17°S) Great Barrier Reef shelf ecosystem

Seasonally averaged N and P box model budgets were constructed for two regional-scale sections of the Great Barrier Reef (GBR) shelf, one in the near-pristine far-northern GBR (13.5–14.5°S) and the other in the central GBR (17–18°S) adjacent to more intensively farmed wet tropics watersheds. We were unable to simultaneously balance shelf-scale N and P budgets within seasonal or annual time frames, indicating that magnitudes of a number of key input and, especially, loss processes are still poorly constrained. In most cases, current estimates of system-level N and P sources (rainfall, runoff, upwelling, N-fixation) are less than estimated loss processes (denitrification, cross-shelfbreak mixing and burial). Nutrient dynamics in both shelf sections are dominated by the tightly coupled uptake and mineralization of soluble N and P in the water column and the sedimentation–resuspension of particulate detritus. On an area-averaged basis, internal cycling fluxes are an order of magnitude greater than input–output fluxes. Denitrification in shelf sediments is a significant sink for N while lateral mixing is both a source and sink for P.