Abstract
Nitrate and nitrite (NO x ) and phosphate (PO 4 ) dynamics over Ningaloo Reef, Western Australia, are shown to depend on oceanographic forcing of coupled mass transfer limited (MTL) gross uptake and gross release from remineralized oceanic particulate organic matter (POM). Estimates of gross release rates increased significantly with increasing POM uptake and were of the same order as gross uptake rates. Gross uptake rates increased significantly with increasing oceanic concentrations and wave energy dissipation, were 35–80% higher over the reef crest (7–9 mmol NO x m −2 d −1 and 4–5 mmol PO 4 m −2 d −1 ), and were significantly correlated with independent estimates of POM‐mediated gross NO x uptake, supporting both MTL uptake and the strong role of oceanic POM supply. The relative supply of NO x and POM was linked to the seasonal dynamics of a regional current system. In late spring, upwelling associated with seasonally strong equator‐ward winds led to increased NO x concentrations (0.71 ± 0.2 µmol L −1 ), POM x and the reef was a net nutrient sink (5390 mmol NO x m −1 d −1 and 270 mmol PO 4 m −1 d −1 ). In contrast, during the autumn, NO x was low (0.16 ± 0.06 µmol L −1 ), but POM > NO x and the reef was a net nutrient source (−7060 mmol NO x m −1 d −1 and −730 mmol PO 4 m −1 d −1 ). The autumn enhancement of oceanic POM supply to the reef can be attributed to a regional phytoplankton bloom associated with acceleration of the oligotrophic Leeuwin Current, which may result in a significant supply of dissolved nutrients to downstream communities.