Benthic uptake of phytoplankton and ocean-reef exchange of particulate nutrients on a tide-dominated reef

Benthic fluxes of chlorophyll a (Chl a) and particulate organic carbon (POC) and nitrogen (PON) were quantified on Tallon reef, a strongly tide-dominated (spring range > 8 m) reef located in the Kimberley region of northwestern Australia, over a 2-week period. Extensive hydrodynamic observations were used to construct a reef-scale mass balance to estimate material exchange between the reef and ocean over individual tidal cycles. Additionally, a one-dimensional control volume approach was used to estimate fluxes of Chl a in waters traversing the reef platform. Particulate material was delivered to the reef platform in a pulse during flood tide, and benthic uptake of Chl a declined to negligible values toward the end of ebb tide. On the scale of tidal cycles, a net uptake of Chl a was observed on the reef platform (on average 1.3 mg Chl a m⁻² d⁻¹), which was lower than previous studies of many reef communities. Fluxes showed variability depending on the magnitude of individual tidal cycles, which was likely related to volumes of oceanic Chl a inputs. Tallon reef was a net source of detrital POC and PON to the surrounding coastal ocean, with average POC exports ∼ 3% of the reef's benthic gross primary production. Seasonal measurements of water quality reported here are among the first records for the coastal Kimberley, and suggest that reefs in the west Kimberley may experience naturally elevated levels of phytoplankton and particulate nutrients, especially during the wet season.