Particulate nutrient fluxes over a fringing coral reef: relevant scales of phytoplankton production and mechanisms of supply

Abstract

Seasonal observations of phytoplankton uptake at Ningaloo Reef, Western Australia, reinforce the importance of particulate organic nitrogen (PON) and carbon (POC) in reef nutrient budgets and identify wave action and the dynamics of regional currents (over a range of temporal and spatial scales) as important factors determining plankton supply to the reef. Phytoplankton uptake rates, calculated from declining chlorophyll a concentrations as water moved over the reef, appeared to be near the physical limits of mass transfer. Phytoplankton-derived PON flux of 2 to 5 mmol N m -2 d -1 was on the order of that typical for dissolved N uptake — confirming that particle feeding may supply the N missing in reef N budgets — while POC flux of 14 to 27 mmol C m -2 d -1 was on the order of net community metabolism. Phytoplankton supply was highly variable at daily-to- seasonal time scales in response to the dynamics of a regional current system dominated by the downwelling-favourable Leeuwin Current (LC). Acceleration of the LC in the austral autumn may supply as much phytoplankton to the reef as sporadic upwelling associated with the Ningaloo Cur- rent (NC) in summer. The ocean catchment concept is introduced as a basis for examining the spatial scale of pelagic processes influencing benthic systems: every day, Ningaloo may completely consume the phytoplankton over 87 km 2 of LC water, compared to only 20 km 2 of NC water. Production within this catchment appears insufficient to maintain offshore phytoplankton concentrations, and advec- tion of remotely sourced production into the catchment is required to balance reef uptake. A func- tional dependence by reef organisms on externally sourced ocean productivity increases the poten- tial scale at which human- or climatically induced changes may affect reef communities and suggests that processes such as changes in offshore currents and plankton communities require further con- sideration in reef-level biogeochemistry.