Abstract. Benthic fluxes of dissolved nutrients in reef communities
are controlled by oceanographic forcing, including local hydrodynamics and
seasonal changes in oceanic nutrient supply. Up to a third of reefs
worldwide can be characterized as having circulation that is predominantly
tidally forced, yet almost all previous research on reef nutrient fluxes has
focused on systems with wave-driven circulation. Fluxes of dissolved
nitrogen and phosphorus were measured on a strongly tide-dominated reef
platform with a spring tidal range exceeding 8 m. Nutrient fluxes were
estimated using a one-dimensional control volume approach, combining flow
measurements with modified Eulerian sampling of waters traversing the reef.
Measured fluxes were compared to theoretical mass-transfer-limited uptake
rates derived from flow speeds. Reef communities released 2.3 mmol m−2 d−1 of nitrate, potentially derived from the remineralization of
phytoplankton and dissolved organic nitrogen. Nutrient concentrations and
flow speeds varied between the major benthic communities (coral reef and
seagrass), resulting in spatial variability in estimated nitrate uptake
rates. Rapid changes in flow speed and water depth are key characteristics
of tide-dominated reefs, which caused mass-transfer-limited nutrient uptake
rates to vary by an order of magnitude on timescales of ∼ minutes–hours. Seasonal nutrient supply was also a strong control on reef
mass-transfer-limited uptake rates, and increases in offshore dissolved
inorganic nitrogen concentrations during the wet season caused an estimated
twofold increase in uptake.
Tidal and seasonal forcing of dissolved nutrient fluxes in reef communities