TIDAL FORCING OF NUTRIENT UPTAKE ON CORAL REEFS: FIELD AND MODELLING STUDY FROM A TIDE-DOMINATED SYSTEM

In oligotrophic systems such as coral reefs, the uptake of nutrients by benthic communities is a function of ambient nutrient concentrations and local hydrodynamics. Reefs can be classified as having circulation that is dominated by either waves or tides; wave-dominated systems have a mean annual significant wave height that exceeds their mean tidal range, while tide-dominated systems have tidal ranges greater than their mean wave heights. Approximately 30% of reefs worldwide are tide-dominated, yet physical-biological coupled processes on these reefs have rarely been studied previously. We conducted a field and modelling study in the macrotidal Kimberley region of Western Australia, where large semidiurnal tides (~8m range) cause rapid changes in flow velocity and water depth on reefs. We measured uptake rates of nitrate and phosphate in seagrass and reef communities over a full spring-neap tidal cycle using a 1-dimensional control volume approach. Reef communities released a moderate amount (2.3 mmol m^-2 d^-1) of nitrate, which was likely derived from remineralisation of organic material. Nutrient uptake at the limits of mass-transfer was estimated using simple modelling, which showed that tidal forcing can cause uptake to vary by an order of magnitude over a semidiurnal tidal cycle. We broaden these results to discuss the importance of tides in regulating nutrient uptake in other tide-dominated systems, such as the southern Great Barrier Reef and Indo-Pacific.