Dynamics of Tidally Driven Nutrient Transport into a Semi-Enclosed Isolated Coral Reef Atoll

The hydrodynamics within and surrounding coral reefs directly influence ecosystem productivity, community structure and diversity. Most studies on reef atolls have focused on shallow reef flats forced by surface waves, but fluxes of nutrients and heat driven by internal waves can be important within the deeper outer slopes and lagoons of many atolls. At present the number of studies into the effect of internal waves on reef environments are geographically limited, and principally for sites with relatively small tidal ranges.

Scott Reef rises steeply from depths of 2500 m on the edge of Western Australia’s continental slope. A combination of complex shelf topography, stable thermal stratification, and a large barotropic tide (>3m) contribute to the strong internal wave climate around this isolated atoll. The emphasis of this study is on the physical environment associated with the rich ecosystem in the large (300km²) and deep (~50 m) semi-enclosed lagoon of South Scott. A multidisciplinary research cruise took place during April 2015. To investigate the dynamics of the internal waves influencing the lagoon, a mooring was deployed at a lagoon entrance with an array of current meters, temperature sensors and pressure sensors over a 15 day period. A simultaneous ship-based sampling programme of water column profiles, including water quality analysis, provided a comprehensive understanding of the how the physical environment contributes to maintaining this productive reef system. Results reveal that steep topography forces the internal waves to shoal into the north-east of the lagoon, transporting nutrient-rich deep water that results in a 3-fold increase in nitrate concentrations. Our field measurements indicate that cold water intrusions cause a baroclinic velocity response of 0.3 m s^-1 and previous modelling studies reveal that the cold water can propagate up to 9 km into the lagoon at flood tide. The ecological implications for this replenishment of nutrients will be discussed.