Nutrient fluxes into an isolated coral reef atoll by tidally driven internal bores

Abstract

Scott Reef is a remote yet biodiverse coral reef atoll, rising from depths > 1000 m on the edge of the continental shelf of northwestern Australia. An intensive 2‐week field study was conducted to assess how physical oceanographic processes influence reef communities in South Scott's deep (~ 50 m) and expansive (~ 300 km 2 ) semi‐enclosed lagoon. The study covered a spring–neap tidal cycle, during which moored instruments measured temperature and velocities at a lagoon entrance and ship‐based vertical profilers measured water quality parameters. Tidally driven internal bores advected cooler and deeper offshore waters (up to 4.5°C, mean 2.4°C) into the lagoon, which were also richer in nutrients and chlorophyll a than lagoon waters. This offshore water originated from approximately 75 m depth. We used the strong observed relationship between in situ measurements of nitrate and temperature to estimate nitrate concentrations from high temporally and spatially resolved temperature measurements at the lagoon entrance, and then estimated the horizontal nitrate fluxes into the lagoon using the velocity measurements. The depth‐integrated, time‐averaged nitrate flux during spring tides (1.46 kg m −1 d −1 ) was over three times larger than during neap tides (0.46 kg m −1 d −1 ), despite a passing tropical storm that temporarily deepened the offshore thermocline and reduced the magnitude of the fluxes. Our observations indicate that colder, deeper, and nutrient‐rich water associated with internal tidal bores during spring tide is likely to be the primary mechanism through which allochtonous nutrients are delivered to benthic and pelagic communities within the lagoon of this isolated atoll.

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