Breathing of a coral cay: Tracing tidally driven seawater recirculation in permeable coral reef sediments

Abstract

[1] Coral reefs are characterized by high gross productivity in spite of low nutrient concentrations. This apparent paradox may be partially reconciled if seawater recirculation in permeable sediments over large (meters) and long (hours to days) scales is an important source of recycled nitrogen and phosphorus to coral reefs. In this paper we use radon ( 222 Rn, a natural tracer) to quantify tidally driven pore water (or groundwater) exchange between (1) an offshore coral cay island and its fringing reef lagoon and (2) a reef lagoon and the surrounding ocean. As seawater infiltrates Heron Island at high tide, it acquires a radon signal that can be detected when pore waters emerge from carbonate sands at low tide. A nonsteady state model indicated that vertical pore water upwelling rates (or saline submarine groundwater discharge) were >40 cm/d within the reef lagoon and >100 cm/d outside the lagoon at low tide. Within the lagoon, tidal pumping and temperature-driven convection were the main driving forces of pore water advection. At low tide, the reef lagoon level is about 1 m higher than the surrounding ocean. As a result, a steep hydraulic gradient develops at the reef edge, driving unidirectional filtration through the reef framework. Groundwaters were highly enriched in nitrate (average of 530 mmol, likely influenced by bird guano) relative to lagoon waters (1.9 mmol). Rough but conservative estimates indicated that groundwater-derived nitrate fluxes (7.9 mmol/m 2 /d) can replace the entire lagoon nitrate inventory every <19 days. We speculate that as offshore coral islands "breath" (inhale seawater at high tide and exhale groundwater at low tide), they release nutrients that lead to sustained productivity within coral reefs.

Tidal variability of nutrients in a coastal coral reef system influenced by groundwater

“…5a). This pattern reflected the variation in the groundwater discharge induced by tidal pumping in this coral reef system (Wang et al, 2014), which is also observed in other coastal regions (Burnett and Dulaiova, 2003;Santos et al, 2010).…”

“…Groundwater flux onto coral reefs was found to fluctuate with the tidal cycle (Lewis, 1987;Santos et al, 2010). The contribution of groundwater discharge to the nutrient budget of adjacent marine waters of coral reefs varies greatly from one site to another around the globe and at each site varies from one tidal state to another (Paytan et al, 2006).…”

Seasonal, daily and diel N&lt;sub&gt;2&lt;/sub&gt; effluxes in permeable carbonate sediments

“…The chambers were stirred at 40 RPM, which induced a sediment percolation rate of approximately 43 L m −2 d −1 . Although this is at the low range of advective rates estimated for permeable sediments in the Heron Island reef lagoon (0 to 600 L m −2 d −1 ; average 150 L m −2 d −1 ; Santos et al, 2010), these higher rates include all advective processes (e.g. tidal pumping), not just the small-scale advective processes in the sediments (e.g.…”

Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

“…The variable SGD flow rates between high tide and low tide are known as tidal pumping, and are the main driving force of pore water advection. SGD-induced tidal pumping has previously been described as the "breathing" of offshore coral islands, where seawater is inhaled at high tide and nutrient rich groundwater is exhaled at low tide, leading to sustained productivity within coral reefs [57].…”