Nutrient excess and the demise of coral reefs and carbonate platforms

Growth rates of corals on Holocene reefs indicate that carbonate platforms should easily keep pace with long-term subsidence and sea-level changes, yet drowned reefs and platforms are common in the geologic record. Recognition of the negative influence of nutrients on reef communities provides a clue to that paradox. The primary carbonate-sediment producers of the coral reef community are highly adapted to nutrient-deficient environments. Input of nitrates and phosphates stimulates growth of plankton, which reduces water transparency, limiting depth ranges of zooxanthellate corals and calcareous algae and thereby reducing carbonate production. Higher nutrient concentrations and plankton densities also stimulate growth of fleshy algae and ahermatypic suspension-feeding animals in the benthos. Besides displacing hermatypic algae and corals, many of these fast-growing competitors are bioeroders that actively destroy the reefal structure. Because rates of carbonate production and bioerosion are similar, even modest increases in nutrient availability can shift a reef community from net production to net erosion. In the geologic record, drowned reefs and carbonate platforms typically exhibit evidence of nondeposition, bioerosion, and reduced redox potential, which indicate excess nutrient availability during drowning. Drowned reefs overlain by shales are possible victims of nutrients in terrestrial runoff that suppressed reef growth before arrival of siliciclastic sediments. Other drowned platforms may have succumbed during rapid pulses of sea-level rise that flooded previously subaerial platforms. Nutrients in the soils of the flooding platform were mixed into surface waters, suppressing reef growth. The reef drowned if submergence proceeded beyond the critical depth before the excess nutrients were exported from the system. Other mechanisms for reef drowning by excess nutrients include changes in local or regional upwelling patterns or mid-ocean overturn.