Episodic nutrient transport to Florida coral reefs

Abstract

This study documents the changes in nutrient fluxes associated with internal tidal bores arriving on Florida Keys coral reefs and points to biological use of subthermocline nitrate brought onshore by this mechanism. Internal bores on Conch Reef, Florida Keys, are associated with concentrations of 1.0–4.0 µmol L−1 nitrate (NO3− ) and 0.1–0.3 µmol L−1 soluble reactive phosphate (SRP) and onshore flow velocities of 0.1–0.3 m s−1. The arrival of internal bores causes 10–40 fold increases in nutrient concentrations and 1–2 orders of magnitude increases in nutrient flux relative to ambient, nonbore conditions. The magnitude and duration of cool‐water nutrient transport events increases significantly with increasing depth on reef slopes. In June 2001, the gradient of increased exposure to subsurface water with depth corresponded to increased percentage of N and δ15N and decreased C:N ratio in a common benthic macroalga, Codium isthmocladum. Internal tidal bores are widespread throughout the Florida Keys reef tract, with cool‐water episodes influencing reefs up to 10%–25% of the time during summer months and with significant variability among years. Estimated inputs of nitrogen and phosphorus by internal tidal bores to Florida Keys reef slopes are as much as 40‐fold larger than published estimates of inputs to near‐shore waters from waste water and storm water runoff. Internal tidal upwelling represents an important, previously underestimated, episodic source of nutrients on the Florida Keys reef tract. In order to assess nutrient availability in this system accurately it is essential to understand natural sources of high‐frequency variability.