Episodic nutrient transport to Florida coral reefs

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 mmol L21 nitrate (NO ) and 0.1–0.3 3 mmol L21 soluble reactive phosphate (SRP) and onshore flow velocities of 0.1–0.3 m s21. 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 d15N 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. Concern over the recent, rapid decline of corals and the widespread shifts from coral to macroalgal dominance on many coral reefs has produced a focus of research attention on anthropogenic factors associated both with reductions in herbivore populations and the input of dissolved nutrients to reef environments (e.g., Done 1992; Lapointe 1997, 1999; Hughes et al. 1999; McCook 1999). While patterns of herbivory appear to exert proximate control over macroalgal abundance in many instances (Hughes et al. 1999; McCook 1999; Edmunds and Carpenter 2001; Jompa and McCook 2002), the availability of dissolved nitrogen and phosphorus 1 Corresponding author ([email protected]).