Effect of Nutrient Enrichment and Elevated CO2 Partial Pressure on Growth Rate of the Zooxanthellate Coral Acropora cervicornis.

Trends of increasing coastal eutrophication and atmospheric pCO2 require investigation to predict the combined effects on coral and reef condition and growth. Increases in nutrient concentrations have been observed over the past several decades in a number of reef systems, and it has been predicted that this will adversely affect coral growth rates. The species targeted by this research, Acropora cervicornis, is among the most important reef-builders in the Caribbean and has suffered widespread mortality in southern Florida. Approximately 192 branch tips were harvested from two local populations of A. cervicornis and maintained in the laboratory, where the growth rate was measured before, during, and after exposure to elevated levels of nitrate (N) (5 µM and 10 µM), phosphate (P) (2 µM and 4 µM), and/or pCO2 (CO2) (~700-800 µatm). During the control period, there was no significant difference in growth rate between the treatments, with a mean growth rate of 4.84 mg d-1. During Period 2, the first enrichment period, the largest decrease in growth rate was caused by the combined NPCO2 treatment (mean growth rate of 0.51 mg d-1). The least effect on growth rate was caused by the N treatment (mean growth rate of 4.39 mg d-1). During Period 3, the second enrichment period, the lowest growth rate was again found in the NPCO2 treatment tanks (mean growth rate of 0.03 mg d-1). The highest growth rate among the treatments was in the N treatment tanks (mean growth rate of 1.90 mg d-1), however there was no significant difference between the growth rates of the N, P, NP, CO2, NCO2, and PCO2 treated corals during this period. The mean growth rates in the Control tanks were 7.59 and 7.32 mg d-1 during Periods 2 and 3, respectively. During Period 4, the recovery period, the lowest growth rates were found in the combined NCO2 and PCO2 treatment tanks (mean growth rates of 1.00 and 1.02 mg d-1, respectively). The highest growth rate among the treatments was again observed in the N treatment tanks (mean growth rate of 2.75 mg d-1). The mean growth rate in the Control tanks was 7.40 mg d-1 during this period. The results of this study demonstrate that increased levels of nitrate, phosphate, and pCO2 cause statistically significant decreases in the growth rate of A. cervicornis. The effects of nitrate and phosphate appear to be concentration dependent, and the effect of pCO2 is greater than that of nutrification. Therefore, continued eutrophication and projected pCO2 increases can be expected to have negative impacts on the survival and reef building potential of this species