Effect of nutrient enrichment on growth and photosynthesis of the zooxanthellate coral Stylophora pistillata

The effect of prolonged (9 week) nutrient enrichment on the growth and phot osynthetic rates of the zooxanthellate coral Stylophora pistillata was inve stigated. The main questions were: (1) what is the exposure time needed to induce measurable change in growth rate? (2) which are the concentrations o f nitrogen and phosphorus required to cause changes in these rates? (3) wha t is the recovery potential of the corals after the nutrient stress? For th is purpose, three tanks (N, P, NP) were enriched with ammonium (N), phospho rus (P) or both nutrients (NP), respectively. A fourth tank (C) served as a control. The growth of 40 nubbins (10 in each tank) was monitored during f our periods: period 1 (nutrient-poor conditions), period 2 (10 mu M NH4 and /or mu M PO4 enrichment), period 3 (20 mu M NH4 and/or 2 mu M PO4) and peri od 4 (nutrient-poor conditions). Period 4 was performed to study the recove ry potential of corals after a nutrient stress. During period 1, growth rat es remained constant in all tanks. In the P tank, growth rates declined dur ing the two enrichment periods, with a total decrease of 60% by the end of period 3. In the N tank, growth rates remained nearly constant during perio d 2 but decreased in period 3 (60% decrease). In the NP tank, 50% and 25% d ecreases were observed during periods 2 and 3. At the end of the recovery p eriod, a regain in growth rate was observed in the N and NP tanks (35 and 3 0% increase, respectively, compared with the rates measured at the end of p eriod 3) and growth rates returned to 60% of the initial rates. By contrast , in the P tank, there was no regain in growth and a further decrease of 5% was observed. Rates of photosynthesis were often higher during the enriche d than the nutrient-poor period (up to 150% increase). Corals with the high est percent increases in maximal gross photosynthetic rate (P-max(g)) had t he smallest decreases in growth rate due to nutrient enrichment. In conclus ion, high ammonium (20 mu M) and relatively low phosphorus concentrations ( 2 mu M) are required to induce a significant decrease in coral growth rate. The largest reduction was observed with both ammonium and phosphorus enric hment. The decrease in growth rate was rapid following nutrient enrichment, since a 10% decrease or more could be observed after the first week of tre atment.