Experimental exposure to elevated nutrient concentrations can induce a number of negative responses of corals such as reduced reproductive success, calcification rates, skeletal density or linear extension [3"¢, 23"¢, 25, 26"¢ and 27]. However, several studies did not reveal direct negative effects of increased nutrient levels on coral physiology or found them only at unnaturally high concentrations [3"¢ and 9"¢]. We have recently shown that increased nutrient levels might not negatively affect the physiological performance of zooxanthellae as long as all essential nutrients are available at sufficient concentrations to ensure their chemically balanced growth [28"¢]. These results could explain why some reefs and the nutritional status and metabolism of their inhabitants do not always show negative responses to eutrophication [29"¢ and 30"¢], at least in the absence of temperature and light stress.
In yet some other cases, corals responded positively to the addition of nutrients, for instance by increased growth [25, 31 and 32"¢] or by a reduced susceptibility to the end-of-summer bleaching, the seasonal loss of corals' zooxanthellae [33]. Refuges from heat stress-mediated bleaching were found in regions with small-scale upwelling [34]. The water from greater depths might not only provide cooling effects [34], but may also supply dissolved inorganic nutrients [35].
A number of studies found that elevated nitrogen levels in the water promoted zooxanthellae growth and resulted in higher zooxanthellae densities without obvious negative effects on the corals (see review by Fabricius [3"¢]). Most recently, however, we could demonstrate that corals exposed to elevated nitrogen levels were more susceptible to bleaching when exposed to heat and light stress [28"¢]. Interestingly, the detrimental effects observed in these experiments could be attributed to the relative undersupply of phosphorus that resulted from the enhanced demand of the proliferating zooxanthellae population rather than to the elevated nitrogen levels themselves
