Nutrient enrichment on coral reefs: Is it a major cause of coral reef decline?

Abstract

Coral reefs are degrading worldwide at an alarming rate. Nutrient over-enrichment is considered a major cause of this decline because degraded coral reefs generally exhibit a shift from high coral cover (low algal cover) to low coral cover with an accompanying high cover and biomass of fleshy algae. Support for such claims is equivocal at best. Critical examination of both experimental laboratory and field studies of nutrient effects on corals and coral reefs, including the Elevated Nutrient on Coral Reefs Experiment (ENCORE) enrichment experiment conducted on the Great Barrier Reef, does not support the idea that the levels of nutrient enrichment documented at anthropogenically-enriched sites can affect the physiology of corals in a harmful way, or for most cases, be the sole or major cause of shifts in coralalgal abundance. Factors other than nutrient enrichment can be significant causes of coral death and affect algal cover, and include decreased abundance of grazing fishes by fishing, and of grazing sea urchins to disease; grazing preferences of remaining grazers; temperature stress that kills coral (i.e., coral bleaching) and creates more open substrate for algal colonization; sedimentation stress that can weaken adult corals and prevent coral recruitment; coral diseases that may be secondary to coral bleaching; and outbreaks of coral predators and sea urchins that may be secondary effects of overfishing. Any factor that leads to coral death or reduces levels of herbivory will leave more substrate open for algal colonization or make the effects of even low-level enrichment more severe. Factors that contribute to an imbalance between production and consumption will result in community structure changes similar to those expected from overenrichment. Over-enrichment can be and has been the cause of localized coral reef degradation, but the case for widespread effects is not substantiated.

Ecological Shifts along the Florida Reef Tract: The Past as a Key to the Future

“…The reason water quality is so often cited as the cause of coral reef decline in the Keys is that the increased abundance of algae observed on reefs over the last two decades supports an intuitive assumption that nutrient enrichment must be the cause (Szmant 2002). Those who advocate nutrient pollution as the primary factor behind coral reef decline in the Keys have proposed connections from agricultural runoff in the Lake Okeechobee region, through the Everglades, into Florida Bay, and ultimately to the offshore coral reefs of the Keys (Brand 2002;Lapointe, Matzie, and Barile 2002).…”

“…What, then, explains the unambiguous increases in algae that are seen Caribbean-wide? Recent reviews of the role of nutrient enrichment on coral reef decline (Szmant 2002) and the relationships among nutrients, algae, and competition between algae and corals (McCook 2001;McCook, Jompa, and Diaz-Pulido 2001) suggest that the increased abundance of algae is not due to nutrients but instead is explained by the availability of new substrate for algal colonization following coral mortality (Aronson and Precht 2001b;Williams, Polunin, and Hendrick 2001;McManus and Polsenberg 2004). The role of herbivores is clearly also a factor in explaining increased algal abundance on reefs in Florida, from the demise of D. antillarum throughout the Caribbean (Lessios 1988), to observations that an inverse relationship exists between algae and herbivorous fish biomass surveyed on 19 reefs throughout the Caribbean .…”

“…Distinctions are typically not made between benthic habitats nearshore (especially hardbottom communities and seagrass beds close to shore) and the main reef tract located 5 to 7 km offshore. While few data exist to suggest that changes in offshore reef communities were caused by local pollution sources in Florida, and although few historical data exist regarding nutrient pollution, algal abundance, or coral cover (Szmant 2002), strenuous debate about the relative importance of topdown versus bottom-up (Lapointe 1997(Lapointe , 1999 processes continues (see Davidson 1998).…”

“…Periodic advection of coastal waters may also deliver pulses of nutrient-rich water to the offshore reefs (Pitts 2002). Szmant (1997Szmant ( , 2002 suggested that the level of topographic complexity on a reef indirectly influences the effects of nutrient input and uptake. In the absence of overfishing or mass mortalities of herbivores, topographic complexity determines the availability of shelter for herbivores.…”

“…Two hurricanes also affected the region during this period, further confounding interpretations. Although estimates of bleaching-related coral mortality are not available for Florida, anecdotal evidence suggests that bleaching has killed large numbers of corals (Causey 2001) and must be considered a significant threat to reefs in Florida (Szmant 2002). The projected continuing increases of bleaching events on Florida's coral reefs related to global warming are likely to decrease coral abundance into the future (Hoegh-Guldberg 1999; Walther et al 2002;Sheppard 2003).…”