Reevaluation of ENCORE: Support for the Eutrophication Threshold Model for Coral Reefs

Abstract

The results from the multimillion dollar Enrichment of Nutrients on Coral Reefs Experiment (ENCORE) on One Tree Island Reef (OTIR) suggest that increased nutrient loads to coral reefs will have little or no effect on the algal growth rates and, hence, on the associated effects that increased algal growth might have on the functioning and stability of coral reefs. However, a comparison of the concentrations of nutrients within the OTIR lagoon with the proposed nutrient threshold concentrations (NTC) for coral reefs suggests that all sites, including the control sites, were saturated with nutrients during ENCORE, and, hence, one would not expect to get any differences between treatments in the algal-growth related measurements. Thus, ENCORE results provide strong support for the proposed NTCs and support the ecological principle that algal productivity and, consequently, the functioning of coral reefs are sensitive to small changes in the background concentrations of nutrients. The principal conclusion of ENCORE, namely that the addition of nutrients did not cause the "pristine" OTIR to convert from coral communities to algal dominated reefs, is contrary to the fact that there was prolific macroalgal growth on the walls and crests of the experimental microatolls by the end of ENCORE.

Herbivory, Nutrients, Stochastic Events, and Relative Dominances of Benthic Indicator Groups on Coral Reefs: A Review and Recommendations

“…As a baseline for healthy coral-reef habitats, herbivore population abundances and diversity should be high and palatable test plants should show at least greater than 50% loss 6 h Ϫ1 (i.e., Ͻ6 h half-life) during a replicated series of midday in situ exposures. Hypothetical nutrient tipping points (i.e., thresholds that sustain algal growth) are thus far indicated to be quite low (i.e., ϳ0.1 M soluble reactive phosphorus [SRP], ϳ1.0 M dissolved inorganic nitrogen [DIN]), as suggested by laboratory growth experiments, case studies for macroalgal overgrowth of coral reef communities, and in situ experimental nutrient enrichment research (Bell, 1992;Bell and Elmetri, 1995;Lapointe et al, 1993, Bell et al, 2007. We further posit that latent trajectories (reduced resilience/ recoverability from phase shifts) are often triggered or accelerated by large-scale stochastic disturbances such as tropical storms, cold fronts, warming events, diseases, and predator outbreaks.…”

“…A dramatic increase in algal biomass resulting from eutrophication, without any simultaneous reduction in herbivore populations, was reported (Fishelson, 1973). The importance of the very low nutrient levels involved in eutrophication (i.e., nutrient threshold hypothesis, NTH), either natural or anthropogenic, has only recently come to light (Bell, 1992;Lapointe et al, 1997;Small and Adey, 2001;Bell et al, 2007) regarding the potential for phase shifts from corals toward macroalgal dominance. These kinds of biotic phase shifts also have been attributed to overfi shing of herbivore stocks (see Hughes, 1994 on Jamaican reef trends), in concert with cultural eutrophication (Goreau et al, 1997;Lapointe et al, 1997).…”

“…Furthermore, nutrient increases are sometimes associated with coral inhibition (Koop et al, 2001) as well as coral diseases (Harvell et al, 1999(Harvell et al, , 2002Bruno et al, 2003), and algal blooms can serve as disease vectors (Nugues et al, 2004). The sophisticated enrichment study (ENCORE) on a large and carefully controlled scale (Larkum and Koop, 1997;Encore Group, 2001) did not produce supportive results because (1) ambient nutrient levels within the lagoon at One Tree Island are well above tipping-point concentrations that may be inhibitory to some corals, while being more than suffi cient to support luxuriant frondose macroalgal growth (Bell, 1992;Larkum and Koop, 1997;Bell et al, 2007) and (2) the test organisms were isolated on raised grids to measure growth rates, precluding natural encroachment, overgrowth, or other competitive interactions crucial to testing the RDM. However, all increases in nutrient levels did adversely affect coral reproduction (Koop et al, 2001).…”

“…Chlorophyll a concentration (determined by fl uorometric or spectrophotometric methods; see Bell and Elmetri, 1995) is an especially useful ancillary indicator of water column enrichment because phytoplankton blooms can rapidly attenuate critical light energy while buffering inorganic nutrient pulses. Along with nutrient levels, chlorophyll a serves as a valuable tipping-point indicator, where levels in excess of 0.2-0.3 g L Ϫ1 indicate approaching overabundances of nutrients (Bell et al, 2007).…”

“…Consequently, a nutrient threshold model based on nutrient concentrations (rather than on nutrient fl uxes) is not only valid but is likely the best index of nutrient status. Low-nutrient tipping points, where increasing nutrients reach hypothetically critical levels that begin to reduce recoverability from phase shifts (i.e., ϳ1.0 M dissolved inorganic nitrogen N U M B E R 3 8 Bell, 1992;Lapointe et al, 1993;Bell et al, 2007) for sustaining macroalgal overgrowth of seagrass beds and coral reefs. The physiological/kinetic basis for such low-nutrient tipping points is the hyperbolic Monod relation (Droop, 1985;Bell et al, 2007), which is also supported by controlled, high-fl ux, continuous-culture laboratory experiments (Caperon et al, 1971DeBoer et al, 1978Lapointe and Tenore, 1981).…”