Quantifying and valuing potential climate change impacts on coral reefs in the United States: comparison of two scenarios.

The biological and economic values of coral reefs are highly vulnerable to increasing atmospheric and ocean carbon dioxide concentrations. We applied the COMBO simulation model (COral Mortality and Bleaching Output) to three major U.S. locations for shallow water reefs: South Florida, Puerto Rico, and Hawaii. We compared estimates of future coral cover from 2000 to 2100 for a "business as usual" (BAU) greenhouse gas (GHG) emissions scenario with a GHG mitigation policy scenario involving full international participation in reducing GHG emissions. We also calculated the economic value of changes in coral cover using a benefit transfer approach based on published studies of consumers' recreational values for snorkeling and diving on coral reefs as well as existence values for coral reefs. Our results suggest that a reduced emissions scenario would provide a large benefit to shallow water reefs in Hawaii by delaying or avoiding potential future bleaching events. For Hawaii, reducing emissions is projected to result in an estimated "avoided loss" from 2000 to 2100 of approximately $10.6 billion in recreational use values compared to a BAU scenario. However, reducing emissions is projected to provide only a minor economic benefit in Puerto Rico and South Florida, where sea-surface temperatures are already close to bleaching thresholds and coral cover is projected to drop well below 5% cover under both scenarios by 2050, and below 1% cover under both scenarios by 2100.

Differential responses of tallgrass prairie species to nitrogen loading and varying ratios of NO3- to NH4.

Worldwide atmospheric N deposition varies in both quantity and composition of inorganic N forms (NO3- and NH4+). Many studies designed to assess the potential consequences of N pollution, however, pay little attention to the relative abundance of N forms. We hypothesized that native species with different physiological properties will show varying responses to different forms and amounts of N deposition. Here, we assessed the responses of six herbaceous species native to tallgrass prairies in North America to increased N loading at differing ratios of NO3- : NH4+. Individual plants of each species were grown for roughly 80 d in a sand culture under field conditions, three N levels (0.1, 1, 3 mM N), and three ratios of NO3- : NH4+ (1:1, 4:1, 1:3). Analysis of total biomass and relative growth rate revealed a significant 3-way interaction between N level, NO3- : NH4+ ratio, and species. We found that the C3 grasses showed a greater relative response to increasing N addition than did C4 grasses or forbs. We also found a strong correlation between specific root uptake capacity for N and growth response to N level, for five of the six species. Specific leaf area of green leaves and % N of senesced leaves both showed a significant N-level by ratio interaction. Cautiously interpreted, these species-specific responses in growth and tissue quality could lead to changes in community-level and ecosystem dynamics as atmospheric N deposition continues to rise. We suggest that future studies addressing the potential impacts of N loading on natural communities use inorganic N compositions that are consistent with those expected from atmospheric deposition.