Coastal Impacts, Adaptation, and Vulnerabilities - Climate ...

Vulnerability and Impacts on Natural Resources 63Delta survival requires equilibrium between sediment supply and rising sea levels.Deltas naturally degrade as sediments subside and get reworked by storms, and, overthe longer term, deltas, like barrier islands, are generally self-sustaining systems undernatural conditions given an adequate supply of sediment. However, the constructionof levees and dams, and navigation activities along major U.S. rivers have alterednatural sediment flows and, in some cases, starved the natural environment of the sedimentsupply necessary to keep pace with changing water levels. Deltas world-wide areexperiencing land-loss due to the combined effects of increased human development,subsidence, and rising sea levels (Nicholls et al., 2007; Syvitski et al., 2009). The Bird’s-Foot Delta of the Mississippi River provides one of the more striking examples of thesecompounding impacts. Here, sea levels are rising at three times the rate of sedimentaccumulation on the delta, which portends the inevitable drowning of this delta system(Blum & Roberts, 2009).Mudflats: Mudflats are susceptible to threshold changes due to the combinedeffects of sea-level rise, temperature, land use, altered flows, and increasednutrient runoff.Sea-level rise is already reducing the intertidal area of mudflat feeding habitat for wadingbirds within estuaries, especially in locations constrained by coastal protective barriers(Galbraith et al., 2005). In a recent assessment of the sensitivities of mudflat communitiesto climate change in the San Francisco Bay estuary (EPA, 2012a), local experts identifiedthe potential for nonlinear losses within benthic communities due to a combination ofnutrient runoff and increased temperatures that drive dissolved oxygen levels below athreshold. They also anticipated precipitous losses of wading bird populations due toreduced prey densities and shrinking feeding habitat resulting from sea-level rise andsediment starvation. Although the potential for such thresholds may be high, patternsof disturbance and recovery in mudflats are extremely patchy and context-dependentand larger-scale behavioral responses of bird populations are unknown, which makespredictions of exactly when and where a threshold change may occur difficult to offer(EPA, 2012a; Norkko et al., 2010).Rocky Shores: Complex interactions between physical and biological factorshave been demonstrated in rocky-shore communities, which makes responses toclimate change difficult to predict.Although the impacts of sea-level rise are obvious for rocky-shore intertidal communities,temperature and ocean acidity are also potentially important. Hawkins et al. (2008)have noted general changes in abundance and range of rocky-shore species that can beattributed to climate warming. Although most of these changes involve range expansions,investigators caution that this may simply reflect the preponderance of studies atthe northern limits of species ranges compared to studies in lower latitudes. At a regionalscale, long-term recruitment records for mussels and barnacles showed a relationshipto large-scale climate and upwelling patterns along the U.S. West Coast (Menge et al.,2011a). The relationship explained approximately 40 percent of the variance in the recruitmentrecord. At the local scale, Harley (2011) determined that climate warming hadthe potential to drive changes in the community structure of rocky shores as it limited