Climate impacts on coastal and estuarine systems take many forms and are dependent on the local conditions, including those set by humans. We use a biocomplexity framework to provide a perspective of the consequences of climate change for coastal wetland ecogeomorphology. We concentrate on three dimensions of climate change affects on ecogeomorphology: sea level rise, changes in storm frequency and intensity, and changes in freshwater, sediment, and nutrient inputs. While sea level rise, storms, sedimentation, and changing freshwater input can directly impact coastal and estuarine wetlands, biological processes can modify these physical impacts. Geomorphological changes to coastal and estuarine ecosystems can induce complex outcomes for the biota that are not themselves intuitively obvious because they are mediated by networks of biological interactions. Human impacts on wetlands occur at all scales. At the global scale, humans are altering climate at rapid rates compared to the historical and recent geological record. Climate change can disrupt ecological systems if it occurs at characteristic time scales shorter than ecological system response and causes alterations in ecological function that foster changes in structure or alter functional interactions. Many coastal wetlands can adjust to predicted climate change, but human impacts, in combination with climate change, will significantly affect coastal wetland ecosystems. Management for climate change must strike a balance between that which allows pulsing of materials and energy to the ecosystems and promotes ecosystem goods and services, while protecting human structures and activities. Science-based management depends on a multi-scale understanding of these biocomplex wetland systems. Causation is often associated with multiple factors, considerable variability, feedbacks, and interferences. The impacts of climate change can be detected through monitoring and assessment of historical or geological records. Attribution can be inferred through these in conjunction with experimentation and modeling. A significant challenge to allow wise management of coastal wetlands is to develop observing systems that act at appropriate scales to detect global climate change and its Estuaries and Coasts (2008) 31:477-491
As in other eutrophied estuaries and coastal embayments, persistent hypoxia now routinely develops during summer in the mesohaline portion of the Neuse River estuary (North Carolina, USA). In response to interannual differences in hydrography, summer 1997 exhibited much more intense and widespread hypoxia than summer 1998, permitting inferences about impacts of hypoxia on food web dynamics by comparing system changes across these two summers. The trophic structure of the Neuse estuary now resembles the generic pattern for a degraded temperate estuary with (1) intense planktonic algal blooms and similarly high production of free-living bacteria, (2) trivial levels of abundance of rooted aquatic plants and benthic macroalgae, (3) depleted apex predators, and (4) functional extinction of the historically dominant benthic grazer, eastern oysters. Detailed carbon-flow models, based on comprehensive field data, demonstrated large differences between the two summers in trophic transfers and system dynamics. Largely because of greater mortality of benthic invertebrates from more intense hypoxia, total biomass of heterotrophs declined over summer by 51% in 1997 as compared to only 17% in 1998. Because net primary production increased over summer and herbivory in this system is predominantly benthic, the fraction of primary production consumed by herbivores declined over summer by 35% in 1997 and 29% in 1998. Influx of juvenile fishes and their rapid growth in the estuarine nursery over summer led to increases in energy demand by demersal fishes of 380% and 507% in the successive summers. Thus, hypoxia-enhanced diversion of energy flows into microbial pathways away from consumers and mass mortality of benthic invertebrates from bottom hypoxia occurred at the season of greatest demand by predatory fishes and crabs using the estuary as nursery. Average residence time of carbon in the ecosystem declined by 51% in 1997 and 29% in 1998. Total system throughput declined over summer 1997 while increasing in 1998, indicating the reduced capacity of the system to transfer carbon to higher trophic levels in the more hypoxic summer. Late-summer trophic pathways were characterized by greater numbers of cycles, but flows became increasingly dominated by microbial loops rather than transfers to consumers. Ecosystem trophic efficiency was only ϳ4%, lower than other estuaries similarly analyzed. System properties indicative of resiliency of system function including development capacity, ascendancy, and flow diversity declined over summer 1997, while increasing or declining less in 1998. Thus, intensification of hypoxia caused dramatic reduction in the ecosystem's ability to transfer energy to higher trophic levels and rendered the ecosystem potentially less resilient to other stressors.
Wetland ecosystems in agricultural areas often become progressively more isolated from main water bodies. Stagnation favors the accumulation of organic matter as the supply of electron acceptors with water renewal is limited. In this context it is expected that nitrogen recycling prevails over nitrogen dissipation. To test this hypothesis, denitrification rates, fluxes of dissolved oxygen (SOD), inorganic carbon (DIC) and nitrogen and sediment features were measured in winter and summer 2007 on 22 shallow riverine wetlands in the Po River Plain (Northern Italy). Fluxes were determined from incubations of intact cores by measurement of concentration changes or isotope pairing in the case of denitrification. Sampled sites were eutrophic to hypertrophic; 10 were connected and 12 were isolated from the adjacent rivers, resulting in large differences in nitrate concentrations in the water column (from \5 to 1,133 lM). Benthic metabolism and denitrification rates were investigated by two overarching factors: season and hydrological connectivity. SOD and DIC fluxes resulted in respiratory quotients greater than one at most sampling sites. Sediment respiration was coupled to both ammonium efflux, which increased from winter to summer, and nitrate consumption, with higher rates in river-connected wetlands. Denitrification rates measured in river-connected wetlands (35-1,888 lmol N m -2 h -1 ) were up to two orders of magnitude higher than rates measured in isolated wetlands (2-231 lmol N m -2 h -1 ), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9-1,888 lmol N m -2 h -1 ) than in winter (2-365 lmol N m -2 h -1 ). Denitrification supported by water column nitrate (D W ) accounted for 60-100% of total denitrification (Dtot); denitrification coupled to nitrification (D N ) was probably controlled by limited oxygen availability within sediments. Denitrification efficiency, calculated as the ratio between N removal via denitrification and N regeneration, and the relative role of denitrification for organic matter oxidation, were high in connected wetlands but not in isolated sites. This study confirms the importance of restoring hydraulic connectivity of riverine wetlands for the maintenance of important biogeochemical functions such as nitrogen removal via denitrification.
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