The Economic Value of Water and Ecosystem Preservation

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2. The Role of Freshwater Inflows in Sustaining Estuarine Ecosystem Health in the San Antonio Bay Region 2.1. Introduction Estuaries are vital aquatic habitats for supporting marine life, and they confer a multitude of benefits to humans in numerous ways. These benefits include the provision of natural resources used for a variety of market activities, recreational opportunities, transportation and aesthetics, as well as ecological functions such as storing and cycling nutrients, absorbing and detoxifying pollutants, maintaining the hydrological cycle, and moderating the local climate. The wide array of beneficial processes, functions and resources provided by the ecosystem are referred to collectively as “ecosystem services.” From this perspective, an estuary can be viewed as a valuable natural asset, or natural capital, from which these multiple goods and services flow (Constanza, d'Arge et al., 1997). The quantity, quality and temporal variance of freshwater inflows are essential to the living and non-living components of bays and estuaries. Freshwater inflows to sustain ecosystem functions affect estuaries at all basic physical, chemical, and biological levels of interaction. The functional role of freshwater in the ecology of estuarine environments has been scientifically reviewed and is relatively well understood. This role is summarized in section 2.2, after a brief overview of the geographical context of the San Antonio Bay Region in the next section. Section 2.3 follows with discussion of the impacts of reduced freshwater inflow to the San Antonio Bay. Section 2.4 concludes with some general observations. 2.2. The Role of Freshwater Inflows to Sustain Estuarine Ecosystems The following list of the roles of freshwater inflows in sustaining estuarine ecosystems is taken from Longley (1994) and is accompanied by a brief explanation of each of these effects. These functions of freshwater inflow apply to estuaries in general and may not apply specifically to the San Antonio Bay. 2.2.1. Functional role of freshwater inflow Dilution of Seawater A primary role of freshwater inflows is the mixing with seawater to create brackish conditions typical of most bays and estuaries. Many commercially and recreationally important species rely on the lower salinity conditions of estuaries for at least some portion of their life cycle. Dilution of contaminants Freshwater inflow into bays and estuaries carries with it contaminants from land surfaces within the watershed. The contaminants are transported into bays and estuaries where they are diluted in the greater volume of water. The dilution 6
effect is limited by assimilative capacity of bays and estuaries and their various habitats. Creation and maintenance of nursery habitats Freshwater inflows are vital to the creation and maintenance of estuarine habitats which provide food and protection to many organisms including finfish, crustaceans, birds, reptiles, and mammals. Reduction of metabolic stresses in estuarine dependent organisms Salinity concentrations in bays and estuaries are naturally variable. To deal with the variability, all estuarine organisms have a range of salinity concentrations that they can tolerate based on their ability to regulate concentrations of internal body salts relative to environmental salinity. Drastic changes in salinity regimes can impair an organism’s ability to maintain osmotic balance triggering metabolic stresses. Metabolic stresses can lead to increased incidence of disease, parasitism and can have a negative effect on the ability of organisms to forage and reproduce. Transportation medium for beneficial sediments and nutrients; cycling, and the removal of metabolic waste Freshwater flows provide a medium for the transport of suspended particulate matter including sediment, detritus (decaying organic material) and organisms such as phytoplankton. Additionally, freshwater inflows transport nutrients (e.g. nitrogen and phosphorus) from watershed point sources (e.g. wastewater treatment) and non-point sources (i.e. runoff) to bays and estuaries. Modification of concentration dependent chemical reactions of particles in the saltwater environment Various compounds adhere to the surface of suspended particles and interact with other chemical constituents in the water column. Freshwater inflows are not only a source of suspended particles, but also influence salinity levels which have a direct effect on the rate of chemical reactions, ion-exchange, coagulation and precipitation of particles. Creation of a resource partitioning mechanism among estuarine plants and animals The combined effects of inflow on salinity, temperature, and turbidity influence the distribution of ecological producers and consumers in the estuary. When foraging, species must very often share one resource such as a specific wetland area or mudflat. Resource partitioning ensures that multiple species (crustaceans, finfish, birds) are able to utilize the same resource, but each in a different way. Freshwater inflows ensure diversity among habitat types as well as the consuming organisms dependent upon them. 7
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