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243 ≋ insights into the similarities and differences between warm-temperate and subtropical estuaries—such as those found in the Gulf—and cool-temperate estuaries on which much of our current understanding of estuarine biogeochemistry is based. Deegan et al. (1986) argue that the Gulf of Mexico is an excellent region to compare the characteristics and processes among estuaries for the following reasons: (1) there are a large number of estuaries (64); (2) the climate ranges from tropical to temperate and from humid to arid; (3) the area encompasses a wide range of riverine influence, from systems with almost no riverine input to the Mississippi River; and (4) the size of estuarine areas (in terms of both water and intertidal area) varies from very small to the largest in North America (Table 13.1). They compared the influence of physical factors among Gulf estuaries, such as river discharge and size of estuary, to vegetation distribution and fishery harvest. Both analyses are predicated upon a similar assumption, that the physical and geomorphological template of an estuary—modified by the evolution of coastal landscapes—constrain the patterns of biological and chemical processes that we observe today. Linkages between the biogeochemistry and biotic resources of an estuary should provide managers with the insights needed to recognize the unique properties that are responsible for sustaining water quality and economic conditions in individual estuaries. For example, Deegan et al. (1986) found that fishery harvest and area of an estuary are strongly related to freshwater input and physiogThe field of biogeochemistry involves the study of how biological, chemical, and geological processes interact to determine the fate and effects of materials that influence the metabolism of ecosystems. An understanding of the role that biogeochemical and physical processes play in regulating the chemistry and biology of estuaries is fundamental to evaluating complex management issues such as those found in the Gulf of Mexico. As we have described (Bianchi et al. 1999), biogeochemistry links the processes that control the fate of sediments, nutrients, organic matter , and trace metals in estuarine ecosystems. Therefore, this discipline requires an integrated perspective of estuarine dynamics associated with the introduction, transport , and either accumulation or export of materials that largely control primary productivity. The metabolism of in situ primary production, and indirectly the utilization of allochthonous organic matter, is also linked to patterns of secondary productivity and fishery yields in estuaries in the Gulf of Mexico. As humans alter the way regional watersheds and local landscapes of estuaries produce and process natural and synthetic chemicals, principles of biogeochemistry will continue to influence how we manage these unique coastal ecosystems. Estuaries in the Gulf of Mexico basin possess physical characteristics that are as diverse as any coastal region in the world, and offer a unique opportunity to compare the role that physical factors play in regulating estuarine biogeochemical processes. Recent research on biogeochemical processes in Gulf systems also provides new 13 Biogeochemistry of Gulf of Mexico Estuaries Implications for Management Thomas S. Bianchi, Jonathan R. Pennock, and Robert R. Twilley 244 ~ Bianchi, Pennock, and Twilley into estuaries along the southwest coast of Florida. Increased agriculture and urban demand for water have also had major impacts on the delivery of freshwater to areas such as the Everglades and estuaries of the western Florida shelf. In addition, this region has some of the largest phosphate mining operations in the world. This industry has historically contributed to severe eutrophication in Tampa Bay but has undergone significant clean up over the past decade. The transition from the Eastern Gulf to the Northern Gulf from the Florida panhandle region to Mississippi includes rivers that drain forested coastal plain watersheds . In recent years, land-use changes in the forested watersheds of Florida, Alabama, Georgia, and Mississippi associated with timber processing has resulted in increased erosion and the introduction of chemical pollutants to estuarine environments in the central Gulf of Mexico. The coastal areas of this region are also becoming increasingly populated with commercial activity, which requires maintenance dredging of ship channels and residential housing. There have been major environmental changes in the watershed of the Mississippi River that have led to significant patterns of sedimentation and nutrient cycling in estuaries in the Northern Gulf. Over the last century there has been a quadrupling of river nitrate concentrations and a decrease in suspended load in the last several decades (Walsh et al. 1981; Turner and Rabalais 1991; Rabalais et al. 2007). Discharge from the Mississippi River is about 577 km3 /yr and enters...

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