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The production and deposition of biogenic carbonate sediments plays a key role in the development of the topographic structure and chemical and biological characteristics of many coastal ecosystems. Production of carbonate sediments and skeletal material in coral reef ecosystems provides the topographic relief that supports biological diversity (Friedlander and Parrish 1998), enables reefs to keep up with rising sea level (Smith and Kinsey 1976), and protects the coastline from erosion (Cesar and van Beukering 2004). While the coastal ocean covers only 7% of the surface of the global ocean, it accounts for 90% of sedimentary remineralization and 50% of the deposition of calcium carbonate sediments (Gattuso et al. 1998).

Rates of biogenic carbonate sediment production are affected by natural and anthropogenic alterations in water quality parameters including temperature, salinity, nutrients, light availability, pCO₂, and aragonite saturation state (Barnes and Chalker 1990; Kleypas et al. 1999; Leclerq et al. 2002). Identifying potential changes in sediment production resulting from perturbations in these water quality parameters is essential for predicting the effects of resource management actions on the health of coastal ecosystems. While anthropogenic effects on natural systems may continue to change and remain apparent for very long periods of time (hundreds to thousands of years), initial ecosystem responses to anthropogenic alteration typically occur on very short time scales (a few months to a few years; McIvor et al. 1994). It is critical to understand both short-term and long-term variability of sediment production to accurately predict ecosystem responses. The aim of this study was to measure diurnal fluctuations in rates of calcification and dissolution in Florida Bay (a shallow, subtropical estuary located along the south coast of the Florida peninsula, Fig. 1) to identify short-term variability in sediment production that may affect long-term rates of carbonate sediment production (measured over yearly time scales) and accumulation (over thousands of years).

Fig. 1. Locations of mud bank and basin study sites in Florida Bay. Open circles indicate basin sites. Open circles connected by lines indicate the beginning and end of mud bank transects. Numbers correspond to numbered locations in Table 1. [larger image]

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TABLE 1. Study site locations and descriptions.

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The production and transport of biogenic carbonate sediments in Florida Bay is responsible for the development of mud banks that alter circulation patterns, surface water chemistry, and the distribution of benthic habitats. Accurate measurements of sediment accumulation and production rates are required to determine how these processes have changed during the last century and to predict how they may change in the future in response to largescale restoration activities in south Florida that will change the flow and quality of freshwater entering Florida Bay (Fourqurean and Robblee 1999; U.S. Army Corps of Engineers 1999). Estimates of both long-term sediment accumulation rates (cm 1000 yr^-1) and annual carbonate sediment production rates based on standing crop and growth rates of various calcifying species are available for Florida Bay from previous research (Stockman et al. 1967; Nelson and Ginsburg 1986; Bosence 1989a; Frankovich and Zieman 1994), but discrepancies occur, not only between long-term accumulation rates and short-term productivity measurements, but also among short-term productivity measurements.

Estimates of annual biogenic carbonate sediment production range from 1.9-238 g CaCO₃ m^-2 yr^-1 (Frankovich and Zieman 1994) to 448-1,042 g CaCO₃ m^-2 yr^-1 (Bosence 1989a). Annual rates of sediment production overestimate long-term rates (thousands of years) of sediment accumulation (Stockman et al. 1967). Differences among annual (short-term) productivity rates determined by standing crop methods have been attributed to variations in methodology, annual changes in seagrass leaf productivity, and the calcifying organisms included in or excluded from studies (Frankovich and Zieman 1994). Standing crop methods for measuring short-term carbonate sediment production account for gross sediment production and do not include the effects of sediment transport or dissolution. Long-term sediment accumulation rates based on sediment thickness account for sediment production, dissolution, and transport. Differences between short-term productivity measurements and long-term accumulation measurements have been attributed primarily to transport of sediment both within and out of Florida Bay (Stockman et al. 1967; Bosence et al. 1985; Bosence 1989a,b; Prager and Halley 1999). Sediment dissolution in pore waters has also been identified as a potential mechanism for loss of sediment in Florida Bay. Very few studies have attempted to quantify porewater dissolution (but see Walter and Burton 1990; Rude and Aller 1991; Walter et al. 1993; Ku et al. 1999). Despite these differences, most investigators agree that the highest production rates are observed in western Florida Bay and are associated with mud banks as opposed to basins (Stockman et al. 1967; Ginsburg 1979; Nelson and Ginsburg 1986; Bosence 1989a).

While long-term and annual carbonate sediment production in Florida Bay are quite well known, seasonal and diurnal variability remain largely unstudied despite the fact that the calcifying organisms responsible for most of the biogenic sediment production in Florida Bay respond to daily and seasonal environmental changes. We have measured short-term, net carbonate sediment production in Florida Bay to examine diurnal and seasonal variability using the alkalinity anomaly technique (Broecker and Takahashi 1966; Smith and Key 1975), which has proven successful for measuring production in carbonate reef and seagrass bed ecosystems (Smith and Key 1975; Chisholm and Gattuso 1991; Gattuso et al. 1993, 1997; Boucher et al. 1998). We define net carbonate sediment production as gross sediment production minus sediment dissolution. The alkalinity anomaly method provides an independent measure of net sediment production that incorporates biogenic calcification by all carbonate-producing species, nonbiogenic calcification, and sediment dissolution.

Florida Bay is a subtropical estuary characterized by numerous mangrove islands that are connected by submerged, shallow mud banks that bound basins averaging 2 m in water depth (Fig. 1). The western zone of Florida Bay is open to the Gulf of Mexico and is tidally influenced with relatively stable marine conditions. Circulation in central Florida Bay is restricted by mud banks with little tidal influence and is controlled primarily by winddriven currents (Fourqurean and Robblee 1999). Salinity in the central zone is variable and frequently exhibits hypersalinity during summer months due to increased evaporation rates and long water residence times (Fourqurean et al. 1993). Water temperature throughout the Bay has a median value of approximately 26°C and shows distinct seasonal signals ranging from a high of approximately 30°C during summer months to a low of 20°C during winter months (Boyer et al. 1999). Dissolved organic matter and N:P ratios are typically higher in the central zone than in the western zone. Banks are comprised of biogenic carbonate mud derived primarily from calcareous green algae, a variety of seagrass epiphytes, mollusks, and stony corals (Stockman et al. 1967; Nelson and Ginsburg 1986; Bosence 1989a; Frankovich and Zieman 1994). Wanless and Tagett (1989) subdivided the Bay into western, central, and eastern zones characterized by different mud bank processes. Western Florida Bay is considered a zone of mud bank formation where mud banks are accreting fast enough to keep pace with sea level rise. Central Florida Bay is a zone where mud banks migrate, erosion and deposition appear to be in equilibrium, and mud banks move as sediments are eroded from one side and deposited on the other. Seagrass communities covering much of Florida Bay consist primarily of Thalassia testudinum, Halodule wrightii, and Syringodium filiforme, with Thalassia being the most abundant.