Seagrass Restoration Varies in Southwest Florida's Estuaries

David A. Tomasko dave.tomasko@swfwmd.state.fl.us

Just like Tampa Bay's once lowly football team, the Buccaneers, the bay has dramatically improved its win-loss record over the past two decades. When the Estuarine Research Federation holds its 16th Biennial Conference in St. Petersburg Beach in November 2001, attendees will have an opportunity to see for themselves the results of decades of resource management-oriented research and restoration activities in Southwest Florida. The ERF conference site is located just a few miles from Tampa Bay, a receiving water body of a watershed that contains more than 2 million inhabitants and home of the United States' third largest port (in terms of domestic tonnage).

In addition to seeing Tampa Bay, attendees can drive a half an hour south of the conference site to Sarasota Bay (Figure 1). Although it is located in one of the fastest growing population and economic centers in Florida, Sarasota Bay is one of the most stunningly beautiful estuaries in Florida. Like Tampa Bay, it is a place where water quality and seagrass coverage have improved dramatically in recent years.

Lemon Bay, south of Sarasota Bay, is a small estuarine system unknown to even those who call Southwest Florida home. Unfortunately, Lemon Bay's future appears to be as cloudy as its waters are becoming.

Further south still, Charlotte Harbor is commonly viewed as one of the most productive estuaries in Florida. With most of its shoreline having been purchased by the State of Florida in the 1970s, Charlotte Harbor boasts miles of unspoiled mangrove fringes and tidal creeks. Famous for fishing, Charlotte Harbor has produced the largest snook (Centropomus undecimalus) ever caught in Florida. The Boca Grande tarpon fishing tournament held there is one of the richest recreational fishing tournaments in the United States.

The recent positive changes taking place in Tampa Bay, Sarasota Bay and in some of the tributaries to Charlotte Harbor, have not been cheap, or easy, to bring about. Hundreds of millions of dollars have been spent on upgrading wastewater treatment plants by private and public utilities. Stormwater retrofits in polluted watersheds, although mostly focused on reducing toxin loads, have resulted in additional nutrient load reductions. Hundreds of acres of intertidal and freshwater marsh habitat have been restored, using the talents of numerous highly talented environmental engineering firms and thousands of citizen volunteers.

To better understand some of the issues involved with managing the resources of Southwest Florida's estuaries, this article will briefly describe the science being used to understand how to best protect and restore a single, yet important, component of estuarine health --- seagrass meadows.

Watersheds and Receiving Water Bodies

The watersheds and open waters of Southwest Florida's estuaries vary in size between Charlotte Harbor "Proper" and Lemon Bay (see Table 1). The ratio between the size of the contributing watershed and the open waters of the receiving waterbody vary between 12.2 (Charlotte Harbor) and 2.9 (Sarasota Bay). Tampa Bay has a watershed:open water ratio of 6.2, with Lemon Bay at 4.9. As a consequence, Charlotte Harbor is more strongly dominated by terrestrial and riverine influences than the other three estuaries, with Sarasota Bay having the lowest relative influence from its watershed.

Salinities in Charlotte Harbor are frequently much lower than those found in Tampa Bay, with brackish and/or oligohaline water extending, at times, out into the nearby portions of the Gulf of Mexico. In contrast, salinities in Lemon Bay and Sarasota Bay seldom drop below 20 ppt. In Charlotte Harbor, extended periods of salinities below 10 ppt have been shown to dramatically reduce the productivity and biomass of seagrass beds, if such events occur during times of elevated water temperature.

Causes of Light Attenuation

Water clarity is usually increased in estuarine systems by protecting and/or restoring seagrass coverage. However, results can be difficult to achieve in areas where pollution is not a major cause of decreases in water clarity. Various techniques exist for developing "optical models" for Southwest Florida estuaries. Optical models are designed to partition light attenuation into its component (e.g., water, phytoplankton, dissolved organic substances and suspended material). When comparing results using similar methodologies, it can be seen that the role of phytoplankton populations in reducing water clarity varies from system to system. Water clarity in Lemon Bay and Tampa Bay has been shown to be strongly influenced by how "green" the water is, with phytoplankton being responsible for 29 and 27 percent of light attenuation, respectively. In Sarasota Bay and Charlotte Harbor, light attenuation caused by phytoplankton is thought to account for 6 and 4 percent, respectively, of total light attenuation. However, values for Tampa Bay and Sarasota Bay were derived after massive nutrient load reduction efforts took place. In effect, phytoplankton populations would have been even more important in their previous, more polluted, conditions. In contrast, Charlotte Harbor's water clarity varies mostly as a function of the amount of dissolved organic matter brought into the estuary from its substantial watershed.

Nitrogen Loads and Nitrogen Yields

Numerous studies have shown, both directly (through manipulative experimentation) and indirectly (through nutrient ratios), that Southwest Florida's estuaries are strongly nitrogen limited. Phosphorus-rich deposits in the area of the "Bone Valley Formation" in the eastern portion of the watershed have been the basis for a phosphate mining industry that dates back more than 100 years. As a result, nutrient loading models typically focus on nitrogen as the nutrient of concern.

Nitrogen loads, minus the contribution from direct atmospheric deposition onto open water, can be used to compare how "polluting" are the various watersheds. When nitrogen loads are normalized for the size of the contributing watershed, nitrogen yields can be derived. Nitrogen yields for Southwest Florida estuaries vary from a low value of 2.6 kg TN/ha /yr in Charlotte Harbor to 10.9 kg TN/ha/yr in Sarasota Bay. It is somewhat surprising to note that the nitrogen yield calculated for Tampa Bay comes out to approximately 5.9 kg TN/ha/ /yr---higher than only Charlotte Harbor, and lower than even Lemon Bay (7.6 kg TN/ha/yr).

This leads to the seemingly strange conclusion that Tampa Bay's watershed is less polluting than are both Sarasota and Lemon Bay's. A possible explanation is that nitrogen loads calculated for Tampa Bay and Sarasota Bay were derived after point source nitrogen loads had been dramatically reduced. Still, the degree of "urbanization" of an estuary's watershed appears to explain much of the variation in nitrogen yields in Southwest Florida's estuaries. After subtracting undeveloped land, wetlands and agricultural areas, the amount of watershed in "urban" land use categories varies between 7 percent in Charlotte Harbor to 48 percent in Sarasota Bay. Tampa Bay's watershed is approximately 24 percent urbanized (a surprise to those who think the watershed is mostly similar to that seen in St. Pete Beach) while Lemon Bay's watershed is 43 percent urbanized.

When comparing watershed nitrogen yields to the percent of land urbanized within the four watersheds, a positive correlation is found.

Implications for Future Population Growth

Seagrass coverage in Southwest Florida's estuaries has increased in recent years, mostly as a result of improvements in water quality that are, in turn, related to massive (and expensive) reductions in human-induced nitrogen loads. Seagrass acreage in Tampa Bay is estimated to have increased by approximately 25 percent between 1982 and 1996, whereas Sarasota Bay's acreage has increased 19 percent between 1988 and 1996. Charlotte Harbor's increase has been more moderate, a 6 percent increase between 1982 and 1996, and may be simply natural variability. Lemon Bay's coverage appears to have slightly decreased between 1988 and 1996.

It is possible that the relationship between nitrogen yields and the degree of urbanization of the watershed evident when comparing Southwest Florida's estuaries against each other also holds true when comparing single systems over time. If that is true, then recent increases in seagrass coverage might potentially be offset in the future by increased nitrogen loads associated with continued population growth in the watersheds of these beautiful estuaries.

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