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USGS Research Shows Value of Tampa Bay's Tidal Wetlands at 5th Bay Area Scientific Information Symposium
Scientists from the U.S. Geological Survey (USGS) in St. Petersburg, Florida, presented papers at the 5th Bay Area Scientific Information Symposium (BASIS) in Florida in late October 2009. This latest in a series of meetings organized approximately every 5 years revisited the science from the first symposium nearly three decades ago. It is a forum for bringing the science community together to discuss current and future environmental conditions and new ways of balancing current ecosystem and human needs in the Tampa Bay region. Resource managers and scientists from disciplines spanning geology, hydrology, and fish ecology participated in presentations and panel discussions on recent scientific studies, environmental regulations, and issues impacting area ecosystems. The Tampa Bay Regional Planning Council, the Tampa Bay Estuary Program, and a steering committee of Tampa Bay experts hosted the 4-day BASIS 5 meeting.
The first BASIS meeting was organized in 1982, at a time when local newspapers were claiming that Tampa Bay was dead. The bay was heavily polluted with nitrogen; water clarity was poor; algae blooms were rampant; and seagrasses, mangroves, and wetlands were disappearing. BASIS 1, which addressed the state of the bay and conditions crippling its environment, led to some of the first concerted efforts to improve the ecosystem.
Twenty-seven years later, Tampa Bay is a shining example of a recovering estuary. Seagrass and mangrove habitats are increasing, water quality and clarity are improving, and species of bay scallops and spotted seatrout are returning, according to the Tampa Bay Estuary Program. These successes are largely the result of greater regulatory control of pollutants, such as nitrogen, and greater emphasis on sustainable management of resources, such as fisheries, with many such policies derived from previous BASIS meetings.
Besides looking back to update science from the first meeting, BASIS 5 also served as a platform to highlight new scientific studies that examine the performance of the estuary ecosystem in the face of greater urbanization and population growth. One of the major problems facing Tampa Bay is continued coastal development, which threatens tidal streams and rivers. The conversion of coastal land to impervious surfaces surrounding natural tributaries is bringing about immediate physical and chemical changes and setting the stage for eventual biological changes related to deteriorating habitat quality. Modified for years and woefully understudied until recently, tidal tributaries were the focus of a half-day session led by USGS fish ecologist Justin Krebs and featuring collaborative research by USGS scientists and others from various agencies around Tampa Bay.
One such study sought to determine the extent to which coastal development is affecting the quality of tidal tributaries as fish habitat. Krebs, working with USGS scientist Carole McIvor and University of South Florida Chair of Integrative Biology Susan Bell, collected fish, shrimp, and crabs from tidal wetlands during a 2-year study. "By comparing the community structure—the identity and abundance of species—and the health of common species, we hoped to assess the relative quality of these understudied but potentially critical habitats for fish populations in Tampa Bay," said Krebs.
The scientists sampled tidal tributaries in urban, natural, industrial, and mosquito-ditched sites. Urban sites were the most altered habitat, having, on average, less than 20 percent natural cover. Non-urban sites—industrial, mosquito-ditched, and natural sites—retained, on average, approximately 60 percent or more natural vegetation. Among the more common organisms collected were grass shrimp and fish species like sailfin molly, Atlantic silversides, and killifishes.
The study revealed that the total abundance of all fishes and crustaceans was independent of the type or degree of development in the surrounding watershed. However, the nekton community—mobile marine organisms—in urban creeks was noticeably different from those in non-urban tributaries. Many common fish and shrimp species were underrepresented in tributaries in urban areas. Those that were found were in poorer condition, having 10 to 15 percent smaller body size than nekton from natural, industrial, and mosquito-ditched tributaries.
"Natural habitats typically have diverse fish communities made up of many different species, but when these habitats are developed, the natural processes like hydrology can be altered, eventually degrading water quality and habitat," said Krebs. "For example, changes in water quality can cause a reduction in the number of species in the fish community and lead to certain, more tolerant species becoming more numerous. Though these tolerant species may be able to persist, they may still suffer in terms of reduced body size due to the stresses of a degraded habitat." Differences found in fish condition and community structure among tidal wetlands are likely due to shoreline modification, channel dredging, and construction of impervious surfaces, all of which degrade the water quality in urban tributaries.
Coastal development is also likely to impact large numbers of popular sport fish in Florida that use tidal wetlands for nursery habitat. An example is the common snook (Centropomus undecimalis). It is one of the most highly sought species in a sport-fishing industry that generated $4.4 billion for the Florida economy in 2006, according to the U.S. Census Bureau. Along the Gulf Coast alone, an estimated 1.5 million anglers target common snook today. USGS fishery technician Adam Brame presented a paper at BASIS 5 that characterized the preferred habitat for juvenile snook within a tidal creek of the Tampa Bay watershed.
"The economic impact of the snook fishery on the State of Florida is easily in the millions of dollars," said Brame. "There is a pressure and desire to catch these fish, but without fully understanding their life history, and especially their early life history, where there is a lack of data, activities such as fishing and coastal development can have adverse impacts on the population in the long run."
Brame collected more than 450 juvenile snook, along with water-quality and habitat data, from two habitats (ponds and creeks) at two locations (upstream and downstream) in Frog Creek in the Terra Ceia Aquatic Preserve in southern Tampa Bay. He collected a variety of data, including length, weight, abundance, and isotopic signatures of carbon and nitrogen.
Snook abundance varied by habitat type, with a larger number found in ponds. "There are a couple of possible explanations for this," said Brame, "such as lower predation or higher prey availability in ponds, but more likely it is related to energetics." Juvenile snook expend less energy in ponds—where water flow is largely negligible—than in creeks that are constantly flowing.
Water-flow characteristics also explain why snook found in ponds were isotopically enriched in 15N, a stable isotope of nitrogen that is heavier than the much more abundant 14N. Brame explained: "In these tidal creeks, there is a lot of organic-matter input, such as leaves. In a creek most of this matter washes away, but it stays in ponds and is broken down more slowly, causing a nutrient-recycling effect," which leads to 15N enrichment. Distinct differences in the isotopic composition of snook between ponds and the creek indicate little movement by snook between the two habitats, again pointing to the value of the pond habitat as primary nursery grounds.
Contributions from these two studies involving tidal creeks are building upon research that will be used in the future to better protect and conserve these habitats, just as science from previous BASIS meetings has improved habitats throughout Tampa Bay. To view the program and abstracts from the October meeting, download the Tampa BASIS 5 Program (full version) from the Web page at http://www.tbrpc.org/events/basis5/.
in this issue:
Tampa Bay Area Scientific Information Symposium
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