The global phenomenon of burgeoning coastal population growth has led to coastal watershed landscape transformation and ecosystem degradation, prompting policy-makers to set limits on freshwater withdrawals and labile nutrient loads. Important components of Florida's economies lie in the state's expansive coastal zone; the organisms driving the billion-dollar recreational fishing industry are rooted in coastal habitats, while the agriculture and real-estate industries sprawl throughout numerous coastal watersheds. This study aimed to identify the connections between anthropogenic land use and essential juvenile fish nursery habitats within the coastal zone, which is the first critical step for sustaining the ecology and related economies of the region.
The need for this study arises from the fact that these economies are interconnected through nitrogen, and therefore nitrogen management can influence their prosperity or collapse. Juvenile fish nursery habitats are located in waters that receive nitrogen from adjacent landscapes. Runoff delivers nitrogen derived from human nitrogen use and processing within the watersheds to the juvenile fish nursery habitats. Ecosystem managers must understand that although copious amounts of nitrogen applied to land may ultimately support nursery habitat foodwebs, overwhelming nitrogen loads may also create algal blooms that decay and cause lethal hypoxic events leading to ecosystem degradation. This study aims to pinpoint the specific nitrogen sources that support primary production and ultimately fish production in watersheds dominated by agricultural landscapes and residential neighborhoods.
Stable isotopes are versatile tools used to identify these connections. The nitrogen and carbon compounds that make up the moieties of an ecosystem inherently carry information on major nitrogen sources, trophic structure as well as the crucial information concerning dominant nitrogen removal and transformative processes that occur within sediments. Specifically in this study, the stable isotopes of carbon and nitrogen of dissolved inorganic nitrogen, primary producers, and fish were used to identify 1) the connections between urban and agricultural landscapes and the nutrients that percolate through the foodweb, 2) the primary producers that support fish biomass, 3) the origins of sedimentary organic matter that can provide new nitrogen via recycling, and 4) the heterogeneous function of fish nursery habitats in polluted systems. This study was conducted during the region's wet and dry seasons and in over thirty watersheds that differ from each other in terms of size and anthropogenic influence.
In agricultural watersheds, nitrogen derived from row crops and tree crops ultimately supported fish production during the wet season. Convective afternoon thunderstorms coupled with runoff delivered nitrogen from the landscape to receiving waters. These nutrients supported phytoplankton which deposited into the sediments and supported benthic foodwebs. During the dry season, nitrogen derived from row crops and nitrogen transformation in the sediments ultimately supported fish production. In this case, irrigation water used for agriculture delivered nitrogen from lands covered with row crops to the nursery habitats in receiving waters.
The dry season was characterized by the nitrogen transformation process known as dissimilatory nitrogen reduction to ammonium (DNRA), where biologically available nitrate is converted to biologically available ammonium. Phytoplankton deposits, most likely delivered during the wet season, were recycled through the slow burning DNRA processes, which provided nitrogen for the benthic microalgae that dominated in the dry season. These organisms in turn supported benthic communities which ultimately supported dry season fish production.
In small urban watersheds, nitrogen derived from septic tanks, lawn irrigation, leaky sewage pipes, and atmospheric deposition ultimately supported fish production via phytoplankton, but unlike the nitrogen sources in agricultural watersheds, these sources (with the exception of atmospheric deposition) were seasonally consistent because a mechanisms to deliver nitrogen derived from septic tanks, lawn fertilizer, and leaky sewage pipes were, at least to some extent, available during both seasons.
In polluted, tidal, fish-nursery habitats, the specific mechanism that allowed nursery habitats to decrease the ratio of mortality over growth rates of juvenile fish was not consistent among systems. These mechanisms were likely dependent on physical-chemical parameters and stream geomorphology. If the geomorphology or physical-chemical characteristics of nursery habitats are not adequate to set up an efficient nitrogen transfer process to fish, these habitats become more of a haven from predators rather than a source of food for fish.
This study has several implications for management. Managers must first recognize that microalgae are dominant supporters of tidal nursery foodwebs. Managers must define the relationship between nitrogen loads and fish abundance. If this relationship is unknown, the results of increasing nitrogen loads on fish production will remain uncertain; foodwebs in nursery habitats may collapse due to eutrophication, or fish abundance may increase due to increases in food supply. Connectivity factors derived from stable isotope mechanistic mass-balance models can be used as measurable targets for groups of watersheds. The use of wetlands as nitrogen remediation tools may not be effective at removing nitrogen; nitrogen transformation processes such as DNRA likely outweigh removal processes in wetland soils.
|Advisor:||Hollander, David J., Peebles, Ernst B.|
|Commitee:||Byrne, Robert, Hollander, David J., Kroeger, Kevin, Mann, David, Peebles, Ernst B.|
|School:||University of South Florida|
|School Location:||United States -- Florida|
|Source:||DAI-B 72/02, Dissertation Abstracts International|
|Subjects:||Biogeochemistry, Environmental economics, Environmental management, Environmental science, Aquatic sciences|
|Keywords:||Fish nurseries, Land use, Nitrogen cycle, Nitrogen recycling, Stable isotopes|
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