Degree Granting Department
David J. Hollander, Ph.D.
Ernst B. Peebles, Ph.D.
Robert Byrne, Ph.D.
Kevin Kroeger, Ph.D.
David Mann, Ph.D.
Stable Isotopes, Land Use, Foodwebs, Nitrogen Recycling, Fish Nurseries
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.
Scholar Commons Citation
Malkin, Elon M., "The Economically Important Nitrogen Pathways of Southwest Florida" (2010). Graduate Theses and Dissertations.