Graduation Year

2010

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Marine Science

Major Professor

David J. Hollander, Ph.D.

Co-Major Professor

Ernst B. Peebles, Ph.D.

Committee Member

Robert Byrne, Ph.D.

Committee Member

Kevin Kroeger, Ph.D.

Committee Member

David Mann, Ph.D.

Keywords

Stable Isotopes, Land Use, Foodwebs, Nitrogen Recycling, Fish Nurseries

Abstract

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.

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