Publication Date

2017-08-03

Availability

Open access

Embargo Period

2017-08-03

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Biology (Arts and Sciences)

Date of Defense

2017-06-06

First Committee Member

Leonel Sternberg

Second Committee Member

Don DeAngelis

Third Committee Member

Carol Horvitz

Fourth Committee Member

Robert Stephen Cantrell

Abstract

In southern Florida, conservation of coastal wetlands is facing two challenges under sea level rise (SLR): (1) How to estimate and predict encroachment of mangroves and other halophyte vegetation into the areas previously covered by freshwater species? (2) How to allocate more freshwater from Lake Okeechobee in middle Florida to the coastal wetlands (e.g., Everglades) to counteract increasing saltwater intrusion? Fundamental to the above challenges is saltwater intrusion associated with SLR, which can increase the soil pore water salinity to levels where freshwater species cannot survive. The overall aim of my dissertation is to access the potential changes in vegetation community structure based on different methods such as: remote sensing, using stable isotopes as a tracer and modelling the flow of these tracers through coastal wetlands. In the second chapter of my dissertation, I investigated the persistence of mud islands in Florida Bay. Mud islands in Florida Bay are probably the most sensitive land formation in southern Florida to be affected by SLR, because they have small area and low elevation. However, surprisingly, there is no study to estimate the historical vegetation changes in these islands. More importantly, there are few human activities in these islands. Therefore they provide an appropriate setting to examine how these small island ecosystems responded to the impacts of SLR without the confounding anthropogenic factors such as road building and land management. I used high-resolution 61-yr historical aerial images and 27-yr time-series of Landsat images to estimate changes in island areas and mangrove coverage for 15 mud islands of Florida Bay. I found, surprisingly, that these islands actually increased in their area and showed mangrove expansion under the local SLR. In addition, I observed a positive relationship between island area increase and mangrove area increase in these islands, and it indicated the contribution from the biogeomorphic feedbacks between mangroves and sedimentation to the island survival. Large spatial scale vegetation shift from freshwater vegetation to mangroves can be estimated and even predicted by remote sensing data using the same techniques as in my second chapter. However, a more detailed prediction of vegetation shift at a relatively smaller spatial scale and even at an individual scale is required for local ecological conservation efforts. In my third chapter, I attempted to find an appropriate individual based predictor for the potential vegetation shift under SLR. I incorporated stable isotope 18O abundance of water as a tracer for various hydrologic components (e.g., vadose zone, water table) in a previously published individual based model describing ecosystem shifts between hammock and mangrove communities in southern Florida. My modelling efforts showed that freshwater hammock trees that were to be replaced by mangroves had higher δ18O values in their plant stem water than those which remained despite SLR. These tracer differences could be detected as early as 3 years before their eventual replacement by mangroves. Much of the susceptibility of the Everglades to SLR is exacerbated by the decrease of freshwater flow into the system. To mitigate this water shortage, the water conservation areas (WCAs) of southern Florida were constructed to serve two functions: 1) provide water to the Everglades on a regular basis and 2) remove high nutrient content from the water before the entering the naturally oligotrophic Everglades ecosystem. One critical problem of the WCA is loss of water by evaporation as they move from the Lake Okeechobee area and on to the Everglades. However, quantifying evaporation in a wetland is challenging, because it is difficult to separate evaporation from transpiration. My fourth chapter addressed this knowledge gap by using oxygen and hydrogen isotope ratios of water as a tracer for evaporation. I used a deuterium excess method based on oxygen and hydrogen stable isotope ratios (δ18O and δD) of reservoir water to calculate the remaining fraction of water after evaporation in the Water Conservation Area-1 (WCA-1). My results showed that both vegetation coverage and distance to the discharge gate had significant effects on the remaining fraction, however the depth of the water column had no effect.

Keywords

Sea Level Rise; Mangrove; Stable Isotope; Ecohydrology; Mathematical Modeling

Share

COinS