Voss, Joshua

Coastal development shifts natural hydrology through water redirection, increased impervious surfaces, and increased connectivity to the coastal ocean through inlets. In Southeast Florida, watershed alterations can cause flash-freshening in nearshore coastal habitats, threatening coral reef ecosystems. This study assessed the hyposalinity tolerance threshold of two prominent scleractinian corals in Southeast Florida. In a series of experiments, we determined that Montastraea cavernosa corals have an LC50 of 19 PSU but can survive for at least 21 days at an intermediately stressful salinity of 25 PSU. Porites astreoides corals demonstrated an LC50 of 19 PSU, but experienced mortality when exposed to 25 PSU for 17–18 days. Prior to mortality, corals displayed decreased polyp activity, altered coloration, and decreased tissue integrity. These data suggest that lower-volume, longer-duration releases of freshwater from reservoirs may preserve coral health in Southeast Florida.

Coral reef ecosystems worldwide are facing increasing degradation due to
disease, anthropogenic damage, and climate change, particularly in the Tropical Western
Atlantic. Mesophotic coral ecosystems (MCEs) have been recently gaining attention
through increased characterization as continuations of shallow reefs below traditional
SCUBA depths (>30 m). As MCEs appear to be sheltered from many stressors affecting
shallow reefs, MCEs may act as a coral refuge and provide larvae to nearby shallow
reefs. The Deep Reef Refugia Hypothesis (DRRH) posits that shallow and mesophotic
reefs may be genetically connected and that some coral species are equally compatible in
both habitats. The research presented here addresses key questions that underlie this
theory and advances our knowledge of coral connectivity and MCE ecology using the
depth-generalist coral Montastraea cavernosa. Chapter 1 presents an overview of the
DRRH, a description of MCEs in the Gulf of Mexico (GOM), and the framework of
research questions within existing reef management infrastructure in the GOM. Through microsatellite genotyping, Chapter 2 identifies high connectivity among shallow and
mesophotic reefs in the northwest GOM and evidence for relative isolation between depth
zones in Belize and the southeast GOM. Historical migration and vertical connectivity
models estimate Gulf-wide population panmixia. Chapter 3 focuses on population
structure within the northwest GOM, identifying a lack of significant population
structure. Dominant migration patterns estimate population panmixia, suggesting
mesophotic populations currently considered for National Marine Sanctuary protection
benefit the Flower Garden Banks. Chapter 4 quantifies the level of morphological
variation between shallow and mesophotic M. cavernosa, revealing two distinct
morphotypes possibly representing adaptive tradeoffs. Chapter 5 examines the
transcriptomic mechanisms behind coral plasticity between depth zones, discovering a
consistent response to mesophotic conditions across regions. Additionally, variable
plasticity of mesophotic corals resulting from transplantation to shallow depths and
potential differences in bleaching resilience between shallow and mesophotic corals are
identified. The dissertation concludes with a synthesis of the results as they pertain to
connectivity of shallow and mesophotic corals in the Gulf of Mexico and suggests future
research that will aid in further understanding of MCE ecology and connectivity.

Coral reefs on Florida’s Reef Tract (FRT) are susceptible to many anthropogenic
influences including controlled freshwater discharges and agricultural runoff as well as
high natural environmental variability from seasonal rainfall, runoff and upwelling. To
better understand coral population structure and responses to sublethal stressors,
populations of the scleractinian coral Montastraea cavernosa in the northern FRT were
examined using a combination of genomic and transcriptomic techniques. Microsatellite
genetic markers identified high local retention among sites and a slight southward gene
flow. An in-situ temporal gene expression analysis utilizing a tag-based sequencing
transcriptomic approach was used to analyze baseline coral health at St. Lucie Reef
(SLR), off Stuart, FL. Temporal variation had the greatest influence of differential gene
expression among M. cavernosa at SLR. Results will be shared with local resource
managers and coupled with a complementary ex-situ experimental trial.

Coral reef declines, particularly in coastal zones, have been linked to thermal
stress and anthropogenic impacts on water quality. St. Lucie Reef near Stuart, Florida
receives increased estuarine efflux as a result of watershed changes and management
policies that have substantially altered historic, natural flows. This research used ambient
and elevated temperatures (25°C and 30°C, respectively), and offshore versus St. Lucie
Estuarine discharge water to investigate the individual and interactive effects of thermal
and water quality stress on Montastraea cavernosa, a dominant scleractinian coral
species at St. Lucie Reef. These goals were accomplished using ex-situ, factorial,
experimental design that was supplemented with existing in-situ monitoring on St. Lucie
Reef. Zooxanthellae density and chlorophyll content were evaluated to determine effects
on the corals and their symbionts. Zooxanthellae populations were significantly affected
by thermal stress. Significant interactions between temperature and water treatment were observed, suggesting that the impacts of discharge water may be supplanted when corals
are exposed to thermal stress. In a supplement to the experiment, M. cavernosa colonies
transplanted from Palm Beach to St. Lucie Reef demonstrated resilience despite exposure
to more variable environmental conditions. Collaborative partnerships with multiple state
agencies and local government offices facilitated data sharing to inform decision making
for South Florida’s resource management strategies. Creating effective resource
management is crucial for the conservation of coastal ecosystems impacted by land-based
sources of pollution both locally and globally.

Montastraea cavernosa is a widely distributed scleractinian coral found in temperate and tropical waters globally. Because it occurs in both shallow and mesophotic reefs, it is often referred to as a depth generalist. This study examined the morphometics of shallow-water and mesophotic cohorts in various locations in the Gulf of Mexico and used measurements of skeletal characteristics to assess morphological variation. Results from multivariate analyses showed that these characteristics differed significantly by site, not by depth, though there existed a significant site-depth interaction. Additionally, sites evaluated using a pairwise comparison showed that there were significant morphological variations as well, however among fewer characteristics. Overall resulting showing morphological difference in M. cavernosa suggested that there is morphological acclimation to different depths or separate populations who demonstrate morphological traits may be adaptive for different depths.

Mesophotic reefs represent biodiverse ecosystems that may act as a refuge for
depth-generalist coral species threatened in shallow habitats. Despite the importance of
coral-algal symbioses, few studies focus on mesophotic zooxanthellae assemblages and
their influence on connectivity. This study compared zooxanthellae in Montastraea
cavernosa at shallow and mesophotic depths at Flower Garden Banks National Marine
Sanctuary and McGrail Bank. Mesophotic corals contained more zooxanthellae and more
chlorophyll a and c2 per unit area coral. Increased zooxanthellae within mesophotic
corals may represent an adaptive strategy to optimize light capture in low-light
environments. Genetic profiles for zooxanthellae assemblages from shallow and
mesophotic corals showed similar diversity across banks and between depths. The
dominant sequence making up assemblages was identified as Symbiodinium type C1.
Similar assemblage diversity suggests that zooxanthellae assemblages will not limit
connectivity potential between shallow and mesophotic corals at these reefs.

Mesophotic reefs have historically been understudied, leading to a dearth of information on the corals that compose these reefs. Recent advancements in technology have enabled researchers to have greater access to reefs at mesophotic depths, generating a greater interest in the microbial communities in both mesophotic and shallow reefs, in order to determine the relative bacterial community differences. Samples of coral mucus from Montastraea cavernosa were collected from Flower Garden Banks National Marine Sanctuary (FGBNMS) in the summer of2010 and 2011. Bacterial community DNA was extracted from the coral mucus and amplified using LH-PCR. Length Heterogeneity PCR is a technique used to amplify the hypervariable Vl and V2 region of the 16s rRNA gene in order to detennine relative base pair length and abundance of microbial connnunities. During the PCR product verification, contamination became apparent in the gel. Through contamination troubleshooting, the source was determined; however, not soon enough to continue with analyzing the samples. This project will be continued in the future in order to add to the mesophotic bacterial community database. The DNA extraction protocol frequently utilized in the laboratory will be modified to increase DNA yields, PCR conditions will be optimized, and the samples will be fully analyzed.

Algal symbionts, commonly called zooxanthellae, living in within coral tissues provide
energy to hermatypic corals through photosynthesis. Zooxanthellae density, as well as
photosynthetic pigment concentration, can be used as an indicator of coral health. This study compared zooxanthellae densities and concentrations of chlorophyll a and c between four sites and two species of coral, Montastraea cavemosa and Diploria clivosa, found at the St. Lucie Reef to establish baseline levels for comparison to future levels. No significant differences were found among sites despite increasing depth farther from the inlet, suggesting reduced light penetration closer to the inlet. Significant differences were found for zooxanthellae densities and chlorophyll concentrations per zooxanthellae cell between coral species. However, there was no significant difference for chlorophyll concentrations per unit area of coral tissue between species. This suggests that all site locations experience similar conditions despite differences in depth. It also suggests potentially different types of zooxanthellae, which is being addressed by ongoing research.

The coral reef habitat at St. Lucie Reef Stuart, FL persists despite environmental instability resulting from extensive freshwater discharges, summer upwelling, and seasonality. By examining the symbiotic algae, or zooxanthellae, that reside in corals, we can gain insight to overall coral physiology during stress events. Two hermatypic corals, Montastraea cavernosa and Pseudodiploria clivosa were sampled over a year and a half, representative of wet and dry seasons. Zooxanthellae were isolated from each coral fragment and responses to water quality fluctuations were quantified by measuring cell density and chlorophyll a and c2. Both coral species varied in the amount of zooxanthellae harbored as well as chlorophyll concentrations. Pseudodiploria clivosa had a higher density of cells than Montastraea cavernosa, but chlorophyll concentrations per cell were greater in M. cavernosa. Morphological differences between both coral species and their zooxanthellae concomitant with the inverse relationship between cell density and chlorophyll could be indicative of photosynthetic capabilities. Genotypes of zooxanthellae within each host species were evaluated by amplifying the internal transcribed spacer region of ribosomal DNA and sequencing on Illumina’s MiSeq platform. Patterns in fine-scale diversity and genetic variation may explain the observed physiological differences among corals at St. Lucie Reef.