The Importance of Seagrass Epiphytes in Big Lagoon, Florida, With Emphasis On the Diatoms (Bacillariophyceae)

Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Marine Science

First Advisor

Steven Lohrenz

Advisor Department

Marine Science


Seagrass epiphytes have a vital role in the trophodynamics of coastal marine systems, contributing significantly to total system primary productivity and serving as the primary energy source for numerous meio- and macrofaunal grazers. Among the epiphytic microalgal groups, diatoms are numerically dominant and are preferentially utilized by grazers within seagrass beds. A study was conducted to determine the contribution of dislodged and suspended seagrass epiphytes to planktonic chlorophyll a concentration and diatom diversity. Additional work focused on the role of the environment on influencing the structure of epiphytic and planktonic diatom assemblages. There was no overall difference in chlorophyll a or diatom diversity in the water column over seagrass relative to nonvegetated bottom but individual sites did exhibit temporal differences in chlorophyll a between the two substrata. Dislodgement and suspension of epiphytes occurred regularly, as evidenced by the presence of epiphytic diatoms in the planktonic assemblage on all sampling dates. The ambient turbulence regime in Big Lagoon may consistently be suspending epiphytic taxa in the water column, where currents then transport suspended cells into nonvegetated areas. This supplement to the planktonic assemblage would be widely disbursed throughout Big Lagoon, increasing the quantity and quality of organic carbon for primary consumers near the base of the trophic web. Canonical correspondence analysis indicated that diatoms in the two ecological groups were correlated with different environmental variables. The planktonic assemblage demonstrated a seasonal shift in the taxonomic composition and a strong correlation with water temperature. The epiphytic assemblage was temporally stable and was highly correlated with dissolved nutrients. Planktonic diatom diversity was high on all sampling dates, suggesting effects of eutrophication in Big Lagoon were minimal during this study. However, coastal development is likely have anthropogenic impacts that include effects on diatom species diversity. The role of diatoms in monitoring water quality is discussed. Prior to the inclusion of diatoms in monitoring programs, further research is needed to examine the responses of selected taxa to specific environmental conditions.