Seagrass, Submerged Aquatic Vegetation, Macroalgae, & Prop-Scarring



Milbrandt, E.C., L. Reidenbach, and M. Parsons. 2019. Determining the sources of macroalgae during beach stranding events from species composition, stable isotope analysis, and laboratory experiments. Estuaries and Coasts 42(3):719–730.

Large accumulations of stranded macroalgae on the beaches of Sanibel Island from 2004 to 2007 were unusual and thought to be driven by high rainfall and river flows stemming from a high frequency of hurricanes. The southwest Florida shelf was thought to be isolated from far-field effects such as high river flows and urbanization but field- and laboratory-based studies suggest that nitrogen enrichment, fragmentation, and species composition of macroalgal communities on the shelf and in the estuary contribute to beach stranding events. Macroalgae were sampled using a belt transect method to determine species distribution and abundance. Macroalgae were abundant (1) in the lower estuary with abundant seagrass and (2) on limestone outcroppings in the Gulf of Mexico. An MDS analysis of the quadrat samples indicated two distinct macroalgal community types, a “Sound” assemblage around Pine Island Sound and a “Gulf” assemblage, associated with a live bottom and patch reef in the Gulf of Mexico. Peak abundances for the two community types differed with the Gulf having peak abundances from July to November, while peak abundances in the Sound occurred from January to July. Sound macroalgal tissue had significantly enriched δ15N compared to Gulf tissue when all species were combined and in five of six species collected at both locations suggesting that stable isotope analysis could be useful in combination with species composition in determining the source of macroalgae during stranding events. In addition, a laboratory study was conducted on four species that were sampled and frequently collected as a result of stranding events. Laboratory growth experiments demonstrated the potential for three of four common species (Solieria filiformis, Gracilaria tikvahiae, Agardhiella subulata) to fragment and grow significantly more under elevated nitrate conditions.


Milbrandt, E.C., and J. Siwicke. 2016. Leaf growth rates (Thalassia testudinum, Banks ex Koning) as an indicator of seagrass responses to regulated freshwater discharges. Gulf of Mexico Science 2016(1):38–46.

In southwest Florida, changes in hydrology have fundamentally changed the timing and amount of freshwater delivered to the estuarine ecosystem. Biological indicators such as oyster and submerged aquatic vegetation distribution and abundance have been used to establish minimum and maximum discharges to the estuary. These indicators are robust long-term indicators for comparing interannual and climatological changes; however, they lack sensitivity to variable freshwater flows that occur over the course of months or seasons. Seagrass leaf growth rates could provide an integrated biological response for evaluating events caused by climatological shifts (e.g., El Niño) or to evaluate the biological responses to management actions (e.g., flood control releases of freshwater from Lake Okeechobee). Leaf growth rates for Thalassia testudinum were determined monthly across a gradient of increasing distance from the mouth of the Caloosahatchee estuary. Leaf growth at sites near the Caloosahatchee (within 5 km) had significantly lower growth rates during the April–June period. Salinity was also significantly lower, while light attenuation and temperature were not significantly different. High discharges for flood control caused lower salinities and significantly slowed leaf growth rates. Leaf growth can be a sensitive indicator to water management and climatological events and can show an integrated biological response to high flows.


Bartleson, R.D., M.J. Hunt, and P.H. Doering. 2014. Effects of temperature on growth of Vallisneria americana in a sub-tropical estuarine environment. Wetlands Ecology and Management 5:571–583.

The submersed aquatic vegetation (SAV) species Vallisneria americana Michx. (tape grass) is a valuable resource in the Caloosahatchee estuary and in many other aquatic systems. Given the variable nature of freshwater inflows and environmental conditions in the Caloosahatchee, it is necessary to understand how tape grass will respond to high and low salinity conditions at different light and temperature levels. Specifically, quantitative information is needed as input to modeling tools that can be applied to predict growth and survival of tape grass under a range of environmental conditions present in the estuary. We determined growth rates for small and medium sized tape grass plants obtained from the Caloosahatchee estuary, southwest coastal Florida, USA in freshwater (0.5 psu) under high (331 μE m−2 s−1) and low light (42 μE m−2 s−1) and at 10 psu under high light conditions. We ran six treatments at five temperatures spanning 13–32 °C for 8–9 weeks. The optimum temperature for growth was roughly 28 °C, with a minimum threshold temperature of 13 °C and a maximum threshold temperature of 38 °C. Plants grew fastest in freshwater, at high light and temperatures greater than 20 °C. The slowest growth rates were observed at 13 °C regardless of salinity, light or plant size. Our results suggest that tape grass growth is strongly influenced by water temperature and that additional stressors such as low light and elevated salinity can reduce the range of temperature tolerance, especially at colder water temperatures.


Provost, K., A.J. Martignette, E.C. Milbrandt, and J. Siwicke. 2013. Quadrat vs. video assessment of macroalgae cover: a methods comparison. Florida Scientist 76(2):249–258.

In 2004–2007, the beaches of Sanibel Island, Florida, were affected by large drifts of macroalgae that accumulated on shore. The adjacent seafloor is composed of three ecoregions: inshore, nearshore and offshore. To measure attached and unattached macroalgae on various substrata, two field survey methods were compared from 2008 to 2010 along thirteen 100 m long transects. One method used visual assessments of macroalgae in 1 m2 quadrats along the transects and one used an underwater camcorder to capture video footage along the transects. Results of each method were converted to percent cover of the transect and compared for each sampling date. The results showed that digital video observations were not significantly different than diver assessments for two of three ecoregions. Sparse amounts of macroalgae were underestimated nearshore using visual SCUBA, due to the inconsistent, but high percentage, of algae cover. Uses, advantages, disadvantages, and time-effectiveness of the two methods were compared. Natural resource managers can choose which survey method meets their scientific, management and budget needs. Based on these results, it is suggested that video analysis of bottom cover is a practical method for rapid, widespread assessment of macroalgae abundance surrounding Sanibel Island.


Milbrandt, E.C. 2009. The effects of root zone manipulation of microcosm Turtlegrass (Thalassia testudinum) transplants. Florida Scientist 72:406–419.

The objective of this research was to determine whether the survival and growth rate of transplanted seagrasses is hindered by sediment sulfide addition and bacterial community disruption. A microcosm experiment was designed to control for temperature, salinity, and light availability. Bare root transplanted seagrass shoots were exposed to one of four treatments; plus sulfide, plus autoclave; minus sulfide, plus autoclave; plus sulfide, unmanipulated (not autoclaved); and minus sulfide, unmanipulated. Bare root transplants had less than half the rate of growth of the control that was transplanted as a plug with sediments, demonstrating the sensitivity of root disturbance in Thalassia testudinum. Bare root transplants in autoclaved sediments grew slower than in unmanipulated sediments regardless of sulfide treatments. The greatest amount of extractable DNA was measured in bare root transplanted treatments that had not been autoclaved. Standard diversity indices along with a Bray-Curtis similarity index of Terminal Restriction Fragment Length Polymorphism in a MDS were used to assess community composition. The MDS showed no significant differences, while comparisons of diversity indices indicated differences between transplants and control. The results support the conclusion that an intact sediment bacterial community increases transplant success, but the nature of the interaction (e.g., functional, structural) remains unclear.


Milbrandt, E.C., J.M. Greenawalt, and P.D. Sokoloff. 2008. Short-term indicators of seagrass transplant stress in response to rhizosphere and bacterial community disruption. Botanica Marina 51:103–111.

Bacterial communities in sediments engage in activities that determine the pathways and rates of organic matter remineralization. Changes in bacterial community composition might result in greater restoration success if interactions among seagrasses, sediment, and bacterial communities were elucidated. A manipulative experiment that disrupted the sediment bacterial community examined the response of Thalassia testudinum transplants. Planting units were transported to a suitable location and replanted with either autoclaved, transplant, or donor site sediments. Sediments from the rhizosphere were sub-sampled for analysis of the 16S rDNA gene. The autoclave treatment group had significantly lower bacterial diversity than the other treatments, as measured with terminal restriction fragment length polymorphism. Significantly higher mortality of the transplants was observed in the autoclave treatment group, while mortality in the donor and transplant treatment groups was low relative to the control group. Rhizosphere disruption through the removal of surrounding sediments did not increase mortality, while manipulating the composition of the bacterial community increased mortality in transplants. A native bacterial community was a critical component for minimizing short-term stress associated with a seagrass transplant, which suggests a tighter coupling between seagrasses and sediment bacterial communities than previously thought.


Greenawalt-Boswell, J., J.A. Hale, K.S. Fuhr, and J.A. Ott. 2006. Seagrass species composition and distribution trends in relation to salinity fluctuations in Charlotte Harbor. Florida Scientist 69(S2):24–35.

Seagrass species composition and distribution reflect environmental changes, making these measures potentially useful estuarine indicators. An annual seagrass transect and quadrat monitoring survey program including 50 locations in Charlotte Harbor, Florida, began in 1999. This six-year data set was analyzed in conjunction with a monthly water quality monitoring program covering the same time period to examine trends in seagrass species composition and distribution. Analyses of the maximum depth of seagrass distribution for each transect did not indicate any large-scale changes in seagrass depth distribution. This suggests a stable overall area of seagrass distribution in the Charlotte Harbor area during the study period. However, abundance of Halodule wrightii and Thalassia testudinum has significantly declined, along with the overall frequency of seagrass occurrence among quadrats. Finally, the distribution of the three dominant seagrass species, H. wrightii, T. testudinum, and Syringodium filiforme, appear to be influenced by low, wet-season salinity and high variation. This study highlights the value of research into seagrass species abundance and distribution on a meter-to-meter scale to recognize the effects of water quality or environmental variables such as salinity on a small scale, prior to large scale loss.


Books, Reports, & Symposia


Milbrandt, E.C., M. Thompson, R.E. Grizzle, and K. Ward. 2018. The effects of long-term mooring and anchoring by vessels in shallow water on the benthic environment. A report to the West Coast Inland Navigation District. SCCF Marine Laboratory. Grizzle Coastal Engineering LLC.


Thompson, M.A. 2015. Before after control intervention (BACI) analysis of prop scars in Wulfert Flats as a result of implementation of a No Motor Zone, October 2015. SCCF Marine Laboratory report to Ding Darling National Wildlife Refuge. Sanibel, FL.


Thompson, M.A. 2015. Prop scar density in Ding Darling NWR, Sanibel; a brief survey of the wildlife refuge and Ladyfinger Lakes. SCCF Marine Laboratory. Sanibel, FL.


Milbrandt, E., R. Bartleson, A.J. Martignette, M. Thompson, and J. Palmer. 2014. The cumulative effects of regulated discharges on seagrass density and community composition around Sanibel Island. Charlotte Harbor Watershed Summit, March 25-27, 2014, Punta Gorda, FL.


Bartleson, R.D., E.C. Milbrandt, and M.A. Thompson. 2014. Restoration of propeller-scarred seagrass beds near Sanibel using Halodule wrightii Year 3 Progress Report to Keewaydin Island Community Association, and Humiston and Moore Engineers, Inc. November, 2014 Naples.

Flynn, R.L., R.D. Bartleson, and E.C. Milbrandt. 2013. Seston removal by a tunicate Didemnum duplicatum in a seagrass meadow. 2013 Benthic Ecology Meeting, Savannah GA.

Milbrandt, E.C., R.D. Bartleson, D. Fugate, A. Rybak, M. Thompson, and L. Coen. 2011. Reopening a tidal pass: implications for changes in water column optical properties and seagrass habitats FINAL REPORT. SCCF Marine Laboratory, Florida Gulf Coast University. Florida Seagrant. Sanibel, FL.


Bartleson, R.D., E.C. Milbrandt, and Mark A. Thompson. 2010. Feasibility of Ruppia restoration in the Caloosahatchee Estuary using grazer exclosures. Benthic Ecology Meeting, Wilmington, NC, March 2010.


Bartleson R.D., E.C. Milbrandt, and M.A. Thompson. 2010. Feasibility of Ruppia restoration in the Caloosahatchee Estuary. Final Report to Lee County. 52 pp.


Bartleson, R.D. 2009. Caloosahatchee submersed plant restoration using grazer exclosures. CHNEP Science Forum, Arcadia, Florida. July, 2009.


Bartleson, R.D. 2008. Growth and photosynthesis of Caloosahatchee tape grass: effects of light, temperature and salinity. Final Report to SFWMD. 44 pp.


Milbrandt, E.C. 2008. An assessment of the effect of water quality on seagrasses in the Caloosahatchee River/estuary and Pine Island Sound using optical properties to predict light levels and thresholds. Final report to SFWMD, 49 pp.


Bartleson, R.D. 2008. Growth and photosynthesis of Caloosahatchee tape grass: effects of light, temperature and salinity. Final Report to SFWMD, 44 pp.

Bartleson, R.D., and J.G. Guinn. 2007. Submerged Aquatic Vegetation Monitoring – Caloosahatchee Estuary. Report to SFWMD for 6 month extension (December).


Bartleson, R.D., and J.G. Guinn. 2007. Submerged Aquatic Vegetation Monitoring – Caloosahatchee Estuary. Report to SFWMD for 6 month extension (June).


Bartleson, R.D. 2007. Effect of light and salinity on growth and photosynthesis of tape grass (Vallisneria americana) at 32°C. Report to the SFWMD May 2008.


Bartleson, R.D. 2007. Effect of light and salinity on growth and photosynthesis of tape grass (Vallisneria americana) at 30°C. Report to the SFWMD January 2008.


Bartleson, R.D. 2007. Effect of light and salinity on growth and photosynthesis of tape grass (Vallisneria americana) at 20°C. Report to the SFWMD June 2007.


Bartleson, R.D. 2007. Effect of light and salinity on growth and photosynthesis of tape grass (Vallisneria americana) at 25°C. Report to the SFWMD September 2007.


Bartleson, R.D., and J.G. Guinn. 2006. Submerged Aquatic Vegetation Monitoring – Caloosahatchee Estuary. Report to SFWMD Final Report (December).


Bartleson, R.D., E.D. Berris, L.B. Linsmeyer, M.P. Hannan, and J. Siwicke. 2006. Final Report: Macroalgae and seagrass monitoring during spring and summer of 2006. Report to City of Sanibel, 54 pp.


Milbrandt, E.C. 2006. Determining the effects of root zone manipulation on seagrass transplants to enhance restoration projects. Final Report to CHNEP, 34 pp.



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