Effects of excluding sea turtle herbivores from a seagrass bed: Overgrazing may have led to loss of seagrass meadows in Bermuda

Abstract

Protecting a Thalassia testudinum-dominated seagrass meadow from grazing by sea turtles for 1 yr caused an increase in the biomass of seagrasses and an increase in the structural complexity of the seagrass canopy, as the length and width of the seagrass blades increased in comparison to grazed plots. Plots from which turtles were excluded had higher rates of primary production on a per-shoot or areal basis, but the relative growth rate was not affected. The leaves of seagrasses protected from grazing had lower concentrations of nitrogen and phosphorus than grazed blades, but the storage of soluble carbohydrates in the rhizomes increased markedly in the protected plots, suggesting that reduced carbon fixation caused by the removal of photosynthetic leaves is the mechanism for seagrass decline in heavily grazed meadows, not nutrient limitation as has been suggested in the literature. The continued grazing of sea turtles in our plots did not lead to significant changes in seagrass shoot density or nutrient content over the 1 yr duration of our experiments. The decreased canopy cover and the shorter, thinner seagrass leaves induced by sea turtle grazing in our experimental plots suggest that the progressive narrowing and thinning of seagrasses observed before the collapse of 2 offshore seagrass beds in Bermuda during the 1990s may have been in response to repeated and intense grazing of those seagrass beds. KEY WORDS: Caribbean Coastal Marine Productivity Program · CARICOMP · Thalassia testudinum · Nutrient content · Soluble carbohydrates · Productivity Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 419: [223][224][225][226][227][228][229][230][231][232] 2010 (Williams 1988, Bjorndal 1997. Whether to reject the relatively low nutrient content of old seagrass leaves (Bjorndal 1980) or to avoid the calcareous encrusting epiphyte communities on those old blades , green sea turtles often clip seagrass shoots to within approximately 1 cm of the sediment surface, allowing the leaves to float away; they then repeatedly graze the newly-produced, unepiphytized, high nutrient-content seagrass leaves that grow after the clipping (Bjorndal 1997). Thayer et al. (1984) surmised that these repeated grazing events would stress the seagrasses, so that leaf widths would decrease, short shoot density would decline, leaf production would decline, and recycling of nutrients through a local detritus cycle would be interrupted. Further, they suggested that internal stores of carbohydrates and proteins would be mobilized from the rhizomes to grow new leaves, and that the repeated cropping of new leaves would eventually lead to a decline in the nutrient content of the seagrass as stores were depleted, resulting in poor food quality and decreased food production for the turtles. These declines would then cause the turtles to abandon that grazing patch and begin the cultivation of a new grazing site. However, recent experimental work has shown that growth rates (Moran & Bjorndal 2...

Morphological and physiological responses of seagrasses (Alismatales) to grazers (Testudines: Cheloniidae) and the role of these responses as grazing patch abandonment cues

“…The driver(s) behind changes in patch use are elusive, as monitoring of Chelonia mydas patch use through time can be challenging in the field. Contrary to our hypothesis, we did not find that patches with the lowest soluble carbohydrate content in the seagrass rhizomes were more likely to be abandoned (as suggested by Fourqurean et al, 2010), nor did we find that patches with the lowest nutrient content of seagrass leaves were likely to be abandoned (as suggested by Thayer et al, 1984). However, those patches abandoned had the smallest biomass and therefore density of leaves, indicating that a modified GUD or a 'giving up biomass' may be the mechanism by which patches are abandoned.…”

“…The relative physiological (productivity, tissue nutrients and soluble carbohydrates) and morphological effects of grazing on seagrasses were similar to those previously measured from simulated grazing or exclosure experiments (Fourqurean et al, 2010;Moran & Bjorndal, 2005. Leaf widths of fish grazed (4.9-5.4mm) and turtle grazed (5.0-5.9mm) patches in this study were within range of other studies on grazing (5.3-6.3mm, Zieman et al, 1984;5.6-6.3mm, Moran & Bjorndal, 2005;3.3-4.7mm, Williams, 1988;1.4-3.0mm, Fourqurean et al, 2010).…”

“…We expect that summer nutrient content of aboveground biomass would be lower than winter, and that summer rhizome soluble carbohydrates would be higher, because high light availability in summer drivers increased carbon fixation (Lee & Dunton, 1997;Perez, Guevara, & Bone, 2006). Owing to the previous finding that nutrient content of seagrass leaves does not decline as a result of repeated grazing (Fourqurean et al, 2010;Moran & Bjorndal, 2005, we expect that nutrient content of seagrass leaves would not be an explanation for grazing patch abandonment. If reduced seagrass carbon fixation leads to abandonment by C. mydas, we predict that patches abandoned would have significantly lower soluble carbohydrates than patches remaining grazed.…”

“…Thayer, Bjorndal, Ogden, Williams, and Zieman (1984) hypothesized that decreases in aboveground nutrient content, and therefore food quality, of repeatedly grazed seagrasses would lead to abandonment of grazed patches. More recent studies indicate that repeated grazing does not lead to a decrease in the food quality of grazed seagrasses (Fourqurean, Manuel, Coates, Kenworthy, & Smith, 2010;Moran & Bjorndal, 2005). An alternative theory proposed by Fourqurean et al (2010) as an extension of research by Dawes et al (1979), suggests that reduced carbon fixation by repeatedly grazed plants, which results in progressive thinning of leaves and decreased storage of soluble carbohydrates in plant rhizomes, may occur prior to seagrass bed abandonment.…”

“…More recent studies indicate that repeated grazing does not lead to a decrease in the food quality of grazed seagrasses (Fourqurean, Manuel, Coates, Kenworthy, & Smith, 2010;Moran & Bjorndal, 2005). An alternative theory proposed by Fourqurean et al (2010) as an extension of research by Dawes et al (1979), suggests that reduced carbon fixation by repeatedly grazed plants, which results in progressive thinning of leaves and decreased storage of soluble carbohydrates in plant rhizomes, may occur prior to seagrass bed abandonment. Based on this theory, decreased soluble carbohydrates may therefore serve as a predictor of herbivore abandonment of particular grazed areas over others.…”