Simulated green turtle grazing affects structure and productivity of seagrass pastures

Abstract

The seagrass Thalassia testudinum evolved under much greater grazing pressure than exists today because overexploitation by humans caused the severe decline of green turtles Chelonia mydas, the major seagrass herbivore in the Caribbean. Understanding the effects of grazing on the structure and function of seagrass ecosystems is critical to evaluating how ecosystem processes have changed since the major herbivore was essentially removed from modern Caribbean seagrass systems. We evaluated effects of grazing on the structure and productivity of T. testudinum pastures by simulating green turtle grazing in 3 × 3 m plots for 16 mo in the central Bahamas and comparing these clipped plots to adjacent control (unclipped) plots. Simulated grazing affected the physical structure of T. testudinum plots, resulting in a system with reduced structural complexity. Simulated grazing resulted in compensatory growth in T. testudinum. Clipped plots maintained levels of growth comparable to unclipped plots over the 16 mo clipping trial, and specific mass growth was significantly elevated in clipped plots, even without simulating the nutrient return from green turtle feces and urine. The green turtle grazing pattern of re-cropping T. testudinum plots is sustainable for long periodsat least 16 mo. We estimated a range of carrying capacities of T. testudinum for green turtles of 1567 to 3748 kg turtle ha -1. Our study provides a foundation for reconstruction models and restoration plans for Caribbean seagrass ecosystems.

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

“…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.…”

“…In order to determine if this theory is applicable, it is necessary to monitor morphology, growth rate, leaf nutrient content and soluble carbohydrate levels in rhizomes, which are all important to seagrass recovery and growth, as they are abandoned by green sea turtles. There is a lack of studies which consider these parameters in natural rather than simulated grazed seagrass beds (Kuiper-Linley, Johnson, & Lanyon, 2007;Moran & Bjorndal, 2005.…”

Habitat selection by green turtles in a spatially heterogeneous benthic landscape in Dry Tortugas National Park, Florida

“…Experimental mani pulation is particularly difficult for highly mobile and large-bodied marine vertebrates, such as sea turtles. Sea turtles play key roles in coastal and coral reef ecosystem function, with direct links between green turtles Chelonia mydas and seagrass productivity through herbivory (Thayer et al 1984, Moran & Bjorndal 2005 and indirect links for all species of sea turtles to nutrient cycling through nesting activities on beaches (Bouchard & Bjorndal 2000, Bjorndal & Jackson 2003.…”

Turtle herbivory in the seagrass ecosystems in the Lakshadweep atolls: concerns and need for conservation measures

“…2). Seagrass herbivory by turtles is established beyond doubt (Jackson, 1997;Bjorndal and Jackson, 2003;Moran and Bjorndal, 2005;Lal et al, 2010). Reduction in the wet biomass of seagrass above the sediment than the underground parts indicate intense grazing (Fig.…”