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

Abstract

Green sea turtles, Chelonia mydas, are grazers influencing the distribution of seagrass within shallow coastal ecosystems, yet the drivers behind C. mydas patch use within seagrass beds are largely unknown. Current theories center on food quality (nutrient content) as the plant responds to grazing disturbances; however, no study has monitored these parameters in a natural setting without grazer manipulation. To determine the morphological and physiological responses potentially influencing seagrass recovery from grazing disturbances, seagrasses were monitored for one year under three different grazing scenarios (turtle grazed, fish grazed and ungrazed) in a tropical ecosystem in Akumal Bay, Quintana Roo, Mexico. Significantly less soluble carbohydrates and increased nitrogen and phosphorus content in Thalassia testudinum were indicative of the stresses placed on seagrasses during herbivory. To determine if these physiological responses were the drivers of the heterogeneous grazing behavior by C. mydas recorded in Akumal Bay, patches were mapped and monitored over a six-month interval. The abandoned patches had the lowest standing crop rather than leaf nutrient or rhizome soluble carbohydrate content. This suggests a modified Giving Up Density (GUD) behavior: the critical threshold where cost of continued grazing does not provide minimum nutrients, therefore, new patches must be utilized, explains resource abandonment and mechanism behind C. mydas grazing. This study is the first to apply GUD theory, often applied in terrestrial literature, to explain marine herbivore grazing behavior. Rev. Biol. Trop. 62 (4): 1535-1548. Epub 2014 December 01.

Peer Review #2 of "Nutrient availability induces community shifts in seagrass meadows grazed by turtles (v0.1)"

“…The paradigm about N depletion in tissues as a patch abandonment cue is not supported by our data nor to those reported in previous studies (Moran & Bjorndal 2007;Fourqurean et al 2010). Turtles most likely stopped grazing because not much new tissue was formed, as shoot density decreased (this study) or the leaves stopped growing as suggested by Lacey, et al (2014), which was likely driven by the decline in soluble carbohydrates content (table 4, Moran & Bjorndal 2007). Robust seagrass species like T.…”

“…Rotational grazing by green turtles decreases the aboveground community biomass, particularly that of T. testudinum. Grazed patches are maintained for one to two years after they are abandoned most likely because of reduced leaf growth due to internal carbohydrate depletion; (Fourqurean et al 2010;Lacey et al 2014;), and the turtles do not return to abandoned patches (Molina-Hernández & van Tussenbroek 2014). Abandoned patches show thinner and shorter leaves of T. testudinum, with sparser seagrass shoots than ungrazed nearby meadows (Molina-Hernández & van Tussenbroek 2014).…”

“…Manuscript to be reviewed herbivory drives community shifts with faster-growing species replacing slower-growing dominant species (Molina-Hernández & van Tussenbroek 2014, Lacey, et al 2014. After abandonment, the recovery of grazed patches to pre-grazing conditions may take several years (Molina-Hernández & van Tussenbroek 2014).…”

Contribution of seagrass plants to CO2 capture in a tropical seagrass meadow under experimental disturbance

“…However, if the effect persists for too long, the resources will be exhausted, as was demonstrated here. Thereby, the plants cannot compensate and instead have to adapt by reducing shoot density and total leaf area [54]. …”

“…Such an effect has previously been observed in T . testudinum [54] exposed to turtle and fish grazing, while a study of T . hemprichii found no effect of clipping on leaf width [52].…”