Nutrient availability induces community shifts in seagrass meadows grazed by turtles.

Isis Gabriela Martínez López,Marieke M Van Katwijk,Marloes Van Den Akker,Liene Walk,Brigitta I Van Tussenbroek,Tjisse Van Der Heide

doi:10.7717/peerj.7570

Isis Gabriela Martínez López, Marieke M Van Katwijk + Show 4 more

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https://doi.org/10.7717/peerj.7570

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Abstract

In the Caribbean, green turtles graze seagrass meadows dominated by Thalassia testudinum through rotational grazing, resulting in the creation of grazed and recovering (abandoned) patches surrounded by ungrazed seagrasses. We evaluated the seagrass community and its environment along a turtle grazing gradient; with the duration of (simulated) grazing as a proxy for the level of grazing pressure. The grazing levels consisted of Short-term (4 months clipping), Medium-term (8 months clipping), Long-term grazing (8 months of clipping in previously grazed areas), 8-months recovery of previously grazed patches, and ungrazed or unclipped patches as controls. We measured biomass and density of the seagrasses and rhizophytic algae, and changes in sediment parameters. Medium- and Long-term grazing promoted a shift in community species composition. At increasing grazing pressure, the total biomass of T. testudinum declined, whereas that of early-successional increased. Ammonium concentrations were highest in the patches of Medium-term (9.2 + 0.8 μM) and Long-term grazing levels (11.0 + 2.2 μM) and were lowest in the control areas (4.6 + 1.5 μM). T. testudinum is a late-successional species that maintains sediment nutrient concentrations at levels below the requirements of early-successional species when dominant. When the abundance of this species declines due to grazing, these resources become available, likely driving a shift in community composition toward a higher abundance of early-successional species.

Highlights

Changes in the species composition of seagrass communities have been primarily attributed to bottom-up control mechanisms, such as resource availability (Touchette & Burkholder, 2000; Ralph et al, 2007), with top-down mechanisms, such as herbivory, only playing a minor role
We found changes in the species composition and pore-water ammonium content with increasing grazing pressure, suggesting that the continuous removal of above-ground biomass of the dominant T. testudinum enhances nutrient availability, which in turn drives the shift in the species composition of the vegetation (Fig. 5)
When turtles reach a density beyond the carrying capacity for turtle grass recovery, they drive species replacement of a whole seagrass bed toward a higher dominance of faster-growing and early successional seagrasses (Molina-Hernández & Van Tussenbroek, 2014), inducing a pattern shift through meadows in lower successional stages (Kelkar et al, 2013) or cause complete loss of the meadow (Fourqurean et al, 2010)

Summary

Introduction

Changes in the species composition of seagrass communities have been primarily attributed to bottom-up control mechanisms, such as resource availability (Touchette & Burkholder, 2000; Ralph et al, 2007), with top-down mechanisms, such as herbivory, only playing a minor role. Conservation strategies have led to local increases in abundance of green turtles (Chelonia mydas), with noticeable impacts of turtle grazing on seagrass communities (Zieman, Iverson & Ogden, 1984; Kaladharan et al, 2013; Kelkar et al, 2013; Molina-Hernández & Van Tussenbroek, 2014). Herbivores, large herbivores such as green turtles, can alter plant species composition and community structure (Lal et al, 2010; Kelkar et al, 2013; Molina-Hernández & Van Tussenbroek, 2014)

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