Seagrass coastal protection services reduced by invasive species expansion and megaherbivore grazing

1. Seagrasses provide an important ecosystem service by creating a stable erosion-resistant seabed that contributes to effective coastal protection. Variable morphologies and life history strategies, however, are likely to impact the sediment stabilisation capacity of different seagrass species. We question how opportunistic invasive species and increasing grazing by megaherbivores may alter sediment stabilisation services provided by established seagrass meadows, using the Caribbean as a case study.

2. Utilising two portable field-flumes that simulate unidirectional and oscillatory flow regimes, we compared the sediment stabilisation capacity of natural seagrass meadows in situ under current- and wave-dominated regimes. Monospecific patches of a native (Thalassia testudinum) and an invasive (Halophila stipulacea) seagrass species were compared, along with the effect of three levels of megaherbivore grazing on T. testudinum: ungrazed, lightly-grazed and intensively-grazed.

3. For both hydrodynamic regimes, the long-leaved, dense meadows of the climax species, T. testudinum, provided the highest stabilisation. However, the loss of above-ground biomass by intensive grazing reduced the capacity of the native seagrass to stabilise the surface sediment. Caribbean seagrass meadows are presently threatened by the rapid spread of the invasive opportunistic seagrass, H. stipulacea. The dense meadows of H. stipulacea were found to accumulate fine sediment, and thereby, appear to be effective in reducing bottom shear stress during calm periods. This fine sediment within the invasive meadows, however, is easily resuspended by hydrodynamic forces, and the low below-ground biomass of H. stipulacea make it susceptible to uprooting during storm events, potentially leaving large regions vulnerable to erosion. Overall, this present study highlights that intensive megaherbivore grazing and opportunistic invasive species threaten the coastal protection services provided by mildly grazed native species.

4. Synthesis: Seagrass meadows of dense, long-leaved species stabilise the sediment surface and maintain the seabed integrity, thereby contributing to coastal protection. These services are threatened by intensive megaherbivore grazing, which reduces the stability of the surface sediment, and opportunistic invasive species, which are susceptible to uprooting in storms and thereby can leave the seabed vulnerable to erosion.

Methods
This dataset contains data collected in Lac Bay, Bonaire where the critical erosion threshold was measured within vegetation patches of varying seagrass species and different levels of grazing.

Erosion threshold measurements: Measurements were conducted with field flumes that simulated unidirectional and oscillatory flow. The critical erosion threshold was considered the flow velocity at which sediment began to move.

Sediment grain size analysis: Sediment samples were freeze-dried and sieved through a one mm sieve, sediment grain size distribution was measured by laser diffraction on a Malvern Mastersizer 2000 (McCave and others 1986).

Biomass: Five replicate cores of 15 cm (for Thalassia) and 10 cm for (Halophila) diameter and length were taken from each vegetation patch. Sediment was washed from the biomass, and the biomass was separated into above-ground biomass (leaves and shoots), and below-ground biomass (roots and rhizomes). The biomass was dried in a 60°C drying oven and weighed.

Seagrass bendability: Seagrass shoots were placed in the fume with the broadest part of the leaf positioned adjacent to the flow, to allow for the natural bending direction of the seagrass. The flow within the flume was increased at 10 cm s^-1 increments, from 0 - 50 cm s^-1, and a digital photograph was taken of the individuals at each flow speed. Using ImageJ 1.50i, a straight line was drawn from the base of the stem to the most outward part of the thallus, and the angle of this line was recorded as the bending angle of the individual. Bending angles are presented relative to the angle of the thallus at 0 cm s^-1.

Seagrass leaf strength: Three to four undamaged leaves of the four seagrass types were measured with a tensometer (Instron Corporation, Canton, MA, USA, precision ± 0.5%). Seagrass blades were individually clamped into Instron screw side action grips (Cat. No. 2710-102), with the mountings 20 mm apart. The leaves of the seagrass were stretched at a velocity of 5 mm min^-1, while the force (measured in Newton, N) was recorded every 0.1 seconds until the blade broke. The maximum force (N) that the blades could bear before breaking was recorded and reported as the ultimate strength.