DOI: 10.5061/dryad.hmgqnk9d2
title: Data from: Seagrass coastal protection services reduced by invasive species expansion and megaherbivore grazing

description: 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.
description: 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.
description: Readme for: Data presented in the paper “ Seagrass coastal
 protection services reduced by invasive species expansion and
 megaherbivore grazing” ***General
 Introduction*** 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. The dataset is split into csv files containing
 data from within each measured patch type (critical erosion threshold and
 grain size) and data about the dominant vegetation types (biomass, leaf
 bending properties, leaf strength). Seagrass species
 measured were: Ungrazed Thalassia testudinum (T. testudinum),
 intermediately grazed T. testudinum intensively grazed T. testudinum,
 and Halophila stipulacea (H. Stipulacea)  
 ***Individual file information*** File:
 JamesEtAl_SGSedimentStabilisation_Bonaire_Flume General
 Info: Measures of critical erosions thresholds within different vegetation
 types in Lac Bay, Bonaire. Column headings:
 Code: Code used in manuscript Species:
 Dominant vegetation type Replicate: Replicate
 number Flume type: Either unidirectional flow flume or
 oscillatory flow flume water_depth_cm: Depth that
 measurements were conducted (cm) Canopy_length_cm:
 Average height of the vegetation canopy at measurement position in
 (cm) %_plant_cover: Estimated percent cover of all
 vegetation at the measurement position (%) SGVol:
 canopy height (m) multiplied by plant cover (%)
 Uni_CritVel: Unidirectional flow velocity at which sediment began
 to move (cm/s) Wave_CritVel|: Oscillatory flow velocity
 at which sediment began to move (cm/s)  
 File:
 JamesEtAl_SGSedimentStabilisation_Bonaire_GrainSize
 General Info: The grain size distribution of the sediment
 collected within the different measurement positions.<br> Column
 headings:<br> Sample: Dominant vegetation at measuring
 position<br> SD(0.9)_2 (μm): 90% grains are bigger than this
 (μm)<br> SD50MUM_2 (μm): Median grain size (μm)<br> SD50PHIM_2
 (phi): median grain size (phi)<br> SMODE_2 (μm): Modus grain size
 (μm) SCOARSE%_500-1000μm (%): percent of grains
 500-1000 μm in size<br> SMEDIUM%_250-500μm (%): percent of grains
 250-500 μm in size SFINES%_125-250μm (%): percent of
 grains 125-250 μm in size SVFINES%_63-125 (%): percent
 of grains 62.5-125 μm in size<br> SSILT63_0-63 (%): percent of
 grains less than 63 μm in size  
 File: JamesEtAl_SGSedimentStabilisation_Bonaire_Biomass
 General: Biomass of the different parts of the most dominant
 vegetation types within Lac Bay, Bonaire.<br> Column
 headings: Species: Species of vegetation
 Part: The part of the plant rep:
 Replicate g_DW_m2: Dry weight per m2  (g
 ,m^-2)   File:
 JamesEtAl_SGSedimentStabilisation_Bonaire_Bending
 General: The angle of bending of the dominant vegetation types in
 increasing unidirectional flow speeds. Column
 headings: Species: Species of vegetation
 0cm/s: Bending angle of plant at a flow of 0 cm/s
 10cm/s: Bending angle of plant at a flow of 10 cm/s
 20cm/s: Bending angle of plant at a flow of 20 cm/s
 30cm/s: Bending angle of plant at a flow of 30 cm/s
 40cm/s: Bending angle of plant at a flow of 40 cm/s<br>
 50cm/s: Bending angle of plant at a flow of 50 cm/s
   File:
 JamesEtAl_SGSedimentStabilisation_Bonaire_Strength
 General: Breaking point of the leaves of the dominant seagrass
 species at Lac Bay, Bonaire. Column headings:
 Species: Species of vegetation No: Replicate
 number Max load (kgs): Maximum load in kgs that the
 leaf withstood before breaking (kgs) Max load (N):
 Maximum load in Newtons that the leaf withstood before breaking
 (N)