Western Torres Strait Seagrass Survey, Torres Strait, September 2020 (TropWATER, JCU)

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This dataset summarises benthic surveys of Western Torres, Torres Strait in December 2020 into 3 GIS shapefiles: (1) The site shapefile describes (a) seagrass presence/absence, (b) species composition, (c) algae cover and (d) benthic macro-invertebrate cover at 542 sites. (2) The meadow shapefile describes subtidal seagrass communities. (3) The interpolation shapefile describes variation in subtidal seagrass biomass across the survey area. Carter AB, McKenna SA and Shepherd L (2021), “Subtidal seagrass of western Torres Strait”, Centre for Tropical Water & Aquatic Ecosyormastem Research Report no. 21/11, James Cook University, Cairns, 36 pp. This project is a baseline survey of subtidal benthic habitats, including seagrass, algae and coral, in the Western Cluster of Torres Strait. Torres Strait’s Western Cluster is an ecologically important region due to extensive seagrass habitat, and high densities of turtle and dugong. This survey provides essential information to the TSRA, Australian and Queensland governments for dugong and turtle management plans, complementing dugong and turtle research studies in the region. The sampling methods used to study, describe and monitors seagrass meadows were developed by the TropWATER Seagrass Group and tailored to the location and habitat surveyed; these are described in detail in the relevant publications (https://research.jcu.edu.au/tropwater). 1 Location Sites were surveyed by helicopter. At each site latitude and longitude was recorded by GPS. Sediment type was recorded. 2 Seagrass metrics At each site observers estimated the percent cover of seagrass, then for three quadrats within each site using boat-based free diving or camera drop equipment, ranked seagrass biomass and estimated the percent contribution of each species to that biomass are provided. Seagrass above-ground biomass was determined using the “visual estimates of biomass” technique (Mellors 1991) using trained observers. This involves ranking seagrass biomass while referring to a series of quadrat photographs of similar seagrass habitats for which the above-ground biomass has been previously measured. Three separate biomass scales are used: low biomass, high biomass, and Enhalus biomass. The percent contribution of each seagrass species to total above-ground biomass within each quadrat is also recorded. At the completion of sampling each observer ranks a series of calibration quadrats. A linear regression is then calculated for the relationship between the observer ranks and the harvested values. This regression is used to calibrate above-ground biomass estimates for all ranks made by that observer during the survey. Biomass ranks are then converted to above-ground biomass in grams dry weight per square metre (g DW m-2). 3 Benthic macro-invertebrates At each site a visual estimate of benthic macro-invertebrate (BMI) percent cover was recorded each site according to four broad taxonomic groups: • Hard coral – All scleractinian corals including massive, branching, tabular, digitate and mushroom. • Soft coral – All alcyonarian corals, i.e. corals lacking a hard limestone skeleton. • Sponge. • Other BMI – Any other BMI identified, e.g. hydroid, ascidian, barnacle, oyster, mollusc. Other BMI are listed in the “comments” column of the GIS site layer. 4 Algae A visual estimate of algae percent cover was recorded at each site. When present, algae were categorised into five functional groups and the percent contribution of each functional group was estimated: • Erect macrophyte – Macrophytic algae with an erect growth form and high level of cellular differentiation, e.g. Sargassum, Caulerpa and Galaxaura species. • Erect calcareous – Algae with erect growth form and high level of cellular differentiation containing calcified segments, e.g. Halimeda species. • Filamentous – Thin, thread-like algae with little cellular differentiation. • Encrusting – Algae that grows in sheet-like form attached to the substrate or benthos, e.g. coralline algae. • Turf mat – Algae that forms a dense mat on the substrate. Geographic Information System (GIS) All survey data were entered into a Geographic Information System (GIS) developed for Torres Strait using ArcGIS 10.8. Rectified colour satellite imagery of Western Torress (Source: ESRI, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community), field notes and aerial photographs taken from the helicopter during surveys were used to identify geographical features, such as reef tops, channels and deep-water drop-offs, to assist in determining seagrass meadow boundaries. Three GIS layers were created to describe spatial features of the region: a site layer, seagrass meadow layer, and seagrass biomass interpolation layer. Site layer This layer contains data collected at each site, including: • Temporal details – survey date. • Spatial details – latitude/longitude. • Habitat information – sediment type; seagrass information including presence/absence and above-ground biomass (total and for each species); percent cover of seagrass, algae, hard coral, soft coral, sponges, other BMI, and open substrate; percent contribution of algae functional groups to algae cover. • Sampling method and any relevant comments. • NOTE: the column “exclude from biomass” describes sites that were not included in the main report and analysis of subtidal seagrass. These sites were located in intertidal waters. Seagrass meadow layer Seagrass presence/absence site data, mapping sites, field notes, and satellite imagery were used to construct meadow boundaries in ArcGIS®. The meadow (polygon) layer provides summary information for all sites within the meadow, including: 1. Habitat information – seagrass species present, meadow community type, meadow cover, mean meadow biomass + standard error (SE), meadow area + reliability estimate (R), and number of sites within the meadow. 2. A meadow identification number and reef name; this allows individual meadows to be compared among years. 3. Sampling methods. Meadow community type was determined according to seagrass species composition within each meadow. Species composition was based on the percent each species’ biomass contributed to mean meadow biomass. A standard nomenclature system was used to categorize each meadow (Table 1). This nomenclature also included a measure of meadow density categories (light, moderate, dense) determined by mean biomass and the dominant species within the meadow (Table 2). Mapping precision estimates (R; in metres) were based on the mapping method used for that meadow (Table 3). Mapping precision estimates ranged from 10-50m for intertidal seagrass meadows and up to 100m for meadow mapping precision estimates based on the distance between sites with and without seagrass. Mapping precision estimate was used to calculate an error buffer around each meadow; the area of this buffer is expressed as a meadow reliability estimate (R) in hectares. Meadow area and error buffers were determined in hectares using the calculate geometry function in ArcGIS. Table 1. Nomenclature for seagrass community types. Community type, (Species composition) - Species A, (Species A is 90-100% of composition) - Species A with Species B, (Species A is 60-90% of composition) - Species A with Species B/Species C, (Species A is 50% of composition) - Species A/Species B, (Species A is 40-60% of composition) Table 2. Density categories and mean above-ground biomass ranges for each species used in determining seagrass community density. - Species: H. uninervis (thin), Categories: Light (<1), Moderate (1 - 4), Dense (>4) - Species: H. ovalis, Categories: Light (<1), Moderate (1 - 5), Dense (>5) - Species: C. serrulata, C. rotundata, T. hemprichii, Categories: Light (<5), Moderate (5 - 25), Dense (>25) - Species: E. acoroides, Categories: Light (<40), Moderate (40 - 100), Dense (>100) Table 3. Mapping precision and methods for seagrass meadows. Mapping precision: Mapping method 10-20 m: - Meadow boundaries mapped in detail by GPS from helicopter - Intertidal meadows completely exposed or visible at low tide - Relatively high density of mapping and survey sites - Recent aerial photography and satellite imagery aided in mapping 50-100 m: - Meadow boundaries determined from helicopter and camera - Inshore boundaries mapped from helicopter - Offshore boundaries interpreted from survey sites and satellite imagery - Relatively high density of mapping and survey sites Seagrass biomass interpolation layer An inverse distance weighted (IDW) interpolation was applied to seagrass site data to describe spatial variation in seagrass biomass across Western Torres meadows. The interpolation was conducted in ArcMap 10.8. An assessment of seagrass condition for all of Torres Strait, can be found in this publication: Carter et al. (2021) Torres Strait Seagrass 2021 Report Card. TropWATER JCU, Report no. 21/13 Format of the data: This dataset consists of 3 shapefiles with a spatial reference of GDA94. Meadow shapefile 1. Western Torres seagrass community type 2020.lpk Includes 27 individual seagrass meadows across Western Torres mapped in 2020 with information including individual meadow ID, meadow mean seagrass biomass (g DW m-2) + SE, number of sites surveyed, seagrass cover, meadow area + R, seagrass community type, seagrass species present, survey dates, survey method, and data author. ESRI and Landsat satellite image basemaps were used as background source data to check meadow and site boundaries, and re-map where required. 2. Western Torres seagrass biomass interpolation 2020.lpk An inverse distance weighted (IDW) interpolation was applied to seagrass site data to describe spatial variation in biomass across Western Torres. Site shapefile Includes information including latitude/longitude, seagrass presence/absence, algae and benthic macro-invertebrate percent cover, percent cover of algae functional groups, individual seagrass species biomass, survey date, survey method, and data author. These shapefiles have been presented as 4 layer packages based on symbology from specific columns: 1. Western Torres seagrass present absent 2020.lpk 2. Western Torres seagrass composition 2020.lpk 3. Western Torres algae cover 2020.lpk 4. Western Torres benthic macro-invertebrates 2020.lpk