Dugong and Turtle seagrass habitats in the North-West Torres Strait

Brief description

This dataset describes seagrass at 34 individual meadows from surveys of Dugong and Turtle habitats in the North-West Torres Strait for November 2015 and January 2016. The data includes information on seagrass species, biomass, diversity, and BMI and algae percent cover.

This meadow (polygon) layer provides summary information for all survey sites within the 34 individual seagrass meadows mapped in 2015-2016 with information including individual meadow ID, meadow location (intertidal/shallow subtidal/subtidal), meadow density based on mean biomass, meadow area, dominant seagrass species, seagrass species present, survey dates, survey method, and data custodian. ESRI and Landsat satellite image basemaps were used as background source data to check meadow and site boundaries, and re-map where required.

The data described by this record is current as of 01/12/2016 for use in the Seamap Australia project. Newer versions of the data, additional 'point' data for 853 sites, and alternative download formats are available from eAtlas. http://eatlas.org.au/geonetwork/srv/eng/metadata.show?uuid=034ce816-0777-4bbd-aefc-8b73bd540245

Lineage

Statement: The sampling methods used to study, describe and monitor 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 - Latitude and longitude was recorded by GPS at each site. Depth was recorded when sampling by boat and converted to depth below mean sea level (dbMSL) in metres.

2 Seagrass metrics - Above-ground biomass was determined using a “visual estimates of biomass” technique (Mellors, 1991) using trained observers. A linear regression was calculated for the relationship between the observer ranks and the harvested values. This regression was used to calculate above-ground biomass for all estimated ranks made from the survey sites. Biomass ranks were then converted into above-ground biomass estimates in grams dry weight per square metre (gdw m-2). Observers ranked seagrass biomass, and the percent contribution of each species to that biomass, using video transects, grabs, free divers, and helicopter:
* Video transect: Commonly used for subtidal meadows sampled from larger boats. At each transect site an underwater CCTV camera system was lowered from the vessel to the bottom. For each transect the camera was towed at drift speed (less than one knot) for approximately 100m. Footage was observed on a TV monitor and digitally recorded. The video was paused at ten random time frames and an observer ranked seagrass biomass and species composition. On completion of the video analysis, the video observer ranked five additional quadrats that had been previously videoed for calibration. These quadrats were videoed in front of a stationary camera, then harvested, dried and weighed.
* Helicopter: Commonly used for intertidal surveys. At each site seagrass above-ground biomass and species composition were estimated from three 0.25 m2 quadrats placed randomly within a 10m2 circular area. Seagrass percent cover and sediment type were recorded at each site. The “visual estimates of biomass” technique when applied to helicopter surveys (and free diving/camera drops – see below) involves ranking while referring to a series of quadrat photographs of similar seagrass habitats for which the above-ground biomass has previously been measured. Three separate biomass scales were used: low-biomass, high-biomass, and Enhalus-biomass. The relative proportion (percentage) of the above-ground biomass of each seagrass species within each survey quadrat was also recorded. Field biomass ranks were converted into above-ground biomass estimates in grams dry weight per square metre (gdw m-2). At the completion of sampling each observer ranked a series of calibration quadrats as per video transect surveys.
* Camera drop/free diving: Commonly used for shallow subtidal meadows sampled from a small boat. Sampling follows the same protocol as helicopter surveys but the three quadrats were either assessed by a free diver with quadrat, or by an underwater CCTV camera system camera attached to a frame. Video footage was observed on a TV monitor and seagrass ranked in real time, with the camera frame serving as a quadrat.
* van Veen grab: Commonly used for shallow subtidal meadows sampled from a small boat in conjunction with camera drops, or to record seagrass presence/absence where visibility was too poor for camera drops. A sample of seagrass was collected using a van Veen grab (grab area 0.0625 m2) to identify species present at each site. Species identified from the grab sample were used to inform species composition assessments made from the video drops (Kuo et al., 1989).

3 Benthic macro-invertebrates - A visual estimate of benthic macro-invertebrate (BMI) percent cover was recorded at each shallow subtidal and intertidal site according to four broad taxonomic groups:
* Hard corals – All scleractinian corals including massive, branching, tabular, digitate and mushroom.
* Soft corals – All alcyonarian corals, i.e. corals lacking a hard limestone skeleton.
* Sponges
* Other BMI – Any other BMI identified, e.g. hydroids, ascidians, barnacles, oysters, molluscs. 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 shallow subtidal and intertidal site. When present, algae were categorised into five functional groups and the percent contribution of each functional group was estimated:
* Erect macrophytes – 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.

Statement: Data Dictionary: * Meadow location was classed according to whether meadows were intertidal (all sites surveyed by helicopter), shallow subtidal (generally an extension of an intertidal meadow into shallow waters <5m deep), or subtidal (no intertidal sites adjoining the meadow). * Seagrass community types were determined according to species composition within a 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 (see Table 1 "Nomenclature for seagrass community types" in final report). This nomenclature also included a measure of meadow density categories (light, moderate, dense) determined by mean biomass of the dominant species within the meadow (see Table 2 "Density categories and mean above-ground biomass ranges for each species used in determining seagrass community density" in final report). Further information can be found in this publication: Carter, A. B. and Rasheed, M. A. (2016) Assessment of Key Dugong and Turtle Seagrass Resources in North-west Torres Strait. Report to the National Environmental Science Programme and Torres Strait Regional Authority. Reef and Rainforest Research Centre Limited, Cairns (40 pp).