WAMSI 2 - Dredging Science Node - Project 5.3 - Natural Dynamics

Brief description

The Theme 5.3 study was undertaken to improve our understanding of the spatial and temporal patterns in seagrass composition, abundance and reproductive phenology in the Pilbara region. Key environmental parameters, especially light, that influence seagrass survival and can be altered by dredging were also characterised.

From August 2013 to March 2015 (18 months), surveys of seagrass abundance were undertaken in the Exmouth Gulf region. The locations surveyed (South Muiron Island, Bundegi and Exmouth Gulf) encompassed a range in water clarity from clear to turbid. Less frequent surveys were undertaken at other locations in the Pilbara: Thevenard Island, Rosemary Island and Balla. Measurements of seagrass abundance were also obtained from monitoring conducted as part of the dredging and dredge-spoil management plan for Chevron Australia’s Wheatstone Liquefied Natural Gas (LNG) Project.

During each survey measurements were made in the field, photographs were taken or samples were collected in order to measure the following variables:
1. percentage cover of seagrass and other benthos;
2. above- and below-ground biomass, shoot density, leaf length, leaf width and number of flowers;
3. density of seagrass seeds;
4. sediment grain size;
5. stable isotope ratios (13C and 15N) of seagrass leaf tissue; and
6. water quality (light, conductivity, temperature, salinity, nutrients, suspended particulate matter and chlorophyll).

Lineage

Surveys were conducted every three months from August 2013 to March 2015 at three locations: South Muiron Island, Bundegi, and Exmouth Gulf. At each location, two sites separated by about 200 m were surveyed. In November 2014, the surveys were expanded to include Thevenard Island, approximately 100 km northeast of Exmouth. Opportunistic surveys at Rosemary Island and Balla Balla during November-December 2013 provided an opportunity to evaluate whether species composition and abundance at places we surveyed regularly were similar to those found elsewhere in the Pilbara.

At each site the following activities were conducted:
1. Photographic transects to estimate percentage cover of seagrass and other benthos.
2. Cores (11 cm diameter) to provide estimates of above and below ground biomass, shoot density, leaf length, leaf width and flower counts.
3. Representative samples of each seagrass species for stable isotope analysis;
4. An RBR Concerto submersible Conductivity, Temperature and Depth recorder with a Licor 192SA PAR sensor (fitted with a Zebra-Tech Hydro-Wiper H) was moored approximately 15 cm above the substrate, and programmed to record parameters every 30 seconds; data were downloaded during each survey.
5. Additional core samples (11 cm diameter to 5 cm depth where possible) to provide estimates of seagrass seed density;
6. Core samples (2.5 cm diameter to 5 cm depth) for sediment grain size analysis;
7. Water samples were collected for measurement of total suspended solids (TSS), nutrients, and chlorophyll.
Data Workflows:
Photographic transects
* 5 x 50 m transect
* photographs taken of 0.25mx0.25m quadrat every 2m
* photos uploaded and georeferenced and renamed using JetPhoto Studio software
* A 20 point dot grid (4x5 dots) was transposed onto the photographs in TransectMeasure ® and attributes identified. (attribute table was constructed and applied to the photos. Attributes included appropriate substrates, seagrass spp, macroalgal type, biota, bioturbation)
* TransectMeasure ® output (txt file) uploaded into open source software, R Studio, for graphing and statistics
Biomass
* 5 x11cm cores of all present seagrass species were collected
* each sample was labeled and frozen (-20oC)
* samples were defrosted in the laboratory at ECU
* seagrass was cleaned of sediment and epiphytes collected by careful scrapping of the leaves
* each individual leaf was laid out and a photograph of all the sample leaves taken
* the sample was divided into its different plant parts (above ground biomass (leaves, petioles and sheaths), vertical rhizome (if appropriate), horizontal rhizome and roots)
* plants parts were dried at 60oC for 24 hours or until dry and weighed
* the photos were uploaded to Jetphoto Studio and renamed
* using ImageJ software, the length, width and area of each leaf was measured.
* All information was input into an excel spreadsheet for graphing and statistical analysis in the open source software, R Studio
Nutrient/Isotopes
* 5 hand grabs of each present seagrass species were collected
* each sample was labeled and frozen (-20oC)
* samples were defrosted in the laboratory at ECU
* seagrass was cleaned of sediment
* cleaner, younger leaves were chosen and placed in a microtube
* if the leaves had epiphytes, they were scraped clean using a razor blade
* the samples were dried in an oven at 60oC for 24 hours or until dry
* each sample was then ground using a MM-200 ball and mill grinder to a fine powder
* 1.1-1.2 mg of sample were weighed out into a tin capsule and sent to UWA Isotope facility for analyses.
* All information was input into an excel spreadsheet for graphing and statistical analysis in the open source software, R Studio
Sediment Grain Size /LOI
* 5 x 2.5-5cm depth sediment cores were collected
* each sample was frozen (-20oC)
* samples were defrosted in the laboratory at ECU
* each sample was dried at 60oC for 48hours or until dry
* 3 of the 5 replicates wer