Benthic habitat characterisation of Montgomery Reef, Kimberley region, Western Australia

Brief description

The abundance and distribution of benthic organisms and communities at both broad and fine scales were surveyed at Montgomery Reef in March and July 2009. Due to the logistical constraints placed on sampling techniques by the macrotidal changes around Montgomery Reef, three sampling techniques were employed to obtain imagery of the benthic environment between the depths of 0 and 53 metres.Reef Walks:With much of the reef edge exposed at low tide, the shallowest sections of the reef (0 to ~6 metres) could be sampled whilst walking across the exposed reef. Digital cameras (Ricoh GX100) were held at 1~1.2 metres above the reef with 1 photograph taken approximately every 2 metres. Teams of researchers walked in parallel, with 3-4 camera deployed simultaneously at a site. Each camera was synchronised with a GPS providing accurate positional information (2 - 5 metres) from which each image could later be georeferenced. Reef walks were conducted to the northeast, southeast, northwest and southwest. Tripod Camera:A simple tripod camera system developed by A. Heyward was trialled to sample the turbid shallow waters between 2 and 16 metres. Each system consisted of a camera tripod with a downward facing digital camera (Ricoh GX100) attached 600mm above the ground and set to photograph every 5 seconds. Each camera was synchronised with a GPS providing accurate positional information from which individual images could later be georeferenced. The tripod systems were attached to 20 metres of rope and lowered off the side of a tender to the bottom and held for a minimum of 5 seconds to allow the camera to complete at least one image while the tripod was located firmly on the substrate. It was noted that using underwater strobes generally reduced image quality. As a result the strobes were turned off and only natural light was relied on for photography. During retrieval the tender was allowed to drift with prevailing wind and tide, with images of the benthos effectively taken every 2 to 5 metres. The start of each transect position allowed the length of each drift to be monitored, so that transects of a nominal length, often around 200m, could be completed before moving the tender to a new location. The advantage of the technique is that even in very turbid water, high resolution digital stills are obtained with the stability afforded by the tripod allowing for cameras to be set with a longer exposure letting in maximum light.Towed Video:The AIMS towed video system was used to survey the deeper water (between 10 and 53 metres) to the north and south of the reef. The system was flown at approximately 1 metre above the sea floor with real time analysis of the video footage (AIMS Towvid) used to examine the broad scale distribution of dominant benthic biota. In addition to video footage, a downward facing digital still camera was attached to the bottom of the towed body. The camera was set to photograph the benthos approximately every 5 metres. The high resolution images allowed for a more detailed assessment of the benthic community. The objectives of the research were three-fold:1. to provide a broad- and fine-scale characterisation the benthic communities and substratum at Montgomery Reef2. to identify similarities and differences in the benthic communities examined and3. to undertake a detailed spatial analysis to examine and map the distribution of these benthic organisms and communities.This research was the first detailed quantitative survey of the benthic communities of Montgomery Reef. Univariate, multivariate and spatial statistics are employed to examine both the similarities and differences in the distribution and relative abundance of these benthic communities. This study is part of a broader regional survey of the Kimberley fringing reefs initiated by AIMS in 2009.

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Statement: Processing and georeferencing digital images: The digital images from all three sampling techniques were sorted with unusable (i.e. poor resolution) images removed. The remaining 12,011 images were processed using ACDSee Pro v 8.1 to optimise exposure. Reef walking images and tripod camera images were georeferenced using RoboGEO v5.6.6. This program automatically links the GPS coordinate from a corresponding tracklog to the image time stamp providing positional accuracy of between 2 and 5 meters. Images collected using towed video were georeferenced using the GPS position of the boat. The towed body itself was positioned between 39.8 to 81.8 metres behind the boat. These positions will require post processing to adjust for the layback in the towed video cable before data may be used for precise spatial modelling, but are indicative of the general survey locations.Sampling strategy:Of the 12,011 digital images collected at Montgomery Reef, 992 were selected for more detailed analysis. These included 324 reef walk images, 308 tripod camera images and 360 towed video images. The selection of reef walk images and tripod camera images was based upon a stratified random selection, as no previous information was available on the benthic communities sampled. The selection was stratified by site, with 25 images randomly selected within each reef walking site and 20 images randomly selected within each tripod camera site. In contrast to the reef walks and tripod camera transects, the real time towed video analysis provided information on the substrate, benthos and organisms present. These three categories were combined with a maximum of 30 images randomly selected from each unique combined benthic group to be analysed.Substrate classification used for the analysis:Consolidated substrate:Reef (high relief > 1m)Reef (low relief)Sand inundated reefUnconsolidated substrate:Rocks (>15cm)Rubble (1.5-15.0cm)Coarse sand (0.5-1.0cm)Fine sand (0.2-0.5cm)Silt (mud) (Sand ripplesBioturbatedHierarchical benthic classification used for the analysis:Live coral cover:Branching AcroporaDigitate AcroporaTabulate AcroporaBranching non-AcroporaFoliaceousMassiveSubmassive (flattened)Submassive (protrusions)EncrustingMushroomNew recruitsRecently dead (white)Filter feeders:Soft coralSpongesSea whipGorgonian (branching/fan)Sea penSea anemoneAscidianHydroidBryzoanSea starAlgae:Red macroalgaeGreen macroalgaeBrown macroalgaeHalimedaCalcareous red macroalgaeEncrusting coralline algaeTurf algaeFilamentous algaeRhodolithsSeagrass

Notes

Credit
Heyward, Andrew J, Dr (Principal Investigator)