Depletion of suspended particulate matter over coastal reef communities dominated by zooxanthellate soft corals, Great Barrier Reef

Measurements were carried out at the southern ends of the Pelorus and Fantome Channels (between 2 adjacent islands in the Palms Group) where the reefs are 50-150 m wide and over the sand bottom (more towards the middle of the channels).

Current meters were deployed at 6 m depth (averaging 120 readings every 10th minute over 3-4 days. Relative flow was measured using plaster blocks: 2 blocks beneath each current meter, and 10 blocks across each reef perpendicular to the flow left for 24 hr (2 tidal cycles).

Water samples were taken from 5-8 m depths upstream of the reefs, repeated downstream, and at the sandy sites (these at 7-10 m). Sampling was repeated 64 times in 4 sets of 5-6 days each month. September and November samples were affected by the cyanobacterium Trichodesmium. September samples were not prefiltered and standard deviations between duplicate chlorophyll samples were much higher than other months. November samples were filtered through a 10 µm mesh to remove Trichodesmium - particlate matter data therefore was <10 µm in size. Chlorophyll and phaeopigments were analysed. In July, additional filtering processes allowed the contributions of pico (<2 µm) and nanoplankton (2-10 µm) to be assessed.

Percent cover and abundances of individual taxa were assessed with 4 x 50 transects over both channel reefs. Rapid ecological assessments (REAs, 48 swims of 10-15 mins) were used for comparison of the macrobenthos and physical parameters on reef (25 REAs) and sand (23 REAs). Percent total cover of alcyoniid and other soft corals, hard corals, turf and macro algae, sand and unconsolidated rubble were estimated. Inventories of genera of soft corals, gorgonians, sponges, tunicates and zoanthids (growth forms and phyla) were complied and abundances estimated using a ranked scale (0=absent, 1=1 or few colonies, 2=uncommon, 3=moderately common, 4=common, 5=dominant).

The standing crop of soft corals was assesed by measuring the projected areas and colony volumes of individuals in each of the 6 most common soft coral genera. Soft coral biomass per m2 was estimated by collecting 15 specimens each of the 2 dominant soft coral species (Sinularia flexibilis and S. capitalis) after measuring their projected areas and height. These were then dried to constant weight ashed and then the ash-free weight determined (the relationship between the AFDW and colony area was linear in both taxa).

Concentrations of phytoplankton pigments and suspended particulate nutrients were measured (in µg 1-1). Net removal of chlorophyll (as a marker for phytoplankton), particulate organic carbon (for determining energy gain), and particulate phosphorus and nitrogen were estimated.