This study was carried out at Davies Reef in the central Great Barrier Reef region, and measured for algal biomass and primary productivity and compared to surrounding epilithic algal communities. At Davies Reef the distribution and comparative cover of damselfish territories were examined in detail.
Three automatic data-logging respirometers were deployed for 24 h in the field to measure productivity parameters - light (as irradiance, Li-cor), temperature and oxygen concentration at intervals of 1 to 255 min. Primary productivity was estimated from diel patterns in oxygen flux using data from: photosynthesis-irradiance relation (Ik, Icomp and alpha); rates of photosynthesis (P) and respiration (R); primary productivity expressed as the area1 gross (Pg) and net (Pna) rates and as the net turnover (Pnb) of community biomass (B, g C m-¹). These results were compared to the parameters: damselfish species (Plectroglyphididon lacrymatus, Stegastes apicalis, S.fasciolatus, S. nigricans); reef location; date; inside/outside fish territories.
At Davies Reef two experiments examined damselfish territoriality impacts on algae in detail. Algal productivity inside and adjacent to the territories of 2 species was measured using square blocks (8 x 8 x 2 cm) of Porites which had been attached to the respective reef substrata for a period of 12 months prior. In the second experiment, coral blocks with established algal covering from non-territory substrata were caged with wire mesh to exclude all large grazers for 30 d, then the productivity of the increased algal biomass on these blocks was measured by respirometry, uncaged blocks were measured at the same time. The surface area of Acropora sp. branches was calculated. Algal biomass was determined by drying coral rock samples at 60°C and scraping the surface to a depth of 2 mm. Scrapings were ground, redried, and then analysed for total organic carbon.
Four major substratum categories were surveyed: fish territories with dense mats of turf algae; closely-cropped filamentous and encrusting coralline algae; bare sand; organisms other than algae.
Fourteen sites were established along each of three sections of Davies Reef (northern, central and southern): front reef slope at depths of 10 (#l) and 5 m (#2); reef crest (#3); reef flat at 20 (#4), 60 (#5), 80 (#6), 110 (#7) and 145 m (#8) behind the crest; reef flat at lagoon edge (#9, 190 m from the crest); lagoon slope at 5 m (#10) and 10 m (#11) depths; back reef flat (#12); and back reef slope at 5 m (#13) and 10 m (#14) depths. At each of these sites, a 100 m tape transect was surveyed perpendicular to the section lines and the length of intercept to the nearest 1 cm for each major substratum category and damselfish territory was recorded.
The aims of this study were to:
To quantify differences in reef epilithic algal community metabolism.
To assess the significance of these in the context of total reef primary productivity.
Epilithic algal communities include coralline algae, turf algae (filamentous forms and thick turfs), corallines mixed with short turf.
