Candidate bioindicator measures to monitor exposure to changing water quality on the Great Barrier Reef. Interim Report

Spatial variation of water column characteristics, sediment and irradiance on coral reefs was examined in the Whitsunday region of the Great Barrier Reef during five sampling events between 2004 and 2006 (Fig. 2.1). Sampling locations were selected along a transect from outer reefs distant from terrestrial inputs, to coastal reefs near the Australian mainland. Most of the water column variables (especially chlorophyll a, total suspended solids, particulate organic carbon and particulate nutrients) and irradiance variables (Secchi and optical depth) changed significantly along the transect. For example, concentrations of chlorophyll a and total suspended solids increased twoand three-fold, respectively, from outer to coastal locations, while sediment inorganic carbon decreased and sediment colour became darker. Similarly, Secchi and optical depth, two measures of water transparency, decreased approximately three-fold towards the coast. Most of these gradients were persistent over the five sampling events, but for several parameters, the magnitude and steepness of the gradient changed. The maximum depth of zooxanthellate corals increased five-fold along the gradient and was related significantly to a water quality index derived for the Whitsunday Islands. Our data of the maximal depth limit for reef development at locations where suitable settlement substrata were available suggest that the absolute minimum of light required for a reef to persist is in the range of 6-8% of surface irradiance in the Whitsunday Islands.