Phyto-pigment composition in a GBR World Heritage Area affected by Fitzroy outflow

• Research undertaken in southern Great Barrier Reef World Heritage Area.
• Profound water quality changes at Capricorn Coast associated with Fitzroy River outflows.
• Phyto-pigment composition related to water quality using multivariate analyses.
• Phyto-pigments depict presence of cyanobacteria, diatoms, and green flagellates.
• DSi:DIN foremost in defining phytoplankton assemblages indicated by phyto-pigments.

This research reports on the phyto-pigment composition (by High Performance Liquid Chromatography) within the southern Great Barrier Reef World Heritage Area at the end of a 15-year dry spell, and then during and after an intervening flood event. The study period covers the wet season of 2007–08 and involves four main estuaries of the Capricorn Coast, namely Corio Bay, Ross Creek, the Causeway Lake and Cawarral Creek. These flow into the southern Great Barrier Reef World Heritage Area (GBRWHA) and are situated about 20–60 km north of where the Fitzroy River estuary meets the sea in north eastern Australia. The four estuaries were sampled on three occasions, during the study period, while one estuary was sampled continuously on a fortnightly basis for water quality and phyto-pigment concentrations. Phyto-pigment concentrations indicated the relative presence of different phytoplankton functional groups such as diatoms (fucoxanthin), cyanobacteria (zeaxanthin) and green flagellates (chlorophyll bb). Trends in phytoplankton assemblages were analysed using multivariate analyses, including the relationship between phyto-pigment composition and water quality. Overall, cyanobacteria were dominant in spring and summer before large river outflows stimulated diatoms, and green flagellates followed in the cooler and lower-nitrogen waters of autumn. We propose that large Fitzroy River outflows have profound biogeochemical importance for the Capricorn Coast and can cause significant phytoplankton shifts in this world heritage area. Moreover, this study provides a valuable contribution towards understanding the presence/absence of phytoplankton functional groups with extreme climate conditions in this southern GBRWHA region.