Phytoplankton Response to Changes in Water Quality and a Flood Event in Hervey Bay, Queensland
Authors: Milham-Scott Deborah 
Published: June 2021

Dynamic, integrated and complex processes dominate nearshore marine ecosystems. Industrial, agricultural and population growth delivers pollution and excess nutrient and sediment loads to nearshore marine zones. Climate change also poses serious threats to the delicate balance that exists between and within marine ecosystem processes. Methods to monitor, characterise and model nearshore marine systems have been developed to predict responses to changes in the physical, chemical and biological parameters that constitute elements of a model. Such models are useful to management authorities as they provide a tool to mitigate adverse impacts to marine ecosystems. Phytoplankton community structure provides a sensitive early warning for change to marine ecosystems. It is also a vital component of models derived to determine light attenuation and requirements for healthy seagrass and coral habitats. The multifactorial physicochemical drivers of change to marine microalgae abundance and community structure were investigated in Hervey Bay following two floods, a cyclone and destruction of 1000 km2 of seagrass. A water quality gradient from point sources to offshore sites was identified. Correlations between chlorophyll a (Chl a) and both soluble reactive phosphate (SRP) and total suspended solids was revealed. Diatoms dominated the phytoplankton community structure with seasonal and site differences detected amongst 150 phytoplankton species. Chl a, Secchi depth, SRP and pH maximised the rank correlation with phytoplankton assemblage structure (ρ = 0.63) to best ‘explain’ the link between water quality and phytoplankton (BIOENV). Indicator species include Cylindrotheca closterium for the nearshore degraded sites and Rhizosolenia sp. and Guinardia sp. (G. flaccida and G. striatula) for the offshore more pristine habitats. Following the flood in February 1995, Cylindrotheca closterium, Pseudonitzschia sp. and Skeletonema costatum bloomed at Pulgul Creek (a treated sewage effluent site), and Thalassionema frauenfeldii, Cerataulina bicornis and dinoflagellates bloomed at the mouth of the Mary River (agricultural runoff). Discharge-driven flood impact and trigger values were analysed to assist management authorities in their decision making around potential seafood biotoxin problems associated with harmful algal blooms (HAB’s). Results highlight the importance of species determination rather than Chl a assessment when evaluating the health of an ecosystem. Spring blooms of Trichodesium sp. indicate an additional nutrient source during high traffic of commercial whale watching vessels. Flood impacts highlight the need to schedule dredging, construction and other land management and development activities outside of flood periods. Commercial fishing and swimming should be avoided following a flood due to the health risks associated with toxic phytoplankton species (HAB’s) and for the health and safety of swimmers, consumers and other aquatic species. Future research to replicate this study in Hervey Bay and in other estuaries holds much value to further explore the phytoplankton species identified as bio-indicators of ecosystem health in the present study. Research yet to be published, addresses the links between water quality and seagrass epiphytes, light attenuation and the habitat requirements of seagrass in Hervey Bay.