Macrophyte communities in the Peel-Harvey Estuary: Historical trends and current patterns in biomass and distribution

Estuaries are significant coastal environments and amongst the most productive ecosystems. However, anthropogenic activities have led to widespread degradation of estuaries and loss of ecosystem function. Eutrophication, a major driver for these changes, caused widespread loss of seagrass and significant blooms of macroalgae and phytoplankton. This study determined the spatial and temporal dynamics of macrophyte communities over a forty-year period (1978- 2018) in the Peel- Harvey Estuary, a hydrologically modified eutrophic estuary in south-western Australia. Analyses revealed a progressive decline in macroalgal biomass and an associated increase in seagrass biomass over the examined periods. The seagrass Ruppia became the dominant macrophyte in the system and expanded into previously unvegetated areas in the southern Harvey Estuary. The observed changes in macrophyte community composition were correlated with declining total nitrogen concentrations over time in those regions of the estuary furthest from the rivers. While these effects partly reflect improved water clarity and flushing of nutrients following the opening of an artificial channel to the ocean, they are likely also influenced by changes in river flow patterns caused by climate change. Although the overall seagrass expansion and decline in macroalgal biomass can be perceived as signals of improved estuary health, areas close to the river mouths remain exposed to higher nutrient loads. Significant accumulations of the green algae Willeella brachyclados (formerly Cladophora montagneana) in these areas underpin the potential vulnerability of the system to algal blooms and seagrass decline. Future research and monitoring is required to understand the potential threats to macrophyte communities in the Peel-Harvey Estuary from ongoing climate change