The Queensland Government has identified three river systems – the Flinders, Gilbert and Mitchell rivers in the Gulf of Carpentaria region – where water development for irrigated agriculture is already occurring or likely to occur. Information on the water needs of estuarine ecosystems is needed to ensure that water development is done in a sustainable way, and that economic and social trade-offs between agriculture and the downstream environment are well understood and made transparent.

The southern Gulf has been identified as a key Australian site for migratory shorebirds (Garnett, 1989; Driscoll, 1997, 2001, 2014). Migratory shorebirds use the Gulf intertidal flats for feeding on macrobenthos during summer months. However, there have been very few surveys of shorebirds in the Gulf (Driscoll, 2014). Additionally, little is known of the scale of the macrobenthic food supply, and the likely impact of water development on the sustainability of this food supply.

This National Environmental Science Program (NESP) project therefore examined the links between freshwater flow and estuarine and coastal productivity as a mechanism for understanding how changes in flow may impact shorebirds and their food supply in estuaries and adjacent coastal environments. A range of approaches were used, including measuring coastal and estuarine primary productivity and macrobenthos densities, based on sampling during a number of field trips. The premise is that this productivity is the fundamental underpinning of the resource needs of shorebirds. Additionally, shorebird surveys were conducted to assess the importance of the Flinders, Gilbert and Mitchell estuaries and nearshore as feeding sites for birds. This study complemented NESP Northern Australia Environmental Resources (NAER) Hub project 1.4 (Burford et al., 2020) which examined freshwater inputs to the three Gulf estuaries in the context of fisheries species, and some productivity data overlaps with outputs from project 1.4. The primary productivity and macrobenthos components of the research are presented as published (Burford & Faggotter, 2021) and submitted papers (Lowe et al., submitted ECSS; Venarsky et al., submitted ECSS).

This study showed that freshwater flows had a number of impacts on primary productivity in the Flinders, Gilbert and Mitchell river estuaries and nearshore environment. In the short term, when wet season flooding was occurring, chlorophyll a concentrations in the water column and intertidal flats decreased as salinities decreased. This is because low salinity has negative effects on the physiology of the marine algal species in estuaries. However, low salinity water also contributed significant nutrient loads to estuaries and the nearshore. In our in-situ experiments, addition of nutrients stimulated mudflat primary production in all estuaries at multiple sampling times, suggesting chronic nutrient limitation. Therefore, we conclude that catchment nutrients are critical to promoting primary productivity across all three estuaries.