Changing water flow regimes regulate nutrient retention and habitat quality in a complex coastal landscape

The nature of nutrient transformations and redistribution in coastal aquatic landscapes varies in response to short- and long-term changes in hydrologic connectivity, which are mediated by river flow regimes and ocean dynamics. Coastal lagoons with topographically restricted connection to the ocean and regulated river flows are particularly sensitive since poor flushing and evaporation promotes nutrient retention and degraded water quality. Resolving nutrient dynamics is important for their sustainable management, yet being able to accurately resolve nutrient budgets remains challenging due to complex hydrological regimes and habitat heterogeneity. In this study, we undertake a systematic nutrient budget and habitat assessment of a large shallow lagoon (Coorong, South Australia), using a high-resolution coupled hydrodynamic-biogeochemical-habitat model.  The interplay between hydrological drivers and biogeochemical processes was quantified using an adjusted Damköhler number, comparing the timescales of nutrient flushing versus processing. This showed the general transition from hydrologic to biogeochemical control with increasing distance from the main ocean connection, modified by episodes of increased flows and external loads. Whilst water age was a useful indicator of the factors controlling the overall rate of net nutrient retention, interannual variability in retention between areas of the lagoon was explained based on the river inflow regime and changes in mean sea level. We demonstrate that high rates of evapo-concentration and limited water connectivity have led to a persistent accumulation of nutrients, and poor water and sediment quality within the lagoon, which has also been associated with a decline in habitat availability for key species. Scenarios are used to provide evidence that increasing environmental water allocations through the river to offset recent declines in flows reaching the river mouth would reduce the nutrient retention, and we discuss the potential for net nutrient export to the ocean and habitat recovery under sustained high flows.

Presentation Preference: Oral

Primary Presenter: Matthew Hipsey, The University of Western Australia (matt.hipsey@uwa.edu.au)

Authors:

Matthew Hipsey, The University of Western Australia  (matt.hipsey@uwa.edu.au)

Peisheng Huang, The University of Western Australia  (peisheng.huang@uwa.edu.au)

Sherry Zhai, The University of Western Australia  (sherry.zhai@uwa.edu.au)

Daniel Paraska, The University of Western Australia  (dan.paraska@uwa.edu.au)

Justin Brookes, The University of Adelaide  (justin.brookes@adelaide.edu.au)