Saltpan primary producer and inorganic nutrient dynamics in response to inundation with nutrient-rich source waters

Abstract

This study made use of a microcosm experiment to inform a planned restoration project that aims to simultaneously address two global issues – stormwater management and saltpan abandonment, which leads to the loss of unique wetland habitats and the ecosystem services they provide. To reinstate the wetland function of an abandoned saltpan at the Swartkops Estuary, South Africa, an opportunity has been identified to fill it with stormwater which is currently a major source of nutrient pollution to the nationally important estuary. This study investigated the primary producer communities that may arise in response to the inundation of the saltpan with stormwater as well as the potential for the saltpan to remove nutrients from the stormwater. The microcosm experiment contained stormwater treatments and an estuary treatment consisting of water from the adjacent Swartkops Estuary to mimic the previous management regime of the saltpan. Initially, all treatments were eutrophic – the stormwater treatments had exceptionally high concentrations of dissolved inorganic nitrogen (DIN ≥ 99 μM), while the Estuary treatment was rich in dissolved inorganic phosphorus (DIP = 25 μM) and were characterised by diatom blooms (> 20 μg Chl-a l−1). However, these blooms quickly assimilated inorganic nutrients and shifted the water column to an oligotrophic state, before subsequently collapsing. By the end of the study, diverse primary producer communities, dominated by benthic microalgae and submerged macrophytes, were able to assimilate the studied nutrients and all treatments reached an oligotrophic, clear water state. A phytoplankton (Tetraselmis sp.) bloom occurred in the Estuary treatment, but no evidence was found to support the potential for nuisance blooms if stormwater is pumped into the abandoned saltpan. This study highlights the potential for converting defunct saltpans, which have been abandoned in many parts of the world, to stormwater retention ponds that can reduce the anthropogenic nutrient loading of estuaries.