Eutrophication, one of the leading global water pollution challenges, can be addressed with nature-based solutions (NBSs). NBSs use or mimic natural processes (ecosystem services) to improve water quality, for example microorganisms, plants and filter feeders have already been shown to mitigate eutrophication. However, burrowing endobenthic organisms have not yet been considered as a NBS, despite their potential to greatly impact water quality. Therefore, the purpose of this study was to determine whether these endobenthic organisms could be effective as NBSs, more specifically whether the activities of the common sandprawn (Callichirus kraussi) can improve water quality in terms of pelagic nutrient, microalgal and suspended solids concentrations. C. kraussi is a benthic ecosystem engineer that, like other endobenthic crustaceans, affects water flow, sediment biogeochemistry and benthic-pelagic fluxes through bioturbation and bioirrigation. Their elaborate burrow systems may act as biological filters for pelagic microalgae, but they may also be a source of nutrient-rich excrement inputs into the water column. To determine the effects of C. kraussi on water quality, a laboratory mesocosm experiment and comparative in situ analysis were used in this study. In the mesocosm experiment, water quality at 3 increasing C. kraussi densities (0 [control], 100 and 200 individuals·m-2 ) were compared. At the end of the experiment, the water column NH4 + concentration in the high density C. kraussi treatment was 2.6 times greater compared to the controls. Despite this increase in the NH4 + concentration, the water column chl-a concentrations were 42.8% (50% density treatment) and 44.3% (100% density treatment) lower compared to the controls. Additionally, microalgal biomasses in the sediment were less concentrated on burrow mounds created by C. kraussi compared to the adjacent sediment between mounds and within the burrows. However, the overall chl-a concentrations in the sediment did not differ between treatments. This suggests that C. kraussi has the effect of breaking up microalgal mats on the sediment surface with their burrow mounds without changing the overall sediment surface chl-a concentration, thereby maintaining benthic-pelagic coupling. C. kraussi did not affect suspended solids concentrations in the water column over the 12-day study period. For the in situ component, water quality was compared at sites where C. kraussi was present and the adjacent sections where it was absent in the Zandvlei Estuary Nature Reserve. The findings from the mesocosm experiment were reflected in situ: Chl-a concentrations in C. kraussi habitats were reduced by 43.1% from upstream areas without C. kraussi, and again increased by 32.9% from C. kraussi habitats to downstream canalised areas without C. kraussi. Differences in suspended solids concentrations were also not explained by C. kraussi presence in situ. The conclusion from this study is that ecosystem engineering by C. kraussi has an overall positive effect on water quality in urban estuaries, principally by reducing chl-a concentrations in the water column and maintaining benthic-pelagic coupling. Therefore, C. kraussi should be conserved for its water purification ecosystem services. This can be achieved by maintaining soft sediments in estuaries, rather than constructing concrete canals in which C. kraussi cannot burrow. Regulation of C. kraussi extraction for fishing bait is also necessary to prevent the loss of their filtration function. Furthermore, this study supports the use of C. kraussi as a NBS to eutrophication and encourages the consideration of similar endobenthic organisms for this purpose. Endobenthic ecosystem engineering should not be underestimated in addressing the global challenge of eutrophication.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/30843 |
| Date | 29 January 2020 |
| Creators | Venter, Olivia |
| Contributors | Pillay, Deena |
| Publisher | Faculty of Science, Department of Biological Sciences |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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