Assessing dependence on groundwater of nearshore coastal habitats in south-east South Australia

Author: Rohan Henry

Henry, Rohan, 2017 Assessing dependence on groundwater of nearshore coastal habitats in south-east South Australia, Flinders University, School of Biological Sciences

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The ecological function that groundwater has in influencing ecosystems in Australia is poorly understood and there is virtually no literature that gives evidence about the hypothesised dependence that nearshore coastal ecosystems may have on groundwater outflows. Specifically, nothing is known about what effects groundwater has on the ecological components of nearshore marine environments, nor the implications of any measurable dependence on groundwater that this may have for natural resource managers and water allocation planning. Thus, as a first step, this study set out to assess whether animal and plant assemblages in coastal waterbodies such as small estuaries, drains, creeks, ponds, springs, beaches and other nearshore marine environments in the South East Region of South Australia are influenced by groundwater discharge.

Waterbodies were first grouped by their type, regional location and observed flow regime. Analyses of salinity, temperature, [TIN], [NOx], [NO3], [NO2], and 2H and 18O isotopes found that: Eastern drains were different from Western Drains; seasonal-flow drains differed from continuous-flow drains; and Western drains were significantly different from Eastern drains, groundwater ponds and springs, which were all similar. Thus Eastern drains are predominantly maintained and fed from groundwater resources. Waterbodies associated with groundwater inputs tended to be fresher, cooler, had more nitrogen (especially NO3) and were more enriched with hydrogen and oxygen isotopes.

Continuous groundwater outflow in coastal creeks appeared to restrict any marine water influence so that estuarine gradients were non-existent. Macrofaunal total abundance and species richness was dominated by insects suggesting that continuous groundwater inputs might have a level of influence shaping macrofauna assemblages in these continuous-flow groundwater-fed streams, as opposed to a more estuarine character in intermittent streams.

Beach springs feature a sand slurry caused by constant outflow of groundwater. Meiofaunal abundance and species composition was lower in groundwater-driven beach springs than on marine intertidal (open coast) beaches. Animals unique to each sampling location were observed and two possibly new nematode species were collected from within the beach spring.

A locally common bivalve, Paphies elongata, was collected for 15N flesh analyses from an area under direct influence of discharging groundwater but showed no difference in the values from different sample groups. Thus groundwater-influenced 15N accumulation was not evident in the tissue mass of Paphies elongata, suggesting that the spatially-removed individuals collected were from the same localised population. Nitrogen concentration in the flesh of P. elongata was approximately 10% of their dry weight; this is likely to be the first published data relating to N concentration in this species.

Standing stock biomass of flora in coastal SE SA drains ranged on average from 0 to >263 g DW.m-2 and daily mean productivity of periphyton ranged from <1 to 18 g DW.m-2.d-1. Continuous-flow drains had about 10 times the average biomass of seasonal-flow drains, and mean productivity in seasonal-flow drains was approximately twice that of continuous-flow drains. Continuous- and seasonal-flow drains had approximately similar mean δ15N values.

The findings and methodological learnings from this study can be built upon and applied by other researchers within this field as well as managers of these water resources.

Keywords: Springs, ponds, streams, meiofauna, isotopes, beach, macofauna,

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2017
School: School of Biological Sciences
Supervisor: Professor Peter G. Fairweather