Spencer Gulf - Assessing cumulative impacts

 

Management of marine areas is often based on the effect of individual stressors. The cumulative impact caused by multiple stressors is seldom considered, usually due to the challenges in collecting data and the uncertainty associated with current assessment methods (Halpern and Fujita 2013; Bevilacqua et al.2018). Current cumulative impact assessment is mostly based on expert elicitation, with scores assigned to areas depending on the level of threat, the sensitivity of the system to the threat, and the potential for the system to recover. This year, I will be comparing the health of marine habitats within Spencer Gulf, with cumulative assessment scores of the same habitats calculated by Jones et al. (2018). 

Spencer Gulf is Australia’s largest estuary (Jones et al.2018), but it is actually termed an inverse estuary as salinity decreases as we move outwards towards the open sea (Nunes-Vaz 2012). Given its importance to South Australia’s seafood and tourism industry (Carrick and Ostendorf 2007; Robbins et al. 2017), it’s no surprise that Spencer Gulf is subjected to multiple stressors in order for humans to benefit from its resources. So what effect have these stressors had on the state of Spencer Gulf?

 

After spending last year battling the mangroves, this year I conducted my field work in the (nearly) open seas of Spencer Gulf, from the comfort of an Environment Protection Authority (EPA) research vessel. During my time at sea, I was lucky enough to hang out with the same pod of dolphins every day, and learn how to quantify marine ecosystem condition with the group who protect South Australia’s seascape. 

 

The three weeks I spent completing the fieldwork yielded many lessons and experiences. First and foremost, anybody who wants to take measurements on pH, temperature and/or salinity should invest in a sonde multi-probe. They log hundreds of lines of data in minutes and require minimal supervision. I was also able to see how the EPA videos and characterises benthic habitat, and how they process and analyse water samples.

 

As well as the lessons, I was also able to witness first hand some of the natural occurrences which characterise Spencer Gulf. For example, last year I learnt how increased evaporation and little water exchange between the open sea and the gulf caused increased salinity within the gulf (Nunes-Vaz 2012). One of our sites was located in the northern-most region of the gulf, near Port Augusta. Salinity levels here surpassed 60ppt, with average salinity in other sites being between 39-42ppt. It was a day of dry hands, crystallised clothing, and intense thirst, but showed how changes in salinity can influence the environmental conditions.  

 

The development of comprehensive cumulative impact assessment is important to inform management of priority areas for conservation. By not considering the effect of cumulative impacts, ecosystems are at risk of passing ecological tipping points and shifting to a state from which it may never recover (Andersenet al.2015). With data collected on benthic structure, water chemistry and physical characteristics, we now await the health condition index. We will then compare this index with the current cumulative impact scores to establish what kind of (if any) relationship occurs.

 

 

 

References

  • Andersen, JH, Halpern, BS, Korpinen, S, Murray, C, Reker, J (2015) Baltic Sea biodiversity status vs. cumulative human pressures. Estuarine, Coastal and Shelf Science161, 88-92.

  • Bevilacqua, S, Guarnieri, G, Farella, G, Terlizzi, A, Fraschetti, S (2018) A regional assessment of cumulative impact mapping on Mediterranean coralligenous outcrops. Scientific Reports 8.

  • Carrick, NA, Ostendorf, B (2007) Development of a spatial Decision Support System (DSS) for the Spencer Gulf penaeid prawn fishery, South Australia. Environmental Modelling and Software 22, 137-148.

  • Halpern, BS, Fujita, R (2013) Assumptions, challenges, and future directions in cumulative impact analysis. Ecosphere 4.

  • Jones, AR, Doubleday, ZA, Prowse, TAA, Wiltshire, KH, Deveney, MR, Ward, T, Scrivens, SL, Cassey, P, O'Connell, LG, Gillanders, BM (2018) Capturing expert uncertainty in spatial cumulative impact assessments. Scientific Reports 8.

  • Nunes-Vaz, RA (2012) The salinity response of an inverse estuary to climate change & desalination. Estuarine, Coastal and Shelf Science 98, 49-59.

  • Robbins, WD, Huveneers, C, Parra, GJ, Möller, L, Gillanders, BM (2017) Anthropogenic threat assessment of marine-associated fauna in Spencer Gulf, South Australia. Marine Policy 81, 392-400.

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