Today, let us start on a new mini-series on the Great Barrier Reef. Unlike the previous mini-series which looks into macroscopic pollution, this mini-series aims to focus on chemical pollution and other microscopic pollution.
The Great Barrier Reef, one of the seven wonders of the natural world. A World Heritage Site, with the largest coral reef system, so large that it can even be seen from space! However, much of the Great Barrier Reef has been lost. While global warming is a major factor in this, let us look instead at the presence of other pollutants which affect the reef.
Australia is an agricultural country and sugar cane is a major agricultural crop that is being grown within the Reef’s catchment areas. With approximately 3000 farms spread of over a wide area, vast amounts of fertilisers are being washed away into rivers and out into the ocean. The presence of nitrogen in these fertilisers that get washed out promotes algae growth (Kroon et al., 2012) which in turn is a source of food for crown-of-thorns juveniles (Brodie, Fabricius, De’ath and Okaji, 2005). When juvenile crown-of-thorns settle in a reef environment, they begin life by eating coralline algae. Under normal, natural circumstances, algae populations would be limited due to insufficient nutrients like nitrogen which in turn limits the number of juvenile crown-of-thorns from reaching maturity. However, with the introduction of fertilizers, nutrients no longer become a limiter for algae and in turn, more crown-of-thorns reach maturity and switch their diets to eating corals. These starfish pose a dire threat to the Great Barrier Reef!
A short introductory video on the boom in crown-of-thorns starfish population:
Another group of pollutants that can be found at the World Heritage site are pesticides and herbicides. Samples collected at the site show high levels of concentrations of both herbicides and pesticides. It was found that about 43 pesticide residues were identified and of which, 7 pesticides exceeded the ecological water quality guidelines (Smith et al., 2012)! Due to the long half-life of these chemicals, such chemicals are often washed out and make their way into marine waters. This problem is further exacerbated by the lack of ground cover due to the clearing of land for agricultural use. Such land use creates further problems such as sedimentation.
Sedimentation is yet another source of pollution. The lack of ground cover due to agriculture makes the land highly vulnerable to erosion. Without plant roots to hold the soil together and leaves to soften the impact of raindrops on soil surface (Vaezi, Ahmadi and Cerdà, 2017), large amounts of soil are quickly washed away into rivers and out into the ocean as sediments. In large amounts, these sediments block out sunlight thereby preventing photosynthesis. Furthermore, these sediments would eventually settle and may cover and smother entire reefs!
What do you think? What are the impacts of these pollutants? What might be the best way to deal with the crown-of-thorns? Can enough change be done before it is too late? Please share your thoughts in the comments below!
Reference:
Brodie, J., Fabricius, K., De’ath, G. and Okaji, K., 2005. Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish? An appraisal of the evidence. Marine Pollution Bulletin, 51(1-4), pp.266-278.
Kroon, F., Kuhnert, P., Henderson, B., Wilkinson, S., Kinsey-Henderson, A., Abbott, B., Brodie, J. and Turner, R., 2012. River loads of suspended solids, nitrogen, phosphorus and herbicides delivered to the Great Barrier Reef lagoon. Marine Pollution Bulletin, 65(4-9), pp.167-181.
Smith, R., Middlebrook, R., Turner, R., Huggins, R., Vardy, S. and Warne, M., 2012. Large-scale pesticide monitoring across Great Barrier Reef catchments – Paddock to Reef Integrated Monitoring, Modelling and Reporting Program. Marine Pollution Bulletin, 65(4-9), pp.117-127.
Vaezi, A., Ahmadi, M. and Cerdà, A., 2017. Contribution of raindrop impact to the change of soil physical properties and water erosion under semi-arid rainfalls. Science of The Total Environment, 583, pp.382-392.
