Water Quality – how healthy is the sea in Torres Strait, and what are the risks?

The Torres Strait marine environment contains a relatively shallow (<20m) and highly productive stretch of seawater between the tip of Queensland and Papua New Guinea. It straddles the juncture of the Indian Ocean (Arafura sea) with the Pacific Ocean (Coral Sea), resulting in complex patterns of influence from the two ocean systems, including complicated tides and currents, and high biodiversity. Torres Strait contains over 100 islands, and a diversity of marine habitats including coral reefs, seagrass meadows, and rich benthic garden communities.

The importance of a healthy Torres Strait marine environment to its indigenous inhabitants, the Torres Strait Islanders, is immense. Besides immeasurable cultural significance and spiritual connection stemming from a rich and ancient maritime history, the marine environment provides Torres Strait Islanders with a medium for transport, food, and livelihoods. It supports important commercial and traditional fisheries in which Torres Strait Islanders participate, based on many species including pearl oyster, trochus shell, beche-de-mer, crayfish, prawns, barramundi, and other finfish such as reef-fish and Spanish mackerel. It also supports a range of subsistence and culturally important fisheries based on dugong, turtle, amongst other species. The consumption of seafood by Torres Strait Islanders was estimated and found to be amongst the highest in the world!

Due to this fundamental connection to and reliance on the ocean, a healthy Torres Strait marine environment based on good water quality is essential. A recent study examined the risks to water quality in Torres Strait, by examining existing knowledge and published literature about the Torres Strait marine environment. It was conducted by Jane Waterhouse, Jon Brodie and others from TropWater at James Cook University, with funding from the National Environment Research Program. Once their report is available, it will be linked to this article here. The remainder of this article summarises their findings.

Threats to Torres Strait water quality from the PNG-Irian Jaya landmass:

Torres Strait experiences a mean annual rainfall of 1.75m, which is only a fraction of the 10-13m experienced to the north on the PNG landmass. In the past, several studies have been conducted to address potential threats to Torres Strait from mining activities in Papua New Guinea, such as the gold and copper mine on the Ok Tedi River (which is a tributary of the Fly River). Studies have looked at heavy metals in sediments, indicator organisms and seafood species , and patterns of water movement +others. Overall, these studies indicate that the influence of the Fly River is limited to the far north-east Torres Strait, while other smaller coastal rivers flowing from Irian Jaya also influence the water quality in the north-central areas. Thus, the entire northern area of Torres Strait can be considered to be under the influence of river runoff from the PNG/Irian Jaya land mass. This is consistent with water circulation studies, which show that most of the freshwater coming out of the Fly River enters the Gulf of Papua and then travels eastwards, with only a small amount entering Torres Strait in the North-east +others. Arsenic, chromium and nickel are heavy metals considered to be coming from PNG, which were high enough to be of concern in some areas in northern Torres Strait from a sampling study undertaken in 1998. In comparison to inputs from the PNG/Irian Jaya landmass, land run-off influences to Torres Strait from the Australian mainland have been considered by studies to be negligible.

Flood Plume from Fly River (Landsat 5)

This satellite image (Landsat 5) shows a fairly typical flood plume from the Fly River in PNG on 11th Feb 2009. The plume mostly extends northward away from the Torres Strait, however a percentage hugs the PNG coastline reaching Saibai Island. 

by Eric Lawrey, NASA

Torres Strait’s extensive seagrass beds, coral reefs and other benthic habitats are considered to be highly biodiverse. However, extensive runoff from terrestrial areas is known to negatively impact shallow water tropical marine envirnonments , thus the ongoing health of Torres Strait environments, particularly in the north, may be tied to future runoff inputs from PNG.

There have been episodic die-backs of Torres Strait seagrass meadows, particularly in north-west Torres Strait. These have been attributed to high turbidity and increased sedimentation from flooding PNG rivers (Poiner and peterken 1996, Marsh et al 2004). It has been suggested that if sedimentation from these rivers increases in future due to mining and other activities on the land, seagrass diebacks will also increase due to the smothering effect of the sediment and the filtering of light available for seagrass (Heap and Sbaffi 2008). Similarly, higher rates of sedimentation has the potential to negatively impact on the health of coral reefs in Torres Strait, both directly and through facilitating the persistence of Crown of Thorns Starfish.

Many large developments with the potential to increase sedimentation and contaminant inputs to Torres Strait, are in the pipeline for PNG. These include gas platforms, oil palm expansion, Daru port dredging and development, and extension of the Ok Tedi mine for a further ten years. While the extent of these impacts are likely to be contained in northern Torres Strait (due to water and sediment transport patterns, as discussed earlier), they are cause for concern and worthy of a new monitoring program to compare current and future levels of contaminants to those recorded in the Torres Strait Baseline Study over 20 years ago. Further, development of remote-sensing MODIS tools to map and monitor the actual extent of river plumes has been recommended.

Heavy metals in seafood species

Studies have analysed heavy metals in the mangrove cockle Akul because it is known to a good indicator organism for metals, and is widely distributed through Torres Strait. It is also commonly eaten, so results have a double value by also informing the impact of heavy metals on humans through consumption of seafood. Copper is one metal associated with land-runoff which was much higher in northern Akul compared with southern samples, while nickel varied between years, with 1998 being a year with elevated levels.

Interestingly, some metals that are not associated with land run-off, also showed elevated levels in sediments and some seafood species in Torres Strait. Cadmium was found to be associated with carbonate sediments of marine origin, which are more prevalent as you move south away from the PNG coast and therefore very unlikely to be coming from PNG rivers. In fact, crayfish samples collected closer to the Fly River had lower concentrations of cadmium than those reported from the south. Regardless of the source being completely natural, cadmium was found to occur in high concentrations in some food species, including dugong, turtle, crayfish and prawns. In crustaceans such as crayfish and prawns, cadmium was mostly found in the head (and an organ called the hepatopancreas), so avoiding consumption of these tissues, and instead consuming just the tail flesh, is recommended.

Potential for pollution from within Torres Strait

Island waste management systems, including sewage management and waste disposal, have the potential to contribute pollution into the Torres Strait marine environment. Fortunately, only islands with small populations rely on septic systems, and the design standard of sewage treatment plants on other islands is relatively high. Thus, when these systems are operating well, the standard of effluent into the marine environment can be expected to be good. However, some of these systems don’t always operate properly due to maintenance and repair issues. Waste management relies primarily on land-fill, and some of these are located close enough to the coasts to allow for seepage and saltwater inundation. Overall, the potential for contamination of the marine environment as a result of waste management systems is localised and minor.

By contrast, the threat posed by shipping and associated risk (oil spills, groundings, ghost nets) is far more significant. This has been addressed in another article (link to Kate’s shipping article). Due to the limited net water movement in and out of Torres Strait, a significant pollution event from shipping would probably remain in Torres Strait for a long time and cause prolonged impacts on the marine environment.

Understanding ocean circulation

Torres Strait faces threats to water quality from outside (eg runoff from PNG) and inside (eg shipping) the Torres Strait. Many of these potential pollutant issues are large scale, so an accurate oceanographic model capable of predicting circulation (and carriage of pollutants) in the extremely complex physical and ecological environment of Torres Strait, is essential if the impact of pollution is to be predicted and possible management actions optimised. Such a model has been prepared, and will be published and reported on shortly.

References

1. Waterhouse J, Brodie J, Wolanski E, Petus C, Higham W (2013) Hazard assessment for water quality threats to Torres Strait marine waters and ecosystems. TropWater, James Cook University.

2. Dight I, Gladstone W (1993) Torres Strait Baseline Study: Pilot Study final report. Townsville: GBRMPA.

3. Gladstone W (1996) Trace metals in sediments, indicator organisms and traditional seafoods of the Torres Strait.

4. Evans-Illidge E (1997) Heavy metals in commercial prawn and crayfish species in Torres Striat. 111 p.

5. Woolfe (1997) Receiving Marine Environment: Offshore Pipeline. Environmental Impact Statement - PNG LNG Project.

6. Taylor HA, Rasheed MA (2010) Torres Strait dugong sanctuary seagrass baseline survey, March 2010. 22 p.

7. Pitcher R, Haywood M, Hooper JNA, Coles R (2007) Mapping and characterisation of key biotic and physical attributes of the Torres Strait ecosystem.: CSIRO, QDPI, Queensland Museum.

8. Haywood M, Pitcher C, Ellis N, Wassenberg T, Smith G, et al. (2008) Mapping and characterisation of the inter-reefal benthic assemblages of the Torres Strait. Continental Shelf Research 28: 2304-2316.

9. Brodie J, Kroon F, Schaffelke B, Wolanski E, Lewis S, et al. (2012) Terrestrial pollutant runoff to the Great Barrier Reef: An update of issues, priorities and management responses. Marine Pollution Bulletin 65: 81-100.

10. Weber M, de Beer D, Lott C, Polerecky L, Kohls K, et al. (2012) A series of microbial processes kills corals exposed to organic-rich sediments. PNAS 107: E1558-E1567.

11. De'ath G, Fabricius K, Sweatman H, Puotinen M (2012) The 27 year decline of coral cover on the Great Barrier Reef and its causes. PNAS 109: 17995-17999.

12. Fabricius K (2005) Effects of terrestrial runoff on the ecology of corals and coral reefs. Marine Pollution Bulletin 50: 125-146.

13. Fabricius K, Okaji K, De'ath G (2010) Three lines of evidence to link outbreaks of the crown-of-thorns seastar Acanthaster planci to the release of larval food limitation. Coral Reefs 29: 593-605.

14. Haynes D, Kwan D (2002) Trace metals in sediments from Torres Strait and the Gulf of Papua: concentrations, distribution and water circulation patterns. Marine Pollution Bulletin 44: 1296-1313.