Backgrounder: Impact of land runoff


Increased runoff of sediment, nutrients and contaminants from the land has lowered coastal water quality and marine ecosystem health across northern Australia, including the Great Barrier Reef. Increased sediment runoff ultimately increases sedimentation on coastal reefs, reduces the clarity of coastal waters and restricts the growth of light-dependent plants and animals. Increased nutrient inputs stimulate algal growth on reefs and in reef waters, increase the occurrence of coral disease, and may influence crown-of-thorns starfish outbreaks.

Well documented by 25 years of AIMS research on the Reef, the increased sediment and nutrient loads to coastal waters:

  • smother coral reef organisms due to the settling of suspended sediment
  • reduce light availability for coral and seagrass photosynthesis due to increased turbidity
  • favour the growth of macroalgae at the expense of corals due to high nutrient availability.

More recent work on contaminants such as agricultural pesticides has demonstrated that several reef foundation species are highly sensitive to acute exposure of herbicides. The potential build-up of contaminants can weaken the health and resilience of corals and other organisms, making them more susceptible to disease outbreaks or climate impacts.

Our Reef water quality research has informed the development and implementation of the Reef Water Quality Protection Plan 2013, which aims to improve the quality of water entering the Reef. Our contributions can be found in the Reef Plan Scientific Consensus Statement, published in June 2013.

More broadly, tropical marine ecosystems across northern Australia face a growing threat of exposure to oil and gas spills, due to the increased extraction operations and shipping traffic planned over coming decades, especially on the North-West Shelf. Other coastal ecosystems, especially those close to urban centres, ports and processing facilities, are exposed to contaminants such as metals, coal dust, endocrine-disrupting chemicals and microplastics.

The combined impact of land runoff makes it hard for researchers to predict the risk posed to tropical marine ecosystems by increasing agricultural and industrial activities in northern Australia.

To address this challenge, AIMS is:

  • developing new toxicity tests for tropical marine species
  • quantifying dose-response relationships for acute and chronic exposures to current and emerging contaminants
  • developing new molecular-based biomarkers for the detection and monitoring of the effects of land runoff.

An ongoing key research challenge is to understand water quality variability in order to reliably detect trends, and to forecast the responses of key organisms and communities to changing water quality in the context of other disturbances that affect the marine environment.

Nutrients

AIMS estimates that average yearly inputs of nitrogen from the land have nearly doubled from 23 000 to 43 000 tonnes over the past 150 years, while phosphorus inputs have tripled from 2400 tonnes to 7100 tonnes. In wetter years, these inputs can be many times higher. AIMS scientists believe that, while most of this material is eventually flushed out of the Reef lagoon, transferred to the atmosphere or buried in coastal sediments, net levels in Reef waters may be slowly increasing.

The lagoon is so large and complex that we still only partly understand the processes that control the fate of nutrients: how long they remain in the lagoon, what organisms exploit them and where they go. If the Reef is to survive the threats of runoff and climate change—to be there for future generations to enjoy—we need to improve our understanding of the sources and fates of nutrients.

One of our most vital missions is to understand and interpret the nutrient cycle in the Reef and in Australia's tropical northern waters. The scientific insights and data gathered by AIMS researchers are critical inputs to Australia's Reef Water Quality Protection Plan, acknowledged as setting a world-best standard for minimising human water quality impacts in reef waters.

Murky water

Light is an important factor for the growth and survival of coral reefs. While corals can grow and form reefs over a wide range of water clarity conditions, the nature of the reefs and their continued survival depends on getting sufficient light. Measurements suggest that coastal waters in some parts of the Reef are becoming more turbid due to increased loads of fine sediment and organic particles, continually resuspended by waves and currents. Research by AIMS oceanographers and reef scientists is defining how reef organisms are affected by fine sediment and how coastal reef communities are responding to enhanced sediment runoff.