The Great Barrier Reef is one of Australia’s most important environmental and economic assets. It is estimated to contribute Australian dollars 56 billion per year and supports about 64,000 full-time jobs, according to the Great Barrier Reef Foundation. However, the reef is under increasing pressure.
While much public attention is focused on the impacts of climate change on the Great Barrier Reef and the debate around its endangered status, water quality is also crucial to the reef’s health and survival. Our new study, published today in the journal Environmental Science and Technology, found that previously unquantified groundwater inputs are the largest source of new nutrients to the reef. This finding could potentially change how the Great Barrier Reef is managed.
Too much of a good thing
Nutrient enrichment can also promote the breeding success of crown-of-thorns starfish, whose increasing populations and voracious appetite for corals have decimated parts of the reef in recent decades. What are the sources of nutrients driving the degradation of the reef? Previous studies have focused on river discharge.
According to one estimate, there has been a fourfold increase in riverine nutrient input to the Great Barrier Reef since pre-industrial times. This past focus on rivers has emphasised reducing surface water nutrient inputs through changing regulations for land-clearing and agriculture, while neglecting other potential sources. However, the most recent nutrient budget for the Great Barrier Reef found river-derived nutrient inputs can account for only a small proportion of the nutrients necessary to support the abundant life in the reef. This imbalance suggests large, unidentified sources of nutrients to the reef. Not knowing what these are may lead to ineffective management approaches.
New nutrient source found
Our research team decided to try and track down this missing source of nutrients. We used natural tracers to track groundwater inputs off Queensland’s coast. Our team collected data from offshore surveys, rivers and coastal bores along the coastline from south of Rockhampton to north of Cairns.
We used the natural groundwater tracer radium to track how much nutrient is transported from the land and shelf sediments via invisible groundwater flows. We found that groundwater discharge was 10–15 times greater than river inputs.
This meant roughly one-third of new nitrogen and two-thirds of phosphorous inputs came via groundwater discharge. This was nearly twice the amount of nutrient delivered by river waters. Past investigations have revealed that groundwater discharge delivers nutrients and affects water quality in a diverse range of coastal environments, including estuaries, coral reefs, coastal embayments and lagoons, intertidal wetlands such as mangroves and saltmarshes, the continental shelf and even the global ocean
A paradigm shift needed?
Our results suggest the need for a strategic shift in management approaches aimed at safeguarding the Great Barrier Reef from the effects of excess nutrients. This includes better land management practices to ensure fewer nutrients are entering groundwater aquifers. We can also use ecological (such as seaweed and bivalve aquaculture, enhancing seagrass, oyster reefs, mangroves and salt marsh) and hydrological (increasing flushing where possible) practices at groundwater discharge hotspots to reduce excess nutrients in the water column. The reuse of nutrient-rich groundwater for agriculture also needs to be explored as it represents an untapped and inexpensive nutrient source.
