Is agricultural run-off a significant problem for the Great Barrier Reef?
Here’s an extract from the Queensland government’s “Frequently Asked Questions” (FAQs) supporting its 2017 Scientific Consensus Statement and the Reef 2050 Reef Water Quality Improvement Plan (with my underlines but original bold formatting) [1, 2, 3]:
“The current poor state of Great Barrier Reef ecosystems is from the combined impacts of land run-off from development in the catchment now and in the past, coastal development, extreme weather events, and climate change impacts such as coral bleaching events.
The environment is changing with more extreme weather events occurring, and ocean warming and ocean acidification predicted to intensify.
This means it is even more important to increase and accelerate efforts to mitigate local stressors such as land-based sources of pollution, coastal development and the management of direct uses including fishing.”
The extract highlights the detrimental role of land-based pollution sources and the need to address them.
Of those pollution sources, livestock grazing (the largest single land use in the reef’s catchment area) is a major contributor but is rarely mentioned by prominent environmental groups that otherwise have much to say about the reef. Such groups almost invariably focus on coral bleaching caused by global warming and increased ocean acidification. However, the impact of bleaching is relatively recent.
The chart that follows (updating earlier versions) adds some context to the loss of coral cover since 1960, noting that more than half the loss had already occurred thirteen years before the first major bleaching event of 1998. Crown-of-thorns starfish (COTS) are likely to have been the main cause of coral loss between 1960 and 1985.  Between 1985 and 2012, COTS and cyclones were responsible for 90 per cent of coral loss, with bleaching responsible for 10 per cent. 
The adverse impacts of nutrients and sediments
Sediment blocks the sun, smothers coral and promotes the excessive development of algae, making the coral less resilient than it would otherwise have been to the impacts of other stressors, such as warming and more acidic waters.
Nutrients such as nitrogen and phosphorus increase the population of phytoplankton, which are a source of nutrition for COTS larvae. COTS eat nothing but coral and have had a devastating impact, with major outbreaks commencing in the 1960s, late 1970s, early 1990s and 2010. The most recent outbreak is still under way.
Livestock grazing’s contribution to sediment
Modeling indicates that fine sediment loads from rivers to the Great Barrier Reef’s waters have increased around 5-fold since the beginning of European settlement.
In the FAQs referred to earlier, the Queensland government reported that livestock grazing (49 per cent) and streambank erosion (34 per cent) were the main sources of anthropogenic fine sediment (under 16 micrometres) in the reef’s waters. Fine sediment is the type most likely to reach the reef’s waters.
Streambank erosion caused by livestock grazing has not been directly attributed to that activity within the latest Scientific Consensus Statement. However, after allowing for its role in such erosion, livestock grazing’s overall contribution to fine sediment load may be close to its contribution to sediment as a whole, at around 70 per cent.
Such a possibility is supported by the fact that the 2013 statement indicated (similarly to the more recent statement) that grazing land’s share of total sediment was 75 per cent, while its share from gully and hillslope erosion was 45 per cent, with streambank erosion from all causes accounting for 39 per cent. 
It is also consistent with feedback from Dr Jon Brodie, Professorial Research Fellow at the Centre of Excellence for Coral Reef Studies at James Cook University, who co-authored the 2013 and 2017 consensus statements. He has indicated that relevant estimates are unlikely to have changed significantly between the two. 
Livestock grazing’s contribution to nutrients
Of the various nutrient types, dissolved inorganic nitrogen and dissolved inorganic phosphorus are of greatest concern because they are immediately and completely available for uptake by marine organisms.
However, particulate nutrients, of which the main source is grazing land, are also a major problem. They are mostly deposited close to river mouths from where they can be broken down for years by bacteria into dissolved inorganic nutrients. The dissolved nutrients may then travel further into the reef’s waters, where they are consumed by phytoplankton, algae and bacteria. Phytoplankton are a key source of nutrition for crown-of-thorns starfish larvae.
The latest consensus statement indicates that particulate nitrogen contributes 45 per cent of total nitrogen, and particulate phosphorus 76 per cent of total phosphorus, with rangeland grazing dominating overall particulate nutrient loads. (p. 11)
It also indicates that “erosion processes (hillslope, gully and streambank) in grazing lands are likely to be contributing higher bioavailable nutrient loads than currently estimated using models”. (p. 4)
The ABC may have been incorrect in reporting nutrient sources
It appears the Australian Broadcasting Corporation (ABC) may have been incorrect in relation to comments made in a special report aired in August 2018 by the “7.30” program.  Contrary to those comments, sugarcane farming is responsible for 78 per cent of dissolved inorganic nitrogen, not total nutrients, in the reef’s waters. 
The Queensland government issues report cards which measure progress towards the Reef Water Quality Protection Plan’s goal and targets. The most recent report card, issued in October 2017 and showing the status as at June 2016, rated overall grazing management as “D”, indicating “poor”.  That result reflected 36 per cent of grazing lands being subject to best management practice systems.
If we are committed (as we must be) to giving the Great Barrier Reef’s corals and the myriad of organisms that depend on them the best possible chance of survival, then major contributors to their demise must not be ignored.
Further background material from Planetary Vegan
 Frequently Asked Questions: Reef 2050 Water Quality Improvement Plan and 2017 Scientific Consensus Statement, State of Queensland, pp. 7 – 8, https://www.reefplan.qld.gov.au/about/assets/faqs-reef-2050wqip-2017scs.pdf
 Bartley, R., Waters, D., Turner, R., Kroon, F., Wilkinson, S., Garzon-Garcia, A., Kuhnert, P., Lewis, S., Smith, R., Bainbridge, Z., Olley, J., Brooks, A., Burton, J., Brodie, J., Waterhouse, J., 2017. Scientific Consensus Statement 2017: A synthesis of the science of land-based water quality impacts on the Great Barrier Reef, Chapter 2: Sources of sediment, nutrients, pesticides and other pollutants to the Great Barrier Reef. State of Queensland, 2017, https://www.reefplan.qld.gov.au/about/reef-science/scientific-consensus-statement/
 Queensland Government, Reef 2050 Reef Water Quality Improvement Plan 2017-2022, https://www.reefplan.qld.gov.au/about/
 Brodie, J., “Great Barrier Reef dying beneath its crown of thorns”, The Conversation, 16th April, 2012, http://theconversation.com/great-barrier-reef-dying-beneath-its-crown-of-thorns-6383
 , , “The 27–year decline of coral cover on the Great Barrier Reef and its causes”, PNAS 2012 109 (44) 17995-17999; published ahead of print October 1, 2012, doi:10.1073/pnas.1208909109, http://www.pnas.org/citmgr?gca=pnas%3B109%2F44%2F17995
 Kroon, F., Turner, R., Smith, R., Warne, M., Hunter, H., Bartley, R., Wilkinson, S., Lewis, S., Waters, D., Caroll, C., 2013 “Scientific Consensus Statement: Sources of sediment, nutrients, pesticides and other pollutants in the Great Barrier Reef Catchment”, The State of Queensland, Reef Water Quality Protection Plan Secretariat, July, 2013, Ch. 4, http://www.reefplan.qld.gov.au/about/scientific-consensus-statement/sources-of-pollutants.aspx
 Email correspondence of 2nd October 2018
 Australian Broadcasting Corporation, 7.30 special, Part 2: “Could a change in farming practices help save the Great Barrier Reef?”, 21st August 2018, https://www.abc.net.au/7.30/could-a-change-in-farming-practices-help-save-the/10149466
 Frequently Asked Questions, op. cit., p. 6
 Queensland Government, Reef 2050 Reef Water Quality Improvement Plan, Report Card 2016 – Management Practices, https://www.reefplan.qld.gov.au/measuring-success/report-cards/2016/management-practices/
Tracey Winholt, “School of Sweet-lips, Great Barrier Reef, Australia”, Image ID 265177598, Shutterstock
Brian Kinney, “Wonderful and beautiful underwater world with corals and tropical fish”, Image ID 260385482, Shutterstock