As the human population has increased, so has the invention of new technologies to support our food systems. Little did we know that these inventions could result in environmental damages that may or may not be reversible. We began thinking about our actions on land without understanding how our land works. Pesticides and fertilizers allow farmers to have a profitable growing season while also feeding the world, but these applications have had unintended consequences for our waterways and oceans. Our biggest indicator of agricultural harm is seen in the bleaching of our coral reefs.
How exactly does agriculture affect coral reefs?
With 30 years of unique data, researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute and other collaborators have discovered that coral bleaching isn’t just a result of global warming. Concentrations of nitrogen fertilizers have been more devastating. When the nitrogen: phosphorus (N:P) ratio is elevated due to nutrient runoff, phosphorus becomes limited and is known to cause metabolic stress and eventual starvation in corals1. Experimental results have indicated that moderate increases in NO−3 (nitrate) and PO3−4 (phosphate) could alter the growth-erosion balance of coral reefs4. Local stressors, such as chronic nutrient run-off into coastal waters, can destabilize reefs by shifting dominance away from corals and towards fleshy algae and bio eroding invertebrates and endoliths (organisms that thrive inside rocks, coral, or animal shells)4.
Changes in water pH have a greater impact on coral reef health
Eutrophic conditions (increased nutrients and decreased oxygen) can reduce growth rates in many coral species as nutrients can indirectly impact net reef growth by altering the pH environment which changes photosynthesis and respiration rates4. Daytime photosynthesis and nighttime respiration have opposing effects on pH, increasing or decreasing it by changing the concentration of CO2 in the water. Net Community Calcification of coral reefs has significantly decreased with the addition of nutrients in coral reefs worldwide. Coral has seen the greatest decline, with a 56% decrease in Net Community Calcification4.
Great Barrier Reef experiences greatest impacts from agriculture
The Great Barrier Reef is the world’s largest coral reef system, extending over 2000 km along the north‐east coast of Australia2. Over the last 30 years, coral cover in the Great Barrier Reef declined by 50% overall and 70% along developed coastlines with the majority being used for agricultural production2. Pressures such as land-based nutrient run-off impact the health of the Great Barrier reef, even with interventions and investments by the Queensland and Australian governments. Agricultural land uses are the primary source of nitrogen, pesticides, and sediments that enter the Great Barrier Reef, which flows out through the river systems, particularly during major flooding events3.
More than 80 percent of land in the Australian water catchment areas supports agriculture with cattle grazing being the most extensive3. Although intensive agriculture occurs in only five percent of the coastal catchment area, the operations that exist in the lower coastal floodplain result in a more direct impact on the Great Barrier Reef. Fertilizer and pesticide river loads have also increased, resulting in greater amounts of nitrogen, phosphorus, and pesticides being released into the Great Barrier Reef lagoon2.
What does the research say about land use impact on coral reefs?
Researchers gathered data from 1984 to 2014 which included seawater collection during wet and dry seasons1. Collaborating researchers from the University of Georgia and the University of South Florida also monitored the living coral and collected seaweed (macroalgae) for tissue nutrient analysis1. The goal was to better understand how nitrogen traveled from the Everglades to the coral reefs of the Florida Keys National Marine Sanctuary. Data analysis revealed that the annual rate of coral loss varied during the study but increased from 1985 to 1987 and again between 1996 and 1999 which followed periods of heavy rainfall1.
“Citing climate change as the exclusive cause of coral reef demise worldwide misses the critical point that water quality plays a role, too,” said James W. Porter, Ph.D. (Odum School Of Ecology, University of Georgia). “While there is little that communities living near coral reefs can do to stop global warming, there is a lot they can do to reduce nitrogen runoff. Our study shows that the fight to preserve coral reefs requires local, not just global, action.”1
Australia commits to coral reef regeneration
The 2013 Scientific Consensus Statement on Land use impacts on Great Barrier Reef water quality and ecosystem condition found that2:
higher nitrogen availability increases phytoplankton biomass, promotes outbreaks of crown‐of‐thorns sea star, and rapid reproduction of macroalgae on inshore reefs
fine sediments and nutrient inputs lead to reduced light availability for photosynthesis of inshore seagrasses and coral reefs
In 2017, the Scientific Consensus Statement also identified agricultural land as the largest contributor to elevated pollutants from nutrients, sediments, and pesticides3.
In response to scientific findings, the Australian and Queensland governments released the Reef 2050 Long-Term Sustainability Plan in March 2015 which aimed to improve water quality through coordination and collaboration between farmers and industry professionals2. Additional programs under the Reef Sustainability Plan include research, planning, regulation, and ecological restoration.
The Reef 2050 Long‐Term Sustainability Plan hopes to achieve an 80 percent reduction in nitrogen, a 50 percent reduction in sediment deposits, and a 60 percent reduction in pesticides3. Growers have since undertaken a wide range of initiatives to improve water quality through actions such as precise fertilizer and pesticide applications, laser leveling of farmland, and establishing water recycling pits3.
It’s important to be forward-thinking when creating products such as pesticides and fertilizers. As history has shown, without it there is potential for significant ecological impacts. By asking ourselves a simple question such as, ‘and then what happens’, we are forced to examine the full ecological impacts of agricultural management applications.
3. Rolfe, J., Windle, J., McCosker, K., & Northy, A. (2018, March 30). Assessing cost’effectiveness when environmental benefits are bundled: agricultural water management in Great Barrier Reef catchments. Retrieved from https://onlinelibrary.wiley.com/doi/full/10.1111/1467-8489.12259
4. Silbiger, N. J., Nelson, C. E., Remple, K., Sevilla, J. K., Quinlan, Z. A., Putnam, H. M., … Fox, M. D. (2018, June 13). Nutrient pollution disrupts key ecosystem functions on coral reefs. Retrieved from https://royalsocietypublishing.org/doi/10.1098/rspb.2017.2718#d3e1211
