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Coastal pollution is causing significant increase in dead zones in oceans around the world

August 31, 2020

A new study by University of Maryland Center for Environmental Science Professor Emeritus Tom Malone and Professor Alice Newton of the University of Algarve spotlights the current global occurrence of "dead zones" within the coastal ocean, where eutrophication has caused significant oxygen depletion resulting in mass mortalities of marine animals.

"The threats posed by eutrophication include reduced water clarity, oxygen depletion, and toxic algal events that result the loss of critical habitats including coral reefs, seagrass meadows, and mangrove forests," said Tom Malone, a world leader in assessing the health of the Earth’s oceans. "Other serious consequences include mass mortalities of marine animals, loss of biodiversity, and threats to human health."

Conservative estimates indicate that there are now over 700 such areas, including Chesapeake Bay. These harmful effects can also be seen amongst the natural treasures of the ocean, such as the Great Barrier Reef, where eutrophication is partly responsible for a 70% reduction in hard corals over the past century.

By enriching the land to feed a growing population, we are impoverishing the sea.  Nutrients such as nitrogen and phosphorus, found in synthetic fertilizers applied to agricultural land can leach into rivers where they are transported to estuaries and the coastal ocean. While not inherently toxic, excess inputs of these nutrients often cause an explosion of algae that disrupts delicate marine ecosystems. This process is called cultural eutrophication, and it can have serious consequences for marine life.

Oxygen depletion also exacerbates ocean acidification, a process driven by the ocean’s assimilation of large quantities of the carbon dioxide released to the atmosphere via the combustion of fossil fuels, which  increases seawater acidity. Nutrient pollution stimulates the production of algal blooms, much of which sinks and decomposes in bottom water. During decomposition, bacteria consume oxygen and release carbon dioxide, a harmful greenhouse gas that can negatively affect organisms by weakening their calcium carbonate skeleton, for example corals, shellfish and some plankton.

Given that the problem is multifaceted, Malone suggests that the solution is too. An important aspect of this involves better global scale monitoring of nutrient inputs to coastal watersheds by humans, the pathways by which these nutrients are transported to coastal ecosystems, and the responses of ecosystems to nutrient pollution.

“Understanding how marine ecosystems are structured and how all of its components (primary producers, bacteria, viruses, animals) interact with each other is critical to effectively informing policy makers and managers responsible for environmental protection and resource management,” said Malone.

Informed by these data, effective nutrient management must consider watersheds and their receiving bodies of water as a whole and, in this context, land-use practices that integrate land-based controls to manage nutrient releases and transports to coastal ecosystems. Each part of the watershed plays a role in contributing to nutrient pollution, which is modulated by soil type, land use practices and land cover. Identifying critical source areas for cost effective nutrient control must be part of such an integrated approach. The development of such a monitoring-management strategy must be guided by a community of stake holders, including scientists, policy makers, environmental and resource managers, and the public.

Since retiring 10 years ago, Malone’s primary work has involved meta-analysis to synthesize information on the health of the world’s oceans. He served as director of the Horn Point Laboratory from 1990 to 2001. Malone is the senior author of two chapters in the UN’s Second World Oceans Assessment (WOA ll), to be published later this year.  His findings from WOA l in 2016 indicated that the oceans’ carrying capacity for natural resources is near or at its limit. He is presently co-editing a book, “Water Quality in Our Changing World” to be published by the American Geophysical Union. 

“The Globalization of cultural eutrophication in coastal ocean: causes and consequences” was published in Fronteirs in Marine Science.