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Living with multiple stressors in the marine environment

When asked to think about the ocean, most people imagine a pristine habitat in the tropics with golden sands and clear blue waters, or the diversity of fish associated with coral reef communities. Yet, the oceans of the world are being influenced by an array of human activities that are not necessarily visible, from the input of chemical pollutants and nutrients, to the absorption of carbon dioxide and heat, and the effects of UV radiation and noise. All of these have increased over the last few decades, raising concerns about the effects that multiple co-occurring stressors may have on marine ecosystems and their levels of biodiversity.

There is a difficulty, however, in studying the responses of marine organisms and communities as the effects of individual stressors may not be expressed at the same time and in the same way—and may be context dependent. In addition, the expression of these effects may take many years to emerge. Understanding these interdependencies has become a major theme in contemporary environmental science, but demands an interdisciplinary focus that can be difficult to achieve.

Traditionally, the biological effects of human activities have been studied independently in the laboratory. More recently, attention has switched to the combined effects of multiple stressors in order to more fully appreciate the changes that may occur in the natural environment. However, there are a number of challenges associated with studying the effects of multiple and simultaneously acting stressors in a ‘real world’ scenario.

Plastic and trash on the beach by Fabio Achilli. CC0 BY 2.0 via Flickr.
Plastic & Garbage! A Bunaken Beach by Fabio Achilli. CC0 BY 2.0 via Flickr.

Humans can influence marine ecosystems in a number of ways. By first studying the physiological effects of single and multi-stressors on individual species, biologists can learn about the relative vulnerabilities of marine species to environmental change. Variations in their responses at different life stages can influence the survival of populations and communities, and inform on their ecology. Changes in the ability to respond to environmental change can have a negative effect on growth and development, as demonstrated by marine invertebrates exposed to lowered salinity, increased UV radiation, and/or increases in CO2 levels in the seawater. Different strategies are used to survive areas of low oxygen and several mechanisms are influenced by chemical contaminants. All of these responses can be altered by climate change, and in particular, ocean warming. Coastal eutrophication can occur in regions of high nutrient inputs (nitrogen) with early life stages being more sensitive to nitrogen stress. Finally the stress of human activity can come in the form of acoustic pollution and its effect on hearing and the integrity of the auditory system. All of these factors can trigger changes in biological interactions, community structure, and trophic dynamics, ultimately leading to a loss in biodiversity and alterations in ecosystem structure and function.

Complex interactions between various stressors further complicate the identification of generic processes or patterns, but they also have the potential to help towards the development of strategies for the management and conservation of marine habitats. Developing sufficient knowledge and collating information from several fields of science, however, can be particularly challenging. At present, only a handful of studies have gone beyond the traditional domains of a scientific discipline, combining physiological responses with changes in what species do, when they do it, and how efficiently they continue to do so. Gaining such interdisciplinary knowledge is going to be vital if we are to understand and mitigate the effects of human activity on our natural heritage.

Featured image credit: Coral reef by Jan-Mallander. CC0 public domain via Pixabay.

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