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Determining thresholds of combined human impacts to seagrass in Atlantic Canada

Long grass growing by a body of water with trees also reflecting on the water at the top of the photo.
Eelgrass, a type of seagrass, growing by the shore. Photo attribution: Melisa Wong

Seagrass is a marine plant that shelters small invertebrates and juvenile fish, protects soft seafloor areas from erosion, and stores large amounts of carbon, which helps slow climate change.

Seagrass grows like a meadow in shallow waters very close to shore. Being near the coast makes it vulnerable to human activities that occur on land, as well as in the sea.

Human activities such as agriculture, urban land use, coastal construction, or aquaculture can add excess nutrients, sediments, invasive species, and/or pollution to the water, which can have negative impacts on seagrass.

Read this open access paper on the FACETS website.

Seagrass habitat is important to protect because of the many benefits it provides in support of a healthy and thriving ocean.

To properly protect seagrass, researchers need to know how much seagrass is being affected by the many human activities occurring at the same time and in the same place — known as cumulative effects.

We calculated a cumulative effect score for seagrass in 187 different places in Atlantic Canada by combining information on the strength of six different human activities with how vulnerable seagrass is to each of the activities.

To determine at what level the combined impact of these six human activities may cause harm, we also developed a cumulative effect threshold.

This threshold is an upper limit that indicates at what point there could be a measurable change to seagrass if exposed to high levels of all six human activities. Cumulative effect scores that are greater than the threshold indicate a greater potential for damage.

We found a large range in cumulative effect scores — from very low to very high — for seagrass across Atlantic Canada.

When we compared these scores to the threshold, we found that 26% of the seagrass habitats we examined are considered at risk of being damaged from multiple human activities.

Most of the habitats at risk were in places with high levels of human activity but some were in areas that are not typical hot spots of human activity.

Highlighting these areas can help researchers focus conservation and restoration measures where they could have the most successful outcomes.

Our research shows that to identify seagrass habitats at risk of harm, it is better to look at the collective impact of multiple human activities than the strength of each human activity on its own.

By using the cumulative effect threshold to determine which seagrass habitats are most at risk, we can help determine the best plan to protect these important coastal marine ecosystems for the future.

Read the paper — Incorporating anthropogenic thresholds to improve understanding of cumulative effects on seagrass beds by Grace E.P. Murphy, Noreen E. Kelly, Heike K. Lotze, and Melisa C. Wong



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