New tool measures intensity of human activities affecting seagrass

It may look like pretty ordinary stuff — that long, tickly grass that lurks under the water when you go swimming.

But seagrass plays a crucial role in marine ecosystems, and it's on the decline, thanks in large part to human activity.

Until recently, there wasn't a way to measure those human activities, like run-off from farming, shellfish aquaculture and coastal development.

Researchers in Nova Scotia at Dalhousie University and the Bedford Institute of Oceanography have changed that. They analyzed data for a number of factors, including nutrient pollution, population density, coastal land protection, land use, invasive species, commercial fishing and aquaculture, and figured out how to quantify those activities.

"Seagrass beds are incredibly important," said lead author Grace Murphy, a post-doctoral researcher in the biology department at Dalhousie. "I like to think of them as kind of an underwater forest."

Seagrass provides habitat for fish — including commercial species such as herring and cod — and stores carbon and helps prevent land erosion.

"They're kind of providing a habitat that's supporting the whole food web," said Murphy.

A new study by the researchers published this month in the journal FACETS reveals that nitrogen loading — run-off from fertilizer, wastewater effluent and waste from finfish farms — is one of the most significant factors affecting seagrass in the Maritimes.

The scientists studied 180 seagrass beds in P.E.I., Nova Scotia's South Shore, Eastern Shore and Northumberland shore, and along the coast of New Brunswick between Miscou Island and Shediac.

They found that 64 per cent of the beds are at risk of degradation from nitrogen loading, including all the P.E.I. sites and 25 per cent of the sites along Nova Scotia's South Shore and Eastern Shore.

The P.E.I. sites also had the highest risk of eutrophication, which is when an excess of nutrients causes plant life to grow, killing animal life by depriving it of oxygen.

Among the other findings are that structures on the water such as docks, wharfs and causeways are also posing a threat, as they produce shade in seagrass beds and can change the water circulation, dredging up sediment and further blocking sunlight.

On the Atlantic coast of Nova Scotia, coastal fisheries and invasive species such as tunicates were more prominent issues than in the Gulf of St. Lawrence areas.

Some of the other findings include:

• Seagrass beds in Port Joli, N.S., Cable Island, N.S., Taylors Head, N.S., and Kouchibouguac National Park, N.B., were minimally impacted by human activities.
• Second Peninsula near Bridgewater, N.S., was highly affected by urban and agricultural land use, poor water quality and commercial fishing.
• St. Margarets Bay, N.S., was affected by riparian land alteration, poor water quality and commercial fishing.
• Sambro, N.S., was affected by high population density and urban land use.
• Port Mouton, N.S., was affected by commercial fishing and structures such as docks and marinas.
• Mill River, P.E.I., had a lot of riparian land alteration.
• Musquodoboit, N.S., was affected by nutrient pollution.
• Pictou, N.S., was highly impacted by poor water quality and structures over the water.

Murphy said the metric could also be applied to other ocean plants such as rockweed and kelp.

The scientists said their work could be useful to identify high-risk areas where management strategies should be implemented and low-impact areas that should be conserved.

For example, Murphy said the sites studied along Nova Scotia's Eastern Shore were found to have the lowest level of human impact and the highest level of land conservation, with comparatively healthy seagrass beds.

"This indicates to us that this area would be a really good area to protect," Murphy said.

Canada has committed to protecting 10 per cent of its coastal areas by 2020.