Mayne Island eelgrass declined 26% between 2009 and 2021
My most memorable marine experience this past year was snorkeling in the middle of a big ball of herring, a real National Geographic moment. I found if I avoided sudden movements, I could ease myself into the middle of the herring ball. Once there, surrounded by countless thousands of fish all swimming tightly together, I lost all sense of direction and space. My world became limited to herring, scales flashing in the sun and moving together like a huge flock of birds. As I swam among the herring, I was reminded of a scene from the BBC series Blue Planet, where the humpback whales ambush the herring from below like hungry, living freight trains of muscle and mass. This memory made me a little nervous, but on I swam, just another herring following the school.
Pacific herring are just one of the many types of fish and other marine life that use eelgrass meadows for part of their life cycle. Since the meadows provide food and habitat for so many different creatures, they are considered a vital part of the marine environment. Eelgrass also plays an important role in capturing carbon dioxide from the earth’s atmosphere. For these reasons, the Mayne Island Conservancy chose eelgrass as the target of a long-term monitoring program that began in 2009. In this article, we will share our observations from the past 12 years. Unfortunately, our results show an alarming decrease in eelgrass around the shores of Mayne Island during that time.
During the past twelve years we have recorded an overall decline of 26% in the area covered by eelgrass meadows around Mayne Island. Some individual meadows have declined by as much as 58%. We will review our observations at eight locations, including Miner’s Bay, Naylor Bay, Campbell Bay, Bennett Bay, Horton Bay, Gallagher Bay, Dinner Bay, and Village Bay.
Miner’s Bay contains four distinct eelgrass beds. The two beds in the middle are located on either side of Miner’s Bay dock and were likely once connected prior to the installation of the dock many years ago. The two middle beds are the focus of analysis because they have been repeatedly monitored. A decrease of 27% in area covered by eelgrass was recorded between 2009 and 2021, equal to 2.28 acres. Of that area, 0.75 acres was lost north of the dock where the eelgrass grows in shallow sediments over shale bedrock. We suspect loss of sediment due to wave energy (storms and/or large vessel traffic) at low tide to be the cause of eelgrass loss in that specific site. There are a number of private mooring buoys at the edge of the eelgrass beds on either side of the dock with bare patches below them where anchor chains have scoured the bottom.
Eelgrass in Campbell Bay occurs in one large meadow in front of the beach (the one you swim over on the way to the dock), with narrower beds running along the south and north edges of the bay. The large bed in front of the beach was surveyed in 2009, 2018, and 2021 and a decrease in eelgrass area of 24% was recorded during that time. The reason for that decline is unknown.
Bennett Bay boasts the largest continuous eelgrass bed around Mayne Island, though the density of eelgrass is low on the deeper edge. This eelgrass meadow was surveyed in 2009, 2017, and 2021 and a decline in area of 32% was observed during that time. Nearly all the loss has occurred at the deep edge, where growth of eelgrass is typically limited by light penetration. Decreases in light exposure due to increased sediments or phytoplankton blooms are possible causes here.
Our surveys show that the area of eelgrass in Horton Bay has declined overall by 38% since 2009. Between 2009 and 2015 the area covered by eelgrass showed a modest increase of 11%, but there was a 46% decrease between 2015 and 2021. We suspect decreases in water clarity to be the main cause of recent eelgrass loss in Horton Bay. Reduction in water clarity can be caused by a combination of upland development (soil erosion from development and tree cover loss), increased nutrients (septic fields and agricultural runoff), and warming sea temperatures. It is unclear to what extent the Fraser River runoff and associated sediments plays a role in Horton Bay.
Five years of data are available for Gallagher Bay showing a steady decline in eelgrass area from 2009 to 2019 totaling 32%. Much of the decline has occurred along the shallow intertidal edge. Possible causes of decline in these shallow areas include grazing from invasive Canada Geese and extreme air temperatures during low tide events (freezing in winter, heat waves in summer). There are also private mooring buoys in the eelgrass bed where anchor chains have created bare patches.
A 58% decrease in eelgrass was recorded in Dinner Bay since 2009, with most of that decline occurring in the past four years. Like Gallagher Bay, much of the area lost was in the shallow part of the bed. There is also a big hole in the bed towards the deeper edge.
There are just two years of data available for Village Bay (2009 and 2019). This is the only survey site where there was an increase in area covered by eelgrass, with area increasing by 23% between 2009 and 2019. While the large eelgrass bed at Village Bay was not surveyed in 2021, the smaller bed located off the beach north of the ferry terminal was surveyed. That bed did not change significantly since it was last surveyed in 2009 and the eelgrass was lush and healthy in that location.
Like Village Bay, there is only two years of data available (2009 and 2020). During that time, there was a decline in area covered by eelgrass of 30%.
Why is Eelgrass Declining?
It is much easier to record changes in eelgrass than it is to conclusively show the reason for those changes. There are some known causes for decline that we can directly observe such as grazing by Canada Geese, scouring by anchor chains, and shading by docks. While mooring buoys and temporary anchoring have a negative impact on eelgrass beds, and we strongly encourage boaters to avoid anchoring in eelgrass, that impact explains only a very small portion of the loss we are seeing.
Impacts to shallow eelgrass such as grazing by geese and extreme weather events offer possible causes of decline in specific areas, but don’t explain extensive losses in deeper water. Decreases in water clarity is likely a major contributor to declining eelgrass extent. As mentioned, water clarity can be impacted by increases in sediment loads, either from local land development or runoff from the Fraser River. Land development can increase nutrient flow into shallow bays from septic fields, compost piles, and agriculture. Phytoplankton blooms also reduce water clarity and can be caused by a combination of nutrients and warm waters. Increased ocean temperatures are expected with climate change, with associated phytoplankton blooms expected to increase as well.
Interactions among the biological communities connected to eelgrass meadows may also be contributing to changes in eelgrass extent. Many types of algae grow on eelgrass blades and are in turn eaten by creatures like bubble snails. You may have noticed the bright yellow eggs of the bubble snails on eelgrass in the summer. The impact of biological interactions can be hard to predict, especially as they are strongly affected by environmental stressors such as climate change.
What can we do?
We will continue our long-term monitoring surveys. In the summer of 2022, we plan to survey Village Bay, Gallagher Bay, Piggott Bay, Naylor Bay, David Cove, Kadonaga Bay, and north Miner’s Bay. The data we collect is shared annually with local, regional, provincial, and federal governments as well as academic researchers and First Nations. Long-term monitoring programs are very rare, and our program is an invaluable source of information to document changes in the marine environment. If you are interested in access to our geospatial data, it is available on the Strait of Georgia Datacentre here: Link to Geospatial Data
To help prevent the further loss of eelgrass beds, community members on Mayne Island can focus on preventing the negative impacts we know of by taking the following actions:
- Prevent physical damage from mooring chains by anchoring outside of eelgrass beds.
- Reduce erosion from development by retaining as much forest cover as you can on your property and by planning developments for springtime when vegetation has at least one growing season to establish before heavy rains.
- Properly maintain your septic system to prevent nutrient runoff and keep your compost piles covered.