Being underwater during our annual eelgrass monitoring surveys is one of my favorite parts of my job as a biologist with the Mayne Island Conservancy. The nearshore marine environment is rich in energy and nutrients, so everywhere you look is teeming with life, nowhere more so than amongst the waving blades of an eelgrass meadow.

Unfortunately, eelgrass meadows continue to decline around Mayne Island. Since 2009, when we began monitoring, an area the size of the Mayne Island School field has been lost on average every year. This represents a significant loss of habitat for marine organisms, and a loss of the ecological services eelgrass meadows provide us. We have completed all of our annual eelgrass monitoring surveys for 2023. We surveyed nine locations over eleven days this year. Read on for a review of our observations and maps showing the change in the extent of eelgrass over time.

How do we complete an eelgrass survey?

Our surveys are completed by a team of three. This year, our team members were Rob Underhill, Biologist, Justine Apostolopoulos, Stewardship Coordinator, and Alistair Mar-Paine, Ecosystems Technician.

Two of the surveyors don wetsuits, masks, snorkels, and fins, and take to the water. The third team member paddles a kayak while recording observations and GPS waypoints. The snorkelers guide the kayaker to the edge of the eelgrass bed, where the kayaker records a waypoint with a GPS. Waypoints are recorded every 3-5m around the entire perimeter of the eelgrass bed to record its extent. Then, a video is recorded following the same route as in previous years. This allows a visual comparison to past years, so we can see the change in the eelgrass bed over time. The surveyors also record the depth of water at the shallowest and deepest edges of the eegrass bed, and estimate the overall percent cover. 2019 Eelgrass Video.

Mayne Conservancy eelgrass survey team 2021. Maddy Litster, Katie Kushneryk, and Rob Underhill. Photo taken by Jillian Lynn Lawson.

Back at the office, the team converts the recorded waypoints to a map by “connecting the dots.” This data for the current year is added to a database that contains all of the results the Conservancy has recorded since 2009. This database is shared broadly with our partners, including resource managers and academic institutions throughout the world. Though the Mayne Conservancy is well placed to observe and record local conditions, other larger organizations and institutions look at ecosystem change at a regional or global scale, and our data is an important contribution to these broader efforts.

Dinner Bay

After declining significantly in area between 2017 and 2021, the eelgrass bed at Dinner Bay has remained relatively constant since then. The loss that occurred between 2017 and 2021 was most likely a result of competition with sea lettuce, which has formed a thick mat in the area where eelgrass was previously recorded.

Watch the slideshow below to see the change in eelgrass extent.

Village Bay

The area covered by eelgrass in Village Bay continued to decline, though at a slower rate than was observed between 2019 and 2022. We recorded a 15% decrease in extent in the past year. The eelgrass bed has now decreased in area by 3.71 acres (60%) since 2017. Like Dinner Bay, the decrease here is in the nearshore edge where a thick mat of sea lettuce has been observed to smother the eelgrass shoots.

Campbell Bay

The eelgrass bed at Campbell Bay has also continued to decline, with the rate of decline increasing slightly since the last survey in 2021. The site has decreased in area by 0.9 acres (40%) since 2009, with a 20% decline observed in the past two years. The cause of decline at Campbell Bay is not entirely clear. Possible causes include competition with sea lettuce and physical damage from recreational boaters visiting for the day.

Miner’s Bay

The two eelgrass beds (south and north of the Miner’s Bay Dock) remained relatively constant in extent since they were last surveyed in 2021. There has been a gradual decline of 29% since 2009. Three causes of decline have been observed at these locations: increased wave action from ferries and as a result, cloudier water and less light; a section of eelgrass that grew in thin sediments over shale bedrock and washed away between 2009 and 2017; and damage from the increasing number of boats mooring in the eelgrass bed.

Seal Beach

Despite significant reductions in light availability every time a ferry passes, with resulting wake stirring up sediments along the beach, the eelgrass bed at Seal Beach has been relatively stable in extent since 2009, with only a 14% decline. We recorded a 9% increase in extent between 2021 and 2023.

Naylor Bay

This is our favorite eelgrass bed to survey because it is always full of fish and a diversity of different marine creatures. The shoreline is nearly pristine, thanks to the Tsartlip First Nation, who have stewarded the land and waters since time immemorial. The eelgrass bed here is lush, and has only declined by 12% since 2009. We were very pleased to record an 18% increase in eelgrass extent at Naylor Bay between 2020 and 2023, following a significant decline between 2009 and 2020.

David’s Cove

2023 was the first time since 2009 that the eelgrass extent in David’s Cove was surveyed. We observed a 36% decrease in area covered. Many boats are moored in the eelgrass here, and damage from dragging moorage chains is the most obvious cause of decline at this location.

The effects of “green tides” on eelgrass

This is a good time of year to talk about green tide, which is a name given to large green algae blooms such as sea lettuce (Ulva spp.). Since we’ve just reached the end of the peak growing season, green macro algae like sea lettuce are at their peak of abundance and have washed up in a thick mat on many beaches around Mayne Island. Green tides have caused significant ecological problems, not only on Mayne Island, but throughout the world, from Venice Lagoon to Chesapeake Bay.

Although green algae play an important role in marine ecosystems, they can negatively affect other species in the environment when their growth exceeds normal patterns. The three most important factors determining the growth of green algae are light, water temperature, and nutrient availability. When all three are high, green algae grow very quickly, resulting in a large amount of organic material accumulating in the nearshore environment. When that material decomposes, the oxygen in the water is converted to carbon dioxide, resulting in suffocation of fish and other marine organisms, while also lowering pH (ocean acidification) which makes it harder for shellfish like clams to make their shells.

Eelgrass competing for light with sea lettuce at Gallagher Bay.

In addition to reducing oxygen and contributing to ocean acidification, there are other negative impacts associated with green tides. On Mayne Island for example, we have observed sea lettuce smothering eelgrass, and we suspect increases in sea lettuce are the primary reason for decreases in eelgrass extent along the nearshore edge of eelgrass beds. Village Bay is the most significant example of this, where 3.71 acres of eelgrass (60%) has been lost in just the last four years.

Eelgrass and climate change

Considering eelgrass is known to be a valuable habitat for many marine organisms, as well as to play a role in erosion control and carbon sequestration, I was motivated to review the scientific literature to better understand the possible causes of green tides around Mayne Island. The questions I started with were: To what extent has the abundance of sea lettuce in the Salish Sea been monitored over time? What studies exist identifying the causes of sea lettuce increase in the Salish Sea? If nutrient increase is a driving force, are land-based sources of nutrients such as septic systems and agriculture important?

I found that though there have been a lot of studies in various places in the world describing the negative impacts of green tides on sea grasses (and cases where human nutrient inputs drive said increases), few studies have monitored sea lettuce abundance in the Salish Sea. My findings suggest land-based nutrient input in shallow bays with high flushing rates and relatively small watersheds, such as those around Mayne Island, is unlikely to be the primary cause of green tides here. The Salish Sea is naturally a nutrient-rich body of water, and green tides here are more likely a result of increases in ocean temperatures than increases in nutrient input from local land sources. Our work to monitor change in eelgrass meadows continues to document important changes in our environment that would otherwise go unnoticed. While there are some things we can do locally to prevent damage to these important environments (such as avoiding anchoring or mooring within them) some of the causes of decline are a result of climate change. Observing local impacts of climate change is a reminder that this crisis impacts us all, and will require action at all levels—from individual behaviour change to government policy— in order to overcome.

If you are a researcher or natural resource manager and would like a copy of our geospatial data please contact Rob Underhill at


Herbie Rochet · October 12, 2023 at 2:55 pm

Very interesting article, Rob, and excellent illustrations! Thank you and your team for monitoring Mayne’s eelgrass beds so carefully across the years.

    Rob Underhill · December 5, 2023 at 4:37 pm

    Thanks Herbie!

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