Eelgrass Update and Results Summary 2025
The Mayne Island Conservancy has been conducting eelgrass mapping since 2009 around the shores of SḴŦAḴ | Mayne Island. In 2019 and 2020, we also mapped 20 select locations throughout the Southern Gulf Islands around the shorelines of Valdes, Galiano, Parker, Samuel, and S’DÁYES | North Pender Islands. Most of the sites we have mapped have been surveyed just once, but 15 eelgrass beds around SḴŦAḴ | Mayne Island and one bed at S’DÁYES | North Pender Island have been surveyed multiple times, allowing us to observe changes in the area occupied by eelgrass over time. The monitoring results from these 16 sites were compiled this year in a Results Summary, which includes images showing the years of greatest and least extent, and a graph showing the change in area occupied by eelgrass over time.
The Summary of Results is Available Here.
The geospatial data for all survey sites and years is publicly available and can be accessed on the Strait of Georgia Data Centre here, or by request to biologist@mayneconservancy.ca.
What We Have Learned
Based on observations collected between 2009 and 2024, we have observed a decline, on average, of 32.5% in the area covered by eelgrass. This is calculated using the oldest and most recent year of data for each of 16 monitoring sites. This represents a decrease in area covered by eelgrass of 19.8 acres. Most of the loss has been observed along the nearshore, shallow edge of eelgrass beds, with less loss observed in deeper areas.
Causes Of Decline
There are several known and suspected causes of eelgrass decline. It can be a challenge to figure out which ones are the most important. With each year of data we get closer to figuring it out, but there are still a lot of questions unanswered. Below we discuss some of the things we think are driving eelgrass loss and why.
Competition For Light With Sea Lettuce (Ulva spp.)
In the shallow nearshore edge of eelgrass beds, we have observed sea lettuce competing directly with eelgrass for light. Often, sea lettuce creates a thick mat covering the eelgrass, and we suspect this is the leading cause of eelgrass decline along the shallow edges of eelgrass beds within our study area. Sea lettuces, of which there are several species known to Mayne Island, are annual species of green algae that can create extensive, thick mats under high nutrient conditions. These conditions are most likely to occur in bays with shallow, slow-moving water, and where there is a significant input of nutrients from the land, such as from agriculture or septic systems. We have observed sea lettuce covering eelgrass in dense mats in the following locations: Village Bay, Gallagher Bay, Dinner Bay, Navy Channel Beach, and Grimmer Bay, North Pender Island. Observations at Village Bay are particularly interesting, where sea lettuce was removed each year from 2000 to 2019 by the Village Bay Improvement Association, and where eelgrass decline occurred during 2019-2022, a period of time when considerably less sea lettuce was being removed. Increases in Ulva spp. and resulting seagrass decline has been well documented in other parts of the world.
We are currently collaborating with SeaChange Marine Conservation to better understand what might be driving increases in sea lettuce. Preliminary water sample data collected by Susan Anthony, Biological Researcher with SeaChange, suggests that nutrient inputs from local watersheds are not likely driving increases in sea lettuce. Increases may possibly be driven by increased nutrients from regional sources such as the Fraser River, increases in water temperature, or a combination of the two.
Reduced Light From Shoreline Development
Erosion from development along shorelines and in watersheds results in lower light availability for eelgrass. With increased severity and frequency of heavy rainfall events such as the one we experienced in the fall of 2021, the impact of sedimentation on eelgrass beds is expected to increase. Retaining vegetation along shorelines and waterways is the best way to reduce erosion. When surface water is drained and directed into ditches, it typically moves faster and in greater volume, increasing its erosion potential and retaining suspended sediments. You can help mitigate this issue by, when possible, allowing water to slow down and spread out like it would naturally in an undeveloped ecosystem. Retaining natural wetlands and wet forests that help slow down surface water is a great way to do this.
Boat Wake Stirring Up Sediments
During our dive surveys, we have observed that water clarity (and light availability for eelgrass) greatly decreases within 3-8m of the shoreline every time a boat (usually a ferry) passes by because the waves stir up the sediments along the shore. In some locations, the boat traffic is frequent enough that this leads to a significant decrease in light availability for eelgrass in the shallow edge. We suspect this is a contributing cause of decline in eelgrass beds located near high traffic areas, particularly ferry routes.
Physical Damage From Anchor Chains
In some locations, mooring buoys are located in eelgrass beds. As the moored boats move back and forth in the wind and current, the heavy chain drags along the bottom, creating a circle on the seabed where no eelgrass grows. These patches are easily observed during our dive surveys. Please don’t anchor or install mooring buoys in eelgrass beds. Of the known human impacts to eelgrass, this is the easiest to identify and the easiest to avoid.
Biological Factors
European green crab and seagrass wasting disease are two biological impacts to eelgrass that have been described in the region. We haven’t observed either in our survey sites to date therefore we don’t think either of them explain the losses we have observed in the past 16 years. A third biological impact is grazing pressure and nutrient input by non-migratory Canada geese, which has greatly increased around Vancouver Island and the Gulf Islands since they were repeatedly and intentionally introduced between 1926 and 1991.
How You Can Help
- Avoid anchoring boats or installing docks and other structures in eelgrass meadows. Properly maintain your boating infrastructure to prevent derelict docks and boats. As we get into storm season, check that everything is secure.
- Maintain forests and vegetation along creeks, streams, and shorelines to prevent erosion.
- Minimize sedimentation during construction activities by completing work in late spring or summer and initiating a re-vegetation plan immediately after construction.
- Ensure your septic system is working properly and not leaking into nearby water bodies such as streams or the ocean. Make sure your compost and manure piles are covered to avoid excess nutrients and resulting algae growth.
- Use water-permeable surfaces on driveways and patios to reduce water flowing directly into the ocean.
- Maintain and create wetlands in order to capture and slow surface water and allow water to naturally filter into and replenish groundwater reservoirs rather than draining it straight into the ocean through ditches and drainpipes.
- Volunteer with us as a Marine Citizen Scientist
2 Comments
Rosemary Cornell · 02/10/2025 at 9:12 am
Rob, thanks for the informative article. You point out that one reason for loss of eel grass is sediment accumulation. At Dinner Bay there is massive shoreline erosion. In Dec 2021 a large section of the bank on the north side of the beach collapsed bringing down a dozen or so large trees. The disc gulf area has resulted in compacted soil all the way up the slope above the collapse area, reducing water absorption. In December, 2024 I witnessed gushing streams carrying soil down to the beach on the east side of the bay adjacent to the grassy area of the park. I took video of it. Discussion on this issue with Parks and Recreation is needed.
Rob Underhill · 03/12/2025 at 9:05 am
Hi Rosemary. Thank for your comment. I don’t think soil compaction was the primary reason for the erosion at that site, or that the erosion can be blamed on the disc golf course. There was a redirection of water prior to the bank failing at the site which was likely the cause. Compaction at that site has been caused by past logging, the hiking trail, and disc golf course users. Efforts have been made by the Parks and Recreation Commission to mitigate compaction from park use and slow surface water by laying wood chips along paths and fairways and retaining large rotting logs.