Eelgrass

The following section gives an overview of design, management, and monitoring recommendations for eelgrass transplant projects.

Ecological Context

Eelgrass meadows are a limiting habitat along BC’s coast that create structured and stabilized habitats to support a large diversity of species. They recycle nutrients, stabilize sediments, produce oxygen, sequester carbon, and provide refuge and foraging opportunities for multiple life stages of hundreds of invertebrate, fish, mammal, and bird species (Fig. 11). Coastal development, such as dredging, infilling, aquaculture, and logging activities, has significantly reduced eelgrass meadow distribution along BC’s coast through habitat alteration and introduction of invasive species, which in turn reduces marine life biodiversity and productivity. Since these habitats carry strong ecological and economic value, they are an important focus of habitat protection and shore zone restoration efforts in coastal British Columbia.

Biological Context

Eelgrass is a perennial rooted vascular plant that forms communities in shallow subtidal and marine to estuarine systems. Accordingly, eelgrass meadows have strong interrelationships with subtidal, intertidal and backshore communities and environments. There are two dominant species along British Columbia’s coast: the native species, Zostera marina, which tends to grow in the subtidal and intertidal regions (+1.8 to -6.6 metres CD); and the introduced species, Zostera japonica, which grows higher in the intertidal zone (+1.2 to +2.4 metres CD). Eelgrass communities are predominantly located in areas of uniform relief and can be found rooted in a range of sediment types.  Their rhizomes bury from 3 to 20 cm below the sediment surface, with deeper burial generally associated with loose or unconsolidated deposits.

Eelgrass absorbs nutrients from the sediments, then deposits them back to the seabed each season when the leaves decompose. In addition, shoot density forms a rhizome mat that stabilizes the substrate materials and over time affects sediment size and nutrient cycling. This process exemplifies how physical characteristics determine biological communities, which in turn alter and shape the physical environment.

Due to this cycle of environment and community alteration, it is important to understand what will happen to the habitat that eelgrass restoration efforts may replace. Productivity and species assemblage of the transplant site need to be characterized before eelgrass transplants occur to avoid compromising ecosystem functions and services, such as areas of high bivalve productivity.

Figure 11

Figure 11

Physical Context

Previous research has indicated the following conditions as providing suitable habitat for eelgrass colonization:

  • Water depth range between +1.8 m and -6.6 m relative to chart datum (CD)
  • A generally flat seabed
  • Quiescent brackish water
  • Salinity range between 10 and 30 psu
  • pH between 7.3 and 9.0
  • Sediments types ranging from unconsolidated silts and clays to dense sands, although colonies have also been found in gravel and cobble substrates
  • Wave-induced bottom orbital velocities less than about 1.8 m/s

Seasonal variation in water temperature and light affect eelgrass meadows’ net primary productivity (NPP), which is maximized at photosynthetic photon flux density (PPFD) 300-350 μm, and saturates at 350-500 μm. Light penetration is a function of depth and water quality, and therefore varies by site. Seasonal and interannual temperature fluctuations also impact NPP, with increasing temperatures triggering stress, decreased NPP, and a shift to sexual reproduction.

Transplant and Monitoring Method

The following transplant method has achieved success in meeting or exceeding eelgrass density of donor beds within two to three years of transplanting at suitable transplant sites.

  • Site selection: Areas with low habitat value located at the wrong elevation for eelgrass can be modified by constructing a rock berm and filling it in to the right elevation for eelgrass, or dredging areas that are too shallow to make better eelgrass habitat. Alterations to the physical habitat such as berm construction and/or dredging should only be undertaken under the guidance of appropriately qualified and licensed professionals. Refer to Chapter 3 of this report for other considerations for projects where changes to the physical environment are contemplated.
  • Transplant sites need to have sufficient light and water quality, minimal macroalgae, and minimal crab or clam bioturbation. Dungeness crabs and Canadian geese tend to uproot or graze upon eelgrass, therefore avoid or deter these species from transplant sites.
  • On BC’s south coast, eelgrass prefers sandy mud substrate, while on the central and north coasts it has been found in cobble and gravel substrate. This regional variation indicates the importance of understanding each region and site’s ecological history and physical characteristics.
  • Harvest shoots that have rhizomes that are at least eight cm in length with a minimum of three nodes.Store them in fresh seawater and keep them cool and shaded.
  • Anchor each transplant shoot individually with a plain steel washer, which will hold down the shoot until it can grow new roots.
  • Plant at high density. Each square metre should contain clusters of ten shoots planted at a high density.Dense patches are more resilient to erosion or detachment generated by current and wave action. This phenomenon is comparable to a single tree in a lot in contrast to the strength of a forest.
  • After six months check that plants are surviving and multiplying. Check for erosion and plants whose growth is restricted due to being buried in sediment.
  • 100 percent plant coverage is usually reached within two to three years. Monitoring should occur annually for five years following the transplant.

This transplant method considers transplants successful if plant density meets or exceeds pre-development or donor bed levels and seabed coverage approaches 100 percent with continuous and dense plant growth.It has been speculated that when the steel washers rust, they chelate with sulfides in the sediment and enhance overall sediment oxygen content. This may facilitate the transplanted shoots growth and survival. Additional research into this phenomenon is required for validation purposes.

The biggest obstacle in eelgrass transplant methodology is locating appropriate transplant sites. Ideal site conditions include quiescent estuarine waters and marine locations with favourable substrate conditions and low to moderate wave exposure. Insufficient analysis of on-site wave energy can impair the effectiveness of shore zone habitat restoration projects. Another important consideration for eelgrass transplants is that Atlantic eelgrass transplant methods are unsuitable foruse as a template for Pacific transplants, since the eelgrass species are often smaller and annual, and the characteristics of the available transplant sites may be very different than those in coastal BC waters.