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COASTAL

Photo: eelgrass © Gabriolan.ca
Eelgrass

Eelgrass meadows: a coastal ecosystem at risk

Eelgrass meadows are among the most productive ecosystems in the world,44 and among the most threatened.45 They are declining globally, with mixed and often uncertain status along Canadian coasts. Seagrass meadows, which include eelgrass, have declined at an average rate of 7% per year around the world since 1990, an acceleration from an annual decline of less than 1% prior to 1940.46 Declines are most often associated with stressors, such as eutrophication and increased turbidity of coastal waters, mainly related to the growth of coastal human populations. The global analysis on which these rates of decline are based45 does not include Canada due to lack of adequate trend data.

Major regional declines have occurred in the past. In the early 1930s, thousands of hectares of eelgrass disappeared in eastern North America,46 attributed to eelgrass wasting disease, although climatic conditions may also have played a role.47

Eelgrass, a flowering marine plant that forms extensive subtidal beds in sand and mud along coastlines, traps particulate matter and plankton and provides habitat for invertebrates, fish, and marine mammals. Eelgrass is an important food for migrating and wintering waterfowl, and provides foraging areas for other birds.48-50

Pacific On the Pacific coast, where eelgrass beds are spawning grounds for herring and rearing habitat for salmon, some declines may be due to the Pacific oyster, which was introduced for oyster farming and has spread into the wild. Oysters alter habitat physically and may also cause sulphide to accumulate in sediments – the net result is that eelgrass is typically absent seaward of oyster beds.51, 52 Other declines are related to development of coastal areas, for example for log storage and harbours.41 A nonnative dwarf species of eelgrass that thrives higher up in the intertidal zone than does native eelgrass has taken hold in some areas of southern B.C., with mixed ecological consequences. Colonization of mudflats by dwarf eelgrass meadows on Roberts Bank in the Fraser river estuary53-55 has displaced migratory shorebirds that graze on the thin film of organic matter covering the mud.55

James Bay Eelgrass beds along the east coast of James Bay were among the most extensive in North America, covering 250 km2 prior to their rapid decline around 1998.56 Since the decline, eelgrass has shown signs of recovery,57 but neither the cause of the decline nor the present status are well understood.48 Alternative explanations for the decline in James Bay have been put forward, such as:

  • an outbreak of eelgrass wasting disease triggered by a year with unusually high summer and winter temperatures, along with changes to habitat from coastal uplift and climate change;57
  • impaired growth and survival due to reduced salinity of water in James Bay resulting from larger and more frequent discharges of fresh water via the La Grande River, due to diversions.50
Decline of eelgrass in James Bay
Dry leaf biomass (g/m2)
Graph: eelgrass decline in James Bay. Click for graphic description (new window).
Note: samples were taken at several depths at six
sites – this figure shows results typical at all depths
for five of the six sites. The sixth site showed no
change. No data for 1992 and 1996-1997.
Source: adapted from Hydro-Quebec and
GENIVAR Group Conseil Inc., 200557

Atlantic Coast and Gulf of St. Lawrence Compiling results from a number of mainly short-term studies provides a picture of a general decline in eelgrass and some abrupt die-offs, along with some areas with stable to increasing trends.44, 49 One factor in declines on the Atlantic coast is the spread of the invasive green crab, which can uproot eelgrass plants.53 Some study results:

LocationYearsEelgrass trends
Lobster Bay, N.S.1978 to 2000estimated losses of 30% and 44% in two areas58
4 Nova Scotia inlets1992 to 2002loss of 80% of total intertidal area occupied by eelgrass59
13 southern Gulf of St. Lawrence estuaries2001 to 2002biomass decline of 40%60
Antigonish Harbour, N.S.2000 to 2001biomass decline of 95% followed by 50% decline in geese and ducks that feed on the eelgrass61
Newfoundlandpast decadeincrease in abundance, based on local knowledge, possibly due to milder temperatures and changes in sea ice44
Gulf of St. Lawrence in QuebecvariousManicouagan Peninsula distribution expanded (1986 to 2004); generally also expanding or stable in other areas62
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