The ecology of eelgrass meadows in the Pacific Northwest: A ...

hydrocarbons and other herbicides, into
our shallow coastal zone. Additions of
toxic materials are known to affect animal
communities (Thayer et al. 1975a), but
little has been done to document their
direct effect on eelgrass. More research
is needed, only on the bioaccwnulation
of metals and toxic chemicals by the
plants but also their accumulation and
possible transfer thruugh the grazing and
detritus food chains and nutrient cycles.
In an attempt to decimate eelgrass in Nova
Scotia to enhance oyster growth, Thomas
(1968) found that the herbicide,
butoxyethanol ester of 2, 4-Dl was most
effective in killing the plants. This was
applied to the plants in the field.
Correll and Wu (1982) found that the
herbicide atrazine, commonly used in corn
production, stimulated photosynthesis at
75 ug/liter in eelgrass, but inhibited it
at 650 ug/liter. This herbicide was
tested following a gradual decline in
populations of many species of submerged
vascular plants in the upper and
midreaches of Chesapeake Bay. They noted
that the temporal and spatial use pattern
of atrazine arourul the bay correlated well
with the observed decline in the estuarine
plant populations. It is clear that
herbicides can be extreme1 y damaging.
Runoff froin waterways that drain
agricultural areas can severely damage
eelgrass systems by sdirnent transport and
by herbicide contents. These waters
should be monitored for these chemicals.
6.4 RQAT USE
Impacts to eelgrass meadows in the Pacific
Northwest do not normally result from
physical disturbance involving cuts made
by boat propellors. In south Florida,
Zieman (1982) stated that these cuts are
the most common form of disturbance to
seagrass beds.
The numbers of black brant are declining
in the Pacific Northwest, owing to an
increase in human use and development of
the coastal area (Reiger 1982). Between
the 1940's and 1981 brant stopping in
Washington declined by 74%; in Oregon, by
90%; and in California, numbers declined
by almost 99%. Reiyer attributed this to
the draining of coastal marshes, the
conversion of bays to marinas, and the
impact of an increasing number of weekend
bters, who have driven wintering brant
to Mexico where populations have grown
from 80,000 in the early 1950's to as many
as 130,000 tcday.
Even if these numbers are only
approximately correct or merely indicate
trends, an aggressive program is needed to
create protected zones around the large
eelgrass beds which harbor the brant.
6.5 TEMPERATWE AND SALINITY
Temperature is probably the most critical
of the suite of environmental factors
that affect marine life. ~t is a
controlling factor. In the case of
seagrasses, it affects growth,
development, and phenol- ical cycles.
McMillan (1978) and Phillips et al.
(1983a) regorted that eelgrass populations
have upper and lower thermal tolerance
levels and that temperature regimes at
local sites along broad latitudinal
gradients on both coastlines of North
America control the occurrence and timing
of flower and seed production.
McMillan (1978) subjected three different
eelgrass populations from Puget Sound,
each with different leaf widths, to three
different temperature treatments. After 4
mo, each population maintained its
original leaf width, indicating that local
populations maintained distinct genetic
l i m i t s of ecoplasticity to their
environments, These tolerance levels vary
with the local area. Biebl and McRoy
(1971) not only found a difference in the
thermal tolerances of intertidal pool and
subtidal eelgrass in I zembek Lagoon,
Alaska, but also found that both forms
could withstand a range of water
temperatures from -6O C to 27O C, while
plants from Puget Sound and California
were killed at -6O C. McMillan and
Phillips (1979) found that eelgrass in
Alaska had more heat resistance than
plants from Puget Sound or California,
owing to the selective influence of a
greater environmental variability in
Alaska.
In the Bering Sea and along the Atlantic
coast of North m-erica, there is a fairly