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

great annual range of water temperature
(-6' C to 27' C in Izembek riagoon; Nova
Scotia: -2O C to 2 4 O C; Rhode Island:
-2' C to 27' C; North Carolina: 0' C to
27O C) m hilli ips unpubl. data). There is
evidence that eelgrass at the northern and
southern limits of distribution on the
Pacific coast and on the Atlantic coast
may have much greater thermal tolerances
than that in the midportion of the Pacific
coast range (Phillips et al. 1983b). In
Puget Sound, the normal annual range of
water temperature is 6O C to 13O C.
Occasionally, during low tides on sunny
days in summer, there may be brief periods
of elevated water temperatures over
eelgrass (up to 18O C), but this is rare.
In 1974, Phillips warned that heated water
released into eelgrass habitats could
disrupt the reprductive cycle, presumably
interfering with the normal temperaturedependent
periodicity of flowering and
germination. Considering the present
contributions of McMillan (1978) and
McMillan and Phillips (1979), we now know
that the developmental cycle of eelgrass,
as well as its presence in an area, is
temperature related. Populations of the
plant have specific thermal adaptive
limits. The Pacific Northwest does not
have a significant problem with thermal
water release from power plants or
industry. We know from work done in
Biscayne Bay on Thalassia (reviewed by
Zieman 1982) that plants were killed when
the water temperature was elevated C
above ambient and were harmed by an
elevation of 3O C. If the Northwest
should encounter such thermal releases, we
must consider the impacts on eelgrass.
McRoy and Helfferich (1980) noted the
susceptibility of eelgrass in the North
Atlantic to a- very slight increase in
water temperature in 1931. During that
period over 90% of all eelgrass died.
Recent evidence shows that these strains
have a greater thermal tolerance range
than do Pacific coast stocks.
Considering its worldwide distribution,
eelgrass grows in a wide range of
salinity. It is euryhaline. Biebl and
McRoy (1971) reported that eelgrass in
Izembek Lagoon, Alaska, maintained an
osmotic resistence to salinity changes
from freshwater to 93 ppt. At 124 ppt
leaves were killed. Positive net
production was maximum at 31 ppt, as was
photosynthetic rate, but was found in a
range from freshwater to 56 ppt
(photosynthetic rate was zero in
freshwater and at 62 ppt). Respiration
was depressed in freshwater but only
slightly affected from 31 ppt to 93 ppt.
Ostenfeld (1908) considered that a
salinity range for eelgrass in Denmark
from 10 ppt to 30 ppt was optimum for
growth. In Japan, Arasaki (1950) reported
that eelgrass growth was best from 23.5
ppt to 31 ppt but poor at 18.0 ppt and
stop@ below 9.1 ppt, although plants did
not die. Tutin (1938) observed eelgrass
in a bay in England in 42 ppt with no
damage. He grew plants for a considerable
period in the laboratory in salinities
ranging from 10 ppt to 40 ppt without
harm. It is known that eelgrass can
acclimate to a changing salinity regime.
Often extensive meadows grow off the
mouths of streams where the salinity drops
to freshwater level at low tide. The
plants appear to flourish. Low salinities
appear to enhance seed germination in
spring (Tutin 1938; Arasaki 1950; Phillips
1972).
6.6 MANAGENENI! NEEDS
Wilson (1981) noted that in order to
manage our coastal estuaries properly, we
need a better understanding of the ecology
of eelgrass, which requires knowledge of
the causes of its distributional patterns.
These patterns are affected in the short
term by the increasing demands by modern
man on the coastal and estuarine
environments. Sedimentation, dredging,
storms, currents, sewage, power-plant
effluents, and factors such as
adaptational tolerances of the plants as
regulated by their genetic patterns, all
affect these distributional patterns.
Continued monitoring of our estuarine
areas is necessary if we desire to remain
aware of biological and environmental
changes. This awareness is needed for
intellisent manaqement of this resource.
S arti-na alternif lora (saltmarsh
9---T
cordsrass was introduced into Willapa Bay
dur&g the 1940's or early 1950's: The
species has rapidly spread into upper
intertidal and mudflat communities and is
displacing the more productive native