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

Washington State Game Dept., pers. comm.,
1982), with spring counts averaging 47,392
birds (up to 58% oE the Pacific Flyway
passes through Padilla ~ay). An average
of 50,rilBB ducks winter on Padilla and
nearby Samish Bays (over 6,LMB are diving
ducks: canvasbacks, scaup, go ldeneyes,
buffleheads, and scoters on ~adilla Bay
alone; several of these species feed on
eelgrass or animals within the system).
In northern Puget Somld, Sitnenstad et al.
(1979) recorded that the principal
wintering sites for black brant were
Padilla Bay, Samish Ray, Discovery Bay,
and Sequiln Bay. In April up to 18,00U
brant were recorded in Siunish Bay, 55,Om
in Padilla Bay, and 6,000 in Discovery
Bay
At Grays Harbor an estimated 5(21,00U ducks
and 6,000 brant use the estuary during
winter (Proctor et al. 1990b). Grays
Harbor also supports more than 150,01dU
shorebirds during peak migrations and is
an important link in their migration (ACOE
1977b). In Willapa Bay, 50,000 black
brant overwinter (proctor et al. 1980b).
Only 1,500-2,2l30 brant overwintered in
1981-82 (Washington State Game Dept.,
pers. comm., 1982). At peak periods
Willapa Bay harbors 200,O(a0 or more
waterfowl. It is known rmw that Willapa
Bay and Humboldt Bay are important
wintering areas for the canvasback duck
(850 winter in Willapa Bay with peak
populations at 1,4II0 to 1,600; Proctor et
dl. 1980a,b). Wintering populations of
ducks in Humboldt Bay are 124,000 with an
additional 35,000 black brant.
In the Yaquina estuary, black brant arrive
in late October-ear 1 y November, increase
in numbers to 350-525 birds in mid-
January, and further increase to 780-914
birds in late March (R.D. Bayer, pers.
ccmn., 1983).
Tfius, there is a large list of birds that
directly eat or are dependent on eelgrass
and its food webs ee able 14). The
nutrient and energy flows that these birds
control and drive are emrimus in scope.
4.8 TROPHIC RELATIONSHIPS AND GENERAL
mum
As with all the world's ecosystems, the
eelgrass system supports both grazing and
detrital food webs. Owing to the high
prductivity of eelgrdss and its sloughing
of leaf material from the shoots, a third
pathway of energy flow is evident, that of
leaf and detrital exprt.
Recent work in the tropics has documented
the increasing importance of the direct:
use of leaves and epiphytes (grazing,
herbivory), but in seayrasses generally
and eelgrass in particular, the detrital
food webs within the eelgrass meadows are
still the primary pathway of trophic
energy transfer (Kikuchi 1980).
Very few organisms use the fresh eelgrass
plants in the Pacific Northwest as food.
Table 14 lists the waterfowl which are
overwhelmingly the dominant grazers on
eelgrass in the region. McRoy and
Helfferich (1980) listed only the green
urchin, Stronglyvcentrotus drobachiensis,
which consma2 fresh eelgrass.
It would be useful to have abundant data
on the amount of energy or production
which is channeled through direct
herbivory, the detritaL E d webs, and the
export route. Unfortunately, only the old
study done by Petersen (1918) in Denmark
and the relatively recent one conducted by
Thayer et al. (1975b) have documented the
energy flows quantitatively. Thayer et
al. (1975b) concluded that the eelgrass
system exports to adjacent systems, on the
basis of a net incrase of sedirnent carbon,
but no direct work was done. They also
calculated that the macrofauna in the
eelgrass bed in North Carolina consumed
energy equivalent to 55% of the net
prduction of eelgrass, phytoplankton, and
benthic algae in the bed. 'hey calculated
a gross-growth ecological efficiency of
12% for the macrofauna in the bed and 24%
for the fish community, suggesting a
fairly efficient system with a surplus of
energy to support the food webs. The high
efficiency for the fish, in large measure
a result of high proportions of juveniles
in the community, suggests that the
eelgrass system provides resident fish
with superior shelter, food, and
protection. These fish probably spend a
relatively small proportion of their
energy coping with environmental extremes,
searching for food, and escaping from
predators, and can use a relatively large
part of their consumed energy for growth