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

plant density as a result of their effect
on water clarity. The reduction in
seagrass density could result in an
increased silt load, due to a reduction in
sediment trapping, and increased erosion
of bottom sediments. Where turbid water
prevails, eelgrass populations are limited
to less than 1-3 m (3-10 ft) deep
(Burkholder and Doheny 1968; Thayer et al.
1975a). Eelgrass has been observed down
to 3EI m (98 ft) where the water is clear
(Cottarn and Munro 1954). Dredging
reverses the normal redox potential (h%)
of the sediments of a seagrass system,
which reverses the entire nutrient-£ low
mechanics of the ecosystem.
Hydraulic clam dredges have been used in
Puget Sound. The dredge blasts sediment
to a depth of 45 cm (18 inches) and a
width of 1 m (3 ft). Since eelgrass
rhizome mats are located at a maximum
depth of 15 cm (6 inches), this activity
removes the entire ecosystem. The State
of Washington requires leaving strips of
eelgrass between the dredged strips. In
one location, however, I dbservd that all
the eelgrass had been removed. Several
years ago, the use of these dredges was
banned in Florida and in the Chesapeake
Ray after it was determined that they
directly removed the seagrasses or led to
their removal by erosion.
In North Carolina, bay scallop larvae
attach directly to eelgrass blades (Thayer
and Stuart 1974). Commercial f isherlnen
use hand rakes and bar dredges to gather
the scallops. The bar dredges denude the
bottom of eelgrass over large areas,
stirring up sediments and promoting
sedilnent oxidation so that recolonization
of eelgrass and thus that of the bay
scallops was impeded. In the Niantic
River, Connecticut, the eelgrass is too
dense for the settlement of bay scallops.
When the eelgrass disappeared in the
193EI8s, allowing better wave circulation,
the bay scallop flourished, attaching to
small algae (Marshall 1947). e
logging, rnalntendnce drecxylng, and
agricultural lr~lpacts on water quallty in
this sect~on), while bloinass ciecllned by
96hfter tttree ciredglngs. The mean
percentage reduction of stloot density
rtlnged frotn 33mn beds ~ i t no h dredging
to 719 on beas that were dredged tnree
times. 'i'hr greatest reductiori occurred
after the first dredglriy operiitlori. In
some cases, the eelgrass td~lea to rccwver
following dredging.
Proctor et al, (19dab) documenteu the
various studies done In estuaries of the
Pdcific Northwest where dredging has
occurred. In Oregon, a nutriber of
landfills have resulted in tile loss of
eelgrass habit& either by ciirect dredyiny
or by seuiment deposition. Parlpheral
erosion around the larrifill and diking to
protect the la~lfill Itdvi. also reultd in
loss of eelcjrass in sorne areas. 'i'hey
stated that kiillapa Uay required
raaintenance dredging of 23rr,dkJU m 3
(33d,dlc)cd yd3) per year up to 1977, wlien
the Ariily Corps of Engineers decided to
discontinue it. Yaquina estuary, Orqon,
contained abut 5% of its sediments in the
form of wood chips, sawdust, and plant
material. 'l'hese ntateridls are significant
fractions in estuaries where wood
processing industries dre located. Coos
Bay, in southern Oregon, is heavily
impdcted by industrialization, logy ing,
and shipping. klumboldt Bay is heavily
sedii~ieritect. The high tide area decreas
from lUt3 km2 (3.3 mi2) to 67 km2 (2b mi
Y
in the last 12kJ years. This decrease is
related to hunan activity in the area, in
particular, agriculture and logging.
Eclgrdss seems relatively resistant to
substances that can polson other forms of
narlne life (~lc~oy and elf ferich 198ld).
It appears to concentrate metals without
damage (t3arsdate and Nebert 1971;
Brinkhuis et al. 19dd), but Zlclnan (1975)
postulated that this nakes the metals
dvailable for movemmt up the food chain.
Waddell 11964) documented the impacts of
6. OIL FND ORI;ANIC CHEMICALS
harvesting oysters from selgrass beds in
Humboldt Bay by I-~ydraulic dredges and a Several studies made on eelgrass following
modifled draglrrle-typ dredge. uarvestiny oil spills llave shown only temporary
procedures had severe llnpcts on eelgrass. damage to blades I£ the oil contacts the
Under corrditions of no dredyiny, eelgrass blades in alr (~alby 1968). If the leaf
biomass declined 38% {cf. discussion on remains covered with water, there is no