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

ate and percent organic carbon (14 mm/yr;
1.8%, respectively) in an eelgrass bed
were intermediate between an open
estuarine site and a protected island
site. In Izembek lagoon, Alaska, sediment
composition was find to be quite variable
(McRoy 1966; Figure 7). Orth (1977a)
reported that sediment stability resulted
inhigh infaunal diversity within eelgrass
beds.
It is possible that the sediment texture
may influence the eelgrass growth form.
In Denmark, Ostenfeld (1908) found a
narrow-leaved form on wave-exposed coasts
on firm sand, a narrow-leaved form in
shallow water on mixed sand and mud, and a
wideleaved form in deeper water on soft
mud. He concluded that leaf width and
length were directly related to the nature
of the substrate.
Gross effects of eelgrass on sediment
stabilization have been observed. Sand
banks, formerly covered by eelgrass, were
lowered by 30 cm (12 inches) almost
overnight in Salcombe Harbor, Great
Britain, after the plants disappeared in
1931 (Wilson 1949). Many species of
filter-feeding invertebrates, mollusks,
and several flatfishes also disappeared.
Up to 29 cm (8 inches) of sediment eroded
from unvegetated sand banks following a
single storm in Chesapeake Bay, while
little, if any, sediment disappeared from
within a nearby eelgrass meadow (Orth
1977a).
Current Velocity
Moderate current speeds appear to enhance
eelgrass growth. In Puget Sound, Phillips
(1972, 1974) observed the most luxuriant
eelgrass where tidal currents reach 3.5
knots. Conover (1958) found that the
optimum current speeds were about the
average neap and spring tidal current
speeds in the range of 0.6-0.0 knots (30-
40 cm/sec) . Inferential evidence suggests
that rapid currents break down diffusion
gradients across the leaf surface and make
more C02 and nutrients available to the
plants (conover 1968). Conover also found
that eelgrass biomass and production were
strongly influencedby currentvelocity.
If currents are too slow, eelgrass grows
poorly and algae tend to dominate (Proctor
et al. 1900b). Too much current tears
leaves from the plant or erodes the
substrate.
Several studies, especially the work by
Fonseca on eelgrass (Ginsburg and
Lohenstan 1958; Fonseca 1981; Fonseca et
al. 1982a) docunented tb effect of seagrass
leaf canopies on reducing current
flow velocity and turbulence. This
effect, coupled with the dense network of
rhizanes and roots, established an environment
of deposit ion within the boundaries
of an eelgrass meadow, leading to
large p ls of nitrogen in the sediment.
Figure 7. Sediment composition of four
eelgrass beds in an Alaskan lagoon.
Percentages are by weight (a£ ter McRoy
1966 ).
Eelgrass does not grow where wave shock is
regular (Ostenfeld 1908; Phillips 1974).
Tutin (1938) observed only patchy growth
of eelgrass on the south coast of England
that was exped to fairly heavy seas.