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

storms erode and dislodge some eelgrass,
but most is very persistent (~igure 2).
Gallagher et al. (In prep.), however,
found that ~eriodic storms do dislodge a
large amount of eelyrass in winter and
summer and deposit the material in
adjacent marshes in Netarts Bay, Oregon.
The eelgrass litter constituted between
14% and 35% of the dead material in the
marshes. Their conclusion is that in
estuaries where seagrass beds ad join
marshes, the trapping of eelgrass litter
in the marsh provides a mechanism for
retaining and recycling nutrients within
the wetlands and preventing their loss to
the oceanic system. These nutrients )nay
be passed back and forth between the
eelgrass and marsh systems. Also, in
Netarts Bay, Kentula (1983) calculated a
total annual leaf loss which varied from
25 to 111 g dry wt/m2. While the belowground
biomass was retained within the
meadow, she observed that some of the
;Iboveground biomass was carried shoreward
and trapped by the salt marsh and some was
transported out of the estuary. The
presence of eelgrass material in cores
taken in Oregon salt marsh sediments
implied the net transport of eelgrass
(Jefferson 1975).
Probably normal Leaf defoliation and
replacement is the source of most of the
detached leaf material observed in the
Pacific Northwest. Short (1975) estimated
that eelgrass experienceda70% seasonal
defoliation rate. Proctor et al. (1980b)
used. this number and based on standingcrop
estynates from Phillips (1974; 580 g
dry wt/m , 260 tons/acre) and others, to
calculate that eelgrass meadows in the
Pacific Northwest produce annually about
30,000 kg (66,0(110 tons, dry matter) of
eelgrass leaf material, of which 20,909 kg
(46,000 tons) dry weight is defoliated
annually to become detritus: 8,400 ha
(21,000 acres) of eelgrass in Washington;
2,000 ha (5,BBB acres) in Oregon; and
1,600 ha (4,000 acres) in northern
California.
The only record of eelgrass blades in the
deep sea was made by Pearcy and Ambler
(1974) who found the material as
accidental contents in the abyssal
rattail, Coqphaenoides armatus. -
In the Carribbean, tropical seagrasses are
exported and found at great depths
(reviewed by Zieman 1982). At St. Croix,
U.S. Virgin islands, 60%-1B0% of the
Syringcdium f ilifonne daily production was
detached and exported, whereas only 1% of
the Thalassia production was exported by
bedload transport (Zieman et al. 1979).
Leaves and rhizome pieces of ~halassia
were collected in 3,160 m (10,368 ft) of
water off North Carolina (Menzies et dl.
1967); at 3,580 m (11,484 ft) deep off the
Virgin Islands (Raper and Brundage 1972);
and from 1,326-8,339 rn (4,376-27,489 ft)
deep in the Puerto Rican and Cayman
trenches (Wolff 1988). Wolff reported
consumption of the leaf and rhizome
material by numerous invertebrates.
Finally, the important mechanis~n that
leads to detritus production and either
entrapment or export from the seagrass
system is the decomposition and
inineralization of the Leaf material that
becomes detached frorn the plants. Only
two field studles have been conducted on
seagrass leaf decay rates (litter bay
analyses): one on Thalassia (~iemdn 1968)
,md om on eelgrass (Burkholder and Dofwny
1968). Two studies were conducted in the
laboratory (~arrison and Mann 1975a;
Wshalk and Wetzel 1978). Zieman (L%8)
observedthat Thalasei* leaves in litter
bags anchored in areas subject to
alternating perids of drying and wetting
lost weight three times as fast as those
incubated in tanks where they were
constantly wet. Predried ~ hal ass ia leaves
nlaced in tanks lost weiqht five tirnes
;aster than leaves not predried. Drying
is considered to destroy the cellular
integrity more rapidly and allow a more
rapid microbial attack. Zieman also found
a 50% weight loss of leaves after 5 wk of
incubation in tanks of circulatinq
10% loss was
~n ~ ~ Carolina, ~ t mayer h et al. (1979) seawater. Only an additio~l
reported that up to 25% of the production Observd the next wk*
in- open water-beds is exported to the
adjacent estuary, while in embayment-type In New York Burkholder and Doheny
beds over 90% of the production decays in (1968) placed fresh eelyrass leaves in
the bed or is washed up onto adjacent litter bags in two locations. At one site
beaches and marshes. only 23% of the material remained after 56