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

There are seasonal differences in leaf
dimensions of eelgrass. Intertidal and
subtidal leaves are longer and wider in
summer than in winter (Table 3) (Tutin
1938, Burlholder and Doheny 1968, Phillips
1972, Kentula 1983). These changes in
dimension not only affect functional
aspects of the ecosystem (productivity,
reduced eipiphyte load, amount of litter
for decomposition), but also the
structural aspects (density of leaf cover
for the refuge and nursery functions of
the animal communities) .
Setchell (1929) noted that eelgrass growth
was seasonal and correlated with 5O C
intervals of temperature. While eelgrass
activity in the Pacific Northwest does not
fall neatly into these intervals, it does
appear that eelgrass shows distinct
seasonal patterns of activity,
particularly in the case of vegetative
growth and reproductive cycles. Phillips
(1976) created a Phenolcgical Index of 14
separate characteristics of seagrass
activity, but shortened the list to three
reproductive phases following a 4-year
collection program of turtle grass
(Thalassia testudinum Banks ex Konig) arid.
eelsrass from a wide distributional range
in iorth America (Phillips et al. 1983~).
The three phases were significantly
correlated with water temperature. The
three events were initial date of flower
buds, initial date of anthesis, and
initial date of visible fruits. Dates of
maximum and minimum biomass were analyzed,
but no significant correlations with water
temperature appeared. T data also
indicated that both species may include
genotypes with different temperature
requirements for reproductive activity
that are selectively adapted to different
habitats. This study does not rule out
the influence of other factors which could
control phenology; namely, day length
(photoperiod) and nutritional status of
the plants.
Despite a lack of correlation of
vegetative activity with environmental
variables , eelgrass demonstrates seasonal
regularity in the Pacific Northwest
hilli ips 1972, Puget Sound; Kentula 1983,
Netarts Bay, Oregon). Leaves typical of
winter growth (narrower, shorter) appeared
in September in Netarts Bay, with a
complete change to the winter forrn by
December. In Puget Sound the winter
leaves begin to appear in November. In
both areas new vegetative growth events
occur before the water cools and warms,
respectively. Table 4 lists the field
observations made on eelgrass phenology in
the Pacific Northwest.
In Denmark, Ostenfeld (1908) calculated
that a shoot of eelgrass produced four to
six new leaves annually. Petersen (1913)
stated that eelgrass shoots in Denmark
each produced ten leaves in summer and
five more in winter. In Puget Sound,
Phillips calculated that eelgrass shoots
produced an annual crop of 15 leaves
Table 3.
Seasonal changes in leaf dimensions of eelgrass in Puget Sound, Washington.
Season Tidal Zone Width in mn (inches) Length in an (ft) NO. of leaves/
shoot (X)
Winter Intertidal 3-5 (0.1243.20) Up to 25 (1)
Slntnner Intertidal 4-7 (0.16-0.28) Up to 58 (1.67) 4 in June; 2.5 in
ALKJUS~~