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

total annual primary prduction and 14% of
their total excreted material into the
estuarine system.
3.4 ORGANIC AND INORGANIC NUTEUENI'
CYCLING
The anatomy of eelgrass, like that of all
other seagrasses, is modified for
metabolism, growth, and reproduction in
the sea.
1. The cutin layer over the leaf is
thin.
2. Leaf blades are flattened and thin
with a very high surf ace-to-volume
ratio.
3. There is an extensive lacuna1 system
which gives buoyancy (they store and
transport a great quantity of O2 and
a2'
4. Chloroplasts are densely packed in
the epidermal layer suroundingthe
leaf.
5. There is an aerenchyma composed of
large, thin-walled cells which adjoin
the lacunae and facilitate gas and
solute diffusion.
6. There is a reduced amount of
mechanical support, allowing the
leaves to f Lex and bend on an ebbing
tide, which maintains their wetness
and guarantees exposing as much
photosynthetic surface to solar
radiation as possible. At the same
time the leaves are strong enough to
resist breaking when they are whipped
during wave action e erg us on et al.
1980; Zieman 1982) .
The major problem with procurement of
certain nutrients, such as carbon, by
seagrasses is that rates of gaseous
diffusion in water are several orders of
magnitude lower than in air. Also, when
the pH of seawater rises during very
active photosynthesis (from 8.2 to 8.9 or
higher), free C02 in the water is greatly
reduced or becomes unavailable, as does
the KO3-ion. It is now known that
seagrasses obtain most of their nutrients
from the sediments, which maintain a much
lower pH in the deep anoxic layer beneath
the very thin surface oxic zone.
Numerous papers have documented the uptake
of nutrients (carbon, phosphorus, and
nitrogen, in particular) from the
sediments by the root system,
translocation through the seagrass plant,
and release to the epiphytes and water
column. The plants and epiphytes can also
absorb these nutrients from the water to
release them to the interstitial water in
the sediments (MCRO~ and Barsdate 1970;
Mcmy et al. 1972; McWy and Wring 1974;
Harlin 1975, 1980; Penhale and 3nith 1977;
Wtzel and Penhale 1979; Penhale and
Thayer 1980; Short 1981) . All the available
work done states that sediments are
the preferred source for nutrient uptake.
These laboratory experiments were
reinforced by recent field experiments
using fertilizers. Orth (197733) used
Osmacote (14-14-14; a slow-release
fertilizer), placed in the sediments, and
observed dramatic increases in plant
standing crops. Eelgrass root/rhizomes
increased 30%, the leaf crop was three to
four times higher, and shoot density
increased. Increases in the standing crop
were also observed following fertilization
of eelgrass in mode Island (Harlin and
Thorne-Miller 1981) .
Even though eelgrass needs a variety of
macro- and micronutrients, most of the
research has concentrated on carbon,
phosphorus, and nitrogen. Phosphorus
supply does not appear to be limiting. In
Alaska, McRoy and Barsdate (1970) found
that eelgrass plants were a phosphorus
pump from the sediments to the water
column, but in North Carolina the roots
retained most of their absorbed phosphorus
(Penhale and Thayer 1980). McRoy et al.
(1972) estimated that phosphorus turnover
times in the eelgrass meadows in Izembek
Lagoon, Alaska, vary from two turnovers
per year to one every 2 years.
Eelgrass has three sources of inorganic
carbon (C02, HCO~-) : recycled from
respiration and p otorespiration, the
water column (which is not a likely source
during active photcsynthesis, owing to an