Latest
results for the Outdoor Lab Experiments:
Mature
Zostera plants (eelgrass) were collected from
Middle Marsh, NC in Sept 2002, and returned to the
NOAA Beaufort Laboratory for transplanting. Nine plants
(each with
2 shoots) were planted in plastic tubs (Rubbermaid
dishpan) filled with sieved quartz beach-sand.
Plants and tubs were placed in an outdoor tank
with flow-through seawater, and five different irradiance
treatments established using increasing water depths
(15cm, 30cm, 50cm, 70cm, and 70cm-Shaded). Measurements
were begun on Oct 9, 2002 and are ongoing.
Measurements were taken weekly over the first
month, then biweekly until Week 39 (June 2003), with
currently monthly sampling intervals.
Light levels between the indoor and outdoor
experiments are not directly comparable, however,
based on integrated irradiance measures, plants grown
at 50cm depth in the outdoor tank received approximately
the same amount of light as seedlings grown at the
highest light level in the indoor tank.
Table
1: Five irradiance treatments (=depths) used for the
outdoor light experiment. Measurements were taken
for 2-4 weeks in a) fall = Oct 02, b) winter = Feb
03 and c) spring = Apr 03. Yellow box indicates max
light levels seen by plants in the indoor tank.
Fig
2: Plants were grown for the duration of their growth
season (orange box = Dec - Jun). Branching with spatial
expansion was observed in two shallowest depths (=high
irradiance treatments), branching in the other three
depths did not result in much areal expansion. Fifteen
of 45 plants flowered (33.3%), all at depths except
50cm.
Fig
3: The length of the longest leaf on each surviving
plant was measured as a proxy for canopy height. Leaf growth (linear extension) was greater as depth increased, as
the plants attempted to grow towards the higher irradiances
near the surface.
Leaf length started to decline at weeks 31-33,
corresponding to increased water temperatures in May,
which resulted in leaf fragmentation due to increased
grazer abundance (isopods, amphipods, and a small
gastropod = Bittium).
Fig
4: Leaf area (single surface) was calculated from
maximum length x average width. Width was measured
on alternative sampling dates on a random subset of
5 plants, with linear interpolation for non-sampled
dates. Area is influenced by leaf length, as well
as width, which varied with depth, at first decreasing
and then increasing again over the growing season.
The greatest leaf area was found on the plants in
the deeper depths due to the greatly elongated leaves.
Leaf area declined in May for the reasons outlined
above.
Fig
5: Plant photosynthesis was measured with an OptiSciences
OS-30 PEA (Plant Efficiency Analyzer). Photosynthetic yield, expressed as a ratio Fv/Fm ranged from 0.6-0.75
in healthy leaves, and fell below 0.5 in leaves that
were senescing. No
significant difference in Fv/Fm was observed in the
apparently healthy (live) plants across all five depths,
indicating acclimation of the photosynthetic apparatus
to the ambient light field.
There is a discernable dip in yield (Fv/Fm)
at the end of the growing season in June, related
to high water temperatures.
Acclimation of these temperate Zostera
to the unfavorable summer conditions in NC allows
a subset of the population to persist as very stunted
individuals until conditions return favorable for
growth around December.
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