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The
largest impediment to successful management of these
important marine ecosystems is a lack of mechanistic
understanding of how external forcing reverberates through
the complex physical settings, trophic and biogeochemical
interactions characterizing these systems. Seagrasses,
like phytoplankton are dominant primary producers that
play a central role in the stability, nursery function,
biogeochemical cycling and trophodynamics of diverse
coastal ecosystems. Seagrasses are habitat "architects",
and as such are important for sustaining a broad spectrum
of organisms (Thayer et al. 1984, Hemminga & Duarte
2000). Seagrasses stabilize sediments, which are easily
resuspended if the plants are lost, resulting in increased
and prolonged turbidity, which in turn reduces available
light. For these reasons, seagrasses are widely recognized
as the ultimate, downstream barometers of estuarine
water quality (Dennison et al. 1993), and have accordingly
been called the "canaries of the estuary",
being perhaps the most parsimonious integrator of estuarine
water quality throughout the range of their distribution.
Any significant impacts to seagrass abundance and distribution
has the potential for cascading effects, particularly
with the seagrass-associated fishery resources (Costanza
1998), and creates a situation that is difficult, if
not impossible, to reverse (Harlin & Thorne-Miller
1981, Thayer et al. 1984, Short & Wyllie-Echeverria,
1996 Fonseca et al. 1998, Hauxwell et al. 2001). Generally
speaking, thriving seagrass communities signal a productive,
diverse and biogeochemically-trophically well-coupled
coastal ecosystem. Accordingly, the presence of seagrass
is a useful measure of estuarine condition, but reliance
on presence/absence as an indicator implicitly requires
significant degradation of estuarine water quality (Zimmerman
et al. 1991). By focusing only on presence, we are restricted
to detecting conditions when water quality is so degraded
that there is virtually no time for corrective actions.
Therefore, the ability to detect and predict sub-lethal
stress thresholds in seagrass plants is crucial for
effective conservation of the resource.
The
importance of seagrasses as indicators of estuarine
condition, particularly decreased water clarity was
proposed in the early 1990's (Kenworthy & Haunert
1991a & b, Neckles 1994). Dennison et al. (1993)
summarized these efforts and concluded that seagrasses
were potentially sensitive indicators of declining water
quality because of their high light requirements (15-25%
surface irradiance) compared to that of other aquatic
primary producers (<5%). To develop predictive indicators
of estuarine function, physiological and biochemical
measures of seagrass health need to be assessed (Neckles
1994). These measures need to respond clearly and reliably
to abiotic factors that cause sub-optimal seagrass growth
(e.g., light limitation), and could come from a suite
of approaches including:
1. Bio-optical models of water quality in relation to
habitat requirements (e.g., Gallegos 1994, 2001, Kenworthy
& Gallegos 1996, Zimmerman 2003)
2. Growth measurements and morphology (plastochrone
interval, morphometrics, short-shoot density) that have
traditionally been used (reviewed by Short & Duarte
2001).
3. Biochemical markers of stress (amino acid composition,
reduced sugar content, altered chl. a/b ratios, chl.
fluorescence) that have recently been evaluated (Beer
et al. 1998; Beer & Bjork 2000; Longstaff et al.
1999, Ralph et al. 1998, Ralph 1999).
We are currently
focusing on applying all three approaches to understanding
the physiological and growth responses of seagrasses
to light limitation stress. Our emphasis is to understand
photophysiology to compliment growth and biochemical
metrics, and synthesizing this information by defining
appropriate conditions for seagrass survival and reproduction
for the bio-optical model. Light availability to benthic
seagrasses has been determined to be the major criterion
limiting the distribution of seagrass under otherwise
appropriate conditions. Certain water quality criteria,
particularly the optical water quality needed for the
survival and growth of seagrasses has been the subject
of considerable research (Neckles et al. 1994, Kenworthy
& Haunert 1991b, Kaldy & Dunton 1993, Dennison
et al. 1993, Gallegos & Kenworthy 1996, Kenworthy
& Fonseca 1996). A general conclusion of those workshops
and research programs was that water column transmissivity
needs to be greatly increased in order to provide light
conditions suitable for the survival of most seagrasses
(Dennison et al. 1993, Kenworthy & Fonseca 1996,
Moore et al. 1996, Batiuk et al. 2000). Most current
approaches do not address the integrated light requirements
of seagrass, focusing instead on "instantaneous"
measures of irradiance flux, and seagrass photosynthetic
rates. This approach makes implicit that cumulative
stress effects are disregarded, so that important questions
regarding how the duration of exposure or the frequency
of exposure to a given level of environmental degradation
might influence survival of the seagrasses are overlooked.
Only recently has the frequency and duration of stressful
conditions started to be investigated for the survival
of seagrass (Moore et al. 1996, Onuf 1996). We focus
particularly on light stress and determine integrated
(cumulative) light thresholds for seagrass survival
and growth-to-reproduction, as well as the importance
of the duration and frequency of acute attenuation events;
analogous to storms resulting in turbid runoff plumes,
or eutrophication resulting in increased phytoplankton
blooms.
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