|
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.
|
