New Zealand Journal of Marine and Freshwater Research abstracts
Measuring photosynthetic characteristics of the seagrass Syringodium
isoetifolium: implications for in situ productivity
estimates
PETER C. POLLARD
Faculty of Science
University of the Sunshine Coast
Locked Bag 4
Maroochydore South, Qld 4558
Australia
email: ppollard@usc.edu.au
Abstract The rates of photosynthesis of the seagrass
Syringodium isoetifolium (Aschers.) Dandy were measured by a gas-release
technique with whole shoots incubated in the field. Greater than 95% of the
variation of the photosynthetic data could be accommodated by the
Photosythesis-Irradiance (PI) model presented here. As expected, the shape of
the PI-curves differed between shoots. The different shapes resulted from the
different photosynthetic maxima and the efficiency with which shoots used light
but not the amount of light required by the shoot to reach saturating light
intensities. Irrespective of the units (whole shoot, leaf chlorophyll content,
or leaf surface area) used to generate the PI-curves, there was no significant
(
P > 0.05) difference between saturating light intensities (Ik), the
mean value was 180 +/- 33 umol m
-2 s
-1. The quantity
and quality of light available to these plants can modify leaf surface and
chlorophyll content of seagrass shoots. Presenting the rates of photosynthesis
as a function of chlorophyll or leaf surface area diminishes the ability to
observe differences between plants. Hence, the health of seagrass plants may be
more obvious and quantifiable if seagrass productivity data are expressed as a
function of whole seagrass shoots rather than on the basis of chlorophyll
content or leaf surface. During the 20-day study, the photoperiod ranged from
6.2 to 10.8 h and gross productivity estimates ranged from 10
to 18 g C day
-1 m
-2. These estimates were
more than 10 times greater than the
in situ net leaf daily
productivity of 0.9 g C day
-1 m
-2 that was
measured with a traditional leaf marking technique. This result implies that
most of the production was directed below ground. The health of seagrasses is
related to the productivity of the whole plant not just the tissue that appears
above ground. In comparative studies that are used to determine the habitat
requirements of seagrasses and the impacts of pollutants on seagrass survival
whole plant productivity measurements would appear to provide a better
indicator of seagrass health.
Keywords seagrass; Syringodium isoetifolium;
productivity; photosynthesis; irradiance
M97056
Received 29 September 1997; accepted 23 December 1998
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