256
Jens Borum, Kaj Sand-Jensen, Thomas Binzer, Ole Pedersen and Tina Maria Greve
describe and evaluate the techniques, which have
been used for estimating or directly measuring oxygen variability and transport in seagrasses. Next, we
address the internal and external sources of oxygen,
sinks of oxygen and transport of oxygen between
tissues. We demonstrate the factors controlling internal oxygen variability by means of manipulations
under controlled conditions in the laboratory and
present data on in situ oxygen variability from seagrass stands. Finally, we briefly discuss the potential
coupling between seagrass oxygen dynamics and the
occurrence of die-off events in seagrass beds.
II. Measuring Oxygen Dynamics
and Transport
A. Difficulties in Measuring Oxygen Dynamics
and Transport
Measuring oxygen production, consumption, release
and transport in seagrasses and other rooted macrophytes is a major technological challenge. A number
of different techniques are available and have been
applied, but all methods seem to have potential drawbacks depending on the specific objective. The intrinsic problems are related to the fact that all rates of
oxygen exchange within plants depend on a complex
of factors such as (1) the immediate size of oxygen
pools within specific plant tissues and in the media
surrounding leaves and roots, (2) the resistance to
transport within plants and between plant and media, and (3) the steepness of gradients within tissues
and between tissues and the surrounding media. The
factors vary temporally and spatially under natural
conditions and these changes are difficult to control
and mimic in the laboratory. Assessment of oxygen
dynamics within and around seagrasses under controlled laboratory conditions is, therefore, best suited
for describing relative rates and mechanisms rather
than determining absolute rates as they would occur
under natural conditions.
B. Chamber Techniques
Oxygen release and consumption have traditionally
been assessed by measuring changes in bulk water
Abbreviations
DBL – diffusive boundary layer
KPa – kilo Pascals
Rubisco – Ribulose-1,5-bisphosphate carboxylase-oxygenase.
oxygen concentrations in incubation chambers. This
technique is feasible for measurements of photosynthesis and respiration of isolated leaves, although
potential problems with lacunar oxygen accumulation, and especially with poor simulation of natural
boundary layer conditions around leaves, may interfere significantly with rate measurements. Measurements of respiration in isolated roots and rhizomes
using chamber techniques can be more problematic.
It has been argued that measuring respiration of isolated below-ground tissues under aerobic conditions
may overestimate the respiration that would occur in
anoxic sediments (e.g. Smith et al., 1988; Touchette
and Burkholder, 2000). However, such a procedure
may also underestimate respiration, because the lacunar oxygen supply from leaves to roots and rhizomes is disrupted when the tissues are separated
from the leaves. Hence, respiration has to be fueled
by oxygen diffusing from the bulk water through
boundary layers and through the more or less permeable root and rhizome tissues, and this diffusion
may be too slow to sustain an adequate internal oxygen supply and mimic natural conditions of intact
plant gas phase transport (Saglio et al., 1984).
Chamber techniques provide reliable estimates of
whole plant metabolism if intact plants with leaves,
roots and rhizomes are incubated for longer time
intervals allowing equilibration of oxygen between
lacunae and bulk water (Kemp et al., 1986). In
addition, split chambers with leaf compartments
separated from root compartments by water- and
gas-tight seals have been used to estimate oxygen
transport from leaves to roots and subsequent oxygen release to the sediments (e.g. Sand-Jensen et al.,
1982; Kemp and Murray, 1986). Results based on
this technique have, however, to be interpreted with
caution. Transport from leaves to roots is driven by
gradients between sources and sinks, and the steepness of these gradients depend greatly on the experimental conditions (Sorrell and Armstrong, 1994).
The oxygen gradient from the root to the sediment
is especially important, because it determines the
rate of oxygen loss to the sediment and because it
can vary by an order of magnitude depending on
the oxygen consumption within the root medium
(Sorrell and Armstrong, 1994). To mimic natural
sediments as proper sinks the rooting media must
not only be anoxic but also reducing and oxygen
consuming to generate the sufficiently steep gradients between root and sediment forcing the release
of oxygen. Such conditions can be established by
Jens Borum, Kaj Sand-Jensen, Thomas Binzer, Ole Pedersen and Tina Maria Greve
describe and evaluate the techniques, which have
been used for estimating or directly measuring oxygen variability and transport in seagrasses. Next, we
address the internal and external sources of oxygen,
sinks of oxygen and transport of oxygen between
tissues. We demonstrate the factors controlling internal oxygen variability by means of manipulations
under controlled conditions in the laboratory and
present data on in situ oxygen variability from seagrass stands. Finally, we briefly discuss the potential
coupling between seagrass oxygen dynamics and the
occurrence of die-off events in seagrass beds.
II. Measuring Oxygen Dynamics
and Transport
A. Difficulties in Measuring Oxygen Dynamics
and Transport
Measuring oxygen production, consumption, release
and transport in seagrasses and other rooted macrophytes is a major technological challenge. A number
of different techniques are available and have been
applied, but all methods seem to have potential drawbacks depending on the specific objective. The intrinsic problems are related to the fact that all rates of
oxygen exchange within plants depend on a complex
of factors such as (1) the immediate size of oxygen
pools within specific plant tissues and in the media
surrounding leaves and roots, (2) the resistance to
transport within plants and between plant and media, and (3) the steepness of gradients within tissues
and between tissues and the surrounding media. The
factors vary temporally and spatially under natural
conditions and these changes are difficult to control
and mimic in the laboratory. Assessment of oxygen
dynamics within and around seagrasses under controlled laboratory conditions is, therefore, best suited
for describing relative rates and mechanisms rather
than determining absolute rates as they would occur
under natural conditions.
B. Chamber Techniques
Oxygen release and consumption have traditionally
been assessed by measuring changes in bulk water
Abbreviations
DBL – diffusive boundary layer
KPa – kilo Pascals
Rubisco – Ribulose-1,5-bisphosphate carboxylase-oxygenase.
oxygen concentrations in incubation chambers. This
technique is feasible for measurements of photosynthesis and respiration of isolated leaves, although
potential problems with lacunar oxygen accumulation, and especially with poor simulation of natural
boundary layer conditions around leaves, may interfere significantly with rate measurements. Measurements of respiration in isolated roots and rhizomes
using chamber techniques can be more problematic.
It has been argued that measuring respiration of isolated below-ground tissues under aerobic conditions
may overestimate the respiration that would occur in
anoxic sediments (e.g. Smith et al., 1988; Touchette
and Burkholder, 2000). However, such a procedure
may also underestimate respiration, because the lacunar oxygen supply from leaves to roots and rhizomes is disrupted when the tissues are separated
from the leaves. Hence, respiration has to be fueled
by oxygen diffusing from the bulk water through
boundary layers and through the more or less permeable root and rhizome tissues, and this diffusion
may be too slow to sustain an adequate internal oxygen supply and mimic natural conditions of intact
plant gas phase transport (Saglio et al., 1984).
Chamber techniques provide reliable estimates of
whole plant metabolism if intact plants with leaves,
roots and rhizomes are incubated for longer time
intervals allowing equilibration of oxygen between
lacunae and bulk water (Kemp et al., 1986). In
addition, split chambers with leaf compartments
separated from root compartments by water- and
gas-tight seals have been used to estimate oxygen
transport from leaves to roots and subsequent oxygen release to the sediments (e.g. Sand-Jensen et al.,
1982; Kemp and Murray, 1986). Results based on
this technique have, however, to be interpreted with
caution. Transport from leaves to roots is driven by
gradients between sources and sinks, and the steepness of these gradients depend greatly on the experimental conditions (Sorrell and Armstrong, 1994).
The oxygen gradient from the root to the sediment
is especially important, because it determines the
rate of oxygen loss to the sediment and because it
can vary by an order of magnitude depending on
the oxygen consumption within the root medium
(Sorrell and Armstrong, 1994). To mimic natural
sediments as proper sinks the rooting media must
not only be anoxic but also reducing and oxygen
consuming to generate the sufficiently steep gradients between root and sediment forcing the release
of oxygen. Such conditions can be established by