Fróðskaparrit - 01.01.1999, Blaðsíða 166

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HIGH INORGANIC CARBON EXTRACTION CAPACITY OF SUBMERGED
MACROPHYTES FROM SOFTWATER FAROESE LAKES
tions, often above 500 |iM (Steeman-
Nielsen, 1947; Maberly and Spence, 1989;
Nielsen and Sand-Jensen, 1993; Larcher,
1995). This is more than 30 times the equi-
librium concentration in water, which is
about 16 pM (at 15°C). However, due to
metabolic activity and a slow exchange rate
between water and air, disequilibrium is of-
ten encountered. Thus, in a survey com-
prising 1835 lakes and 4665 samples (Cole
et al. (1994) found that nearly all had C02
concentrations different from air equilibri-
um, with 87% having higher concentra-
tions.
The high C02 requirement for saturation
of photosynthesis by submerged macro-
phytes in relation to the low concentrations
found in most waters suggests that C02
limitation of photosynthesis may be a com-
mon phenomenon, an hypothesis that has
been confirmed in several investigations
(Maberly, 1985; Madsen and Maberly,
1991; Madsen, 1991; Nielsen and Sand-
Jensen, 1993; Frost-Christensen and Sand-
Jensen, 1995). The C02 limitation of pho-
tosynthesis might be circumvented or ame-
liorated by the plants through various struc-
tural and physiological adaptations. These
mechanisms and strategies can be grouped
into two main categories: spatial escape
and biochemical/physiological adaptations.
Plants that rely on spatial escape grow in
habitats rich in C02, have the ability to de-
velop aerial leaves or can use sediment
C02. Biochemical and physiological adap-
tations include bicarbonate use and differ-
ent types of C-4 photosynthesis. The bio-
chemical/physiological adaptations are of-
ten referred to as Carbon Concentrating
Mechanisms (CCM), due to their capacity
to enhance the intemal concentration above
what can be obtained by passive C02 up-
take and C-3 photosynthesis.
Escape Strategies
In the bulk water of lakes, the concentration
of C02 might be higher than atmospheric
equilibrium concentrations, though con-
centrations sufficient to saturate photosyn-
thesis of submerged macrophytes are rare
(Cole et al., 1994). Higher C02 concentra-
tions can also be encountered immediately
adjacent to the surface of organic sediment
with high bacterial activity (Pedersen et al.,
1995). An inorganic carbon source that
might be exploited by low stature plants
growing close to the sediment.
A quantitatively significant use of sedi-
ment-C02 among freshwater macrophytes
is a widespread trait among the isoetids of
which Littorella uniflora, Lobelia dortman-
na and Isoetes lacustris are prominent
members (Wium-Andersen, 1971; Boston
et al., 1987): The isoetids are small plants
with short, stiff rosette leaves, a high root:
shoot ratio and a well-developed lacunal
system. The lacunal system forms a more
or less continuous airspace along the entire
length of the plants through which C02 dif-
fuses from the sediment via the roots to the
leaves and oxygen diffuses in the opposite
direction (Sculthorpe, 1967). The ecologi-
cal advantage of sediment-C02 use lies in
the high C02 concentration in the intersti-
tial water, which may be 50 to 100 times
higher than in the bulk water (Wium-An-
dersen and Andersen, 1972; Roelofs et al.,
1984; Boston etal., 1987). Despite the high