Fróðskaparrit - 01.01.1999, Page 172

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HIGHINORGANIC CARBON EXTRACTION CAPACITY OF SUBMERGED
MACROPHYTES FROM SOFTWATER FAROESE LAKES
nisms that allow exploitation of inorganic
carbon sources other than those in the bulk
water (Table 2). The isoetids (Littorella
uniflora, Isoetes lacustris and Lobelia dort-
manna) use sediment-C02. In addition, Lit-
torella and Lobelia, including the Faroese
specimen, are CAM plants (Keeley, 1996;
Madsen and Bagger, unpubl. results 1999)
and, thereby, have the potential to improve
the carbon balance by allowing inorganic
carbon uptake in both light and dark. How-
ever, the effects of CAM on the carbon ex-
traction capacity appear to be low (Table 3).
Of the remaining non-CCM species, Spar-
ganium angustifolium, Potamogeton poly-
gonifolius and the two Callitriche species
have the ability to develop aerial or floating
leaves allowing acquisition of C02 from
the more readily available atmospheric
pool. Eleocharis acicularis, Juncus bulbo-
sus, Nitella sp. and Fontinalis antipyretica
are not known to have developed traits or
features improving inorganic carbon up-
take (Steeman-Nielsen, 1947; Bain and
Proctor, 1980; Allen and Spence, 1981;
Morton and Keeley, 1990), except for one
report showing that Fontinalis antipyretica
collected in a Spanish river could use bicar-
bonate ((Penuelas, 1985). These species,
however, often grow close to the sediment
and might benefit from the higher C02 con-
centration found there (Maberly, 1985).
The list of species with CCM (final-pH >
9.5, Table 3) agrees with reports in the lit-
erature (Spence and Maberly, 1985), except
for submerged leaves of Potamogeton
natans that have been reported to be unable
to operate a CCM (Maberly and Spence,
1983). The C02 compensation point calcu-
lated for P. natans varied among sites, but
was very low (0.2 pM to 0.6 pM) and un-
likely to be achievable without a carbon
concentrating system. Interference from
epiphytes cannot be ruled out, however, al-
though no visible biofilm was found. The
C02 compensation point reported for P.
natans by Maberly and Spence (1983) for
submerged leaves was 2.1 pM, which is
comparable to the range reported by Frost-
Christensen and Sand-Jensen (1995) for
floating leaves measured in water. For
Utricularia vulgaris no report on CCM has
been found. Another Utricularia species,
U. purpurea, has been reported to be unable
to use bicarbonate, though tested at low al-
kalinity (<0.07 meqv L"1) only (Moeller,
1978). In contrast, Utricularia australis has
a high carbon extraction capacity (L. Kris-
tiansen, pers. comm.).
For most of the species categorised as
having CCM, only three, P. natans, P. alpi-
nus and P. gramineus, have the ability to
explore altemative inorganic carbon
sources, all by the ability to develop float-
ing leaves.
Though not verified directly, it is as-
sumed that the CCM operating in the
species tested was based on HC03' use,
which is the most widespread among fresh-
water macrophytes (Prins and Elzenga,
1989; Madsen and Sand-Jensen, 1991). For
these species, the HC03" compensation
point, calculated from alkalinity and final-
pH, varied from 0.1 to 0.77 mM. This range
is comparabie to the range reported for oth-
er macrophytes measured at similar alka-
linity (0.5 - 2.0 mM), but substantially
higher than the compensation point found