& Walker 1980) and, as such, may
therefore represent a 6'" minimum for aquatic plants.
Localized differences between regions can result in
ambiguity as to the widespread generality of observed
isotopic patterns (Fry & Sherr 1984). Because, on a
global scale, a considerable variability exists in the
6I3C values of both benthic and planktonic marine
algae in relation to environmental differences in light
and temperature, as well as to algal differences in photosynthesis
mechanisms, rates of primary production
and species composition (e.g. Sackett et al. 1965,
Deuser 1970, Raven 1970, Fontugne & Duplessy 1978,
Fageneli et al. 1986, Wienke & Fisher 1990, Fry &
Wainright 1991), it is difficult to reach consensus about
whether benthic-planktonic differences exist in carbon
pathways on the basis of individual studies only. This
occurs because the true 613C variability of autotroph