ious work has shown freshwater macrophytes
from lake littoral zones to be "C-enriched compared
to the same species collected from fast movlng nvers It is
thought that carbon fixation In aquatic plants having thicker,
stagnant boundary layers, such as that wh~ch occurs withln
low turbulence lentic systems, will result in more positive F' 'C
values due to greater d~ffusion resistance and subsequent
assimilation of otherwise normally d~sc~iminated "C The
present study confirms th~s hypothesis by examining 876
algal 6I3c values collected from the l~terature The average
6I3C value for benthic algae in lakes was -26%,), whereas that
for nvenne benthlc algae was -29x0 The gieater water tur
bulence to which planktonic algae are exposed is known to
dramatically reduce boundary layer thickness and was found
to cause even more severe 13C depletion resulting in an average
value of -32% Th~same effect also operates in coastal
environments where the average 613C value for marine phytoplankton
was -22%0 compared to -17%" for marine benth~c
algae When comparisons were made on an individual study
basis, differences of 10%0 or greater in F13C were observed
between plankton~c and benthic algae in both oceans and
lakes These algal differences In S1'C were found to be substant~al
enough to be reflected in the 613C values of consumers
in manne coastal environments