Carbon-13 enrichment in benthic compared to planktonic algae:foodweb implications

Previous 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 w h ~c h 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 t h ~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 a n average value of -32% T h ~s 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 sub-stant~al enough to be reflected in the 613C values of consumers in manne coastal environments