However, the interpretation of d15N data can often be challenging, as the isotope values
fractionate substantially due to preferential retention and uptake by biota.
There is a growing
body of evidence that carbon isotopes may be a useful alternative indicator for eutrophication,
as they may be sensitive to changes in primary production that result from anthropogenic
nutrient inputs. We provide three examples of systems where d13C values sensitively track
phytoplankton production. First, earlier (1980s) mesocosm work established positive
relationships between d13C and dissolved inorganic nitrogen and dissolved silica concentrations
Consistent with these findings, a contemporary mesocosm experiment designed to
replicate a temperate intertidal salt marsh environment also demonstrated that the system
receiving supplementary nutrient additions had higher nutrient concentrations, higher
chlorophyll concentrations, and higher d13C values. This trend was particularly pronounced
during the growing season, with differences less evident during senescence. And finally, these
results were replicated in the open waters of Narragansett Bay, Rhode Island, USA, during a
spring phytoplankton bloom. These three examples, taken together with the pre-existing body
of literature, suggest that, at least in autotrophic, phytoplankton-dominated systems, d13C
values can be a useful and sensitive indicator of eutrophication.