Algal contributions to suspended organic matter: A novel approach applied to the US great rivers and the Amazon

Quantitatively distinguishing allochthonous and autochthonous sources of organic matter (OM) transported by stream and river ecosystems has long been a challenge.  Stable carbon isotope ratios of algal carbon is temporally and spatially variable in freshwaters and thus poorly constrain OM sources.  Here we present a new quantitative approach to estimating algal contributions to suspended particulate OM, based on elemental signatures (%OC, %N) of three particle-type end-members (plant detritus, algae, and mineral-complexed OM).  We then applied the mixing model analysis with Monte-Carlo error estimation to quantify the contributions of these three particle types to suspended OM at ~450 river sites in the US Great Rivers (Upper Mississippi, Missouri and Ohio Rivers) and over 100 sites in the Amazon River system.  We find that algae contribute negligibly to OM in the Amazon but can be very important sources at many sites in the US Great Rivers.  We then try to explain these differences as a function of differing river conditions.