A novel approach to quantifying algal contributions to suspended organic matter from elemental composition

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 are 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), at over 100 sites in the Amazon River system and from ~600 samples from one site in the 3rd-order White Clay Creek in the SE Pennsylvania piedmont. We compare model results with measurements of stable isotopes, chlorophyll, nutrients and other variables at this extreme diversity sites with contrasting climate, land use and hydrological conditions. We find that algae typically contribute negligibly to OM in the Amazon but can be very important sources at many sites in the US Great Rivers, but site-specific exceptions to these overall trends can be very informative. In WCC, algal contributions vary seasonally. We then try to explain these differences as a function of differing basin-scale and landuse conditions.