Size fractionated biogeochemical constituents across adjacent coastal systems informs approaches for integrating small catchment studies into regional models

Observed and predicted hydrological changes in C-rich boreal ecosystems have the ability to change the transport trajectory of biogeochemical constituents from the land to ecologically, economically, and culturally important coastal systems. Yet, most of our current understanding of biogeochemical fluxes and cycling across salinity gradients stem from observations of large and urbanized riverine systems, which overlooks the numerically abundant smaller systems. In this study, we conducted a baseline assessment of the biogeochemical constituents across salinity gradients among two adjacent small systems in the boreal zone. Dissolved iron (DFe) and its ratio with dissolved organic carbon (DFe : DOC) were the most sensitive indicators for small catchment heterogeneity. These parameters were the best indicators of change among coastal systems across regional and seasonal scales. Our results also confirm consistencies in common optical measures (SUVA(254) and S[275-295/350-400]) and DOC to nitrogen ratios that may adequately provide representation of biogeochemical composition on a regional scale. Simultaneous variation in biogeochemical parameters across particulate and dissolved pools during the summer-to-fall transition period indicate this as an important timeframe for targeted investigation of the linkages between biogeochemical parameters and coastal ecosystem functioning. By providing some key spatial and temporal constraints on biogeochemical fluxes among boreal river-estuaries, our findings indicate that DFe and DFe : DOC ratios should be used to design research aimed at capturing regional and coastal ecosystem scale biogeochemical fluxes to inform Earth System Models.