Variation in the Flocculation of Dissolved Organic Matter in Two Contrasting Boreal River-Estuarine Gradients

Rivers and estuaries are important conduits and reactors for organic matter (OM) transported from land to the ocean. Despite a substantial amount of terrestrial (T)OM being exported by rivers, only a small proportion of TOM has been found in oceanic waters. One key mechanism for the removal and transformation of TOM is flocculation. Here, we investigate the role of flocculation in two contrasting river-estuarine gradients in Newfoundland and Labrador, over an intra-annual hydrologic cycle. Dissolved organic matter (DOM) freshwater samples from two rivers, filtered through 0.3 µm glass fibre filters, were ‘salted’ to mid and high salinities, mimicking that of an artificial estuary, and re-filtered after an hour incubating in the dark. Coupled increases in bulk particulate organic matter (POM) and decreases in bulk DOM across all experiments throughout the year confirms the removal of OM from the dissolved phase via salinity-induced flocculation. While flocculation of TOM was consistently observed, the degree of flocculation between mid and high salinity treatments varied temporally and among the two catchments. For example, maximum flocculation of winter and spring DOM occurred at higher salinities as compared with DOM from later summer months. Additionally, DOM from the smaller river with a forest dominated catchment consistently exhibited a greater degree of flocculation overall. This suggests that the estuarine turbidity maximum zone not only shifts with seasonal hydrology but is likely a function of DOM composition at the river endmember. Our findings indicate the importance of hydrologic and catchment characteristics in determining the degree and location of flocculation within estuarine mixing zones, a potentially important control on water column cycling and benthic carbon stores.