Bioavailability of Riverine Dissolved Organic Matter in an Estuarine System: Evidence of the Priming Effect?

As the interface between land and sea, estuaries are highly active zones of biogeochemical cycling driven by material exported from upstream watersheds. Despite large inputs of terrestrial organic matter (OM) from riverine sources, little of that OM is detected in ocean water, suggesting major transformations or losses of OM in estuarine zones. The dissolved organic nitrogen (DON) fraction of terrestrial OM has gained attention as a potential N source for N-limited estuarine phytoplankton, and an understanding of DON bioavailability could provide valuable insight into the fate of allochthonous OM in estuaries. A series of N addition bioassays were used to assess the bioavailability of high molecular weight (HWM, < 1 kDa) river DON to microbial assemblages in the Neuse River Estuary. River DON was isolated from tributary water using tangential flow filtration. Results from these experiments showed that HMW riverine DON additions alone produced no significant short (days) or longer-term (weeks) response in phytoplankton biomass and productivity or bacterial productivity, especially when compared to the effect of inorganic N additions. Phytoplankton biomass and productivity did increase, however, in response to other DON sources, including low molecular weight (< 1 kDa) river DON, waste water treatment effluent, and poultry litter leachate. When inorganic N was added in combination with the HMW river DON, there was greater phytoplankton productivity and biomass relative to nutrient-only controls. This could be evidence of the priming effect, or the enhanced biodegradation of refractory OM by the addition of labile OM. We hypothesize that inorganic N stimulated algal production, and the resulting labile autochthonous OM enhanced river OM mineralization, releasing additional inorganic N for phytoplankton production. On the other hand, the direct stimulation of bacterial metabolism by inorganic N is an alternate possibility and remains to be investigated.