Nutrient dynamics in La Romaine river and reservoir complex

Abstract:

Dams and reservoirs can alter the transport and export of nutrients and organic matter from the river to estuary. In the La Romaine River, a series of four hydroelectric dams have been progressively built between 2015 and 2019 where we have been able to monitor the river through this progression. Here, we present longitudinal patterns in concentrations of total nitrogen (TN), total phosphorous (TP), dissolved organic carbon (DOC), and stoichiometric ratios (C:N:P) within the La Romaine River above the reservoirs, through the four reservoirs, and the river reaching the estuary. We found that TN concentrations varied greatly within the reservoirs and between successive reservoirs, suggesting that reservoir habitats can be both sources and sinks of N, but concentrations below the dams remained on average roughly similar to those in the upstream river. TP concentrations, in contrast, consistently increased longitudinally and were greater below the dams than upstream, suggesting that the reservoirs are net sources of P. DOC concentrations were relatively constant across the river and the reservoir continuum. Stoichiometric ratios varied longitudinally where N:P decreased from above the reservoirs to the estuary, C:N were relatively constant, and C:P declined through the reservoirs and downstream river into the estuary; these stoichiometric shifts were mostly driven by changes in P concentrations. In addition to spatial patterns in concentrations, we have determined relationships between dissolved organic matter (DOM) degradation (by bacterial breakdown and photodegradability) and nutrient concentrations to evaluate how biogeochemical processes may have been influenced by the construction of these dams. Increasing P concentrations in the reservoirs correlated positively with both the photochemical and biological degradability of DOC. This may suggest that photodegradation of DOC may releases P that may also enhance microbial activity to further breakdown and transform DOM. Our data suggest that in these young La Romaine reservoirs there were pulses of nutrients, mostly P, which in turn may have influenced DOC turnover, with potential effects downstream which require further research.