CONICET | Buscador de Institutos y Recursos Humanos

Phytoplankton and heterotrophic bacteria are key groups at the base of aquatic foodwebs. In estuaries receiving riverine water with a high content of colouredallochthonous dissolved organic matter (ADOM), phytoplankton primary productionmay be reduced, while bacterial production is favoured. We tested this hypothesis byperforming a field study in a northerly estuary receiving nutrient-poor, ADOM-richriverine water, and analyzing results using multivariate statistics. Throughout theproductive season, and especially during the spring river flush, the production andgrowth rate of heterotrophic bacteria were stimulated by the riverine inflow ofdissolved organic carbon (DOC). In contrast, primary production and photosyntheticefficiency (i.e. phytoplankton growth rate) were negatively affected by DOC. Primaryproduction related positively to phosphorus, which is the limiting nutrient in thearea. In the upper estuary where DOC concentrations were the highest, the heterotrophic bacterial production constituted almost 100% of the basal production (sum ofprimary and bacterial production) during spring, while during summer the primaryand bacterial production were approximately equal. Our study shows that riverineDOC had a strong negative influence on coastal phytoplankton production, likely dueto light attenuation. On the other hand DOC showed a positive influence on bacterialproduction since it represents a supplementary food source. Thus, in boreal regionswhere climate change will cause increased river inflow to coastal waters, the balancebetween phytoplankton and bacterial production is likely to be changed, favouringbacteria. The pelagic food web structure and overall productivity will in turn bealtered.