Spatiotemporal responses of algal, fungal, and bacterial biofilm communities in Atlantic rivers receiving marine-derived nutrient inputs

Spawning anadromous fishes can enhance primary and secondary production in streams by providing marine-derived nutrients (MDN), but little is known of their effects on individual biofilm constituents (algae, fungi, and bacteria). To address this issue, we measured spatial and temporal changes in the abundance and composition of biofilm algae, fungi, and bacteria in 6 Atlantic rivers with contrasting influx of MDN. Each river had a natural barrier to anadromous fish migration, which established an upstream control and downstream treatment area, with Atlantic Salmon (Salmo salar), Alewife (Alosa pseudoharengus), Rainbow Smelt (Osmerus mordax), or Sea Lamprey (Petromyzon marinus) as the primary anadromous species. In areas receiving MDN, increases in algal and fungal biomass and bacterial density were maintained throughout the season. Algal communities showed the largest (2-10×) increase in abundance over control sites, and Rainbow Smelt spawning streams had the greatest accrual. The river with Alewife showed the largest (2×) increase in fungal biomass during the spawning period. Changes in bacterial density were greatest (2× increase) in the Rainbow Smelt spawning streams. An increase in spawning density (i.e., nutrient load) supported greater biomass accumulation, but the system appeared to saturate quickly. Biofilm in Rainbow Smelt and Alewife streams were enriched up to 2.5% in δ15N during spawning, whereas a 1%enrichment occurred in the Sea Lamprey and Atlantic Salmon streams compared with system baseline values. The degree to which MDN drive productivity and the mechanism of how these nutrients are incorporated to increase productivity and ultimately fish production remain unknown.