Can long term-nitrogen increases affect pelagic food web processes and the vertical structure of biogeochemical processes in Puget Sound?

Analyses of Ecology’s longterm monitoring data has indicated that in the pelagic zone of Puget Sound, nutrient concentrations have significantly increased, and nutrient ratios and phytoplankton biomass have steadily changed over the last 14 years. We have also documented a wide-spread decline in benthic community abundance. The cause and impacts of these trends have potential implications for marine food web structure, energy transfer, particle export, vertical structure of biogeochemical rate processes, and oxygen drawdown at depth. Puget Sound, because of its proximity to the cold, nutrient-rich Pacific Ocean, is thought of as a diatom-dominated marine food web supporting higher trophic levels via a productive, short food chain with high export production to benthic communities. Phytoplankton species respond to the nutrient composition, physical character of the water column, and zooplankton grazers. Through Ecology’s long-term marine monitoring program and aerial surveys, we frequently document extensive algal blooms, Noctiluca blooms at the surface. Many of the phytoplankton blooms show high abundances of autotrophic flagellates. Depth-integrated algal biomass, on the other hand, revealed a steady decline from 1999 to 2013. These seemingly opposing observations – increasing nutrients and high algal biomass at the surface and decreasing phytoplankton biomass below the surface - could provide clues to phytoplankton species and food web shifts resulting in reduced organic material export and weaker benthic-pelagic coupling that could explain the long-term decline in deeper-water benthic communities in Puget Sound by 45%. We will conceptually draw these different and opposing observations into a larger cohesive hypothesis and connect changes in Puget Sound’s nutrient balance to larger scale processes of material and energy cycling in Puget Sound.