Linking aquatic vegetation structure with ecosystem metabolism throughout the Klamath River, California, USA

While terrestrial biomes are routinely defined by their dominant vegetation structure and coupled primary productivity regimes, connecting primary productivity regimes with the properties of aquatic vegetation in rivers is uncommon. Gross primary production (GPP) and ecosystem respiration (ER) indicate riverine ecosystem processes that are useful for monitoring river response to human alterations, global change, and restoration, but how aquatic vegetation structure and biomass influence GPP and ER is poorly known. We related patterns in the time series of daily GPP and ER to submerged aquatic vegetation biomass at 11 reaches on the Klamath River, California, downstream of four dams prior to their removal in 2024. Rooted macrophytes dominated vegetation assemblages at upriver sites and transitioned to filamentous algae downriver. Fluxes of GPP and ER were high compared to those of other rivers, with the magnitude and timing of mean ER, mean GPP, peak GPP, GPP variability, and green-up varying among sites. While total autotrophic biomass correlated strongly with mean summer GPP ( $r=$ 0.80), evidence of macrophytes driving mean summer GPP was weaker ( 0.60). Relationships between mean summer ER and macrophyte biomass and total autotrophic biomass ( $r=$ 0.93 and 0.87, respectively) were stronger than relationships between biomass and GPP. This strong relationship between ER and biomass was due to ER increasing late in summer, possibly because respiration of autotrophic biomass produced earlier in the season, or from vegetation structure creating patches of increased respiration of imported or stored carbon. Assessments of relationships between submerged aquatic vegetation and river metabolism may help inform predictions about changing ecosystem structure and function that influence water quality, including ecosystem response to the removal of four large hydroelectric dams on the Klamath River.