Shifts in Klamath River metabolism following a reservoir cyanobacterial bloom

Sources of C and the location of production in rivers can influence trophic state. Despite major alterations to large rivers, data describing metabolic balance and partitioning in these rivers are sparse. We quantified ecosystem metabolism in the Lower Klamath River, USA, before and after a reservoir-derived cyanobacterial bloom. We calculated daily whole-river metabolism at 3 reaches on the Klamath River below Iron Gate Dam from May-October 2012. We measured planktonic metabolism biweekly from June to October to partition the source (planktonic or benthic) of whole-river gross primary production (GPP_Total) and ecosystem respiration (ER_Total) prior to and during the cyanobacterial bloom. Whole-river ecosystem metabolism in the Klamath River varied seasonally, with low GPP_Total and ER_Total in spring and autumn (May, June, October means = 4.4, -2.9 g O₂ m^-2 d^-1), and high GPP_Total and ER_Total during summer (July-September means = 8.0, -6.8 g O₂ m^-2 d^-1). Within sites, daily variation in ER_Total was coupled with daily variation in GPP_Total, suggesting a dominant role of autotrophs in ER_Total. Average rates of GPP_Total declined from upto downriver sites, driving parallel declines in net ecosystem production. After the bloom, planktonic production and respiration increased 2-4 × over nonbloom rates, whereas whole-river metabolism was relatively stable because of compensatory declines in benthic metabolism. Minimum daily dissolved O₂ concentration (DO) declined with increasing GPP_Total. This pattern strengthened during the bloom, showing that DO, a regulated water-quality variable, was tightly linked to C-cycling processes in the river. The bloom changed the location (planktonic vs benthic) of production and respiration in the river and decreased DO minima, but not rates of whole-river metabolism. Location of primary production had only subtle effects on ecosystem metabolism compared to seasonal changes in metabolism.