Ecosystem metabolism determines its role as a CO2 source or sink. In lakes and reservoirs,
metabolism is conditioned by the phytoplankton community. This relationship was studied
throughout a hydrological year in El Gergal, a Mediterranean mesotrophic and warm
monomictic reservoir used for water supply to the city of Seville, Spain. Aerobic metabolic
rates (respiration and primary production) were modeled using high-resolution data, including
temperature, PAR, oxygen, salinity, and meteorological variables. The size structure of the
phytoplankton community was examined using flow cytometry. Thermal stratification
persisted for 7.7 months. During this period the aerobic ecosystem metabolism was
autotrophic, with a net O2 production of 0.06 ± 0.03 mg l–1 day–1, and ultraphytoplankton (an
intermediate size group between pico- and nanophytoplankton) dominated the community.
Notably, the epilimnion was the only layer exhibiting net O2 production, while the
metalimnion remained almost neutral and the hypolimnion was heterotrophic. During the
mixing period, El Gergal was heterotrophic (net O2 consumption: 0.12 ± 0.03 mg l–1 day–1)
with picophytoplankton (the smallest size group) dominating the community. On an annual
scale, El Gergal reservoir was slightly heterotrophic, raising concerns about its role as a CO2
sink
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