Data from: Seasonal changes in plankton food web structure and carbon dioxide flux from Southern California reservoirs

Reservoirs around the world contribute to cycling of carbon dioxide (CO2)
with the atmosphere, but there is little information on how ecosystem
processes determine the absorption or emission of CO2. Reservoirs are the
most prevalent freshwater systems in the arid southwest of North America,
yet it is unclear whether they sequester or release CO2 and therefore how
water impoundment impacts global carbon cycling. We sampled three
reservoirs in San Diego, California, weekly for one year. We measured
seasonal variation in the abundances of bacteria, phytoplankton, and
zooplankton, as well as water chemistry (pH, nutrients, ions, dissolved
organic carbon [DOC]), which were used to estimate partial pressure of CO2
(pCO2), and CO2 flux. We found that San Diego reservoirs are most often
undersaturated with CO2 with respect to the atmosphere and are estimated
to absorb on average 3.22 mmol C m-2 day-1. pCO2 was highest in the winter
and lower in the summer, indicating seasonal shifts in the magnitudes of
photosynthesis and respiration associated with day length, temperature and
water inputs. Abundances of microbes (bacteria) peaked in the winter along
with pCO2, while phytoplankton, nutrients, zooplankton and DOC were all
unrelated to pCO2. Our data indicate that reservoirs of semi-arid
environments may primarily function as carbon sinks, and that carbon flux
varies seasonally but is unrelated to nutrient or DOC availability, or the
abundances of phytoplankton or zooplankton.