Seasonal Changes in Plankton Food Web Structure and Carbon Dioxide Flux from Southern California Reservoirs

Abstract

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.

Fig 1. Twenty-four major reservoirs in San Diego County, California, USA http://www.sdcwa.org/san-diego-county-water-sources (http://www.sdcwa.org/reservoirs).

Barret (BA); Cuyamaca (CU); Dixon (DI); El Capitan (CAP); Henshaw (HE); Hodges (HO); Jennings (JE); Loveland (LOV); Lower Otay (LO); Maerkle (MA); Miramar (MIR); Morena (MO); Murray (MUR); Olivenhain (OL); Poway (POW); Ramona (RA); Red Mountain (RD MO); San Dieguito (SN DG); San Vicente (SN VI); Sutherland (SL); Sweetwater (SW); Turner (TR); Upper Otay (UP OT); Wohlford (WF). The black points indicate the three reservoirs we sampled.

Fig 2. pCO2 and pH time series.

(A) Time series of partial pressure of carbon dioxide (pCO₂) in ppm for each of the three reservoirs. Current atmospheric concentrations of pCO₂ are ~400ppm. (B) pH for each reservoir for 50 sampling weeks. Symbols indicate the reservoir as shown in the legend.

Fig 3. Partial regression plots of the significant drivers of pH and pCO₂ across all reservoirs.

Drivers were identified by the model averaging procedure pH versus (A) total nitrogen (mg N L^-1) and (B) bacteria abundance (# L^-1), and pCO₂ versus (C) zooplankton community biomass (mg L^-1) and (D) particulate organic nitrogen (mg N L^-1). Symbols indicate the reservoir as shown in the legend.

Fig 4. Time series of estimated CO2 flux (mmol m^-2 day^-1).

The four negative values correspond to CO₂ flux in Lake Poway that were below the range of the graph during strong wind events.

Fig 5. Time series for chlorophyll-a (chl-a), total phosphorous (TP), total nitrogen (TN), and particulate organic nitrogen (PON) for the three reservoirs.

(A) chlorophyll-a (μg L^-1), (B) TP (mg P L^-1), (C) TN (mg N L^-1), and (D) PON (mg N L^-1). The vertical dashed lines represent dates that received >1 cm precipitation. Precipitation during our study period totaled 14.78 cm, and average annual precipitation in San Diego County is 26.26 cm.

Fig 6. Time series for (A) dissolved organic carbon (DOC; mg C L^-1), and (B) particulate organic carbon concentrations (POC; mg C L^-1) for all three reservoirs.

Fig 7. Time series of bacteria abundance (# L^-1) for all three reservoirs.

Fig 8. Time series for (A) Zooplankton community mean body length (mm) and (B) community biomass (mg L^-1) for all three reservoirs.

Fig 9. Surface (1 m) and bottom (16–20 m) water temperatures (°C) for each reservoir from Sept 2013-June 2014.

The bottom depths for Lakes Poway and Miramar was 20 m and the bottom depth for Lake Murray was 16–17 m. The water columns are stratified when the lines diverge and mixed when the surface and bottom temperatures are the same.