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.

全球范围内的水库均参与大气与二氧化碳（CO₂）之间的碳循环，但目前针对生态系统过程如何调控CO₂吸收与排放的相关研究仍较为匮乏。北美干旱西南部地区的水库是分布最广泛的淡水生态系统，但目前尚不清楚这些水库究竟是碳汇还是碳源，因此也无法明确筑坝蓄水对全球碳循环的具体影响。本研究对美国加利福尼亚州圣迭戈的三座水库开展了为期一年的每周采样监测。研究测定了细菌、浮游植物与浮游动物的丰度季节变化，同时检测了水体化学指标（包括pH值、营养盐、离子及溶解性有机碳（DOC）），并基于上述数据估算了CO₂分压（pCO₂）与CO₂通量。研究结果显示，圣迭戈的水库大多处于大气CO₂不饱和状态，经估算其平均CO₂吸收速率为3.22 mmol C m⁻² d⁻¹。CO₂分压在冬季达到峰值，夏季则相对较低，这表明光合与呼吸作用的强度随季节发生变化，且与日照时长、水温及水体补给密切相关。微生物（细菌）丰度与CO₂分压均在冬季达到峰值，而浮游植物、营养盐、浮游动物及溶解性有机碳的丰度与CO₂分压并无显著关联。本研究数据表明，半干旱环境下的水库可能主要发挥碳汇功能，且碳通量存在季节变化，但该通量与营养盐、溶解性有机碳的有效性，以及浮游植物、浮游动物的丰度均无显著关联。