Interaction between carbon cycling and phytoplankton community succession in hydropower reservoirs: Evidence from stable carbon isotope analysis

<p><strong>Abstract:</strong> Carbon (C) cycling and phytoplankton community succession are very important for hydropower reservoir ecosystems; however, whether the former controls the latter or the reverse is still debated. To understand this process, we investigated phytoplankton species compositions, stable C isotope compositions of dissolved inorganic C and particulate organic C (δ<sup>13</sup>C-DIC and δ<sup>13</sup>C-POC), and related environmental factors in seven hydropower reservoirs on the Wujiang River, Southwest China. A total of 36 algal genera from seven phyla were identified, and phytoplankton community exhibited obvious temporal and spatial difference. The δ<sup>13</sup>C-DIC (from -9.96 to -3.73‰) and δ<sup>13</sup>C-POC (from -33.44 to -21.17‰) co-varied with the algal species succession and increased markedly during the shift of dominant species from Bacillariophyta to Pyrrophyta or Cyanophyta. In addition, the strong C fixation in the euphotic layer resulted in great δ<sup>13</sup>C-DIC and CO<sub>2</sub> stratification in the reservoir profile. Statistical analyses and C isotope evidence demonstrate that an increase in water temperature triggers phytoplankton community succession, and that CO<sub>2</sub> availability is a key to drive the succession direction, and in turn, C cycling is enhanced when phytoplankton are dominated by Pyrrophyta or Cyanophyta in hydropower reservoirs. This study confirms that C cycling and phytoplankton community succession interact with each other and evolve synchronously, and will be helpful to systematically evaluate the environmental consequences of river damming.</p><p><strong>Keywords: </strong>Carbon biogeochemical cycling; Phytoplankton community succession; Stable carbon isotope; Reservoir effect; Wujiang River.</p>