Impact of Dissolved Organic Carbon and Temperature on Phytoplankton Biomass in an Outdoor Study

Altered environmental conditions in aquatic ecosystems through Dissolved Organic Carbon (DOC) input and increased temperature influence phytoplankton communities. To assess the impacts of climate change as both single and combined effects of DOC inputs and temperature on phytoplankton growth, we conducted an outdoor experiment including four sets of conditions in triplicate: a) control, b) DOC treatment, c) temperature treatment, and d) DOC and temperature treatment. We added 10 mg/L of glucose (C6H12O6) as a source of DOC. The warming treatment increased the water temperature by 3°C above the control (p-value < 0.05). The DOC treatment significantly affected pH, CO2, and DOC (p-values = 0.004, 0.01, and 0.003, respectively). Dissolved oxygen was impacted by both DOC treatment (p-value = 0.0003) and temperature treatment (p-value = 0.003), whereas NO3-1 and PO4–3 were unaffected by either (p-value > 0.05). Both DOC and warming treatments significantly changed the physicochemical properties of the water, influencing phytoplankton biomass. The phytoplankton biomass(chl-a) showed significant positive correlations with temperature (r = 0.5, p-value = 0.0002), CO2 (r = 0.64, p-value < 0.05), DOC (r = 0.6, p-value < 0.05), NO3-1 (r = 0.5, p-value < 0.05), and PO4–3(r = 0.5, p-value = 0.0001), while being negatively correlated with pH (r = –0.5, p-value = 0.001). Dissolved oxygen also showed a positive correlation with phytoplankton biomass (r = 0.63, p-value < 0.05). We concluded that increasing DOC input is crucial for mitigating the negative effects of warming on phytoplankton growth, as DOC provides a primary source of CO2 necessary for photosynthesis.