“Assessment of potential eutrophication in coastal waters of Gran Canaria: Impact on plankton community under CO2 depletion”

Population growth in coastal tourist areas is leading to enhanced waste production, raising concerns about potential nutrient release increases and the resulting impact on marine ecosystems through eutrophication. Knowledge of the specific impacts of eutrophication on plankton communities in many of these regions is limited, highlighting the need for further research and appropriate environmental management strategies. To help address these gaps, we conducted a 30-day mesocosm study in the coastal waters of Gran Canaria, Canary Islands, a major European tourist destination, and the third most densely populated autonomous community in Spain. With the aim of assessing the effects of nutrient input on biomass, primary production (PP) and recycling processes by phytoplankton, zooplankton, and bacterioplankton, we simulated three nutrient discharge intensities (Low, Medium, and High), with daily additions of 0.1, 1, and 10 μmol L^-1 of nitrate, respectively, along with phosphate and silicate. We observed that PP, chlorophyll a (Chl-a), and biomass increased linearly with nutrient input, except in the High treatment, where CO₂ depletion (<1.0 μmol L^-1) and an alkalinity increase (>2500 μmol L^-1) resulted in reduced PP. Despite limitations in nitrogen (Control, Low, and Medium) or carbon (High) availability across treatments, which led to stabilized or decreased PP rates and dissolved organic carbon (DOC) concentrations, bacterial degradation remained active in all treatments. This microbial activity resulted in an accumulation of recalcitrant chromophoric dissolved organic matter (CDOM), indicating the resilience of carbon recycling processes under varying nutrient conditions. Furthermore, a clear succession was evident in all enriched treatments, transitioning from an oligotrophic condition dominated by pico- and nanophytoplankton to a eutrophic state primarily composed of diatoms. However, under CO₂ depletion, diatoms experienced a decline in the High treatment, leading to the proliferation of potentially mixotrophic dinoflagellates. Microzooplankton was less sensitive than mesozooplankton to the decrease in prey availability and high pH caused by CO₂ depletion. Interestingly, the Medium treatment showed high efficiency in terms of PP, despite reaching CO₂ levels near of 1.0 μmol L^-1 by the end of the experiment. PP rates increased from 10 to 100 μg C·L^-1·d^-1 during the first week and remained stable as diatoms predominated throughout the study period. These findings provide valuable insights into the responses of plankton communities to varying nutrient inputs and emphasize the importance of considering the effects of DIC depletion, along with changes in total alkalinity, in eutrophication scenarios as well as in ocean alkalinity enhancement experiments aimed at reducing carbon dioxide emissions.