In a deep hardwater lake, alkalinity contributes significantly to the annual gross primary production, providing at least a third of the needed carbon for aquatic plant growth. This was discovered by analyzing the daily ratios of CO2-O2 and alkalinity-O2 in the lake. The study found that bicarbonates from alkalinity, not just CO2, are the main source of inorganic carbon fueling plant growth for most of the year in both shallow and deep lake areas. This research highlights the important role of alkalinity in the freshwater carbon cycle throughout the seasons.