The carbon pump supports high primary production in a shallow lake

Aquatic research on primary production and carbon dynamics often ignores calcification that supports photosynthesis by producing protons and forming CO 2 . Calcification prevents detrimental pH rise, but causes greater decrease of dissolved inorganic carbon (DIC). Concurrent DIC replenishment is therefore essential to maintain high photosynthesis. Here we show a mean daily DIC loss of 40% (26% to photosynthesis and 14% to calcification) in surface waters during summer periods in a shallow charophyte-lake and replenishment of the DIC pool by respiration and carbonate dissolution in the bottom waters followed by nocturnal mixing. The daytime DIC assimilation in organic matter relative to oxygen production in surface waters was close to 1.0 (molar ratio), while total DIC loss markedly exceeded oxygen production because of calcification. Our results suggest that photosynthesis would rapidly become carbon limited if permanent stratification prevented transfer of DIC from bottom waters to surface waters or if permanent mixing prevented CO 2 accumulation conducive to carbonate dissolution in bottom waters. This vertical transport of DIC effectively functions as a physical and biological pump supporting high metabolism in charophyte-dominated shallow lakes with recurring daily stratification and mixing.