Ecosystem Metabolism and Gradients of Temperature, Oxygen and Dissolved Inorganic Carbon in the Littoral Zone of a Macrophyte‐Dominated Lake.

Dense submerged macrophyte stands in lakes may promote alternating daytime stratification and nighttime convective mixing, driving extensive spatiotemporal variations in water temperature, oxygen, pH, and inorganic carbon (DIC). We set out to investigate environmental conditions and ecosystem metabolism in the macrophyte‐dominated littoral zone (0.6 m depth) of a shallow, mesotrophic lake and compared this with the pelagic zone (3.0 m deep). We found that, during summer, vertical water column gradients only occasionally occurred and were weak in the pelagic zone, while dense littoral macrophyte stands of charophytes exhibited strong diel changes and steep daytime temperature and oxygen depth gradients. Oxygen showed daytime surface supersaturation and bottom anoxia, alternating with nighttime mixing. In spring, before charophytes appeared, the vertical gradients were largely absent. Ecosystem metabolism was primarily positive in spring, but areal rates of daily gross primary production (GPP) and closely related respiration increased 6‐fold in summer. Ecosystem metabolism calculated based on oxygen or DIC was very similar, with a 1:1 M basis. Daytime DIC loss by CaCO3 precipitation on charophyte surfaces averaged 6.3% of total DIC loss, and this loss was restored during nighttime. The contribution of shallow littoral macrophyte communities to whole‐lake summer GPP exceeded that estimated for phytoplankton. Environmental conditions in the macrophyte‐covered littoral zone differed markedly from pelagic waters with steep vertical gradients in temperature and chemistry during daytime including anoxic micro‐habitats in stark contrast to deep pelagic waters. The study highlights the significant influence of the littoral zone and macrophytes on whole‐lake ecosystem processes. Plain Language Summary: In shallow water in clear‐water lakes where light reaches the bottom, dense stands of submerged plants may develop. Dense plant stands promote stratification of the water column resulting in large differences in water temperature, oxygen, pH, and dissolved inorganic carbon between surface and bottom waters. We set out to investigate how dense plant stands in the shallow water of a nutrient‐poor lake influence environmental conditions and compare this to open waters. During summer, we found small differences between the surface and bottom water at the open water site, while pronounced differences developed in dense plant stands in the shallow water site. During daytime, oxygen was high in surface water with high photosynthesis, while respiration consumed virtually all oxygen in dim light below the plant canopy close to the bottom. During nighttime, differences vanished due to surface cooling, which promotes mixing of the entire water column. Two independent methods showed a close 1:1 balance between oxygen production and consumption and complementary carbon consumption and production in photosynthesis and respiration, respectively. Our study highlights the differences between shallow and open waters and the prominent role that plants may play in productivity and environmental conditions in the whole lake. Key Points: Extensive diel and depth variation in temperature, oxygen and pH occurred in dense littoral macrophyte stands in a small mesotrophic lakeMetabolism of oxygen and dissolved inorganic carbon were closely coupled and photosynthesis and calcification peaked before noonSensor measurements reveal the extensive variability of physical mixing, chemical gradients, and metabolism in dense macrophyte stands [ABSTRACT FROM AUTHOR]