A common perception in limnology is that shallow lakes are homogeneously mixed owing to their small water volume. However, this perception is largely gained by downscaling knowledge from large lakes to their smaller counterparts. Here we show that shallow vegetated lakes (less than 0.6 m), in fact, undergo recurring daytime stratification and nocturnal mixing accompanied by extreme chemical variations during summer. Dense submerged vegetation effectively attenuates light and turbulence generating separation between warm surface waters and much colder bottom waters. Photosynthesis in surface waters produces oxygen accumulation and CO ₂ depletion, whereas respiration in dark bottom waters causes anoxia and CO ₂ accumulation. High daytime pH in surface waters promotes precipitation of CaCO ₃ which is re-dissolved in bottom waters. Nocturnal convective mixing re-introduces oxygen into bottom waters for aerobic respiration and regenerated inorganic carbon into surface waters, which supports intense photosynthesis. Our results reconfigure the basic understanding of local environmental gradients in shallow lakes, one of the most abundant freshwater habitats globally.

We investigated the partitioning of carbon, nitrogen, and phosphorus between particulate and dissolved production using 11‐m³ marine mesocosms (bags) in a Norwegian fjord with a salinity of 28.3, a chlorophyll concentration of 0.6 µg L⁻¹, an even biomass among five algal groups, and nitrogen limitation as the initial conditions. The experiment lasted 21 days in August. Addition of silicate (+Si) resulted in diatom dominance, while a more diverse community was present in treatments with no added Si (‐Si). Addition of inorganic nutrients in a N: P gradient from 64 to 4 either conserved the initial N limitation or forced the plankton communities to P limitation. Per added limiting nutrient, the diatom‐dominated bags produced more particulate (POC) and dissolved organic carbon (DOC) than the other bags. However, the relative partitioning of net production to POC and DOC did not differ as a function of the plankton communities. Between 22% and 33% of the net production accumulated as new DOC. The higher values were found in the N‐limited bags. The production of new dissolved organic nitrogen (DON) was variable over time, and short periods of positive production were followed by removal (negative production). Between 6% and 22% of the assimilated N was recovered as new DON in the N‐replete bags, while the DON production was very low during N limitation. The community structure had no effects on nitrogen partitioning. Diatom dominance (+Si bags) resulted in P sequestration to particles and a constant low net production of dissolved organic phosphorus (DOP) across the nutrient gradient. The production of DOP was low in the P‐limited (‐Si) bags; however, with a surplus of inorganic P, most of the assimilated P (74% to 85%) was recovered as new DOP. The consequence was a huge range in stoichiometric ratios for newly produced dissolved organic matter (DOM). With N limitation, the C:N ratio of new DOM was from 40 to 100, but it was below 40 under N‐replete conditions. The C: P ratio of new DOM in the ‐Si bags traced the P availability, and values approached 500 in P‐deficient bags to values between 17 and 58 in the P‐replete bags. The C: P ratio of new DOM in the +Si bags was about 300 at all dosing regimes. Consequently, the range in N: P ratios was also large, with values from below 1 to about 30. Carbon‐rich DOM in oceans and coastal waters is not necessarily a function of a slow diagenetic “maturation’’ process but can be produced almost immediately. Both the nutrient regime and phytoplankton community composition affected the production and composition of new DOM in this experiment.

The world's 1.4 million lakes (≥10 ha) provide many ecosystem services that are essential for human well‐being; however, only if their health status is good. Here, we reviewed common lake health issues and classified them using a simple human health‐based approach to outline that lakes are living systems that are in need of oxygen, clean water and a balanced energy and nutrient supply. The main reason for adopting some of the human health terminology for the lake health classification is to increase the awareness and understanding of global lake health issues. We show that lakes are exposed to various anthropogenic stressors which can result in many lake health issues, ranging from thermal, circulatory, respiratory, nutritional and metabolic issues to infections and poisoning. Of particular concern for human well‐being is the widespread lake drying, which is a severe circulatory issue with many cascading effects on lake health. We estimated that ∼115,000 lakes evaporate twice as much water as they gain from direct precipitation, making them vulnerable to potential drying if inflowing waters follow the drying trend, putting more than 153 million people at risk who live in close vicinity to those lakes. Where lake health issues remain untreated, essential ecosystem services will decline or even vanish, posing a threat to the well‐being of millions of people. We recommend coordinated multisectoral and multidisciplinary prevention and treatment strategies, which need to include a follow‐up of the progress and an assessment of the resilience of lakes to intensifying threats. Priority should be given to implementing sewage water treatment, mitigating climate change, counteracting introductions of non‐native species to lakes and decreasing uncontrolled anthropogenic releases of chemicals into the hydro‐, bio‐, and atmosphere.

The traditional perception in limnology has been that phytoplankton biomass in lakes is limited by phosphorus, nitrogen, and light, but not by dissolved inorganic carbon (DIC) because CO₂ can be supplied from the atmosphere. We tested the possibility of carbon limitation of photosynthesis, growth, and biomass accumulation of phytoplankton communities across an alkalinity and DIC gradient (0.15–3.26 mM) in nutrient‐rich freshwater. During 47‐day long experiments, we measured phytoplankton biomass, organic carbon, calcium, DIC, pH, and oxygen in indoor, constantly mixed mesocosms with either no removal or a 70% weekly removal of the biomass. Photosynthesis was measured in the morning and in the afternoon at high biomass. Maximum biomass and organic carbon production increased two‐ to four‐fold with DIC, which supported 7% of organic carbon production at low DIC and 53% at high DIC concentration, while atmospheric CO₂ uptake supplied the remainder. Weekly biomass removal increased growth rates through improved light conditions leading to enhanced total phytoplankton biomass production at high DIC. Photosynthesis was significantly higher in the morning compared to afternoon due to daily DIC depletion. We conclude that phytoplankton photosynthesis, growth rate, maximum biomass, and organic carbon production can be markedly carbon limited in eutrophic lake waters. Consequently, lakes of high DIC and pH can support a faster primary production by greater DIC use and chemically enhanced atmospheric CO₂ uptake.