PMC

Conceptual illustration of the combined effects of eutrophication and climate forcing on lake trophy. In the holomictic case, stratification first separates epi- and hypolimnion (shades of blue) and transports P from deep waters to the surface upon yearly mixing (green arrow). More P may be transported when larger shares of deep sediments are in anoxic waters. A shift towards warming-induced meromixis could first induce a non-linear response: surface re-oligotrophication together with persistent deep-water anoxia. However, nutrient accumulation in the deep waters bears the potential to trigger episodes of high productivity when meromixis suddenly retreats, allowing surface waters to mix deeper (yellow arrow), as for example during an exceptionally cold winter.

Assessment of the potential for climate-induced meromixis development and the entailing consequences for lake-internal nutrient cycling. The dotted lines illustrate our hypothesis that lakes with a relative depth z_rel between 5% and 0.49% have a non-zero chance to be or become meromictic in a warmer world. (a) A conservative threshold of 5% is assumed to delineate holo- from meromictic lakes but already today the Lakes Iseo, Bourget and Zurich (z_rel 2.90%, 1.98%, 1.49%) feature multi-year deep water stagnation. Rows below show data from Woolway and Merchant who analyzed lakes (n = 85) for their potential to remain holomictic or shift towards meromixis in a warmer future. (b) z_rel of 3,475 global lakes included in the GLDBv2. The red area represents the cumulative water volume retained in lakes with a higher z_rel relative to the global non-glaciated freshwater resources (excluding the Caspian Sea) (c) Lake phosphorus retention in monimolimnetic waters and sediments when a lake shifts towards meromixis. R_P,M was calculated from predicted monimolimnion depths, bathymetry and water residence time of 243 lakes included in both the GLDBv2 and HydroLAKES databases (Eq. ). The black line is a linear model to the data (p < 0.1). Transparent symbols show lakes with a very small chance to turn meromictic (z_rel < 0.49%).

Chlorophyll a abundance over time and depth in Mono Lake, CA. Data were redrawn from publicly available (http://www.monobasinresearch.org/onlinereports/) LADWP reports from 2016, 2017, 2018, and 2019. Chlorophyll a concentrations were measured by fluorometry after collection at 2, 8, 16, 20, 24, and 28 m water depth. Note that LADWP did not collect data in January of each year. The approximate transition from monomixis to meromixis is also indicated, but it should be noted that these regime shifts are gradual, not sudden

Holocene and Late Glacial (past 15,000 yrs) reconstruction of aquatic productivity and lake mixing from Moossee, Switzerland based on HSI-inferred pigments. Meromixis was inferred from the presence of bacteriopheophytin a based on HSI. Meromictic periods (highlighted in purple shading) occurred during periods of closed forest cover (dashed line in the forest cover plot indicates closed forests with 80% tree pollen) and warm summer temperatures. Human-caused forest openings led to periods of greater lake mixing. Modified from Makri et al. []. Summer temperature reconstruction inferred for the Alpine region based on chironomids [].