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Submerged freshwater macrophytes are crucial for the functioning of lakes. Their growth and survival follow environmental conditions like light, temperature, and nutrient availability. Hence, the impending increase in water temperature as well as changes of nutrients and turbidity will lead to changes in macrophyte geographic and depth distribution: Herein, we assess these potential changes. We apply an eco‐physiological macrophyte growth model to simulate biomass growth and survival of virtual species defined by random trait combinations within expert‐derived trait ranges for oligotraphentic, mesotraphentic, and eutraphentic species groups in deep lakes in Bavaria, Germany, which cover clear, moderate, and turbid lake conditions. The emergent potential species richness is compared with empirically observed species richness to evaluate general predictions for current conditions. Thereafter, we apply the model to scenarios of temperature increase and of turbidity and nutrient change to assess potential changes in species richness and the influence of species' traits on being an environmental change 'winner' or 'loser'. We find a cross‐lake, hump‐shaped pattern of potential species richness along depth. This largely reflects observed patterns, although mismatches were also detected and might be explained by missing processes and environmental heterogeneity within the lake. Rising temperature leads to increased richness of potential species in all lake types, species groups, and depths. Turbidity and nutrient change effects depend on depth and lake type. 'Loser species' under increased turbidity and nutrient level are light consumptive and sensible to disturbances, while 'winner species' have a high biomass production. These findings show that the hump‐shaped depth distributions of submerged macrophyte diversity can emerge solely considering eco‐physiology. The differential responses to environmental changes imply that management measures must account for lake type because those responses can have opposite trends depending on lake depth and type. [ABSTRACT FROM AUTHOR]

Investigating diversity gradients helps to understand biodiversity drivers and threats. However, one diversity gradient is rarely assessed, namely how plant species distribute along the depth gradient of lakes. Here, we provide the first comprehensive characterization of depth diversity gradient (DDG) of alpha, beta, and gamma species richness of submerged macrophytes across multiple lakes. We characterize the DDG for additive richness components (alpha, beta, gamma), assess environmental drivers, and address temporal change over recent years. We take advantage of yet the largest dataset of macrophyte occurrence along lake depth (274 depth transects across 28 deep lakes) as well as of physiochemical measurements (12 deep lakes from 2006 to 2017 across Bavaria), provided publicly online by the Bavarian State Office for the Environment. We found a high variability in DDG shapes across the study lakes. The DDGs for alpha and gamma richness are predominantly hump‐shaped, while beta richness shows a decreasing DDG. Generalized additive mixed‐effect models indicate that the depth of the maximum richness (Dmax) is influenced by light quality, light quantity, and layering depth, whereas the respective maximum alpha richness within the depth gradient (Rmax) is significantly influenced by lake area only. Most observed DDGs seem generally stable over recent years. However, for single lakes we found significant linear trends for Rmax and Dmax going into different directions. The observed hump‐shaped DDGs agree with three competing hypotheses: the mid‐domain effect, the mean–disturbance hypothesis, and the mean–productivity hypothesis. The DDG amplitude seems driven by lake area (thus following known species–area relationships), whereas skewness depends on physiochemical factors, mainly water transparency and layering depth. Our results provide insights for conservation strategies and for mechanistic frameworks to disentangle competing explanatory hypotheses for the DDG. [ABSTRACT FROM AUTHOR]