Conclusion

Our study makes a first step towards a generalizable understanding of the Depth Diversity Gradient (DDG) of submerged macrophytes, their regional and temporal heterogeneity as well as the drivers of the DDG shape. Submerged macrophytes richness peaks predominantly at intermediate depth forming a hump-shaped pattern for alpha and gamma richness, but a decreasing pattern for beta richness (Fig. 4). Well-known hypotheses of biogeography that may shape diversity gradients in general, such as mid-domain effect and mean productivity hypothesis. The latter is already supported by our findings on the role of light and temperature as DDG drivers. The key advantage of DDG in contrasting these hypotheses is the logistic feasibility of short-distance scales and the exclusion of confounding effects associated with dispersal constraints. The key drivers of DDG we determined were area influencing the species richness peak hight (R(α,max) ) and light quality, light quantity and layering depth influencing the species richness peak depth (D(α,max) ). Although we found that the DDG in general remained stable over past few years for most lakes, we still found shifting trends for richness metrices for some lakes. However, these trends showed to be diverse across lakes. Whereas climate change might be more ubiquitous, land use change may be lake specific. This suggests that water management strategies should consider besides global warming, also lake characteristics and change in the surrounding land use. The interaction of these aspects also means that although higher temperatures lead to a reduction in the quantity of light available to aquatic plants in lakes, land use measures can be taken to counteract this. Nevertheless, our findings already indicate that warmer water temperatures may still lead to a shift in species along depth dependent on further efforts to hold or increase water quality of lakes.