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.