Limitations and Perspectives
The main limitation is that, in some lakes, the deeper end of the DDG was not clearly quantified. This is, however, most critical for the lakes with increasing DDG (For alpha richness: Eibsee 2016; for beta richness: Eibsee 2011, Grosser Ostersee 2008 & 2014, Tachinger See 2006, Woerthsee 2008, Eibsee 2016, Schliersee 2008; for gamma richness: Eibsee 2011 & 2016, Grosser Ostersee 2008, Tachinger See 2006 and Woerthsee 2008). For these lakes, which are mostly lakes with a high water transparency, it might be interesting to extend sampling to deeper water to quantify a metric termed ‘the lower macrophyte limit’ (Søndergaard et al. 2013). This metric is often used as indicator for water quality and might be useful to further characterize the DDG as it defines the lower limit and the occupied space.
Additional limitations of our analyses can be viewed rather as perspectives for further studies focusing on explaining the underlying causes of the DDG (see previous section) and to disentangle the presented hypothesis, as these limitations require data yet unavailable. This includes 1) depth measurements of the variables that also show depth gradients (i.e. light, temperature or nutrients) and 2) further variables that vary across transects and lakes, such as productivity, slope, soil properties (components, grain size distribution and nutrient content), ice cover duration and different disturbance factors like anthropogenic use intensity (boats, mowing, swimming) or herbivory pressure (fish, water birds). Nevertheless, our analyses already indicate that light quality and quantity may play a main role in forming the DDG in freshwater lakes and will inspire further empirical studies on the DDG as well as comparative studies with other diversity gradients.
A promising direction for future research might be combining eco-physiological experiments with mechanistic modelling to test the different species richness hypotheses. Such an approach might help to clarify the influencing force of disturbances or geometry on DDG on small scales.