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.