Potential and observed submerged species richness distribution
along depth (Q 1.2)
Our results show that the observed hump-shaped pattern of species
richness along depth (Lewerentz et al., 2021) can emerge from an
eco-physiological growth model for macrophytes (Figure 2a and b). This
means that eco-physiological processes alone can emulate general
real-world patterns of depth distribution of macrophytes. However,
patterns specific for particular lakes were not always matched (Figure
2c). A reason for smaller observed than potential species richness might
be missing processes within the model (e.g. interspecific competition or
herbivory) or imperfect detection during field mapping. A larger
observed than potential species richness might be explained by habitat
heterogeneity within the lakes. In our experimental design, we
considered a single set of mean environmental parameters for each lake.
However, large lakes are known to be highly environmentally
heterogeneous (Árva et al., 2015), which must be the case in Bavaria.
This missing representation of habitat heterogeneity within lakes is due
to the low spatial resolution of the measurements. Unexpectedly, we
observe an underestimation of the potential species richness exclusively
in clear and intermediate lakes, but not in turbid lakes. The observed
lack of species richness in the turbid lakes might be the result of a
shift in life forms from complex macrophytes towards “simple” algae.
This shift increases the competition for light and nutrients between
algae and macrophytes (Hilt, 2015). The model is able to capture a
positive, but weak correlation between observed and potential species
richness for the different species groups and lake types, but with the
exception of eutraphentic and oligotraphentic species in clear lakes
(Figure 2c). A reason for this exception might be increased competition
and stress as clear water lakes tend to be nutrient-limited and colder.
Notwithstanding such limitations, our model proved to be useful to
explore distribution patterns of species richness of macrophytes along
the depth gradient of lakes.
Patterns of potential species richness of submerged
macrophytes along depth – changes under different environmental
scenarios