Implications for diversity gradients in general
Looking at different diversity gradients and generalising the findings
might be a chance to get a deeper insight into mechanisms of species
richness. The depth diversity gradient (DDG) of macrophytes brings some
advantages compared the latitudinal diversity gradient (LDG) or the
elevational diversity gradient (EDG). The DDG is formed for shorter
scales (few meters) than the EDG, which implies a lower importance of
dispersal or connectivity processes. Another big advantage looking at
freshwater depth pattern is that it is much easier to use replicates for
DDG than for LDG and EDG. For LDG, the options for replicates are
restricted as there are mainly two replicates for the two hemispheres,
which can be further (pseudo)replicated when studying different taxa or
continents (Pontarp et al. 2019). For EDG, comparative studies require a
high logistic sampling effort. Thus, studies addressing the overall
shape across EDGs and the variables affecting them are not yet clear
(Nogués-Bravo et al. 2008, McCain and Grytnes 2010, Kessler et al. 2011,
Sanders and Rahbek 2012). Hence, the smaller scales of DDG seem an
advantage to do such replicate studies to address the role of energy or
productivity in shaping the diversity gradients. Moreover, a comparative
study across different gradients (EDG vs. DDG) could yield further
insights in understanding diversity drivers.
Besides DDG, there are other small-scale diversity gradients that get
studied. One example is the height or vertical diversity gradient (VDG)
from forest floor to tree crowns following environmental gradients of
light intensity, temperature and humidity. Here, species richness
distribution depends on the study species group. Hump-shaped patterns
for ferns and lycophytes (Acebey et al. 2017) or vascular epiphytes
(Krömer et al. 2007) or decreasing patterns for vascular epiphytes (Wang
et al. 2016) were found. The pattern does not seem to be very clear and
are strongly influenced by host crown characteristics (Wang et al.
2016). Still, VDG could be the most comparable gradient to DDG for
contrasting the role of light and temperature, but paired data with both
diversity and environmental data for VDG is even scarcer than for DDG.
The LDG for species generally peak in the tropics (Pontarp et al. 2019).
However, looking at freshwater plants, the diversity gradient does not
decrease linearly (Alahuhta et al. 2020), but seems to have at
sub-tropical to low tropical latitudes (Murphy et al. 2019). Thus, along
latitude, the species richness peak is at a high to intermediate level
of solar productivity, which is reflected in our analysis of DDG. This
might be explained by the Intermediate Productivity Hypothesis (IPH).
The IPH, stating that at low productivity level (deep waters with low
light quantity and low temperature) only few species can survive and at
high productivity level (shallow waters with high light quantity and
high temperature) only few competitive species survive, is highly
discussed (Adler et al. 2011, Fraser et al. 2015) and mostly used along
LDG. Besides competition, mid-gradient peaks can be explained from an
evolutionary perspective as a maximal difference between speciation and
extinction rates is expected at intermediate productivity (VanderMeulen
et al. 2001). Although quantification of productivity along depth should
be attempted, our findings already indicate a key role of light quantity
and temperature in shaping DDG.
The hump-shaped curve of the DDG might mirror most elevational diversity
gradients (EDG), which often peak at intermediate levels (Kessler et al.
2001, Nogués-Bravo et al. 2008). For both gradients one explanation
might be geometry. The mid-domain effect (MDE) explains richness peaks
in the middle of gradients by species range overlaps simply via
geometric constraints (Colwell et al. 2004). This concept is mainly used
to explain the EDG (McCain 2004, Cardelús et al. 2006, Brehm et al.
2007). Also along the depth gradient niches of single species might
overlap and create an area of enhanced richness. Nevertheless, to
adequately verify the influence of the mid-domain effect, it would be
necessary to study single species in relation to environmental variables
via mesocosm experiments without competition. Potential niche ranges
associated with lake depth (e.g. light or temperature conditions) could
then be determined and the overlap of these ranges for all co-occurring
species of a particular lake would provide at which depth lies the
MDE-predicted maximum number of species. This would make it possible to
compare observed vs. MDE-predicted DDGs. Moreover, another important
explanatory theory of EDG is the Intermediate Disturbance Hypothesis
(IDH). It suggests that species richness is highest at mid-levels of
disturbance as species of early and late successional phases can coexist
(Connell 1978). Whereas disturbances along EDG are caused by enhanced
human activities at lower elevation (Nogués-Bravo et al. 2008),
disturbances along DDG are caused by waves, grazing and water level
fluctuations at shallower depth. These disturbances act on two different
depth patterns: 1) evenly distributed over depth like herbivory by fish
or 2) gradually distributed (loosing strength with depth), which can be
caused by anthropogenic use, herbivory by birds or mammals, wind, waves
(Van Zuidam and Peeters 2015), mechanical disturbance by ice cover, or
water level fluctuations (Evtimova and Donohue 2016). The latter was
already integrated in our study in a very simple way of taking long term
indicators. Further monitoring schemes should target other disturbances,
quantifying herbivory, wave height, ice cover or finer resolved water
level fluctuations. As herbivores are supposed to eliminate on average
40-48% of plant biomass in freshwaters (Bakker et al. 2016) their
influence might be outstanding. The long-term water level fluctuation
showed no strong effect on richness measures and it might not be the
main explanation of the hump-shaped DDG. Nevertheless, although our
findings tend to not support the intermediate productivity/disturbance
hypotheses, considering that human (or natural) disturbances in shallow
waters should be more prevalent in our data, the intermediate
productivity/disturbance hypotheses could still play a role along DDG.