Abstract
Investigating diversity gradients helps to understand biodiversity
drivers and threats. However, one diversity gradient is seldomly
assessed, namely how plant species distribute along the depth gradient
of lakes. Here, we provide the first in-depth characterization of depth
diversity gradients (DDG) of submerged macrophytes across different
lakes. We characterize the DDG for additive richness components (alpha,
beta, gamma), assess environmental drivers and address temporal change
over recent years. We take advantage of yet the largest dataset of
macrophyte occurrence along lake depth (274 depth transects across 28
deep lakes) as well as of physio-chemical measurements (12 deep lakes
from 2006 to 2017 across Bavaria), provided publicly online by the
Bavarian State Office for the Environment. We found a high variability
in DDG shapes across the study lakes. The DDG for alpha and gamma
richness are predominantly hump-shaped, while beta richness shows a
decreasing DDG. Generalized additive mixed-effect models indicate that
the maximum alpha richness within the depth transect
(Rmax) is significantly influenced by lake area
only, whereas for the corresponding depth (Dmax)
are influenced by light quality, light quantity and layering depth. Most
observed DDGs seem generally stable over recent years. However, for
single lakes we found significant linear trends for Rmax and Dmax going
into different directions. The observed hump-shaped DDGs agree with
three competing hypotheses: the mid-domain effect, the mean-disturbance
hypothesis, and the mean-productivity hypothesis. The DDG amplitude
seems driven by lake area (thus following known species-area
relationships), whereas skewness depended on physio-chemical factors,
mainly water transparency and layering depth. Our results provide
insights for conservation strategies and for mechanistic frameworks to
disentangle competing explanatory hypotheses for the DDG.