Effects of shading
Shading significantly reduced the shoot number and biomass, but
increased the internode length of the submerged macrophyte P.
crispus . Light is one of the primary factors determining photosynthetic
rate of plants, and reducing light availability thus is expected to
inhibit growth of submerged macrophytes, which has indeed been
demonstrated in a number of studies (Chou
et al., 2022; Middelboe & Markager,
1997; Riis et al., 2012), and is in line
with our findings. Shading, however, may also promote macrophyte
elongation as a strategy to cope with light limitation as seen in this
and previous studies (Chou et al., 2022;
Riis et al., 2012). Light also indirectly
changed growth conditions of macrophytes in this experiment, as the
average water temperature was for instance almost 2°C lower in the
shaded treatment as compared to the full light treatment. Increased
temperature has been shown to promote growth of macrophytes
(Peiyu Zhang et al., 2016;
Peiyu Zhang, Zhang, et al., 2021), and
thus in part may explain why P. crispus accumulated less biomass
under the shaded colder conditions. No effects of shading could be
detected on growth of V. spinulosa , as the plant grew little in
any of the treatments. Vallisneria species are warm-adapted
species, and the plant thus may have grown little due to the low
temperature in our experiment (Bartleson et
al., 2014; Peiyu Zhang, Kong, et al.,
2021).
Although shading reduced growth of P. crispus , biomass buildup of
the other primary producers was promoted. It could be expected that
reduced light availability may decrease primary producer biomass
(Edwards et al., 2016;
Karlsson et al., 2009). However, in a
naturally complex shallow aquatic ecosystem with macrophytes,
periphyton, and phytoplankton present, macrophytes may be more sensitive
to light limitation, as they cannot win competition for light from
periphyton and phytoplankton (Scheffer,
2004; Yamamichi et al., 2018). Low light
conditions may thus counterintuitively result in increased phytoplankton
and periphyton biomass since they will be released from competition with
macrophytes (Guan et al., 2020;
Yamamichi et al., 2018). Shading also
indirectly increased TN and TP concentrations, which may have benefited
phytoplankton and periphyton. The algal community may have also shifted
towards species that prefer low light conditions
(Schwaderer et al., 2011), thus
maintaining growth rates and biomass
(Mette et al., 2011).