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).