## 4 Discussion  
### Effect on Phosphorus  
The availability of OP in the experiment generally increased as indicated by the rise in OP concentration in the treatment chamber by 18% within \(\sim\)40 days (Figure 5a). The source of this increase was probably the OP supply from the decomposition of dead organic matter initially present in the lake water and its new pool coming from dying phytoplankton when the light access ceased. The OP dynamics observed in the treatment chambers were different and revealed large differences in the efficiency and longevity of laminates and modules (Figure 5a). Although both solutions lowered the OP concentration in the lake water for at least 6 weeks (42 days) compared to no treatment, the laminates were able to  
Figure 6: pH (**a**), concentration of calcium (**b**), alkalinity (**c**) and conductivity (**d**) in the lake during incubation. ‘No treatment’ refers to lake water without any additions. Results were means \(\pm\) SDs from duplicate experiments. Day 0 indicates initial conditions. The laminates and modules were placed into experimental chambers at the end of day 0.  
force continuous OP decline for a relatively short period of about 1 week. This showed that their sorption capacity had still not been exhausted and/or the sorption removed more OP than the native processes delivered. After a longer time, the treatment using laminates was not capable of keeping the declining trend and the OP concentration started to rise, generally corresponding to the dynamics observed in the control mesocosm (Figure 5a). This suggests that the calcite material in laminates reached saturation (the sorption ceased) and/or the OP supply (the source of which was the decomposition of organic material) was higher compared to OP removal via sorption processes. Another possible factor contributing to the reverse in the declining trend in OP availability was the desorption of phosphate ions from the calcite material. OP can be completely desorbed from calcite under phosphate-free conditions [56]. In our experiment, however, in the mesocosm with laminates, the phosphate was still present after 7-14 days (when the OP started to increase) at the concentration of about 0.41-0.49 mg PO4\({}^{3-}\) L\({}^{-1}\) (Figure 5a), so we did not expect a complete or substantial desorption under these conditions. However, even though the OP concentration started to increase after 1 week of incubation, the laminates kept it on a still lower level compared to the control mesocosm (16% lower after 42 days), mitigating the OP supply effect (Figure 5a,b). Modules, in contrast, showed continuous OP elimination throughout the experiment duration. After 42 days, the treatment using modules reduced the OP concentration by 45% compared to the starting conditions and kept it on a 54% lower level compared to the control chamber (where the OP availability in the lake water continuously increased) (Figure 5a). Thus, the modules first effectively eliminated the OP available in the system (initially present in the lake water) and further captured the OP newly supplied by the decomposition processes. The distinctly higher efficiency and longevity of modules compared to laminates in removing OP in our experiment obviously resulted from the much larger calcite content applied using modules. At the surface area of modules being more than two times smaller compared to laminates (741 vs. 1500 cm\({}^{2}\)), they contained 83 times more active agent (300 vs. 3.6 g). Thus, the number of active sites available for OP adsorption was accordingly higher. Consequently, the calcite dose per m\({}^{2}\) of the surface area was 168 times higher in modules compared to laminates (4050 vs. 24 g). This reflected the contrasting technical design--a thicker calcite layer of ca. 2 cm could be applied in modules (which was determined by the depth of trays) while the calcite layer formed onto laminates was only \({}^{\sim}\)2 mm thick (which was determined by the adhesive force of glue).