## 4 Discussion
### Other Effects  
pH in experimental chambers generally increased during the incubation (Figure 6a). Photosynthesis could not be a reason for that in our experimental design due to a lack of light access. Therefore, we conclude the pH increased as a result of the water-air CO\({}_{2}\) exchange due to the mixing employed (twice a day.) In the later stage of the experiment, the pH seemed to stabilize, showing either equilibrium conditions and/or increasing effects of the detritus decay as this process generally lowered the pH [70,72], in our experiment possibly preventing the pH from rising more. Importantly, throughout the incubation duration, the pH was lower in the mesocosms with modules and laminates compared to no treatment, with the lowest pH when using laminates (Figure 6a). In a natural setting, we would expect such an effect to take place due to reduced OP availability and thus limited primary production. In our experimental design, where the lake water had access to light only during short periods of the lake water mixing, this factor had to be excluded. Under our experimental conditions, the lower pH in the presence of laminates and modules could have resulted from the precipitation of calcium carbonate. The calcite layer, present on/in laminates/modules, could have worked as a surface promoting (heterogenous) precipitation of native calcium as CaCO\({}_{3}\). Importantly, the total area of the calcium carbonate layer exposed during the experiment as laminates was about two times higher compared to that exposed as modules (1500 vs. 740 cm\({}^{2}\)). This explains the lower pH in the water treated with laminates (compared to control and modules). However, we did not detect clear signs of CaCO\({}_{3}\) formation such as a loss of calcium or alkalinity (Figure 6b,c). The concentration of calcium was generally stable and changed in a small range between 59 and 62 mg Ca\({}^{2+}\) L\({}^{-1}\) (laminates and modules). Only in the treatment using laminates, a short-time increase in calcium concentration could be observed after about 3 weeks, together with a slight increase in the alkalinity (Figure 6b,c), suggesting, rather, the dissolution of calcite and not precipitation (without, however, any detectable effect on pH; Figure 6a). The saturation index with respect to calcite for the lake water was 0.4 at the beginning of the incubation but it slightly increased due to the pH rise to ca. 0.6 at the end of the incubation. Although theoretical equilibrium exists at the saturation index value of 0, the range between -0.5 and +0.5 is frequently treated as constituting equilibrium conditions, i.e., considering uncertainties resulting from measurement methods [73]. Therefore, the saturation index in our experiment (0.4-0.6) should be interpreted as equilibrium (where precipitation and dissolution balance each other) or slight oversaturation (where the precipitation theoretically prevails). Thus, the CaCO\({}_{3}\) precipitation in mesocosoms treated with laminates/modules is possible, although no other signs of this process, apart from lowering pH (compared to the control mesocosm), could be detected. Another reason for the lower pH observed in the chambers with laminates and modules was that a physical effect occurred. Both solutions could retain the organic matter (settling from the water column). [PERSON] [68], who tested underwater polypropylene sheets for enhanced self-purification in a wastewater reservoir, reported a substantial deposition of detritus on these structures. In our case, the source of the organic material could have comprised both the detritus initially present in the lake water as well as a new pool coming from the dying phytoplankton after the start of the experiment (when the light access ceased). In such a case, the decrease in pH (as affected by the decomposition of the organic material) could be more pronounced in the water column of the treatment mesocosms compared to the control, where the material could easily settle to the bottom.