Effects of nutrient loading
Increased nutrient loading was expected to increase growth of algae, and inhibit growth of macrophytes, as fast growing phytoplankton and periphyton may outcompete macrophytes for nutrient and subsequently light (Scheffer, 2004; P. Zhang et al., 2020). However, we did not find an effect of nutrient loading on the growth of macrophytes and periphyton, which is in accordance with a previous study showing that high concentrations of nitrogen and phosphorus did not have a significant impact on the germination and seedling growth of P. crispus (Gao et al., 2005). This may be a result of algal growth limitation by low temperatures, or due to other interactions with macrophytes such as for instance allelopathy (Sabine Hilt, 2006; Pakdel et al., 2013). P. crispusdid not suffer from low temperature since it is a cold-adapted species that can germinate below 10°C (Ren et al., 1997). Once temperatures rose it was already established and could quickly start growing in early spring, thus potentially outcompeting or partially suppressing growth of phytoplankton. An alternative explanation could be that in our experimental systems P may have limited growth of phytoplankton or periphyton, indicated by the relatively elevated TN:TP ratio in the water (N:P = 25.7 by atoms) (Klausmeier et al., 2004), but not impacting the macrophytes since they can access P from the sediment too. Though no significant direct impact of nutrient loading on the growth of macrophytes was detected, nutrient loading did interact with other treatments to affect P content, C:P and N:P ratios of the plant. These changes in nutrient content and stoichiometry may have impacts on herbivory via changes in food quality and palatability (Elisabeth S. Bakker & Nolet, 2014; Frost et al., 2006).