Species richness change (Q2.1)

Climate change results in higher lake water surface temperatures (Woolway et al., 2020). Within our model, higher water temperatures promote higher potential species richness (Figure 3a). This is expected, as higher temperatures leads to higher metabolic rates, productivity, and, ultimately, also richness (Brown et al., 2004; Z. Wang et al., 2009). Furthermore, higher maximal water temperature means that the vegetation period is also prolonged, which allows slower growing species to mature and reproduce. A reduction in potential species richness was predicted only in particular cases, mainly for deeper depths and oligotraphentic species. The eco-physiological background here might be that higher temperatures increase all biochemical reactions in the macrophytes (respiration and photosynthesis). However, as the photosynthesis might be limited by the available light or nutrients this might result in a netto photosynthesis that is lower than respiration, resulting in the death of the species (Binzer et al., 2006; Ikusima, 1970). In general, metabolic theory alone is a debatable predictor of diversity gradients (Hawkins et al., 2007).
Turbidity and nutrients in lakes can be influenced by different complex processes mostly concerning the whole catchment of a lake. During the last decades turbidity and nutrient levels of most lakes decreased as a consequence of improved wastewater systems and water management (Murphy et al., 2018; Vetter & Sousa, 2012). However, it remains unclear if this trend will proceed in the future. Climate change might lead to a trend reversal, as it enhances eutrophication processes of lakes (Moss, 2012; Moss et al., 2011). The loss of species in increased nutrient and turbidity conditions in currently turbid lakes is caused by light reduction. Alternatively, the gain of species in today’s clear lakes (i.e. nutrient-limited lakes) is a direct effect of nutrient increase. Consequently, the differences of the effects according to the lake types show that there is no generalizable trend for the potential species richness of lakes under turbidity and nutrient change (Figure 5). Overall, we demonstrate that changing environmental conditions will influence the potential species richness of submerged macrophytes substantially.