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.