## 4 Discussion
Years with high macrophyte abundance have been found to increase total bird numbers as well as herbivores, whereas piscivores were lower during such times [77]. This study also found a similar trend, including that of piscivores, which could suggest a benefit in predation in the absence of refuge which is also likely to be a cyclical benefit as macrophytes generally provide habitat and increase the abundance and diversity of fish and macroinvertebrates [89; 90; 91]. Also worthy of further study are the relative roles of invasive crayfish and fish predation on the macrophytes that could also contribute to eliciting a cyclical response in the macrophytes similar to that forwarded for herbivorus birds [19; 92; 93]. It is noteworthy that the levels of coverage and especially density of macrophytes observed in 1997 (particularly in shallower areas) have not been observed since, and that this coincides with the arrival of the invasive crayfish _Orconetes limosus_ in 1998 and _Procamburus clarkii_ in 2003 [93]. Their numbers are also likely to decline in response to a decrease in macrophytes and increasing piscivorous birds observed here. It is likely that no relationships were found between sparse macrophytes (\(\mathrm{S_{RM}}\)) and environmental variables because their variation was dominated by the transition to and from dense macrophytes (\(\mathrm{D_{RM}}\)). Of the indicators of trophic state, both chlorophyll-a and Secchi disk depth were significant in the models for predicting total macrophytes but not total phosphorus. The loss of dense macrophytes in 2019 corresponded to a summer where the highest Chl-a and lowest transparency were recorded. While the seasonal mean of 5.4 m still appears high, a value of 3 m was recorded on 7 May 2019; in comparison, excluding 2019, the average Secchi disk depth in May from 2015-2020 was 10.6 m. Using these extreme values to estimate the depth of colonization of Charophytes would estimate depths of 4.7 m (for 3 m Secchi) to 17 m (for 10.6 m Secchi) [94]. While not an attempt to accurately characterize the light climate over the growing season, it indicates that 2019 was a year with significant pressure that could have contributed to the loss of dense macrophytes. These communities appeared to be lost from areas at a distance off the peninsula in deeper water, indicating that the changing light climate may have been responsible given the exponential decline in light with depth (Figure 2). It is possible that the storm event on 5 May increased both turbidity and released nutrients from the sediment layer via direct resuspension [95]. Such storms may have a double effect of reducing macrophytes in shallower water through wave action, while those in deeper water may suffer from poorer light climate through increased turbidity and nutrient addition causing phytoplankton and epiphytic growth with resultant shading [96]. The higher water level recorded in spring and summer in 2019 will also have had a direct influence on macrophytes through increased attenuation of light alone. For example, in a clearwater oligotrophic lake, if the water above a substrate is increased from 2 to 3 m depth then this would decrease the percentage of light reaching it from 51 to 36%, whereas for a substrate where water is increased from 7 to 8 m depth, then this would decrease the percentage of light reaching it from 9 to 7% [97]. In 2016, similar Chl-a concentrations in the summer (JJA) and comparable water level in spring and summer periods (MAM and JJA) to 2019 were recorded and were reflected in the prevalence of sparse macrophyte meadows also in the summer 2016 (Figure 3). While there are many potential drivers of the interannual variation in macrophytes, the influence of climate change is likely to be prominent. The principal change in Lake Garda is that warming winters are reducing the frequency of winter overturn, with no substantial mixing event since 2005, resulting in altered nutrient cycling and higher nutrients in the hypolimnon [47; 98].