Does connectivity increase resilience of biodiversity against eutrophication in networks of shallow lakes?
Enhanced connectivity between habitats is widely promoted in conservation and restoration science, yet the consequences of connectivity between lakes for plant dispersal and biodiversity are poorly understood. Here, we assess the importance of connectivity for aquatic plant diversity in two lake districts: 41 lakes (2-30ha) in the Upper Lough Erne area, Northern Ireland, UK, and 53 lakes (1-50 ha) in The Broads system, eastern England, UK. Using generalised linear models, we determine whether aquatic plant diversity is driven by connectivity and/or other environmental variables, such as water chemistry and lake morphology, in each lake district for two time periods: 1983-1990 and 2006-2014. We assessed hydraulic connectivity in terms of being: (i) isolated, (ii) connected to a small network of lakes (n=2-5), (iii) connected to a large network of lakes (n>20). We assessed aerial connectivity by measuring the number of water features present within an incremental distance (from 0.1 km to 10 km, at 1 km increments).
We found that connectivity plays a significant and complex role in determining aquatic plant diversity. In the Upper Lough Erne system hydraulic and aerial connectivity acted synergistically and explained a significant (p < 0.01) amount of diversity variation along with other environmental factors. In The Broads, the importance of connectivity changed over time. During the first time period, only water quality variables explained plant diversity. However, during the recent period, hydraulic connectivity had a negative impact (p< 0.01) while aerial connectivity had a positive effect (p<0.05) on plant diversity. In both lake districts, neither connectivity nor species diversity were significantly associated with the presence of non-native species. We suggest that plant colonisation augmented by an increased frequency of plant dispersal can enhance the resilience of aquatic plant communities to eutrophication. However, when water quality is an issue, isolation from hydraulic connectivity may be more beneficial in maintaining diversity. Our findings make a significant contribution to understanding the balance between the positive and negative effects of connectivity on lake biodiversity and associated ecosystem services in freshwater landscapes.