1.1.1 Incorporating Microbial Indicators of Hydrologic Connectivity
One emerging field-based approach that may provide new insights into connectivity is utilizing hydrologic information contained in microbiomes. Recent work has demonstrated that analysis of microorganisms can be a valuable tool in hydrologic research because membership of aquatic microbiomes are intimately coupled with hydrologic processes (Good et al., 2018; Martínez-Carreras et al., 2015; Pfister et al., 2009). Microorganisms are passive dispersers in aquatic systems and dispersal effects are primarily driven by the directional flow of water (Nemergut et al., 2013). As a result, membership of downstream aquatic microbiomes have been shown to be similar to the microbiomes within shallow soils (Crump et al., 2012) and deeper groundwaters (Amalfitano et al., 2014) that generate streamflow. However, as surface water ecosystems become disconnected and residence times of aquatic systems increase, community assembly is increasingly affected by ecological dynamics (e.g., competition, predation) that result in changes in membership of the site-specific microbiome (Crump et al., 2012; Lindström et al., 2006). At any one point in time, aquatic microbiome membership is the balance between immigration and emigration, which are primarily regulated by hydrologic connectivity, and microbial growth and mortality, which are affected by in situ environmental parameters (e.g., resources and temperature) (Crump & Hobbie, 2005; Read et al., 2014; Savio et al., 2015). As hydrologic flow states and aquatic network structure also influence hydrologic connectivity, the membership of aquatic microbiomes has the potential to reflect connectivity status. The recent formalization of analytical techniques and bioinformatic pipelines to characterize microbiome membership have made analyses of environmental microbiomes more affordable and accessible to non-specialists (Thompson et al., 2017). These increasingly routine analyses of microbiome membership, coupled with the ubiquity of microorganisms, make microbial analyses a potentially powerful tool for assessments of hydrologic connectivity.