*4.2. Time*

Most artificial aquatic systems are young, both because most land use change and earth moving has occurred within the past few hundred years [1] and because artificial aquatic systems turn over more quickly than natural ones [93]. Given the timescales over which community assembly occurs in newly formed natural streams and lakes [104], it is likely that limited diversity of some artificial aquatic systems simply reflects their recent origin. Understanding the consequences of recent formation requires that we understand the timescales over which newly created ecosystems develop, whether they arose from anthropogenic or from geologic processes, and potentially attain the characteristics of their older counterparts. At present, we lack both general and system-specific models of these trajectories, as well as criteria with which to judge that an artificial aquatic system has "naturalized".

A relatively limited set of long-term and chronosequence studies indicate that artificial aquatic systems can change in important ways over timescales that are similar to those in natural ecosystems, and that are relevant to decision-making [105–107]. In restored wetlands, for example, ecological structures and functions such as carbon sequestration can improve with time since intervention, and soil characteristics approach natural properties over decades [108]. Re-configured two-stage ditches can achieve soil formation and a geomorphological "quasi-equilibrium" within a decade [109]. Agricultural ditches also undergo a relatively predictable succession of plants and associated invertebrate communities [110]. Accidentally created waterbodies also change over time, often acquiring more 'natural' characteristics. For example, gravel quarries develop more structurally complex and diverse vegetation over several decades [111]. At some point in time, artificial aquatic systems may be difficult to distinguish from natural systems. Many of the small, ephemeral wetlands that sustain populations of amphibians in the Piedmont of the U.S. Southeast are likely legacies of historical human disturbance [26]. Such examples sugges<sup>t</sup> that time eventually erases many signatures of anthropogenic origin, and that this naturalization may change how aquatic systems are perceived and valued.

Better understanding of how time constrains the characteristics of artificial aquatic systems, and the mechanisms by which they evolve, could improve our ability to manage them, individually and as part of the broader aquatic landscape [112]. Properties associated with age may elude newly created waterbodies, and expectations that artificial waterbodies adequately replace natural ones should be tempered accordingly. Goals and expectations of restorations and other interventions might need to reflect the differential responses to the same managemen<sup>t</sup> technique, as has been observed in young and old artificial aquatic systems [113]. Deliberate managemen<sup>t</sup> of successional stages has been used to increase the abundance and diversity of desirable species [107]. Wider adoption of such approaches will require better models of succession and its dependence on design, setting, and management.
