**2. Degrees and Axes of Artificiality**

What does it mean to identify aquatic ecosystems as "artificial"? Designer ecosystems, like rain gardens and ponds, swales, and wetlands conceived and built for water treatment [9], are obviously deliberately constructed for human purposes, often where no waterbody existed before, and thus undisputedly artificial [10]. A stream or lake in a protected watershed far removed from intensive human land use might serve, in a traditional ecological study, as a "completely natural" or "pristine" reference site. However, many, or likely most, ecosystems fall between these extremes. For example, scientists and policymakers often differentiate between a channelized stream and a wholly man-made trench, but either may be colloquially called a "ditch" or "artificial", and the two may look and even function quite similarly, particularly within a highly modified agricultural landscape. All natural waterbodies do not necessarily maintain better structure and function than all artificial ones [11]. Restoration projects similarly blur the bounds of "natural" and "artificial" [10,12]. While restoration typically has the goal of returning an ecosystem to some more natural state [13], the process of restoration necessitates human intervention, which is often sustained through maintenance [14], an implicit acknowledgement that many ecosystems cannot withstand human disturbance without purposeful human assistance. Meanwhile, undirected human actions, like stormwater efflux, can accidentally "restore" natural systems [15–17]. Even mostly or wholly man-made systems, like retention ponds, can "naturalize", or become more biotic and ecosystem-like, in time, without deliberate human effort [18]. Both current character and driving forces behind it are often a combination of human and wild, artificial and natural.

These examples sugges<sup>t</sup> that artificial aquatic ecosystems can be usefully organized along two axes: the degree to which their existence and characteristics depend on human activity, and the degree to which that activity is specifically intended to produce those changes (Figure 1). The first axis spans from moderate alteration of systems with initially geologic origins, to the wholesale creation of waterbodies on formerly dry land, sometimes away from topographic lows. The latter axis ranges from aquatic systems that came to exist as inadvertent or accidental by-products of other human activities on the Earth's surface to deliberate and intentional products of such activity [19]. These two artificiality axes are gradients, not discrete categories, and can be difficult to parse, especially for the many waterbodies with complex, multi-layered histories of modification [20–22]. Depending on the purpose, it may be appropriate to define "artificial" waterbodies broadly or narrowly within this space.

**Figure 1.** Classification of artificial aquatic systems. Level of deliberateness of modification increases from left to right, and degree of modification increases from bottom to top. These two axes combined yield a third axis, likelihood of legal standing as a water for regulatory purposes in the U.S., running from unlikely among deliberately constructed ecosystems like swimming pools and upland farm ponds in the upper right, to likely among accidentally altered ecosystems, like a lake bisected by a railroad causeway, in the lower left. Other potentially influential characteristics, such as size and permanence, may influence regulation in both natural and artificial systems.
