3.1.3. Change

The distribution of artificial waterbodies, like and reciprocally with many natural systems, is dynamic in time, owing to both seasonal and event-driven hydrologic change, as well as longer-term changes in land cover. Some of these changes involve the destruction or reduction of natural waterbodies; net effect of growth in artificial waterbodies includes what they replace and from whence they divert water, including those accidental changes that typically go unmeasured. Between 1984 and 2015, North America as a whole, home to 52% of the world's non-ocean permanent surface water, added a net 17,000 km<sup>2</sup> to this area. The area of permanent surface water in the U.S. as a whole grew 0.5%, even as six of its western states lost 33%, or over 6,000 km2, of their permanent surface water [52]. Farm ponds, in particular, are highly dynamic in use, creation, and abandonment [53]. In commercial and residential developments, ponds and other stormwater features too small to appear in the above analysis, wink in and out of existence too quickly for inventorying, or further scientific study [54].

Emerging technologies and modeling approaches have the potential to improve inventories of small and accidental waterbodies and better characterize the distribution and dynamics of hydroscape change. Advances in remote-sensing technology, such as the increasing availability of high-resolution lidar data, may very soon yield much better maps of small and otherwise over-looked waterbodies over broad extents [55–57].

### **4. The Condition of Artificial Aquatic Systems and Its Drivers**

The perceived poor condition of artificial aquatic systems matches the reality of poor water quality and altered ecological structure in many man-made waterbodies [58,59]. Many artificial

waterbodies support species-poor [60] or otherwise undesirable communities or organisms, including disease vectors [58,61,62], and can spread pest species to natural habitats [63,64]. Some have also contributed to, accelerated, or facilitated flow of excess nutrients and other pollutants [65,66], activation of toxicants [58], interrupted desirable species' movement and dispersal [67], increased greenhouse gas emissions [27,68], yielded bad smells [69], and even concealed crime [70]. Other examples of ecosystem disservices proffered by artificial water bodies appear in Table A1. While natural waterbodies can possess the same undesirable characteristics, we are more likely to assume that artificial waterbodies have a negative influence without investigation [71].

Are artificial aquatic systems intrinsically less biologically diverse and less functional than natural ones? It is at least plausible that humans cannot create a waterbody that supports communities as diverse or provides as many ecosystem services. Certainly, when transforming, altering, or removing a functional natural aquatic ecosystem, one should expect a reduction in current ecosystem services provisioning, unless or until scientific study confirms a better outcome possible from the change. One important constraint on artificial aquatic systems is that with their anthropogenic origin comes a severely shortened evolutionary, ecological, and geophysical history [72,73]. To the extent that diversity and other aspects of ecosystem structure depend on slow processes of physical change and community assembly, the recent origin of most artificial systems will likely limit their function.

There are other potential limits on the condition and value of artificial aquatic systems. First, imperfect understanding of how differing designs and constructions affect ecological outcomes, and imperfect ability to reproduce natural structures and conditions, may constrain the most ecologically-motivated projects, as may be the case for many ecosystem restorations [13]. Second, artificial aquatic systems are often embedded in intensively used landscapes, potentially exposing them to anthropogenic stressors and disconnecting them from diverse natural populations [13]. Finally, and perhaps most importantly, many artificial aquatic systems may support limited diversity and ecosystem function because they are not designed or managed to do so; in many cases, their intended function may preclude or limit the provision of other services [27]. As a result, scientists, policy-makers, and the general public have tended to accept that artificial aquatic systems will necessarily and inherently have limited value. However, these assumptions are often not subject to the same critical assessment and process-based explanations that are applied to explanations of the condition of other aquatic systems.

The poor condition of artificial aquatic systems is far from universal, and at least some, perhaps many, artificial aquatic systems also have clear ecological value. Constructed and transformed aquatic systems, whether agricultural, industrial, urban, or recreational, can sustain biodiversity [74], sometimes including rare and desirable species [18]. In Europe, manmade farm ponds serve as primary or important habitat for amphibians [75], birds [76], invertebrates [77–79], plants, and other species [80,81]. Indeed, European conservation proceeds from the assumption that "artificial", "man-made" ponds are not fundamentally ecologically different from "natural ones" [82]. Some species now apparently depend primarily on deliberate artificial aquatic ecosystems for habitat [83–85]; even species new to science continue to emerge from ditches [86–88]. Equally in the U.S., habitats that we presently tend to overlook, such as stormwater treatment wetlands, can sometimes be the best available sites for reproduction of amphibians and other species with specific hydrologic needs [89]. Artificial aquatic systems, whether designed for the purpose or not, have improved water quality in critical watersheds like the Mississippi River basin [90–92]. Additional examples of ecosystem services provided by artificial aquatic systems appear in Table A1. Without more intensive and systematic study, it remains unclear whether good ecological conditions, and the desirable ecosystem services that derive from them, are a negligible, rare, or even commonplace occurrence in artificial aquatic systems. Similarly, their net impacts, and value relative to natural counterparts, remain undetermined.

Making artificial aquatic systems more functional and valuable will require a mechanistic and predictive understanding of their condition and their capacity to provide ecosystems services and disservices. We propose that the science of artificial ecosystems should entertain and evaluate hypotheses about what drives variation among them as well as their differences from their natural counterparts. Like their natural counterparts, the ecological characteristics of artificial aquatic systems are likely to depend on their physical structure, the characteristics of the watershed and landscape in which they are embedded, their age and trajectory over time, and the ongoing interventions of humans for various purposes (Figure 3). While all of these mechanisms are shaped by human design decisions, they also have clear analogs to factors commonly invoked to explain the condition of natural aquatic systems. This re-casting of rationales for why artificial aquatic systems are assumed to be in poor condition as testable alternative mechanisms allows us to consider how different decisions about the design, placement, and longevity of artificial aquatic systems might improve their condition and value. The poor condition and seemingly inherent limitations of artificial aquatic systems could be simply a syndrome of those decisions. In our exploration of these possible causal variables for the ecological condition of artificial aquatic systems here, we focus on this management-oriented way of examining the causal variables, and barely begin to explore the possible interactions between them. We recognize that scientists with different foci could propose other valid testable hypotheses, and indeed invite them to do so, but we consider setting, time, physical design, and subsequent managemen<sup>t</sup> of artificial waterbodies to be good intellectual places to start trying to understand these ecosystems.

**Figure 3.** Inherent vs. process-based models of the condition of artificial aquatic systems. (**a**) inherent qualities model; (**b**) process-based model. We sugges<sup>t</sup> that scientists and policy-makers have too readily accepted model (**a**), in which the inherent qualities of artificiality negatively impact ecosystem structure and function, rather than scientifically exploring a mechanistic model such as the one we propose in (**b**), which breaks the influence of artificiality down into multiple processes.
