4.3.2. Planning and Construction

The reduced physical complexity of many artificial aquatic systems, such as concrete-lined channels, obviously limits their value as habitat and potential for improvements in water quality [126]. Restorations that seek to improve these values therefore often focus on the (re-) introduction of heterogeneous structures that are more similar to natural systems [120]. Such designs, and their implementation, can be constrained or flawed in ways that limit their ecological value, including by insufficient scientific understanding of how design features and subsequent managemen<sup>t</sup> influence eventual outcomes [35,93,127,128]. However, many such systems are also affected by intensive land use and short lifespans [103]; artificial systems whose structure mimics that of natural systems can support similar biotic communities when water quality is high [100].

Conversely, engineering research on designer ecosystems constructed for a very specific subset of aquatic ecosystem services, such as water quality improvement, clearly demonstrates that design plays a role in how effectively these systems achieve their purpose. For example, plant species choice in wastewater treatment wetlands affects speed and removal efficiency of different forms of nitrogen [129]. In wetlands constructed to remove pharmaceuticals from water, design choices of substrate, plants, and regimes of hydrology, temperature, oxygen, and light all affected removal efficiency, which varied from compound to compound in ways apparently related to microbial processes [130,131]. While much variation remained unexplained even in these relatively controlled systems, they do demonstrate that how an ecosystem is constructed affects its ecological behavior.

Physical, legal, and cultural constraints exert strong control on goals and resulting designs. For example, restoration efforts are typically constrained and otherwise impacted by funding, land ownership, and other social and economic variables [13,93,103]; restorations can have a wide range of intended outcomes [97]. Morphology of stream restorations depends in predictable ways upon funding source and legal purposes, and whether the metric for success is stream length (resulting in very sinuous designs) or some other characteristic [124]. Stream restoration in general has tended towards a single-channel, S-shaped, meandering morphology that conforms to longstanding aesthetic concerns [125], reduces maintenance [123], and maximizes mitigation credits, rather than conforming with local natural history [124]. One indication of the limitations of many restoration projects is the finding that accidental aquatic systems can sometimes provide equal or greater services compared to deliberate, designed systems. For example, "accidentally restored" wetlands at stormwater outflows in the dry bed of the Salt River in Arizona had greater wetland plant richness and cover than comparable actively restored sites, though the reverse was true for birds, non-avian reptiles, and amphibians [15].

Changes in goals often dictate substantial changes in the physical structure of artificial aquatic ecosystems. Two-stage ditches, in which miniature floodplains are constructed alongside existing conveyances [132], can significantly reduce concentrations of phosphorus and other nutrients, turbidity, and total suspended sediments [133,134]. Their nutrient removal efficiency compares well with, and can complement, other farmland best practices, like planting cover crops [90]. When properly constructed according to fluvial principles, these ditches can remain functionally stable, without maintenance, for years [109]. Thus, water infrastructure of agricultural landscapes can be designed, and successfully re-built, to achieve a wider range of goals than water conveyance, though additional land area and design and construction effort may be required.
