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Constructed stormwater wetlands (CSWs) are used to address contaminants in urban stormwater such as nitrogen (N) and phosphorus (P), but their performance is variable. Ephemeral CSWs tend to be less effective than perennial CSWs at removing N and P. We asked: How does wetland vegetation and sediment affect nutrient cycling/release from sediment and vegetation in ephemeral CSWs? We focused on two ephemeral urban CSWs in Pocatello, ID, USA, one densely vegetated and the other nearly bare. We rewetted intact cores of dry wetland sediments and, separately, senesced vegetation for 1 week at the end of the summer dry period to assess whether wetland sediments and vegetation acted as sources or sinks of N and P. For both CSWs, there was a pulse of nutrients immediately following rewetting, but the magnitude of that pulse and subsequent changes in nutrient concentrations suggest different processes dominate at each wetland, driven by differences in wetland vegetation and associated sediment characteristics. There was evidence of denitrification between and during events at the vegetated wetland, but larger fluxes of P at this site suggests a tradeoff between denitrification and P release. While the experimental results suggested specific biogeochemical controls, CSW nutrient concentrations during three events were more dynamic and suggested more biogeochemical complexity than that represented in our experiment, both within events and seasonally. Ephemeral CSWs may create unique biogeochemical conditions and require careful design to ensure N and P retention. Managers will also need to consider whether perennial water sources would improve CSW function.

We critically evaluate some of the key ecological assumptions underpinning the use of nutrient replacement as a means of recovering salmon populations and a range of other organisms thought to be linked to productive salmon runs. These assumptions include: (1) nutrient mitigation mimics the ecological roles of salmon, (2) mitigation is needed to replace salmon-derived nutrients and stimulate primary and invertebrate production in streams, and (3) food resources in rearing habitats limit populations of salmon and resident fishes. First, we call into question assumption one because an array of evidence points to the multi-faceted role played by spawning salmon, including disturbance via redd-building, nutrient recycling by live fish, and consumption by terrestrial consumers. Second, we show that assumption two may require qualification based upon a more complete understanding of nutrient cycling and productivity in streams. Third, we evaluate the empirical evidence supporting food limitation of fish populations and conclude it has been only weakly tested. On the basis of this assessment, we urge caution in the application of nutrient mitigation as a management tool. Although applications of nutrients and other materials intended to mitigate for lost or diminished runs of Pacific salmon may trigger ecological responses within treated ecosystems, contributions of these activities toward actual mitigation may be limited.

Pre-restoration studies typically focus on physical habitat, rather than the food-base that supports aquatic species. However, both food and habitat are necessary to support the species that habitat restoration is frequently aimed at recovering. Here we evaluate if and how the productivity of the food-base that supports fish production is impaired in a dredge-mined floodplain within the Yankee Fork Salmon River (YFSR), Idaho (USA); a site where past restoration has occurred and where more has been proposed to help recover anadromous salmonids. Utilizing an ecosystem approach, we found that the dredged segment had comparable terrestrial leaf and invertebrate inputs, aquatic primary producer biomass, and production of aquatic invertebrates relative to five reference floodplains. Thus, the food-base in the dredged segment did not necessarily appear impaired. On the other hand, we observed that off-channel aquatic habitats were frequently important to productivity in reference floodplains, and the connection of these habitats in the dredged segment via previous restoration increased invertebrate productivity by 58%. However, using a simple bioenergetic model, we estimated that the invertebrate food-base was at least 4× larger than present demand for food by fish in dredged and reference segments. In the context of salmon recovery efforts, this observation questions whether additional food-base productivity provided by further habitat restoration would be warranted in the YFSR. Together, our findings highlight the importance of studies that assess the aquatic food-base, and emphasize the need for more robust ecosystem models that evaluate factors potentially limiting fish populations that are the target of restoration.