Katie Hossler

No‐net‐loss not met for nutrient function in freshwater marshes: recommendations for wetland mitigation policies

Wetlands provide many important services throughout the world, with an estimated economic value that, in comparison to other ecosystems, far exceeds their relatively small global extent. In recognition of their importance, both national and international regulations exist to protect the worlds remaining wetlands. Of growing interest is the “no-net-loss” policy which permits unavoidable destruction of wetlands if compensated by restoration of degraded wetlands or creation of new wetlands. The fundamental assumption of no-net-loss is that wetlands can be created which function equivalently to natural wetlands. One integral function that wetlands perform is cycling of carbon, nitrogen and phosphorus. Here we demonstrate that loss of this nutrient-related function is not being mitigated by creation or restoration of wetlands. We compare indicators of plant- and microbial-mediated functions, as well as abiotic (e.g., soil character, hydrology) and biotic (e.g., plant community composition) structure, between 10 created or restored and 5 natural freshwater depressional wetlands in central Ohio, USA. Nutrient stocks were generally smaller and transformations slower in created wetlands than in natural wetlands, with little development over time. Of particular concern were differences in C- and N-related function. Created wetlands stored 90% less C within litter and 80% less C within soil and processed 60% less N through denitrification, on average compared to natural wetlands. Our study suggests that subversion of natural wetlands into restored or created wetlands could have large-scale environmental consequences such as reduced capacity for nitrate removal and C sequestration.

Ecosystem Structure Emerges as a Strong Determinant of Food-Chain Length in Linked Stream–Riparian Ecosystems

Environmental determinants of fluvial food-chain length (FCL) remain unresolved, with predominant hypotheses pointing to productivity, disturbance, and/or ecosystem size. However, drainage configuration (for example, drainage density, and stream length)—in spite of recent advances demonstrating the significance of catchment structure to habitat and biodiversity of fluvial systems—has yet to be explored in relation to FCL. In this study, we quantified the relative influences of ecosystem size and structure on FCL for linked stream–riparian food webs. At 19 stream reaches distributed within three mountain catchments of northern Idaho, USA, we sampled aquatic and riparian consumers and determined FCL using the naturally abundant stable isotopes 13C and 15N. Food-chain length was then related to reach measures of size and structure using an information-theoretic model selection approach. Model selection was followed by exploratory linear regression of FCL with purported mechanistic factors (that is, resource availability and disturbance regime). FCL ranged from 2.6 to 4.4 across study reaches and was best explained by catchment structure such as number of tributary junctions and distance to nearest downstream confluence. Regression analyses suggested that disturbance regime may mechanistically link number of tributary junctions and FCL, as well as drainage area and FCL. Our results introduce novel evidence that ecosystem structure may integrate the effects of several mechanistic factors and thus be an important predictor of food-web structure.

Fluvial geomorphology and aquatic‐to‐terrestrial Hg export are weakly coupled in small urban streams of Columbus, Ohio

Although mercury (Hg) contamination is common in stream ecosystems, mechanisms governing bioavailability and bioaccumulation in fluvial systems remain poorly resolved as compared to lentic systems. In particular, streams in urbanized catchments are subject to fluvial geomorphic alterations that may contribute to Hg distribution, bioaccumulation, and export across the aquatic-to-terrestrial boundary. In 12 streams of urban Columbus, Ohio, we investigated the influence of fluvial geomorphic characteristics related to channel geometry, streamflow, and sediment size and distribution on (1) Hg concentrations in sediment and body burdens in benthic larval and adult emergent aquatic insects and (2) aquatic-to-terrestrial contaminant transfer to common riparian spiders of the families Pisauridae and Tetragnathidae via changes in aquatic insect Hg body burdens as well as in aquatic insect density and community composition. Hydrogeomorphic characteristics were weakly related to Hg body burdens in emergent insects (channel geometry) and tetragnathid spiders (streamflow), but not to Hg concentrations in sediment or benthic insects. Streamflow characteristics were also related to emergent insect density, while wider channels were associated with benthic insect community shifts toward smaller-bodied and more tolerant taxa (e.g., Chironomidae). Thus, our results provide initial evidence that fluvial geomorphology may influence aquatic-to-terrestrial contaminant Hg transfer through the collective effects on emergent insect body burdens as well as on aquatic insect community composition and abundance.