Ryan S. King

Context: Previous studies developed distance-weighted regression to describe how land use effects on aquatic systems attenuate with arrangement of source areas within catchments. Objectives: We clarify and extend the conceptual foundations of this approach, enhance the spatial and statistical methods, and provide new tools to interpret the results. Methods: We derive the framework from first principles to resolve conceptual issues with how weighting is applied to source area versus total area, and we formalize the requirements for an ideal weighting function. We quantify the spatial distributions of land areas in a way that integrates with model fitting. We adapt non-linear optimization to simultaneously fit regression and weighting parameters. We quantify the spatial distribution of source effects with arrangement and document how different weighting functions alter that distribution. Results: To verify their utility, we applied these methods to a published analysis relating polychlorinated biphenyls in fish to developed land use in catchments. We identified a stronger distance-weighted model and more completely characterized the effects of weighting on where aquatic impacts originate. Conclusions: Our methods enable more comprehensive analyses of the effects of spatial arrangement to better inform a wide range of scientific investigations and applications. Our methods can relate almost any spatially distributed source or driver to an integrated response at a point or along a boundary; and alternate hypotheses about the effects of pattern or proximity on processes can be tested with alternative weighting functions. New applications will generate additional weighting functions that enhance the general approach.

Freshwater ecosystems are exposed to engineered nanoparticles (NPs) through discharge from wastewater and agricultural runoff. We conducted a 9-month mesocosm experiment to examine the combined effects of chronic NP additions on insect emergence and insect-mediated contaminant flux to riparian spiders. Two NPs (copper, gold, plus controls) were crossed by two levels of nutrients in 18 outdoor mesocosms open to natural insect and spider colonization. We collected adult insects and two riparian spider genera, Tetragnatha and Dolomedes, for 1 week on a monthly basis. We estimated a significant decrease in cumulative insect emergence of 19% and 24% after exposure to copper and gold NPs, irrespective of nutrient level. NP treatments led to elevated copper and gold tissue concentrations in adult insects, which resulted in terrestrial fluxes of metals. These metal fluxes were associated with increased gold and copper tissue concentrations for both spider genera. We also observed about 25% fewer spiders in the NP mesocosms, likely due to reduced insect emergence and/or NP toxicity. These results demonstrate the transfer of NPs from aquatic to terrestrial ecosystems via emergence of aquatic insects and predation by riparian spiders, as well as significant reductions in insect and spider abundance in response to NP additions.

One important mechanism governing the temporal maintenance of biodiversity is asynchrony in co-occurring competitors due to fluctuating environments (i.e. compensatory dynamics). Temporal niche partitioning has evolved in response to predictable oscillations in environmental conditions so that species may offset competition, but we do not yet have a clear understanding of how novel anthropogenic stressors alter seasonal patterns of succession. Many primary producers are nutrient-limited, and enrichment may decrease the importance of environmental fluctuations that govern which species are effective competitors under naturally low nutrient regimes. Consequently, elevated nutrient concentrations may synchronize species responses to seasonality. By studying benthic algal assemblages over two years from 35 streams that spanned a wide gradient of nutrient enrichment, we found that compensatory dynamics characterizing seasonal succession under natural nutrient regimes broke down at relatively low levels of total phosphorus (P) enrichment (~ 25 μg L^-1). With increasing P more species were able to coexist at any given time, and seasonal variation in assemblage composition was characterized by synchronous swings in species biovolumes. We also observed much higher instability in assemblage biovolumes with declines in compensatory dynamics, which indicates that anthropogenic alteration of nutrient regimes can affect community stability by changing the dominant mode of seasonal succession. Our findings indicate that compensatory fluctuations of stream algae are driven by seasonality, and provide insight about how nutrient enrichment alters evolved drivers of species coexistence.

Walker, C. M., D. F. Whigham, I. S. Bentz, J. M. Argueta, R. S. King, M. C. Rains, C. A. Simenstad, C. Guo, S. J. Baird, and C. J. Field. 2021. Linking landscape attributes to salmon and decision-making in the southern Kenai Lowlands, Alaska, USA. Ecology and Society 26(1):1. https://doi.org/10.5751/ES-11798-260101

Phosphorus (P) is a nutrient of primary importance in all living systems, and it is especially important in streams and rivers which are sensitive to anthropogenic P inputs and eutrophication. Microbes are accepted as the primary mineralizers and solubilizers of P improving bioavailability for organisms at all trophic levels. Here, we use a genomics approach with metagenome sequencing of 24 temperate streams and rivers representing a total P (TP) gradient to identify relationships between functional genes, functional gene groupings, P, and organisms within the P biogeochemical cycle. Combining information from network analyses, functional groupings, and system P levels, we have constructed a System Relational Overview of Gene Groupings (SROGG) which is a cohesive system level representation of P cycle gene and nutrient relationships. Using SROGG analysis in concert with other statistical approaches, we found that the compositional makeup of P cycle genes is strongly correlated to environmental P whereas absolute abundance of P genes shows no significant correlation to environmental P. We also found orthophosphate (PO₄³⁻) to be the dominant factor correlating with system P cycle gene composition with little evidence of a strong organic phosphorous correlation present even in more oligotrophic streams.

Alder (Alnus spp.) is a woody plant with bacterial symbionts that fix atmospheric N₂ into bioavailable N. We studied 12 North American boreal headwater streams spanning a steep gradient of catchment alder cover (0–27%) to test the hypothesis that increasing inputs of inorganic N associated with increasing alder cover would reduce or eliminate in-stream benthic N₂ fixation. We measured N₂ fixation rates, chlorophyll a, and ash-free dry mass (AFDM) of periphyton in early (May) and late (August) summer 2011. Dissolved inorganic N (DIN) concentrations, composed almost entirely of NO₃/NO₂-N, ranged from below detection limits to nearly 2 mg/L and were strongly predicted by catchment alder cover in both months. Higher N₂ fixation rates were observed in August than in May. N₂ fixation rates declined sharply when alder cover exceeded ~2% of catchment cover, corresponding to 20 to 40 µg/L DIN. This pattern also was evident among 3 streams with contrasting catchment alder cover sampled approximately every 2 wk during the study. The stream with no catchment alder cover exhibited a steady increase in N₂ fixation rates over the summer, whereas the streams with low and high alder in their catchments had very low N₂ fixation rates that did not vary over time. The influence of alder cover on periphyton biomass was not clear. The strong regulation of alder cover on in-stream N availability leads us to suggest that alder presence may be an important terrestrial regulator of stream N cycling.