Study of the inter-annual food web dynamics in the Kuparuk River with a first-order approximation inverse model

We used a long-term observation data set (12 years) of fish, insect and primary producer standing stocks in both reference and phosphate-fertilized reaches of the Kuparuk River located on the north slope of Alaska, USA to test a recently developed first-order approximation model. The model employs a flow analysis-type approach, but uses first-order approximations between annual mean compartment stocks and environmental drivers of temperature, discharge and solar radiation. Consequently, the model is more robust and requires fewer observations than standard process-oriented models, and can utilize observations that are difficult to incorporate into process models. Unlike standard inverse models, we show that our model is capable of prediction provided sufficient data are available for model calibration and environmental drivers are known. The results show that the inter-annual variations of several components in the Kuparuk River ecosystem, including dissolve inorganic phosphate, chironomids, black flies and Arctic grayling, can be accurately approximated as a linear function of temperature, discharge and solar radiation. In particular, the model indicates that changes in river habitat brought about by proliferation of the moss Hygrohypnum spp. in the P-fertilized reach caused a temporary shift in flow paths supporting Arctic grayling from primary producers to detrital-based pathways. However, after moss establishment, primary producer flow paths to Arctic grayling returned and detrital-based pathways weakened.