Title:

Assessing the Transporter/Transformer Hypothesis Using a Nitrogen Budget for the Lamprey River, NH

Rivers play a central role in nutrient transport from terrestrial sources to downstream receiving water bodies. Anthropogenic nutrient inputs alter the balance of nutrient fluxes, often resulting in nutrient impairments downstream. Excess nitrogen (N) and phosphorous (P) can cause eutrophication events that contribute to algal blooms, anoxic conditions, and reduced water quality. We examined the ecological role of the Lamprey River, located in a rural-suburban New Hampshire watershed, in regulating inputs of N and P. Spatial and temporal trends in nutrient loads and nutrient budgets were assessed for total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), and soluble reactive phosphorus across 21 sampling stations and multiple years throughout the Lamprey River network. An annual mass balance approach was used to estimate the quantity of N incrementally stored, or evaded due to denitrification, between mainstem river segments. Median TDN loads across stations ranged from 0.37 to 8.04 kg N/ha/yr, with DIN accounting for 14 to 88% of TDN flux. TDN loads decreased over time, whereas phosphate loads increased. Most river segments had positive incremental loads, indicating downstream N outputs were greater than upstream inputs. Positive loads indicate N production occurs in-situ or that additional, unquantified inputs of N exist. In some cases, downstream river segments showed negative incremental loads for TDN and DIN, suggesting the Lamprey functions as a transformer of N through denitrification or biotic assimilation pathways. By determining when and where a river functions as a transporter or transformer of nitrogen, we can better restore and protect downstream nutrient-impaired coastal ecosystems.

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