Seasonal Patterns of Carbon, Nitrogen, and Phosphorus Concentration and Speciation in Contrasting River–Lake Networks

Zhao, Zongxi and Surridge, Ben (2025) Seasonal Patterns of Carbon, Nitrogen, and Phosphorus Concentration and Speciation in Contrasting River–Lake Networks. Masters thesis, Lancaster University.

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Abstract

In many catchments, lakes and rivers are not isolated from each other but are hydrologically connected as part of integrated freshwater networks which control the transport, fate and impacts of elements, matter and energy. These river–lake networks are potentially important regulators of carbon (C), nitrogen (N) and phosphorus (P) during transport of these critical elements between terrestrial and marine ecosystems. However, despite conceptual development of the river-lake network framework, very little empirical research has examined how natural lakes influence the concentration and speciation of N, P and C in a network context. The research reported in this thesis sought to address this key research gap, focussing on how seasonal change through spring to summer, lake trophic status, and annual average water retention time influenced the impacts of natural lakes on the concentration and speciation of N, P and C in river networks. Based on a programme of coupled field and laboratory research, these questions were explored in exemplar lake-river networks in the English Lake District. Synoptic sampling of inflow river, lake and outflow river sites was conducted within two contrasting river-lake networks between March and July 2025. The resulting data reveal how lakes significantly altered the concentration and speciation of nutrient elements within river networks. Compared to inflow rivers, lakes increased DOC concentrations by an average of 78% (0.52 mg C/L) across both riverlake networks and all sampling dates, whilst also driving changes in spectrophotometric parameters describing the quality of the dissolved organic matter. Within the more eutrophic river-lake network, the natural lake switched between a substantial source (+92% or 0.29 mg N/L) and sink (-78% or 0.34 mg N/L) for nitrate across the spring-summer period, and consistently acted as a sink for soluble reactive phosphorus. Importantly, lake effects on the concentration and speciation of N, P and C persisted for several kilometres in the downstream river of both river-lake networks, demonstrating the potential for natural lakes to impose extensive impacts on river biogeochemistry. These findings highlight the key role that river-lake networks play in the transport of macronutrients through the freshwater interface between terrestrial and marine ecosystems.