The Masked Role of Periphyton in Phosphorus Cycling: Mechanistic Insights Under Large‐Scale Hydrologic and Seasonal Variability

Abstract The role of periphyton in lake phosphorus cycling has long been overlooked or simplified, as water quality models often classify periphyton parameters as non‐sensitive, thereby masking their key roles in phosphorus cycling. Therefore, to quantify how periphyton influence phosphorus cycling and related limnological processes, we conducted over 11,000 sensitivity analyses across a wide range of hydrologic and seasonal variations, comparing simulations with and without periphyton processes (PERI vs. noPERI) in the Spokane River and Lake Spokane model. Results showed that excluding periphyton increased spring average epilimnetic total phosphorus (TP) by up to ∼15% due to orthophosphate (PO 4 ) accumulation, while early summer TP decreased by up to ∼7% because of reduced labile dissolved organic matter phosphorus (LDOMP). Concurrently, the chlorophyll‐a (Chl a ) peak advanced from early July to late May (∼41 days), and minimum volume‐weighted hypolimnetic dissolved oxygen concentration (DO MIN ) decreased by ∼7% in spring. Periphyton regulate phosphorus cycling primarily through two mechanisms: (a) reducing PO 4 via growth‐driven uptake while enhancing LDOMP through mortality‐driven release, leading to seasonally varying contributions to TP; and (b) influencing sediment–water phosphorus exchange and shaping cycling dynamics through direct and indirect competition with phytoplankton. Although sediment oxygen demand parameters were the most sensitive overall and phytoplankton parameters contributed substantially, periphyton parameters that were initially non‐sensitive became sensitive under winter conditions and at both low and high flows. This study shows that periphyton can play an important role in long‐term phosphorus dynamics, and that dynamically incorporating periphyton processes in models of seasonally stratified lakes can improve water quality management.