The Impact of Subglacial Drainage System Evolution and Glacier Lake Outburst on Arctic Fjord Macronutrient Dynamics

Global warming is amplified in the Arctic, accelerating glacier melt and freshwater runoff. At tidewater glaciers, runoff typically enters fjords at depth and generates buoyancy‐driven circulation that enhances glacier‐ocean exchanges of energy and matter, influencing macronutrient delivery and marine primary production. However, most studies lack the temporal resolution to capture low‐frequency, high‐magnitude events, leaving their impacts poorly understood. Here, we combine glacier observations with high‐frequency fjord and glacier‐lake sampling to examine the 2021 glacier lake outburst flood (GLOF) from Lake Setevatnet into Kongsfjorden (Svalbard). We show how evolving subglacial conditions before and during the GLOF shaped macronutrient supply to the inner fjord through both direct runoff and entrainment of bottom waters. Early in summer, nutrient delivery was dominated by direct runoff, supplying nitrate (NO 3 − ) and silicate via an inefficient drainage system. As the melt increased, an efficient system formed, generating a subglacial plume and initiating buoyancy‐driven circulation that entrained nutrient‐rich deep water. Despite high NO 3 − lake concentrations, the flood barely affected fjord NO 3 − levels. Instead, it produced a seasonal maximum in nitrite (NO 2 − ). Comparisons with conservative mixing estimates and nitrogen budget analyses reveal a non‐conservative nutrient signal. Although sedimentary sources cannot be excluded, the timing and spatial pattern of the NO 2 − anomaly suggest subglacial modification during floodwater transit. These findings indicate that Kongsfjorden functions as a summer nitrogen sink, partly shaped by subglacial transformations. Overall, nutrient delivery from tidewater glaciers depends not only on runoff volume but also on the subglacial drainage system characteristics, which evolve during high‐magnitude events such as GLOFs.