Locally enhanced biological production and increased carbon export are persistent features at oceanic density fronts. Studies often assume biological properties are uniform along fronts or hypothesize that along‐ and across‐front gradients reflect physical‐biological processes occurring in the front. However, the residence times of waters in fronts are often shorter than biological response times. Thus, an alternate—often untested—hypothesis is that observed biological patchiness originates upstream of a front. To test these two hypotheses, we explore an eddy‐associated front in the California Current System sampled during two surveys, separated by 3 weeks. Patches of high phytoplankton biomass were found at the northern ends of both surveys, and phytoplankton biomass decreased along the front. While these patches occurred in similar locations, it was unclear whether the same patch was sampled twice, or whether the two patches were different. Using an advection‐reaction framework combined with field and satellite data, we found that variations in along‐front gradients in dissolved oxygen, particle biovolume, and salinity support the conclusion that the two phytoplankton patches were different. They were only coincidentally sampled in similar locations. Backward‐ and forward‐in‐time tracking of water parcels showed that these phytoplankton patches had distinct origins, associated with specific, strong coastal upwelling pulses upstream of the front. Phytoplankton grew in these recently upwelled waters as they advected into and along the frontal system. By considering both local and upstream physical‐biological forcings, this approach enables better characterizations of critical physical and biogeochemical processes that occur at fronts across spatial and temporal scales.

Changing oxygen conditions are altering the distribution of many marine animals. Zooplankton vertical distributions are primarily attributed to physiological tolerance and/or avoidance of visual predation. Recent findings reveal that visual function in marine larvae is highly sensitive to oxygen availability, but it is unknown how oxygen, which affects light sensitivity and generates limits for vision, may affect the distribution of animals that rely heavily on this sensory modality. This study introduces the concept of a "visual luminoxyscape" to demonstrate how combinations of limiting oxygen and light could constrain the habitat of marine larvae with oxygen-demanding vision. This concept reveals the impact of sublethal climate change vulnerabilities in visual marine animals and provides an additional hypothesis for habitat compression under ocean deoxygenation, which we argue deserves attention. Climate change and species' sensitivityManifestations of climate change in the ocean, such as warming, acidification, and deoxygenation, alter the physiological and behavioral responses of marine organisms, and shift their distributions (Somero 2012). Species vulnerability to changing environments is routinely assessed using extreme physiological tolerance limits. "Hypoxia," often defined as 2 mg O 2 L À1 , $ 60 μmol kg À1 , or 5 kPa, for example, is a commonly used oxygen threshold in marine life, though it may not accurately reflect an organism's oxygen limits (Vaquer-Sunyer and Duarte 2008). Studies have used a metabolic oxygen limit, or P crit (oxygen at which an animal's metabolic rate changes), as a threshold for physiological tolerance to oxygen (Seibel et al. 2021). This limit has inspired several indices (e.g., Metabolic Index, Aerobic Growth Index) that integrate metabolic and biogeographic data to predict future changes in species' distributions by examining combinations of oxygen and temperature conditions that are suitable for metabolic function (Penn et al. 2018;Deutsch et al. 2020;Clarke et al. 2021). Here we introduce a novel concept that relates species-specific experimental visual limits to potential changes

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