A key control on the magnitude of coastal eutrophication is the degree to which currents quickly transport nitrogen away from the coast to the open ocean before eutrophication symptoms develop. In the Southern California Bight (SCB), an upwelling-dominated eastern boundary current ecosystem, anthropogenic nitrogen inputs increase algal productivity and cause subsurface acidification and deoxygenation along the coast. However, the extent of anthropogenic influence beyond the coastal band, and the physical transport mechanisms responsible these effects, were not previously documented. Here, we use a submesoscale-resolving numerical model to investigate the transport of anthropogenic nitrogen and its effects on SCB offshore habitats. Anthropogenic nutrient inputs promoted an increase in productivity and respiration offshore, with recurrent oxygen loss and pH decline in a region located 30 – 90 km from the mainland. Over 2013 to 2017, peak losses up to 14.2 mmol m-3 O2 persisted 4 to 6 months of the year over an area of 278,400 km2 (~30% of SCB area). These recurrent features are associated with eddy cross-shore transport of nutrients and plankton biomass, and their accumulation and retention within persistent eddies offshore counteract the dilution and dispersion by mean currents that transport nitrogen and organic matter out of the SCB.