During the summer of 2008, oxygen depleted water, between 5 and 12 m depth, was discovered in Sannasfjord on the Swedish west coast. The resulting sediments were black, benthic macrofauna were absent and Beggiatoa bacterial mats were a characteristic feature. This phenomenon, which was observed several years in a row, appears to be a relatively new phenomenon starting in the mid-1980s. In this study we attempt to find the underlying causes by investigating climatic effects (temperature, wind and precipitation), the local supply of nutrients from land, ecosystem change and the supply of organic material from the open Skagerrak. An analysis of long meteorological time series indicates that climatic effects are contributory, but probably not a dominating factor leading to hypoxia. Results from an advection-diffusion model solving for oxygen show that the observed increase in the river supply of nutrients has a high potential to generate hypoxia. Although complex and more difficult to quantify, it appears that ecosystem changes, with higher abundance of filamentous algae, may have played an important role. It is also possible that an enhanced supply of organic material from the open Skagerrak has contributed.