The Hood Canal Dissolved Oxygen Program (HCDOP) and the United States Geological Service (USGS) are studying Hood Canal circulation, trying to model the tidal circulations and salinity distribution patterns between the canal and Admiralty Inlet. Other factors that, when combined with the constrictive shape, could also influence hypoxia in Hood Canal are:   The picture surrounding hypoxia in Hood Canal is complex; research models point to more than one contributing factor: Nutrient level. Nutrient level is a large issue due to the human impact. The supply of nutrients, primarily nitrates, to the euphotic zone is thought to impact levels of dissolved oxygen. Nutrients feed algae, which under the right conditions, "bloom" and then die and decompose; the entire process requires a large amount of oxygen. This decreases the oxygen in the water column, lowering the dissolved oxygen level. There are both natural and man-made sources of nutrients. The primary natural source is in ocean water that flushes Hood Canal. Man-made sources include leaking septic systems, storm water runoff, agriculture and various other sources causing nutrient pollution. The presence of nutrients leads to algae growth, which consumes oxygen when the algae die and decompose, contributing to the low oxygen conditions in these waters. Another factor mentioned by the HCDOP is the influence of the ocean water. The ocean water that enters Hood Canal is like most estuaries: fresh, warm water flows out at the surface and is replaced by cold, salty water at depth. The cold, salty ocean water that enters Hood Canal comes into Puget Sound from the open ocean and has not recently been in contact with the atmosphere. As a result, this water is initially somewhat depleted in oxygen. Oxygen levels will fluctuate throughout the year; this is due to seasonal changes in nutrient availability, solar radiation, and water column stratification (layers of water of different density, temperature, and salinity). Low oxygen conditions are at their worst in the late summer, after several months of limited flushing and maximum plankton production near the surface. In some years, oxygen becomes sufficiently depleted that animals cannot survive. These kills may occur either locally or over a wide area. Combined sewage overflow (CSO) has been occurring in the Puget Sound. CSO occur during rain events when runoff with the combination of raw sewage overflows the pipes and it carries pollutants, harmful pathogens, and excess nutrients directly into Puget Sound. This is a threat to the biodiversity of marine wildlife as it degrades/damages the marine habitat.