Samantha, UR - Hypoxia and Nutrients leading to a disturbance within the estuaries

While Hypoxia may be permanent or intermittent, it is most commonly manifested as a seasonal disturbance, appearing in mid to late summer after vertical stratification prevents replenishment of deep water dissolved oxygen (DO).  Stratification separates a body of water into layers. This layering can be based on temperature or dissolved substances, like oxygen.  In estuaries there is a signifiant division between freshwater and saltwater resources making up the stratification of the water.   On the surface or upper layer of the canal, saltwater holds less dissolved oxygen than freshwater and the stronger the river flow, the higher the oxygen concentrations.  Our water was hypoxic with a salinity of 9.9 parts per thousand, which is also pretty low.... Which brings me to the conclusion of fresh water being brought  and should be higher in oxygen, this was not the case.... Maybe this fresh water was contaminated with so many nutrients that this is what then caused oxygen depletion due to abundant amount of organisms undergoing respiration, leading to a hypoxic dissolved oxygen measurement.

Image of Hypoxia in Washington Estuaries mid - late summer

Which brings me to the fertilizer ban.  It is said that hypoxia seems to be more common in the late summers, which is around when we noticed our canal was hypoxic... contributing to the idea that these subterranean canals have a water consistency of organisms going through respiration.... this had to have been taking place so rapidly within the fresh water before it even saw the "light of day" that it makes me believe maybe this has to do with that freshwater coming in through the canal is highly contaminated with nutrients such as nitrogen of phosphorus coming from fertilizer possibly.  We know that nutrient levels were high, and oxygen was low... which gives us enough information to come to this conclusion.  There are two approaches you decide whether nitrogen and phosphorus are limiting nutrients in a water body.  Firstly involving the us of TN/TP ratio ( total nitrogen/ total phosphorus ratio).  The other involves the use of Phosphorus threshold value.  Calculating a relatively simple ratio can sometimes provide a useful clue as to the relative importance of nitrogen or phosphorus toward the abundance of algae in a water body.  For information on how to do so... refer to the link below.  Any suggestions ?

http://lakewatch.ifas.ufl.edu/circpdffolder/nutrpt2.pdf