Recently there have been numerous fish kills in the Hackensack River. This is probably due to the cumulative impact of excessive heat, which releases oxygen from the water, excessive influx of sediment run-off, and an overload of nutrients that causes algae blooms which eventually chokes out sunlight and diminishes oxygen as the algae die. It may be acute this year in part due to the lack of rain which cools the river and replenishes the oxygen. This may make for a very grim summer as the temperature gets hotter.

Dissolved oxygen (DO) refers to oxygen gas that is dissolved in water. Fish "breathe" oxygen just as land animals do. However, fish and many other aquatic organisms respire by absorbing oxygen directly from the water into their bloodstream using gills, whereas land animals use lungs to absorb oxygen from the air.

The two main sources of oxygen in the aquatic environment are photosynthesis and direct diffusion from the atmosphere, aided by disturbances of the air-water interface (e.g. wind, waves, rain, and boat traffic). Of these, photosynthesis by submerged aquatic vegetation and phytoplankton (microscopic plants) is pivotal. Unfortunately, due to the turbidity of the Hackensack River, there is very little sunlight penetration into the water to stimulate photosynthesis.

Oxygen depletions occur when oxygen consumption exceeds oxygen production resulting in fish mortality. A concentration of 5 parts per million (ppm) is recommended to maintain fish health. When DO concentrations drop below 3 ppm, fish are severely stressed, and when concentrations fall much lower, fish begin to die. The number of fish that die during an oxygen depletion is determined by how low the DO gets and by the duration of the anoxic conditions. Usually larger fish are affected first by low DO and then smaller fish. A DO reading coincidently taken in the Arthur Kill near Elizabeth as the proliferation of dead fish was noticed in the Hackensack River gave a reading of 1.8 ppm—a value far too low to maintain life.

Although oxygen depletions can occur at anytime, they are most common, and most likely to cause fish kills, during hot summer weather. Warm water is much less capable of holding oxygen gas in solution than cool water. This physical phenomenon puts the fish in double jeopardy because at high water temperatures the metabolic rate is increased, hence their physiologic demand for oxygen is increased.

The only way to know for sure if oxygen depletion has caused a fish kill is to measure oxygen in the affected water while the fish kill is in progress. Indications of oxygen depletion as a probable cause of a fish kill include:

• All fish die at approximately the same time.
• Large fish may be affected more than small fish.
• Fish may be seen at the surface "gasping" for oxygen (this is called "piping").
• Some species may die with their back arched, gills flared, and mouth open.
• The weather immediately prior to the fish kill may have been hot and still.
• An oxygen depletion severe enough to result in significant fish mortality is often observed in water with heavy populations of algae.

Increases in oxygen consumption can be caused by increased organic waste entering the water (i.e., dog and goose feces, combined sewer overflows during moderate to severe rain events, and excess fertilizer), death and decay of organic matter (i.e., algae and animal die-offs), or by certain chemicals (i.e., formalin) that remove oxygen directly from the water column. The dying fish are the canary in the coal mine regarding the river's health. The best thing people can do to help the river in this regard is to spread the word about reducing nutrient loads to the waterways. People can help the river by:

• Pick up after dogs. Fecal matter from dogs (and geese) runs-off into river and increases the nutrient load.
• Maintain at least a 15 foot vegetative buffer along riverbanks by planting trees, etc. Shade reduces water temperature; roots leak oxygen into the river; vegetation slows the runoff of other pollutants.
• Use none or no more than the recommended amount of fertilizer on your lawn.
• Conserve water (so there is greater flow fresh colder water into the river system).
• Be more active in the water (boating, canoeing, waterskiing, etc. help to aerate the water).

Adapted from: University of Florida Cooperative Extension Service, http://edis.ifas.ufl.edu/BODY_FA002.

A bright side? Yes, in this case there is a silver lining. Nature always attempts to recover her losses. All those thousands of dead fish are reclaimed into the food web, providing a veritable feast for many different species of wading birds and for fish and crabs that survived the die-off. However, it still may take several breeding seasons for the system to completely recover from the initial loss.