The Sturgeon City Youth Leadership Development Institute 2000 was held in Jacksonville during June 13 - 16. More than eighty 10th graders from Onslow County high schools attended the four day event. Thursday, June 15, was the environmental field day. One component of the day was held at Wilson Bay Park. There, students performed Secchi disk, temperature, dissolved oxygen, salinity, nitrate, ammonia, and phosphate evaluations. The students were divided into five groups, each of which was at Wilson Bay Park for one hour. Groups 1 through 3 were there before noon, groups 4 and 5 were there during the afternoon.

Secchi disk is a measure of water clarity. The disk is approximately 20 centimeters in diameter and is typically divided into alternating black and white quadrants. The disk is lowered into the water until it disappears. That depth is recorded as Depth 1. The disk is lowered further and then slowly raised until it reappears. That depth is recorded as Depth 2. The average of the two depths is recorded as the Secchi depth. On June 15, the average of the readings obtained by the students throughout the day (10 observations) was 61 +/- 8 centimeters. This clarity is fairly typical. A greater reading would indicate less sediment or algae in the water. A lower reading would indicate possible runoff from a storm or an algae bloom. Secchi depths do not tell the source of the turbidity, just that it is there. Other tests, such as Suspended Solids (debris, sediment, etc.) and Chlorophyll_a (presence of algae), are available for pinpointing the type of material contributing to the turbidity.

The Temperature and Dissolved Oxygen results are presented in the following figures. The Temperature plot clearly indicates that the surface water temperatures increased as the day progressed. The temperature at a depth of 4-ft also increased during the day, but not as much. Also, the deeper water was warmer than the surface at the start of the day (9 am).

The Dissolved Oxygen (DO) plot indicates two important events:

• The surface DO concentration increased between 9 am and 3 pm. This is important because it indicates the daytime impact of algae. Algae produce oxygen during the day; unfortunately, they utilize that oxygen at night, which can cause oxygen levels to become too low for fish to live.
  
  
• At depths greater than 3-ft, the oxygen concentrations dropped to nearly zero. These levels would not support fish life.

It would have been good to obtain a DO reading early in the morning, before the algae began photosynthesis. Such a reading would have shown how low the oxygen concentrations dropped during the night. Several people have called the Cooperative Extension office about fish kills in their ponds. The recent hot dry weather has promoted algae growth, which can lead to low nighttime oxygen levels in ponds. If this is the problem, the DO cencentration needs to be taken VERY early in the morning. An alternative would be to collect a sample early in the morning, keep it in the dark, and bring it to the Cooperative Extension office that morning.

Salinity is the measure of salts in the water. The average of the 17 readings, which were taken from various depths during the day, was 11.7 +/- 0.2 ppt (parts per thousand). The readings were quite consistent throughout the water column and indicated a typical brackish water.

Nitrate is a nutrient that promotes plant growth. It can come from a variety of sources, including fertilizers and septic systems. The EPA has placed a safe drinking water limit of 10 mg/L on nitrate, because of concerns about methemoglobinemia (aka "blue baby syndrome"). The Sturgeon City students collected and analyzed six samples for nitrate. Of those, five samples did not register as containing any nitrate, the sixth sample contained 0.3 mg/L nitrate.

Phosphate is another plant nutrient. It was banned from use in many types of soaps. It is quite common in coastal soils (It's even mined!) and in sea water. The samples analyzed during the field day yielded 0.175 +/- 0.15 mg/L phosphate.

Ammonia can be toxic to fish, even at relatively low concentrations. Toxicity depends on a variety of factors, including: fish species, fish age, water temperature, pH, and other stressors that may be present. The Southern Regional Aquaculture Center has a publication entitled Ammonia in Fish Ponds (SRAC publ. #463), which provides a much more detailed explanation of ammonia toxicity.

The ammonia concentrations in the samples analyzed by the students varied with water depth:

Ammonia, like nitrate, can come from a variety of sources. It is also released during the decomposition of organic materials. Such materials are readily available in the sediments of Wilson Bay. Both the lack of oxygen and the elevated ammonia concentrations at the bottom depths would restrict the ability of bottom-dwelling (aka benthic) organisms from living there.

Contributors:

• Sturgeon City Fellows
• Pamela Wooding, White Oak High School science teacher
• Tanya Barrett, White Oak High School science teacher
• Mike Frinsko, Aquaculture Area Spec. Agent, NC Cooperative Extension

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