Dead Zone Case Study

ENVS 201-02

Dead Zone Case Study

Part II

1. The physical factors that affect the dissolved oxygen in water are temperature, amount of salt content and surface area of the water, evaporation and condensation processes.
2. Temperature increase or decrease will make oxygen levels high and low. The more the surface area of water, the more oxygen is dissolved. When evaporation or condensation happens oxygen might reduce or increase. These factors constantly affect the levels of oxygen dissolved in water.
3. Biological forces include diffusion, aeration, respiration and decomposition, and photosynthesis.
4. The drastic change of weather might cause each identified condition to fluctuate over the seasons.
5. If I had to choose, out of the conditions I’ve identified temperature & excessive freshwater & nutrient discharge is most likely to be the primary cause of hypoxia in the Gulf. I believe this is true because of studies that show this may be the cause of hypoxia in the Gulf.

Part III

1. In April, the water temperature in the top 5 meters ranges between 18oC to 19oC and in August, the temperature at 15 to 20 meters range from 29.5oC at the top to 26oC.
2. In April and September, the difference in salinity at top and bottom waters was greatest. The difference is greater during this period because many events take place like the continuous flowing of river water into the sea, precipitation, and evaporation of water and snow, and melting of ice. These fluctuation events will increase the salinity of seawater.
3. In September, the difference in the salinity between the surface and bottom waters is least. The river water is carried to the ocean and as the river water is fresh it is less dense and stays at the surface resulting in fewer fluctuations.
4. Hypoxia is a condition where the oxygen levels are low. Generally, the water has 2 mg oxygen per liter but if the amount of oxygen is less than 1.43 milliliters of oxygen per liter of seawater, then it will result in saturation where sea animals cannot survive and will die.

Part IV

1. Seaweed and other forms of algae like phytoplankton are the primary producers in the marine aquatic food web. Since these organisms require sunlight and the sunlight reaches as far as 200 meters below the surface, these organisms are found throughout this zone which is referred to as the Euphotic zone.
2. The important limiting factors that affect the growth and succession of phytoplankton and seaweed are light, temperature, organic and inorganic micronutrients, salinity, oxygen concentration, and buoyancy regulation.
3. The fluctuations in the primary productivity in the Gulf of Mexico over the year are due to the dynamic seasonal changes and the effect of fresh water on the salinity at different depths. Since, the density of freshwater is less than sea water, it remains on the surface. Seasonal changes can highly influence the salinity, dissolved oxygen, and temperature of the water at various levels.
4. The major consumers in the Gulf of Mexico food web are sea snail, long head turtle, sea urchin, red starfish, and surgeonfish.
5. The other components of the Gulf of Mexico food web include the primary consumers like jellyfish, small fish, squid and baleen whales which feed on zooplankton, and the predators like large fish, dolphins, sharks that feed on smaller animals.
6. The algae are responsible for the greatest total amount of respiration. The available nutrients provide favorable conditions for algae which utilize the water’s oxygen supply for respiration.
7. The respiration rates peak in the months of July and August. When compared with primary production there is no evident data recorded in the month of July and in the month of August the primary production rate is low. This is due to replenishment of nutrients from deep waters to the surface water. The mixing of river water causes the algae and phytoplankton to move toward the surface so reducing the respiration rate exceeds primary production.

Part V

1. The peak months for nitrate-nitrogen discharge from the Mississippi River into the Gulf are March thru June.