Part V – Why Does the Phytoplankton Population Increase?
Sue could see that photosynthesis by the phytoplankton population increased sharply in the spring months, suggesting their populations had soared. Following the spring peak, photosynthetic rates declined as the population members died and respiration rates in the water rose dramatically. It made sense to Sue that the organic matter from dead phytoplankton was sinking, providing a rich food source for marine bacteria populations at the base of the water column. As the bacterial populations climbed, they depleted the oxygen available in the water, especially at the bottom of the water column.
But why did the phytoplankton population explode in the first place? Sue knew that light and nutrients were the things most likely to limit growth. Day length was increasing in April and May when the populations climbed, but since the population crashed when day length continued to increase in June, light did not seem to be the cause. Nutrients seemed more likely to be the culprit, carried into the Gulf by the Mississippi River. This also made sense knowing that the prevailing current would carry nutrient-rich water from the river to the west of the Mississippi’s mouth, exactly where the Dead Zone was located.
Nitrogen is commonly the limiting nutrient so Sue decided to confirm her suspicions by tracking down data on the monthly change in the nitrogen concentration of the Mississippi River (Figure 6).
Figure 6. Monthly discharge of nitrate and nitrogen from the Mississippi River into the Gulf of Mexico. Values are normalized for the average annual value from 1978 to 1995. (Modified from Turner, et al., 2005.)
Questions
What are the peak months for nitrate-nitrogen discharge from the Mississippi River into the Gulf?
How do the peak months for nitrate-nitrogen discharge compare to the peak months for phytoplankton primary production?
Figure Reference
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Turner, R.E., N.N. Rabalais, E.M. Swenson, M. Kasprzak, and T. Romaire. 2005. Summer hypoxia in the northern Gulf of Mexico and its prediction from 1978 to 1995. Marine Environmental Research 59(1): 65–77.
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