Question

As you saw in "DO or Die", adding nutrients to a lake has direct effects (such as stimulating the growth of phytoplankton) and indirect effects throughout the community (such as drastic effects on fish populations). Suppose you triple (3x) the phosphorus input in a lake where phosphorus is limiting, and phytoplankton populations grow dramatically. Based on your observations, which of the following predictions is most likely to follow the phytoplankton bloom?

a) Decomposition decreases, DO increases, fish populations grow, and zooplankton abundance increases.
b) Decomposition increases, DO decreases, fish populations die off, and zooplankton abundance increases.
c) Decomposition increases, DO decreases, fish populations grow, and zooplankton abundance decreases.
d) Decomposition increases, DO increases, fish populations die off, and zooplankton abundance decreases.

Answer 1

Tripling the phosphorus input likely leads to increased decomposition and DO depletion, causing fish to die off; zooplankton might initially increase but will also suffer eventually.

Step-by-step explanation:

When there is a tripling of phosphorus input in a lake where phosphorus is the limiting nutrient, a large phytoplankton bloom is likely to occur. This bloom will initially support a larger population of zooplankton that feed on phytoplankton. However, once the phytoplankton start to die off, they will be decomposed by bacteria, a process that consumes large amounts of dissolved oxygen (DO), leading to oxygen depletion in the water. As a result, organisms such as fish that require oxygen will likely die off, and the overall health of the aquatic ecosystem will suffer, potentially leading to a dead zone.

Considering this, the most likely prediction following the bloom is: b) Decomposition increases, DO decreases, fish populations die off, and zooplankton abundance increases. However, the increase in zooplankton abundance is temporary, as their population will eventually collapse due to the lack of oxygen and food as the dead algae are decomposed and the system's productivity declines.