What causes a dead zone?You are assisting a friend studying the biodiversity of a local lake. After surveying the lake, your friend looks concerned and says, "there has been a dramatic decline in biodiversity since I last sampled a few months ago". While your friend is typing data into her computer, you take a walk around the lake, write down some observations, and perform some basic biochemical tests. This leads you to hypothesize that the lake has become eutrophic. Which of the following observations and test results would lead you to this hypothesis?Check All That Apply
A. There are a large number of dead snails and fish along the bank of the lake.
B. The water appears very clear and the bottom of the lake is easily visible.
C. Levels of dissolved oxygen are low throughout the water column.
C. Nitrogen levels are high.
D. Phosphate levels are low.

Dead zones are regions of the ocean where oxygen levels are low. These areas are produced in oceans, seas and lakes due to the increase of chemical compounds such as nitrogen and phosphorus. The main responsible is climate change. As surface waters heat up due to rising temperatures, oxygen is more difficult for it to reach the depths of the ocean.