Final answer:
Oxygen levels in seawater increase during glacial advance. This can result in an increase in the amount of algae and phytoplankton, and promote the growth of bacteria that decompose dead organic matter.
Step-by-step explanation:
Oxygen levels in seawater increase during periods of glacial advance. During colder times, more oxygen-16 evaporates and gets stored in ice sheets on continents, leading to an increase in the ratio of oxygen-18 to oxygen-16 in seawater. This increase in oxygen levels can have various effects on marine organisms, such as increasing the amount of algae and phytoplankton and promoting the growth of bacteria that decompose dead organic matter.