Final answer:
Krill indeed migrate vertically on a daily basis, a process called diel vertical migration, to feed on phytoplankton and avoid predators. Phytoplankton populations are essential to the marine food web, and their growth can be affected by nutrient levels and sunlight exposure. Marine ecologists use tools like multibeam echo sounders for non-invasive research on deep-sea organisms.
Step-by-step explanation:
The true statement among the provided choices is that krill migrate vertically on a daily basis. This behavior is known as diel vertical migration, where krill move to deeper waters during the day to avoid predators and surface at night to feed on phytoplankton. Regarding the echosounder, it is indeed a tool used in marine biology to measure the length of large marine animals like blue whales or assess krill densities, but the statement provided that krill patches are uniformly dense and evenly distributed throughout the ocean is incorrect. In reality, krill patches can vary greatly in density and distribution.
Phytoplankton are crucial to the marine food web, and their population can be influenced by factors like sunlight exposure. Nutrients like nitrogen or phosphorus, if added to the ocean in large amounts, could potentially boost the growth of phytoplankton. It's important also to note that deep-sea marine ecologists, such as Dr. Peter Etnoyer, study organisms like corals using non-invasive methods such as multibeam echo sounders to avoid destroying their habitats.