Final answer:
The true statement is that krill migrate vertically on a daily basis. Krill feed at the ocean surface at night and hide in deeper waters during the day. Echosounders show that krill patches are not uniformly distributed, and nutrients can affect phytoplankton growth.
Step-by-step explanation:
The statement a. Krill migrate vertically on a daily basis is true. Krill perform a behavior known as diel vertical migration, where they ascend towards the ocean surface at night to feed on phytoplankton and descend to deeper waters during the day to avoid predation. Contrary to statement b, krill do not remain at the surface during the day; they do so primarily at night.
The use of echosounders in marine research is correctly associated with marine observations, such as the study of blue whale lengths or krill distributions. However, echosounders have shown that krill patches are not uniformly dense or evenly distributed throughout the ocean, which makes statement d incorrect.
The phytoplankton population in an estuary can change based on the intensity and duration of sunlight as well as the presence of nutrients. Elements that could boost phytoplankton growth if added in large amounts to the ocean are typically nutrients like nitrogen and phosphorus (relating to statement B).