1. what is the limiting factor for most animal life in the open ocean?

Biotic: Food availability

Abiotic: Temperature

Explanation:

There are two types of limiting factors for biodiversity: biotic and abiotic. Biotic refers to living things, for example, organisms that are an important food source. Most animal life forms in the ocean highly depend on the availability of a food source. If food is limited or scarce, the populations of a given species could face significant declines.

On the other hand, there are abiotic factors, which refer to factors that are not alive, such as physical factors. For instance, temperature and light. For marine organisms, temperature is a critical factor. Even an increase of 'only' 1 ºC could make a huge difference in the survival of a species as it could disrupt their ability to forage, hunt, or perform physiological processes, e.g. metabolism.

Therefore, if we refer to a biotic factor, food availability is a limiting factor for most animal life in the open ocean, whereas, if the refer to an abiotic factor, temperature (and light) are limiting factors for pelagic life.