Which factor is crucial in defining the biodiversity of a marine ecosystem?

The variety and abundance of species is essential in defining the biodiversity of a marine ecosystem because biodiversity is fundamentally about the range of different organisms and their relative populations within a given environment. High biodiversity indicates a rich variety of life forms, encompassing everything from microorganisms to large marine mammals, which contributes to the resilience and stability of the ecosystem. Biodiversity plays a vital role in ecosystem services, including nutrient cycling, climate regulation, and providing habitats for countless organisms.

While salinity, the size of the ecosystem, and the depth of the ocean can influence the types and numbers of organisms that can thrive in a marine environment, they do not directly define biodiversity itself. Salinity affects what species can inhabit an area, the size of the ecosystem can determine the available space for species, and depth can influence light availability and pressure conditions. However, it is the variety of species and their abundance that ultimately captures the essence of biodiversity.