Final answer:
The arrival of sea otters increases the kelp population due to a trophic cascade. If seals' food supply decreases, the seal population and carrying capacity decline. Mutualism is seen in the Karner blue butterfly larva and ant interaction, and light and nutrient availability affect net primary productivity.
Step-by-step explanation:
In an ecosystem where sea urchins feed on kelp, the arrival of sea otters (which feed on sea urchins) would cause a increase in the kelp population. This would be an example of (a) trophic cascade. If the major food source of seals declines due to pollution or overfishing, the most likely consequence would be that the carrying capacity of seals would decrease, as would the seal population. This is because a decreased food supply cannot sustain the same number of seals. Overfishing has led to trophic cascades in marine ecosystems, causing significant declines in species richness and changes in coastal food webs. This is due to a reduction in predator populations like seals or large marine predators, highlighting the importance of a top-down control in these ecosystems.
A mutualistic relationship is exemplified by the interaction between the Karner blue butterfly larva and ants, labeled as (c) mutualism. The factors that affect net primary productivity, like an algae bloom in a lake, include light and nutrient availability, not predation by primary consumers or the presence of carbon dioxide.