First I would like to note that I have only very basic understanding of ecology, biology and paleontology so I can easily miss something very simple.
I can envision certain ways marine and terrestrial ecosystems influence each other:
- Rivers and surface runoff carries minerals, soil, plants, excrements etc into the oceans. Productive terrestrial ecosystem would mean less minerals, more organic nutrients going into the ocean.
- Terrestrial animals (like birds) eat marine ones and then excrete and are eaten at the land.
- Marine and terrestrial ecosystems absorb and produce atmospheric $O_2$, $CO_2$, $N_2$ that are redistributed globally by atmospheric movements. That also affects climate and therefore water cycle.
Naively it seems to me that the situation is quite asymmetric. While №2 should be of extreme importance to the coastal ecosystems I suppose it's influence drops quickly as you go deeper into continent aggravated by water currents taking everything back into ocean. So (again naively) it seems to me that marine ecosystems should be much more influenced by the terrestrial ones than the other way around.
So imagine sudden significant drop of primary production for some reason constrained to the oceans. Obviously it would be a catastrophe for island and coastal life but in what ways, how much and at what timescale it will affect inland ecosystems? What if we turn it upside down and consider some significant drop of terrestrial primary production?