Although the open ocean is among the least productive ecosystems, it contributes greatly to the overall productivity of the biosphere. how can this situation be explained?

Primary production within the open ocean is mainly through diatoms which are single-celled photosynthesising organisms. Although the concentration of productivity in the open ocean is relatively low, the vast area of the ocean means that this form of productivity contributes greatly to overall primary production on Earth. There are also regions of relatively high production from diatoms in certain parts of the ocean. This is because the productivity of diatoms is limited by a lack of certain essential nutrients in the ocean, such as iron and silica, which tend to sink to greater depths in the ocean than the surface layers that the diatoms inhabit. In these regions, upwelling occurs, bringing nutrients from the deep ocean to the surface, and resulting in diatom blooms. This occurs typically on the west coasts of continents, because upwelling occurs here due to something called the Bernoulli Effect, which relates the the way the Earth spins. This is why very productive fishery areas are usually found on the west coasts of continents, at least in the Southern Hemisphere, and why the seawater is cold in these regions. 

Although i don't know what is the simulated system, starfish are predators that eat mostly  microalgae,  snails, sponges, bivalves, mussels and other small marine animals. removing starfish from any system would definitely increase this animals that they prey.