There is a sudden increase in the number of herbivorous fish due to the migration of some carnivorous fish from the river. what effect will this have on primary producers and npp?

The herbivorous fish will eat more, so the primary producers will be grazed back. They may or may not respond by growing more. We'll assume they don't grow more. So, if the plants are being eaten back, there will be less of them to produce and NPP will drop. This is all assuming that growth is not stimulated by grazing.

Hope this helps!

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.