particulars of a pond will determine whether it can be a self-sustaining ecosystem or if it needs more regular outside intervention.
The larger the body of water (in this case) the less effect, something that enters that body of water, will have on the body of water. ocean for example, if I took a bottle of mtn dew and poured it straight into the ocean, it would slowly dissolve over time, slowly or quickly idk but it will dissolve. Now of course if you crashed 2000000k fighter jets into the ocean carrying nuclear warheads, maybe you'd have a more permanent effect.
Plus the ocean is saltwater and it carries all sorts of things that we haven't even discovered yet that, who knows, maybe those things like mountain dew...
Conclusion of this reply:
Small thing in a big area, little effect
Big thing in a small area, big effect
Little thing in little area, medium effect
big thing in big area, big effect
@Gemma looks like your post has sparked a nice little debate here lol
its a good one.