A freshwater lake is a good example of a localized ecosystem. Why? What are the living organisms that might?

> Why?
It can be localized because it does not exchange its organisms with other locations -- my favorite example is Crystal Lake in California.  No incoming streams, no outgoing flow;  the lake is as isolated as a distant island is from the continents.> What are the living organisms that might populate such a community and how are they inter-connected?
A small lake will have aquatic plants, aquatic snails and insects and turtles that eat the plants, fish that eat the insects, and bigger fish to eat the smaller fish.  And, of course, decomposers assist in the recycling of matter.
A large lake will have the same as the small lake, but phytoplankton and zooplankton will be of greater relative importance to the life in the lake.> what are some human activities that could disrupt the equilibrium of this system?
a) Pumping water away for urban use
b) Introducing non-native predators ("stocking") and organisms that would support the non-native predators ("bait animals") can replace the native ecosystem.  Sadly, this is what happened to Crystal Lake.  The native plants and animals are gone, replaced by plants and animals that are more interesting to people who like to go fishing.  The lake is no longer "crystal" -- it's a green stinking mess, and swimming is no longer allowed.