Cheers from Minnesota! Our state contains the headwaters of the Mississippi river and also the Great Lakes, and waters that flow to the Arctic, so water quality exiting our state is crucial; thus, our State monitors  water quality very carefully--a big task balancing the agricultural and industrial uses, as well as recreational needs. Thank you for presenting this topic!,(reply>), Perhaps you'd be interested in the topic of "invasive exotics", those species which have migrated into new regions, and how they interact with local habitats.  In Minnesota we have a problem of invasive ZEBRA MUSSELS which can clog pipes and  out-compete local mullosks.  Some bodies of water have become cleaner as a result of the zebra mussel's filtering, which is interesting given that these bivalves are much maligned!