Slightly less ELI5 expansion: Nitrogen and Phosphorous are both considered “limiting nutrients” basically every ecosystem on the planet is limited by one of these two nutrients, naturally speaking phosphorous only comes from the weathering of minerals and bio available nitrogen only comes from nitrogen fixings bacteria in anaerobic environments (such as nodes in some symbiotic plant roots) and lightning…until the invention of the Haber-Bosch process the amount of useable nitrogen on earth was functionally fixed….people were scraping guano off of rocks to make TNT…

Anyway, with industrial processes and fertilizers we’ve cranked these nutrients up to 11 to disastrous result on the environment, seriously this might be worse than climate change unless we stop what we’re doing (we won’t).

Nutrient pollution not only causes these aquatic issues, it can also heavily favor invasive species, as native plants are typically adapted well to a specific environment, which includes nutrient availability, invasive plants more suited to higher nutrients can take advantage of higher levels of available nitrogen and phosphorous, grow rapidly and displace the native plants.

This is actually one of the reasons why wetlands are so critically important, not only to they tend to collect the nutrient pollution and prevent it from being washed downstream, they are also a critical site for nutrient cycling, in this case specifically Denitrification.