Algae can also be a product of pollution for example chemical fertilizers

chemical Fertilizers are used often to make farms more fertile. But, they also leach into rivers and lakes and cause Algae blooms among other things which can kill or extinct various plants, animals like Frogs and other species of wild things.

So, though Chemical Fertilizers might help crop yields they also degrade the environment around farms generally speaking and this happens all the way to the oceans from wherever chemical fertilizers are applied.

This is why organic gardening techniques applied to larger farms keep wildlife alive and not going extinct.

So, the less chemical fertilizers are used the more original native species can survive.

They were invented before people were aware of all these chain reactions in the ecology from farm to river from river to lake or pond and from there all the way to the oceans. I'm thinking the main reason frogs are going extinct all over the world is because of Chemical fertilizers, insecticides and herbicides.

And the main reason bees are going extinct (which we need to pollinate most of our crops worldwide) is neurotoxins in pesticides.

And these same pesticides likely are also the main cause of senile dementia and Alzheimers as well in humans in addition to confusing bees so much they cannot find their hives and the hives die as a direct result.