fertilizers have a positive impact on algae growth & runoff contains fertilizers from cropland ->  fertilizers in the runoff from cropland have a positive impact on algae growth  fertilizers in the runoff from cropland have a positive impact on algae growth & algae is a kind of organism & if something has a positive impact on an organism 's growth then that something increases the population of that organism ->  fertilizers in the runoff from the cropland can increase the population of algae  fertilizers in the runoff from the cropland can increase the population of algae & a cropland is a kind of farm -> fertilizers in the runoff from the farm can increase the population of algae