if something enters into something else, then the contents of that something will enter that something else & the contents of something means the things that something contains ->  if something enters something else then the things that something contains will enter that something else  if something enters something else then the things that something contains will enter that something else & runoff enters a lake ->  if runoff enters a lake then the things that runoff contains will enter that lake  if runoff enters a lake then the things that runoff contains will enter that lake & runoff contains chemicals / fertilizer / pollutants / pesticides from cropland ->  if runoff enters a lake then fertilizer will enter that lake  a lake is a kind of body of water & algae is found in bodies of water ->  algae is found in a lake  algae is found in a lake & fertilizers have a positive impact on algae growth ->  fertilizers have a positve impact on algae growth in a lake  if runoff enters a lake then fertilizer will enter that lake & fertilizers have a positve impact on algae growth in a lake ->  runoff entering a lake has a positive impact on algae growth in that lake  runoff entering a lake has a positive impact on algae growth in that lake & if something has a positive impact on something else then that something causes that something else ->  runoff entering a lake causes algae to grow in that lake  runoff entering a lake causes algae to grow in that lake & causing is a kind of affecting -> runoff entering a lake affects that lake by causing algae to grow in it