Why are coastal zones productive?

The surface waters in regions of coastal up-welling are cold and nutrient-rich, promoting robust growth of plants and the animals that feed on them. ... When too many nutrients are released into seas, bays, and estuaries, it can create an overabundance of decaying plants and animals, depleting oxygen from the water.

when [oh-] is 10-5  m, and [h3o+] is 10-9  m, the poh is 5 and ph is 9. again in both cases the sum of ph and poh is 14. however, the hydrogen ion concentration is not always going to be equal to exactly 1 x 10 raised to a negative number. for example, we skipped over the value of 2.0 x 10-7.