I was surprised to read about the blooms on the beach, pleasantly at first, because it has been a question in the back of my mind why the sand and ocean looked so sick. When I read the explanation of diatoms and how it is "a sign of a healthy ocean environment" I was unsure and sought out more about diatoms.
I found a journal paper edited and published by MIT in February 2010 which states that their blooms are causing detrimental harm to the marine environment. However, these are all the diatoms of the genus Pseudonitzschia and you did not give any more detail than "diatom." The paper was studying the effects of iron on these harmful blooms, and found it to increase their occurrence.
"Given the negative effects of DA in coastal food webs, these findings raise serious concern over the net benefit and sustainability of large-scale iron fertilizations." (www.pnas.org/content/107/13/5887.full) These diatoms produce a neurotoxin domoic acid. This does not sound like something we may want our families and animals playing in and perhaps ingesting.
More research needs to be done regarding this possibility and if so what is the source of the fertilizer in our run offs? Who on our Peninsula is feeding these blooms?