Death of a Lagoon

7 06 2010

The Mighty San Mateo

Remember what San Mateo Creek looked like this past winter, after the creek pushed past the sand berm as it reached for the ocean?  Cool, clear water flowing day and night, storm or no storm.

Have you seen it lately?

This is what it looked like this last weekend.  Soon to be a rotting, stinking, choked out, stagnant mess.  The mosquitoes love it.

Green Death

I didn’t have a chance to look for chironomids, back swimmers or other invertebrates that like to hang around the benthic (bottom scum) layer in a healthy wetland.  But the thick mats of algae are a sure sign the water is out of balance.

I couldn’t find any water quality data to verify it for San Mateo Creek, and particularly the lagoon, but it sure looks like something is feeding tons of phosphorus, nitrogen and other fertilizers into the creek.

Could be the El Nino storms mobilizing the phosphorus that is usually tied up in soils, but gets released into streams during high flows.  Or, it could be the “legacy nitrogen” fertilizer still in the old tomato fields of the bottom lands just east of the freeway.

Whatever is feeding it, the “green death” has blossomed with the warm temperatures and full days of sunlight.  Algae is natural, but the green cotton candy currently growing in the lagoon has to be sucking every atom of oxygen out of the water at night when the filamentous plants go into their dark cycle.  Likely there aren’t many fish, or any other oxygen-dependent creatures, left in the lagoon.  Hopefully, they worked their way upstream to more oxygen-rich water.

The Lagoon Cut Off From the Sea

There is hope, in time.  Eventually, the algae will grow itself to death, sink to the bottom of the lagoon, and become a carbon source for bacteria that will release the nitrogen back to the atmosphere where it makes up 78% of the air.  The algae are just a problem right now while there is too much of them.

One last thought.  Let’s just hope there are no algae of the blue green variety.  They and their friends can release domoic acid that can bioaccumulate in shellfish and  make mammals that eat them, like dolphins and sea lions, to lose their memories and sometimes kill them.  A similar thing can happen during a red tide, which most surfers are familiar with.

All this death from well-meaning fertilizers.  So, next time you want that lawn a little greener, don’t over do.  It could be killing a beach or lagoon near you.