Pea soup

There’s a pond near where I live, a relic from a farm long since given over to forest, that often serves as a convenient turn-around point on walks with my dog. If it’s a hot day, sunlight glittering across the still surface of the water, she’ll rush down the steep mud bank and wallow in the shallows.

This morning, as we climbed the narrow dirt trail up to the edge of the pond and the water came into view, it looked like the pond had been replaced with a vat of pea soup – a bloom of algae had spread across the entire surface.

Annual algal blooms are a common summer phenomenon in many places. Depending on the dominant species of algae and the extent of the bloom, they can be harmless, or they can have extreme consequences – fish die-offs, shellfish bed closures, and drinking water bans, to name just a few.

Though researchers have long known that several factors typically control algal blooms, particularly temperature, light, nutrients, and wind patterns, individual bodies of water often respond to these environmental conditions in idiosyncratic ways. Earlier this year, researchers from the Woods Hole Oceanographic Institution reported the results of several years of monitoring blooms of the dinoflagellate Alexandrium fundyense, a microorganism that produces toxins responsible for paralytic shellfish poisoning, in Nauset Estuary on Cape Cod, Mass.

The scientists found that temperature appeared to be the most important factor controlling algal bloom inception in Nauset – the annual blooms there were triggered by a certain amount of warm weather early in the year, meaning that a warm spring might lead to an early explosion of Alexandrium fundyense, necessitating an earlier-than-usual closure of the shellfish beds in the estuary.

In fact, these conditions occurred during one of the years the scientists were monitoring the estuary – as they reported in their study, “the bloom began in Nauset about 1 month earlier in 2012 than in previous years, so the monitoring program for shellfish toxicity had not begun that year. A rapid response by state officials to sample and subsequently close parts of the estuary to shellfishing occurred on the basis of cell concentrations found in our first large-scale survey that year, but without that population sampling, the early onset might have been missed by routine monitoring.”

The scientists go on to point out that something as simple as regularly measuring the water temperature could help civic leaders anticipate when shellfish bed closures are necessary.

There are no shellfish beds on the bottom of the old farm pond where my dog likes to cool down on summer days, of course, but as we turned back toward home this morning I found myself thinking about how common an occurrence algal blooms are, from the coast of Massachusetts to northern Idaho and many places in between. 

A late summer algal bloom in an old farm pond in northern Idaho.

(Image by Emily Benson)