We are all probably aware that the Massachusetts Division of Marine Fisheries recently closed Buzzards Bay and much of the south shore of the Cape along Vineyard Sound to shellfishing. As reported in various news outlets, the closure is due to the presence of a small single-celled plant floating in abundance in these offshore waters. The plant is called Pseudo-nitzschia. The concern is that this plant has the capability to produce a compound (domoic acid) that can accumulate in shellfish and is toxic to humans who might eat those shellfish. The phenomenon is called a harmful algal bloom (HAB), where bloom refers to the growth of one particular species of phytoplankton (in this case Pseudo-nitzshia).
We might ask, Why is this bloom happening here and now? It is not unusual for the autumn to bring a bloom of phytoplankton to the offshore waters of Cape Cod. In fact, it is an annual event that scientists refer to as the autumn bloom. It happens as the water temperature drops, making it easier for the winds of autumn to mix the nutrient rich waters from the deeper parts of the ocean to the still-well-lit surface layers. That mix of sun and nutrients allows a fairly luxuriant plant growth. Each year, that bloom dissipates later in the fall as the hours of sunlight decline and the phytoplankton use up those nutrients.
The difference this year is the species that bloomed. Usually the plants that undergo this autumn growth spurt are one of several common single-celled phytoplankton organisms that biologists have observed offshore of New England every autumn for about 100 years. These common plants were first observed in autumnal abundance by the United States Bureau of Fisheries working from Woods Hole early in the twentieth century. Any scientist who cared to look has observed these same species as blooming at this time of the year pretty regularly since then.
However, this fall a different plant, a species of Pseudo-nitzschia, began to dominate. This is unusual. The terrestrial equivalent might be if instead of seeing mums and other late-blooming flowers in our autumn gardens, we suddenly saw day lillies to the exclusion of nearly all other flowers. It is not that we do not see day lillies on Cape Cod; it is just that we see them in spring and summer, not fall, and we see them along with many other types of flowers. So, what is going on?
In all honesty, marine biologists do not know why this species came to dominate the fall bloom rather than one of the more usual species. Scientists have observed various Pseudo-nitzschia species blooming in other parts of the world, including the Gulf of Mexico, Prince Edward Island, Monterey Bay California and the northwestern coast of the United States. There is no lack of hypotheses about what triggers the robust growth that we are seeing. Maybe this species just does well at low autumn temperatures? Nice try, but laboratory studies show that it has no particular advantage over other species at low temperatures. Maybe it does well in declining sunlight? Again, laboratory experiments show no particular advantage for Psuedo-nitzschia at low light intensity, although sometimes it has an advantage when the hours of light drop to 8 hours per day.
What about nutrients or salinity? Well, as indicated above, there is always an autumnal injection of nutrients from below the surface off Cape Cod. Observations from other areas are mixed. At Prince Edward Island, Pseudo-nitzschia bloomed during high nitrogen runoff from the land. In the Gulf of Mexico, the blooming of Psuedo-nitzschia species occurred when nutrients such as nitrogen and silica were low. High abundance has been observed in Monterey Bay, California, when nutrients are falling. There seems to be no relationship to salinity.
So, we do not really know why this species happens to outgrow other phytoplankton in any particular place or any particular time. Worse yet, we do not know what makes it produce the toxin domoic acid. Sometimes the Psuedo-nitzschia produce it and sometimes not. One hypothesis is that it needs the domoic acid to help it absorb essential nutrients such as iron at low concentrations as might occur toward the end of the bloom. This is why regulatory agencies will be so cautious about re-opening shellfish beds even after the Pseudo-nitzschia appear to stop growing. This is the time when they just might be making the toxin, domoic acid.
So, we do not really know why here or why now. The biology and ecology of phytoplankton blooms, whether toxic or not, are just not well understood. However, what we do know and can be thankful for is that in the face of these uncertainties, our Division of Marine Fisheries and various local shellfish and health departments are keeping a careful and health-protective watch on the progression of this unusual fall phytoplankton bloom.