Volunteer Edgartown shellfishermen worked the tides last week to transfer young bay scallops out of harm’s way at Cape Pogue Pond, after an algae bloom seen a year ago returned.

Cochlodinium polykrikoides, a single-cell dinoflagellate, staged a late-summer comeback in the large, pristine bay that lies north of the Dike Bridge on Chappaquiddick. The algae is not harmful to humans but can be toxic to shellfish.

Edgartown shellfish constable Paul Bagnall said the bloom grew quite large just before the remnants of Hurricane Irene struck last weekend, occupying as much as a quarter of the pond. Mr. Bagnall said winds from the storm may have caused the bloom to break up. This marks the second consecutive summer that the algae bloom has been seen here.

The bloom looks like a cloud of rusty water floating near the surface. Last year the bloom occurred in the outer Edgartown harbor.

Island marine biologists are just beginning to understand the effects of the algae on shellfish; last year when it was first discovered here biologists had a difficult time identifying it due to the fact that they were unschooled in special sampling techniques. Now they have learned how to collect samples for laboratory analysis. Biologists also are just beginning to understand how the algae works against shellfish. Some reports say it starves juvenile shellfish; others say shellfish have a hard time digesting it. Either way it is harmful, so last week before the storm hit a team of Vineyard shellfishermen went out to help move juvenile scallops from Cape Pogue to Sengekontacket Pond, Mr. Bagnall said.

He said this year the bloom appears to be concentrated on the south side of Cape Pogue Pond.

The dinoflagellate has been seen on Nantucket as well. David Fronzuto, marine superintendent of Nantucket’s marine and coastal resources department, said this is the third year it has bloomed there. “It will kill larvae. It depletes the oxygen and the food source,” Mr. Fronzuto said.

There are a variety of Cochlodiniums, some more harmful than others to sea life. Cochlodinium polykrikoides is a late-summer algae. Mr. Fronzuto said this variety is similar to other harmful algae in that it can reside in an area and come to life when the environmental conditions are right. “The problem is that once it is in a pond, it sets in the mud and waits. When the weather conditions are right, it will bloom,” he said.

Rick Karney, director of the Martha’s Vineyard Shellfish Group, said Island shellfish constables are stepping up their sampling of the algae to gain a better understanding of its impacts here.