Phytoplankton Monitoring

Keeping an Eye Out for Blooms
(Published by The Quoddy Tides,
August 23, 2002)

The waters of Cobscook Bay carry a whole host of microscopic organisms that are critical to sustaining life within the bay. Perhaps as many as 200 species of tiny algae called phytoplankton float around in the water column and are consumed by marine animals. They come in a remarkable array of shapes and sizes.

Like many other plants, phytoplankton take up carbon dioxide from the atmosphere during the day and convert it into oxygen. At night, they do just the opposite, using up oxygen in the water to produce carbon dioxide during respiration.

Algal blooms, sudden increases in a population of phytoplankton, are likely to be found from late spring to early fall. While most species of phytoplankton do not cause harm, some can cause illness in humans or cause problems for fish, particularly in aquaculture. In the last Soundings column we wrote about four species of phytoplankton that carry toxins harmful to humans. This month we will focus on phytoplankton blooms that affect fish species important to the aquaculture industry.

Here in Cobscook Bay, Heritage Salmon and the Cobscook Bay Resource Center work together to monitor for phytoplankton blooms at two sites from May through October as part of the Maine Phytoplankton Monitoring Program.

"Occasionally, when algal blooms occur, the algae can strip the oxygen from the water or clog the fish gills, in either case causing the fish to suffocate," says Jennifer Smith of Heritage Salmon. "In natural situations, the fish would typically be able to escape such conditions, but in aquaculture that is not the case."

"We monitor oxygen and temperature on a daily basis at our stocked sites," adds Smith. "While feeding, the fish consume more oxygen (until several hours after for digestion), so, whenever we experience any signs of low oxygen levels, our feeding regime is changed. We have backup oxygen available that can be diffused into the pens, but in Cobscook Bay we have not had to take such measures."

But algae can pose other threats. Some species of phytoplankton grow in colonies and also produce mucus secretions that react with the mucus in salmon gills and cause the fish to suffocate.

"Other species have spines or sharp points that are abrasive and like fiberglass to fish. They will literally cut the gills up," says Heidi Leighton, who handles the phytoplankton monitoring for the Cobscook Bay Resource Center. In the past, Heritage Salmon has seen blooms of a phytoplankton called Chaetoceros, which can cause stress to the fish by abrading the gills.

"We look for the variety and number of species at certain times of the year. By monitoring on a monthly basis, we hope to be able to predict cyclical patterns," says Leighton.

At the St. Andrews Biological Lab in New Brunswick, scientist Jennifer Martin studies the distribution of algal blooms in the Bay of Fundy, the environmental conditions that trigger high concentrations of phytoplankton species harmful to finfish, and the ways in which algal blooms cause harm to fish. Since 1987, the program has checked for phytoplankton species and abundance once a week from April through October, and then typically once a month the remainder of the year.

"There are a lot of species out there in the water. Not very many are harmful, but any species that gets up to very high concentrations is a threat," says Martin. "We monitor the entire phytoplankton community. If we only monitored for a few, we might miss other species that could cause problems. If all of a sudden there are fish dying, it's harder to put the pieces together to explain what happened if we don't have sampling before the fish kill."

In Passamaquoddy Bay, in 1998, a bloom of Mesodinium rubrum dropped oxygen levels to such a low level that fish became stressed and some mortalities occurred, affecting a number of salmon aquaculture sites. Mesodinium can change the water to a red color when it gets to high enough concentrations, which is referred to as a red tide. This is not the species that causes what is commonly referred to as a red tide in Maine. (Here, the term red tide is generally used to refer to Alexandrium, a phytoplankton species that blooms and produces toxins that cause Paralytic Shellfish Poisoning in humans and other consumers of shellfish and rarely causes the water to change color).

Martin states, "A red tide of Mesodinium, which does not contain toxins, may or may not cause fish problems - just having red water going through an area of the bay does not mean fish are going to die."

Sampling for Mesodinium is different than for many other species of phytoplankton, as contact with a net will cause the cells to burst. However, water in a patch of red can be collected in a container at the surface or the desired depth. "Mesodinium is more easily identified while alive," says Leighton. "When placed under a microscope, it will swim around really fast, but then bursts within a few minutes due to the heat of the light source in the microscope." Preserved specimens look like tiny blobs and can be identified, but require experience.

Phytoplankton blooms are part of the natural workings of nature, and important to the life within Cobscook Bay. However, conditions such as sunlight, surface water temperature, salinity, runoff from coastal lands, and the way currents cause the phytoplankton to drift, can contribute to unusual densities of naturally occurring species. Continued monitoring for harmful phytoplankton can help reduce the potential negative impacts on our economically important marine resources.

This column was prepared by Cheryl Daigle. Cobscook Bay Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information.

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