Rhode Island Saltwater Fishing
Monitoring Phytoplankton Blooms in RI Waters
By: David Borkman, RIDEM Water Resources
While you cannot see them without a microscope, marine phytoplankton are the base of the marine food web supporting abundant commercial and recreational fisheries in coastal waters globally. Because they are so small (most are less than one-twentieth of a millimeter in longest dimension), these single-celled organisms usually go unseen as they grow, multiply, and perform photosynthesis converting the sun’s energy into food particles that are eaten by larger organisms. The vast majority of the thousands of species of marine phytoplankton alive today are beneficial and cause no harm to other marine organisms or humans. In fact, through their photosynthesis, these single-celled marine organisms are the source of about half of the oxygen in Earth’s atmosphere.
A relatively small number of marine phytoplankton species can impact human activities, including fishing and the harvest of shellfish. This handful of species can have negative impacts through either accumulation of excessive phytoplankton cells in the water or through the production of organic compounds that act as biotoxins. Too many phytoplankton cells in the water can cause water-discoloring blooms (‘red tides’, brown tides’, ‘rust tides’) and may result in low oxygen levels as the phytoplankton sink to the bottom and decay. The few species that produce biotoxins can result in accumulation of the biotoxins in shellfish as a result of the filter feeding shellfish consuming phytoplankton that are producing biotoxins.
Typically a water sample taken from Narragansett Bay will contain a phytoplankton community comprised of tens to low hundreds of different species and abundance would be about one million cells per liter of seawater or less. Occasionally, under environmental conditions that are not fully known, a single species will increase in abundance and dominate the phytoplankton. This ‘bloom’ or rapid increase in the phytoplankton population can result in millions of phytoplankton cells in a liter of water such that the photosynthetic pigments in the phytoplankton cells discolor the water. The RI DEM Office of Water Resources Shellfish Program monitors marine phytoplankton in Rhode Island’s shellfish growing waters to help ensure that shellfish harvested from RI waters are free of natural biotoxins and meet all health standards. Below, we describe two phytoplankton blooms that occurred in RI marine waters during 2016. One was a water-discoloring bloom that had little effect on shellfishing activities and the other was not visible on the water, but did have an impact on shellfishing activities.
Summer 2016 ‘Rust Tide’
You may have observed this water-discoloring bloom if you were out on the waters of Narragansett Bay during late August and September of 2016. For a few weeks, patches of rust-red water were visible in Narragansett Bay and in some salt ponds. The rust-red patches were hard to miss! Investigation of water samples collected in these patches of discolored water showed that the phytoplankton was dominated by a species of dinoflagellate called Cochlodinium polykrikoides. Cochlodinium is a medium-sized dinoflagellate approximately 30-40 micrometers (30-40 millionths of a meter) in length and is packed with red-brown photosynthetic accessory pigments. Like all dinoflagellates, it has two whip-like flagellae that allow it to swim. Cochlodinum has the ability to form chains of single-cells that give it a larger size and also increases its swimming speed and may aid in its ability to form and maintain dense, water-discoloring patches. In the densest patches observed in RI, up to 8.5 million Cochlodinium cells were present in each liter of water!
Fortunately, Cochlodinium does not negatively impact human health. While its blooms discolor the water, it produces no biotoxins that could enter the food supply and fish and shellfish are safe to consume during a rust tide. The late summer 2016 rust tide appears to have been a regional event. Similar rust tide patches were observed in the waters of Buzzards Bay, MA and in some bays along Long Island, NY. In addition to discoloring the water, Cochlodinium may have negative effects on finfish and shellfish. It produces abundant extra-cellular mucus that appears to at least irritate, and possibly damage finfish gills. Fish will generally avoid the dense, dark red Cochlodinium patches by swimming away from them. In some caged fish culture operations in Asia, penned fish have been killed when a bloom of Cochlodinium moves into a cove and the penned fish cannot escape. Fortunately, the 2016 bloom in Narragansett Bay was extremely patchy, with the patches moving around with the wind and tides. Wild finfish can generally avoid the patches by swimming away from them; because of this, fishing in the dark-red rust tide patches was probably not very good for a few weeks during August and September of 2016 when the rust tide was present.
Photo of dense “rust tide” patch in upper Narragansett Bay taken 9/13/2016. Note the sharp discontinuity between the dark colored Cochlodinium patch and the clear, normal upper Bay water a few feet away. Photo by Heather Stoffel.
Photo of the dinoflagellate Cochlodinium polykrikoides responsible for the late summer 2016 rust tide. This is a 4-celled chain; each cell is approximately 40 micrometers in length; these cells were collected in Ninigret Pond on 8/30/2016. Photo: David Borkman.
October 2016 Pseudo-nitzschia Bloom
The second phytoplankton bloom of interest was completely different than the dramatic red-water rust tide bloom. It would have passed by unnoticed unless you were looking at water samples with a microscope. In late September 2016, during routine phytoplankton monitoring of RI waters an increased abundance of a potentially harmful phytoplankton, Pseudo-nitzschia spp., was detected in the waters of the lower East Passage in outer Newport Harbor. Pseudo-nitzschia is a kind of phytoplankton called a diatom; it is a long (~ 100 micrometers long) but narrow (only a few micrometers wide) cell that is much smaller than the Cochlodinium cells described above. Because it is smaller, a Pseudo-nitzschia bloom is not likely to produce the water discoloration (as in ‘red tide’ or ‘rust tide’ ) oftentimes associated with harmful phytoplankton blooms. Traditionally, diatoms were not thought of as a kind of phytoplankton that had the potential to produce biotoxins that could impact human health. That changed in 1987 when several people became ill after eating mussels grown on the coast of Prince Edward Island, Canada. After much investigation, the mussels were found to have been filter-feeding on a bloom of phytoplankton dominated by Pseudo-nitzschia, and that the Pseudo-nitzschia was producing a biotoxin called domoic acid. Domoic acid is an amino acid that, in high enough doses, can interfere with normal nerve signal transmission. In humans, high doses of domoic acid can cause symptoms such as vomiting, nausea, headaches, dizziness, seizures, and possibly short-term memory loss. In severe cases, usually associated with individuals having previously compromised health conditions, high doses may lead to death. Because of the potential impacts on human health, shellfish monitoring programs now routinely monitor for the presence of Pseudo-nitzschia to ensure that it is not present in harmful levels in shellfish growing waters.
Once elevated Pseudo-nitzschia abundance was detected in Rhode Island during late September 2016, a series of other steps were quickly taken to investigate the extent of the bloom, and most importantly, to determine whether or not the domoic acid biotoxin was present. Unfortunately, the biotoxin was detected, and further, it appeared that the Pseudo-nitzschia bloom was a regional event. The shellfish monitoring program in nearby Massachusetts also detected a Pseudo-nitzschia bloom in the waters of Buzzards Bay. Given the presence of the Pseudo-nitzschia phytoplankton cells in high abundance in the waters of the lower part of Narragansett Bay and nearby Buzzards Bay, and evidence that biotoxin was being produced and that shellfish were taking up the biotoxin, a precautionary shellfish harvesting closure was put in place in early October 2016 to protect public health. During the closure extensive plankton and shellfish sampling tracked the bloom and showed that it was primarily confined to the southern portion of Narragansett Bay and the coastal waters from Point Judith east to Sakonnet Point and the RI-MA state line at Westport. The intensive monitoring performed throughout the bloom detected domoic acid in plankton at four locations and in shellfish at two locations (at levels far below the established thresholds for mandatory closures). Analysis of plankton samples and shellfish samples collected in the salt ponds along the southern RI coast and on Block Island showed that Pseudo-nitzschia abundance was low and that no biotoxin was present, so these areas were not closed to shellfishing. By late October monitoring showed that the bloom had passed, Pseudo-nitzschia abundance in the water had declined, all shellfish tested were free of domoic acid and the shellfish closure was lifted on October 29, 2016. Throughout the closure, shellfish from open areas were tested and found absent of domoic acid, assuring the safety of shellfish in the market.
Photo of the diatom Pseudo-nitzschia spp. taken during the October 2016 bloom. Two types are shown in the photo: three large-type Pseudo-nitzschia cells in a chain running horizontally in the upper portion of the photo and two small-type Pseudo-nitzschia in a chain running vertically on the right side of the photo. Photo: RI Department of Health, Water Micro Lab.
2016 was an interesting phytoplankton bloom year in RI and southern New England waters. Regional phytoplankton blooms, described above, were present and were a challenge to detect, and monitor. A team effort led by RI DEM and the RI Department of Health, with support from colleagues in nearby states and at Federal agencies , RI Universities, and the help and cooperation of RI shellfishers and aquaculturists were able to detect and monitor these blooms and, in the case of the Pseudo-nitzschia bloom, protect public health. If you see unusual water-discoloring phytoplankton blooms during your time out on RI waters, you can call or email the RI DEM Water Resources Shellfish Program (401-222-4700 or DEM.Shellfish@dem.ri.gov) with questions or for information.