The use of clay to remove algal blooms from Chesapeake Bay waters

Algal blooms are becoming increasingly common within the Chesapeake Bay. Nutrients entering the Bay, combined with optimal physical conditions and light, temperature and rainfall, contribute to the formation of these blooms. Certain types of phytoplankton (eukaryotic algae and cyanobacteria) in these accumulations cause problems in local waters, including discolored water, noxious odors and taste, and poor food for zooplankton, benthos, and fish. Some are toxic and may also cause hypoxia/anoxia, with the low oxygen stressful to fish and other aquatic life. Submerged aquatic vegetation can also be adversely affected by the light-scattering cells. Because of increasing concern over the effects of algal blooms on organisms in Chesapeake Bay, laboratory studies were undertaken to examine the potential for removal of phytoplankton from the water column by the addition of kaolin clay as clay has been successfully used in Asia to remove established blooms. Three bloom-forming phytoplanktons were used as test organisms: Prorocentrum minimum, a dinoflagellate common to the Bay which often forms large mahogany tides and can cause oyster mortality; Chattonella subsalsa, a rhaphidophyte observed in the coastal bays of Maryland and Delaware that has been associated with fish kills due to resulting low dissolved oxygen; and a small cyanobacterium from the Patuxent River similar to bloom species in nutrient-rich tributaries of the Bay. The experiments compared phytoplankton in media with and without clay slurry' additions. In vivo fluorescence (IVF), an indicator of the amount of the plant pigment chlorophyll present, was measured on each of 4 replicates for each phytoplankton taxon (control and treated) before clay additions, 2.5 hours after clay addition, and 4 days later. Results showed a significant decrease in IVF in all clay treatments. Largest reductions in IVF were noted in Prorocentrum and Chattonella (99% and 92%, respectively). Single factor ANOVA indicated that significant decreases (p<0.001) occurred after the initial 2.5 hour incubation period. Four days later, there was no further decline in IVF (p>0.05). For the cyanobacterium, two experiments were conducted, one with very high densities typical of laboratory cultures, and a second with lower levels closer to those found in summer Chesapeake Bay waters. At 2.5 hours and 4 days, the IVF of the clay-treated cyanobacterium had decreased significantly (p<0.001). These results indicate the almost complete aggregation and removal of Prorocentrum and Chattonella from the water column with the added clay while the cyanobactium (initially >100 times more dense) remained in suspension longer and was removed by flocculation with the clay through time. The results of the current study suggest that the use of clays in controlling algal blooms looks promising for routine use in many estuaries. As clays are important and common constituents of most soils in coastal plains adjacent to these systems, the control material could be readily available in most basins for routine stripping of phytoplankton blooms from surface waters. Future work includes examination of natural soils as an efficient strategy for inexpensive control of recurring blooms in the Chesapeake Bay and nearby coastal bays.