Using clay to control harmful algal blooms : deposition and resuspension of clay/algal flocs

Author Posting. © Elsevier B.V, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V for personal use, not for redistribution. The definitive version was published in Harmful Algae 4 (2005): 123-138, doi:10.1016/j.hal.2003.12.008.Harmful algal blooms (HABs) may be legitimate targets for direct control or mitigation, due to their impacts on commercial fisheries and public health. One promising control strategy is the rapid sedimentation of HABs through flocculation with clay. The objective of this study was to evaluate flow environments in which such a control strategy might be effective in removing harmful algae from the water column and depositing a layer of clay/algal flocs on the sea floor. We simulated the natural environment in two laboratory flumes: a straight-channel “17-m flume” in which flocs settled in a still water column and a “racetrack flume” in which flocs settled in flow. The 17-m flume experiments were designed to estimate the critical bed shear stress for resuspension of flocs that had settled for different time periods. The racetrack flume experiments were designed to examine the deposition and repeated resuspension of flocs in a system with tidal increases in flow speed. All flume runs were conducted with the non-toxic dinoflagellate Heterocapsa triquetra and phosphatic clay (IMC-P4). We repeated the experiments with a coagulant, polyaluminum hydroxychloride (PAC), expected to enhance the removal efficiency of the clay. Our experiments indicated that at low flow speeds (≤ 10 cm s-1), phosphatic clay was effective at removing algal cells from the water column, even after repeated resuspension. Once a layer of flocs accumulated on the bed, the consolidation, or dewatering, of the layer over time increased the critical shear stress for resuspension (i.e. decreased erodibility). Resuspension of a 2-mm thick layer that settled for 3 hours in relatively low flow speeds (≤ 3 cm s-1) would be expected at bed shear stress of ~0.06-0.07 Pa, as compared to up to 0.09 Pa for a layer that was undisturbed for 9 or 24 hours. For the same experimental conditions, the addition of PAC decreased the removal efficiency of algal cells in flow and increased the erodibility of flocs from the bottom. By increasing the likelihood that flocs remain in suspension, the addition of PAC in field trials of clay dispersal might have greater impact on sensitive, filter-feeding organisms. Overall, our experiments suggest that the flow environment should be considered before using clay as a control strategy for HABs in coastal waters.This project was funded by the Florida Institute of Phosphate Research (Grant # 99-03-138), with facilities provided by the Rinehart Coastal Research Center at WHOI

The importance of human dimensions research in managing harmful algal blooms

Author Posting. © Ecological Society of America, 2010. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Frontiers in Ecology and the Environment 8 (2010): 75–83, doi:10.1890/070181.Harmful algal blooms (HABs) are natural freshwater and marine hazards that impose substantial adverse impacts on the human use of coastal and marine resources. The socioeconomic and health impacts of HABs can be considerable, thereby making a case for “human dimensions” research to support HAB response. Human dimensions research is multidisciplinary, integrating social science, humanities, and other fields with natural science to enhance resource management by addressing human causes, consequences, and responses to coastal environmental problems. Case studies reported here illustrate the importance of human dimensions research. Incorporating such research into the scientific agenda – as well as into management decisions of public agencies concerned with natural resource management, environmental protection, and public health and welfare – requires the development of both strategic guidance and institutional capacity. The recent development of a multi-agency research strategy for HAB response and a strategic plan for human dimensions research represent two important steps in this direction.This paper was developed with partial support from NOAA’s National Centers for Coastal and Ocean Science