MEPS 253:97-109 (2003)  -  doi:10.3354/meps253097

ABSTRACT: An increased incidence of harmful algal blooms (HABs) has been reported in nearshore coastal waters, including economically important aquaculture sites worldwide. This has generated interest in using ecologically inert clays as a means of mitigating HABs at relatively shallow enclosed mariculture sites. This proposed mitigation measure assumes that flocculation and accelerated sedimentation of large algal cells (dinoflagellates) occurs with the application of clays. The objective of the current research was to characterize suspended clay-algal interactions and in situ particle size in a laboratory application of the phosphatic clay IMC-P (1.8 µm mean equivalent spherical diameter, ESD) to a simulated Heterocapsa triquetra (14.7 µm mean ESD) bloom. Flow speed was adjusted to provide 2 contrasting regimes (mimicking possible field conditions). This research was conducted in a recirculating flume and used a novel in situ device, the small volume particle microsampler (SVPM), for particle size characterization. Results indicated that 100% of the thecate dinoflagellate H. triquetra was removed in 48 h by IMC-P in a low-flow (<2 cm s^-1) regime, but remained in the water column in the high-flow (>13 cm s^-1) regime. The mechanism for algal removal is equivocal since SVPM filter photographs indicated that clay aggregated to form particles >3 µm, but did not aggregate with algal cells under the experimental conditions tested. At this particle size, the clay aggregates become available for retention with 100% efficiency on the gills of suspension-feeding bivalves, which could be seriously affected by the increased particle flux to the bottom. The mechanism of cell removal by clays has not been identified; however, it may occur via direct effects of clay on cell surface properties and swimming ability. Further investigations on the removal mechanism are required to predict the fate of cells in a field application of clay.

KEY WORDS: Harmful algal blooms · Clay · Particle size distribution · Small volume particle microsampler · Heterocapsa triquetra · Mitigation · Aggregation