Seeding of Red Tides: Using Artificial Plankton and Biophysical Models to Understand the Mechanics of Karenia brevis Bloom Formation

Populations of the toxic dinoflagellate Karenia brevis that remain near the benthos in deep shelf water in the Gulf of Mexico could be the source for toxic bloom occurrences near shore. A biophysical dynamic simulation model and migrating aquatic drifters were used together to assess whether such "seed populations" could persist in nature. Planktonic vertical migration responses to a solely benthic nutrient source and light limitation would result in benthically oriented behavior in conditions found on the West Florida Shelf (WFS). The biophysical model indicated that flux of nutrients from a 50m deep bottom that had a nutrient concentration ≥mumol nitrate/nitrite in a 2m thick layer above the benthos was needed to permit growth for dark adapted K. brevis in an oligotrophic water column. Growth rates are more dependent on the duration of exposure to nutrients than on concentration; the ability of 1m thick nutrient layer to sustain minimum growth levels was independent of the nutrient distribution for depths down to 40m. In field experiments using Autonomous Behaving Lagrangian Explorer (ABLE) drifters, which exhibit biomimetic vertical migration responses to the external environment, showed a benthically-oriented movement pattern in response to natural light and elevated near-benthic nutrients. Behaviorally controlled migration therefore would result in behavioral trapping of a slowly growing population near the benthos. Average measurements of nutrients and light from the bottom 2m of the water column in a potential bloom-forming region of the WFS were higher than required for growth in our model, suggesting that nutrient distributions in the coastal ocean could support a benthic population offshore. Under upwelling conditions, such populations could be advected inshore to frontal convergence zones and form toxic "red tide" blooms.;Probable retention of near-benthic "seed" populations by bottom currents differed by region (near Sarasota FL, Destin, FL, and in the center of the Big Bend) and was related to regional prevalence of blooms. K. brevis near Destin (simulated by ABLEs) would be transported with bottom water, because they became bottom oriented with ∼10m vertical diel amplitude on the WFS in response to a 1m thick near-benthic nutrient-enriched layer. Modeled cellular division rates combined with transport speeds during ABLE deployments in Destin and Sarasota yielded projected transport distances of up to 89.76km over a K. brevis cell cycle. Such K. brevis populations would be transported into deeper shelf regions with less accessible light in the presence of cross-shelf currents. Based on light availability (SeaWiFS, 2004-2009) is sufficient in these deeper regions for probabilities of year-round population survival to reach 90-100% for Sarasota, 30-80% near Destin, and 50-80% in Big Bend; seasonal probabilities were lowest in winter and summer when blooms are also least likely. Modeled bottom currents indicated that populations near Destin and Sarasota would be transported predominantly alongshore and thus remain at the same bottom depth, supporting a near-benthic population that could later seed blooms, but that this would not occur in the Big Bend region.;The ABLE design was assessed for sensor precision, realistic vertical migration, similarity to transport of "standard" drifter designs, and practicality for future use in other kinds of plankton studies. Comparisons of sensors with organisms' capabilities indicated sufficient resolution to imitate most larval plankton, although higher resolution of pressure and light may be necessary to imitate some species (e.g. C. sapidus.) Vertical migration velocities and depth profiles closely matched the intended (dynamically-calculated) values. ABLEs migrated with the intended water mass both when released with "conventional" drifters with known drift error rates, and when released in an estuary during flood tide. The ABLE is considered to have evolved into a practical field instrument for estuarine and coastal plankton tracking.