Convergent blooms of Karenia brevis along the Texas coast

Abstract

A numerical model of wind‐driven surface flow in the Gulf of Mexico is used to examine physical controls on harmful algal bloom formation along the Texas coast. Karenia brevis, which blooms frequently in the Gulf of Mexico, has a relatively slow growth rate (doubling times of 2–3 days). Increases in K. brevis concentration cannot be explained simply in terms of growth. We hypothesize that the primary mechanism responsible for bloom formation in the western Gulf of Mexico is convergence due to downwelling at the coast. Convergence along the Texas coast caused by seasonal downwelling winds can concentrate the plankton up to 1000 times. This is surprising because the modeled cells do not grow; the simulated increase in concentration is due to physical processes alone. The numerical model qualitatively reproduces both the timing and magnitude of bloom initiation along the coast, but does not predict the details of the migration of the bloom along the coast after it has formed, or the destruction of the bloom. The result of this simulation is significant because it implies that K. brevis blooms may be caused primarily by physical processes and that cell division is not an important factor in bloom formation.

Identifying bloom origins of the toxic dinoflagellate Karenia brevis in the western Gulf of Mexico using a spatially explicit individual-based model

“…Thyng et al (2013) found that a weak downcoast mean alongshore wind velocity for the month of September was required for bloom formation along the coast of Texas. This result was in agreement with the results of Hetland and Campbell (2007) in showing that downwelling at the coast can result in a coastal aggregation of spatially dispersed cells. The model results in this paper suggest subsurface cells are advected north toward the coast of Texas during the summer months and can provide an above background concentration seed population for a fall bloom.…”

“…Previous work revealed no genetic differentiation among blooms from Florida and blooms from Texas and it was hypothesized that bloom-forming cells in Florida and Texas originate from the same source yet the location of the source population remained unknown . Blooms of K. brevis off the coast of Texas appear to be the product of widely dispersed cells accumulating by physical concentration (Hetland and Campbell, 2007;Stumpf et al, 2008). The combination of low background cell concentrations and the physical accumulation of cells suggests blooms could occur randomly throughout the year; however, this is not the case.…”

“…The frequency with which blooms occur near Florida has resulted in extensive research on blooms and bloom development in this region (e.g. Hu et al, 2008;Milroy et al, 2008;Robbins et al, 2006;Vargo et al, 2004) but research into the development of blooms near Texas has remained minimal (Hetland and Campbell, 2007;Stumpf et al, 2008;Thyng et al, 2013). Previous work revealed no genetic differentiation among blooms from Florida and blooms from Texas and it was hypothesized that bloom-forming cells in Florida and Texas originate from the same source yet the location of the source population remained unknown .…”

Karenia: The biology and ecology of a toxic genus

“…This is where tidal mixing is at a minimum (He and Weisberg, 2002) and a transport barrier reduces mixing between inshore and offshore waters Olascoaga et al, 2006;Olascoaga, 2010). In Florida, Texas, and Mexico, K. brevis is most likely to form blooms in the fall months Steidinger et al, 1998;Brand and Compton, 2007;Hetland and Campbell, 2007). Hetland and Campbell (2007) have shown that downwelling during the fall months along the Texas coast can lead to a 1000-fold concentration of K. brevis cells, making it a likely factor in the development of blooms there.…”

“…The most obvious case is downwelling, in which cells at the surface from a large area are transported to the downwelling site and accumulate there as they swim upward to counteract the downward movement of the water. Over time, this mechanism can generate very high concentrations of organisms (Hetland and Campbell, 2007).…”