Origins ofKarenia brevisharmful algal blooms along the Texas coast

The dinoflagellate Karenia brevis is the major harmful algal bloom (HAB) species in the Gulf of Mexico. Given that the rapid appearance of K. brevis cannot be explained by plankton growth alone, advection is likely important in bloom initiation. Forward- and backward-moving numerical surface drifters were employed in a numerical model of the Texas–Louisiana shelf to help determine the basic physical mechanisms explaining sporadic interannual occurrences of K. brevis along the Texas coastline. Results from data analysis from the area show that HAB events occur in years in which there are weaker mean downcoast, along-shore wind speeds. The drifter experiments suggest that southern waters play a role in HAB event initiation, providing an offshore source of cells at the end of summer. As winds switch from upcoast to downcoast in early fall, offshore populations of K. brevis are swept southward by wind-driven currents in years with strong downcoast winds. However, when the downshore wind is weak, shoreward Ekman transport creates a convergent flow near the coast that allows cells to concentrate and initiate a bloom.