Origins of Karenia brevis harmful algal blooms along the Texas coast

Abstract

Harmful algal blooms (HABs) in the Gulf of Mexico are primarily caused by the dinoflagellate species Karenia brevis . During blooms, toxins produced by K. brevis are concentrated by suspension‐feeding shellfish, which, if consumed by humans, can cause neurotoxic shellfish poisoning, and can lead to fish kills. A complication in understanding the underlying cause of the blooms is that the observed rate of growth of the bloom is too high to be explained by algal growth alone. Additionally, blooms occur sporadically in Texas waters, compared with more predictable blooms on the western Florida continental shelf. This study proposes a series of physical mechanisms to explain the interannual variability of bloom occurrences. Wind data along the coast are weaker on average in years in which HAB events occur. We propose that typical northward summer winds bring higher concentrations of cells from the southern Gulf of Mexico and then these cells are transported toward the Texas coastline by the southwestward fall winds. Strong winds wash out the accumulating cells, whereas weaker winds allow algal cells to aggregate. 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.