Brian P. Darrow

Red tides in the Gulf of Mexico: Where, when, and why?

[1] Independent data from the Gulf of Mexico are used to develop and test the hypothesis that the same sequence of physical and ecological events each year allows the toxic dinoflagellate Karenia brevis to become dominant. A phosphorus-rich nutrient supply initiates phytoplankton succession, once deposition events of Saharan iron-rich dust allow Trichodesmium blooms to utilize ubiquitous dissolved nitrogen gas within otherwise nitrogen-poor sea water. They and the co-occurring K. brevis are positioned within the bottom Ekman layers, as a consequence of their similar diel vertical migration patterns on the middle shelf. Upon onshore upwelling of these near-bottom seed populations to CDOM-rich surface waters of coastal regions, light-inhibition of the small red tide of ~1 ug chl l(-1) of ichthytoxic K. brevis is alleviated. Thence, dead fish serve as a supplementary nutrient source, yielding large, self-shaded red tides of ~10 ug chl l(-1). The source of phosphorus is mainly of fossil origin off west Florida, where past nutrient additions from the eutrophied Lake Okeechobee had minimal impact. In contrast, the P-sources are of mainly anthropogenic origin off Texas, since both the nutrient loadings of Mississippi River and the spatial extent of the downstream red tides have increased over the last 100 years. During the past century and particularly within the last decade, previously cryptic Karenia spp. have caused toxic red tides in similar coastal habitats of other western boundary currents off Japan, China, New Zealand, Australia, and South Africa, downstream of the Gobi, Simpson, Great Western, and Kalahari Deserts, in a global response to both desertification and eutrophication.

A simulation study of the growth of benthic microalgae following the decline of a surface phytoplankton bloom

The West Florida continental shelf is an oligotrophic system for most of the year. An episodic chlorophyll plume has previously been observed in satellite imagery on the northern portion of the shelf during the spring months. The fate of the plume upon its decline in the late spring and early summer is unknown. Decreased chlorophyll levels and sustained nutrient stocks may be explained by sediment/water-column interactions, including the presence of benthic microalgae. A one-dimensional model, consisting of 16 state variables, is constructed to simulate the decline of a surface chlorophyll bloom in the northeastern Gulf of Mexico as measured during the Florida Shelf Lagrangian Experiment (FSLE). Results from a baseline simulation of two FSLE studies suggest that remineralized nutrients from the declining bloom are taken up by heterotrophic bacteria in the water-column and by benthic microalgae in the sediments. Perturbation experiments imply that low light levels, due to increased CDOM, do not have significant effects on the benthic microfloral community at mid-shelf locations.

Forecasting and Modeling of Harmful Algal Blooms in the Coastal Zone: A Prospectus

Applications of numerical models to predict and eventually manage the onset, duration, and consequences of toxic phytoplankton events of coastal ecosystems require knowledge of both the ecophysiological properties of the specific organisms and the biophysical processes, which allow them to accumulate large biomass, despite the presence of other competitors. Harmful algal blooms are not always just local events. Furthermore, independent in situ plankton data sets from the Gulf of Mexico (GOM) and Mediterranean Sea over six decades, together with adjunct satellite color data, other nitrogen isotope signals of plankton and sediments, extant circulation models, and phytoplankton biomarkers within sediment cores of this similar ecosystem, all confirm the complex western GOM eutrophied sequence of phytoplankton succession.