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.
River discharge, and its resulting region of freshwater influence (ROFI) in the coastal ocean, has a critical influence on physical and biogeochemical processes in seasonally stratified shelf ecosystems. Multi-year (2010-2016) observations of satellite-derived sea surface salinity (SSS) and in situ water column hydrographic data during summer 2016 were used to investigate physical aspects of the ROFI east of the Mississippi River Delta to better assess regional susceptibility to hypoxia in the summer months. Time series of SSS data indicate that the shelf region impacted by the seasonal expansion of freshwater can be as extensive as the well-known "dead zone" region west of the Delta, and hydrographic observations from a shelf-wide survey indicate strong stratification associated with the ROFI. Peak buoyancy frequencies typically ranged between 0.15 and 0.25 s −1 and were concentrated in a 2-3 m layer around 4-10 m deep across much of the shelf. This ROFI is expected to be influenced by local freshwater sources which, while individually small, make a notable contribution in aggregate to the region (annually averaged daily discharge of approximately 2880 m 3 s −1). The dissolved oxygen (DO) conditions under this freshwater cap were spatially and temporally variable, with areas of hypoxia and nearhypoxic conditions over portions of the shelf, demonstrating the utility of satellite-derived SSS in identifying coastal areas potential vulnerability to hypoxia. These regions of low bottom dissolved oxygen persisted throughout the peak summer season at several sites on the shelf, with the northeastern corner of Mississippi Bight having the most intense and persistent hypoxia.
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