Offshore-centered Phytoplankton Bloom Formation, Probably Triggered by Subterranean, Freshwater Runoff in the Southern Gulf of Mexico.

Riverine runoff is expected to cause phytoplankton blooms near river mouths, fueled by the riverine nutrients added to coastal waters. In the southern Gulf of Mexico, we observed two strong seasonal blooms at the Usumacinta/Grijalva river mouth and approximately 250 km northeast, centered ~25 km offshore. We used monthly MODIS-Aqua normalized fluorescence line height (nFLH) from 2003 to 2016 to document the blooms and to compare them with monthly averaged precipitation data (Mexican Natl. Water Commission). Using nFLH we avoided signal bias due to the high organic load from the river or the shallow coastal region. We defined the core of the blooms by the maximum values of 20 km running average nFLH along 100 km transects. For a north-south transect starting at the river mouth, the bloom maximum is always located at the coast, but further north, along an east-west transect at 20º N, the bloom maximum is always found ~25 km offshore. Linear cross correlation showed a peak bloom formation lagging precipitation by 3 months at the river mouth, and lagging precipitation by 1 month offshore at 20º N, both correlations had: r=0.48, p<0.5. The bloom at the river mouth follows a textbook pattern, but the offshore bloom is more difficult to explain. We considered different explanations for the formation of the offshore blooms: (1) Coastwise transport of the bloom formed at the Usumacinta/Grijalva river mouth, (2) offshore upwelling at the shelf break and (3) offshore subterranean runoff of freshwater. We rejected explanation (1) because the offshore bloom appears before the river mouth bloom. We consider (2) to be unlikely because of the persistent phasing of the offshore bloom with precipitation over all the years. We suggest that explanation (3) is the most likely, considering the existence of numerous nearshore freshwater runoff locations around the Yucatan peninsula. In our case, the runoff would be 25 km offshore, a situation not documented so far.