Alternate Title

Causative Analysis on a Nearshore Bloom of Oscillatoria erythraea (trichodesmium) in the Northern Gulf of Mexico


Physical, chemical, and biological characteristics which preceded and caused a bloom of Osclllatorla erythraea commonly known as trlchodesmium In coastal waters of Mississippi and adjacent waters of the Gulf of Mexico are described. This Is the first report of the blue-green alga occurring in high density near the mainland and In a predominantly estuarine area of the northern Gulf of Mexico. Environmental conditions Immediately prior to and during the bloom were characterized by low rainfall, calm sea, a homogeneous water column, low nitrate-nitrogen (0-5 NO3-N µg-atom/l), no measurable nitrite-nitrogen (0 NO2-N µg-atom/l), high water temperatures (29-30° C), high salinity (270/100), and a basic pH (8.3 to 8.4). Total phosphorus and orthophosphates were also low prior to the bloom (0.1 P µg-atom/l), but increased slightly during the later stages of the bloom (0.7 to 1.5 P µg-atom/l). The alga disappeared with the return of well-mixed sea water, lower salinity, lower temperature and acidic pH, and an increase in combined nitrogen content. Osclllatorla erythraea occurred in bundles of 10 to 25 trichomes or as a single filament, ranging from 8 to 15 µm In diameter and about 0.3 mm in length. The alga occurred in patches with the greatest concentration near the surface. Some entrapment of zooplankton in the dense algal mass was observed, but most of the zooplankton was diverse and unharmed. Harmful effects of the algal bloom on larger animals were not observed nor believed to have occurred.