ABSTRACT. The response of zooplankton, phytoplankton biomass (Chl-a concentration) and nutrients to the oceanographic variability in the Mexican Central Pacific was studied over three oceanographic surveys realized in 2010 (January, May and October). The highest zooplankton biomass (48 mL 1000 m -3 ) was registered in May when nitrites, nitrates, phosphates and silicates were also highest and mean Chl-a was 0.67 mg m -3. The lowest zooplankton biomass was registered in October, with a mean value of 25 mL 1000 m -3. Twenty-nine taxonomic groups of zooplankton were recorded, of which copepods contributed 71% of total abundance followed by chaetognaths (6%) and euphausiids (5%). Multivariate BEST test demonstrated that mixed layer depth and temperature at 25 m depth can explain the distribution of major zooplankton groups. The study area was influenced by El Niño conditions, which caused a seasonal shift. Coastal upwelling was evident until late summer, when geostrophic analysis showed a predominant SW component near the Jalisco coast. We suggest a possible top-down control of zooplankton grazing over phytoplankton, and acclimatization of both plankton groups to variability in environmental conditions induced by the transition from warm to cold ENSO phase.
Gymnodinium bloom events are of concern, since they produce toxins, which have unfavorable consequences to marine ecosystems, human health and the economy. This report describes the physico-chemical conditions that were present during the algal bloom event on May 2010 in Bahía Manzanillo and Bahía Santiago, Colima, Mexico. For this, seawater nutrient analysis, phytoplankton counts, identification, and toxicity tests were undertaken. Nutrients in seawater were determined using colorimetric techniques, the higher concentrations (8.88µM DIN, 0.78µM PO 4 and 24.34µM SiO 2) were related with upwelling waters that promoted the algal bloom that began after registering the year lowest sea-surface temperature, favoring the rapid growth of G. catenatum (up to 1.02 x10 7 cells/L). Phytoplankton counting was carried out using sedimentation chambers and cells enumerated on appropriated area. The bloom persisted in the bays for approximately two weeks and was associated with toxicity (determined with HPLC) in local oysters (1525.8µg STXeq/100g), and in phytoplankton (10.9pg STXeq/cells) samples. Strong variations in cell toxicity (1.4 to 10.9pg STXeq/cells), most likely reflected the availability of inorganic nutrients. The toxin profile of the phytoplankton samples consisted of 11 toxins and resembled those recorded for several strains of G. catenatum isolated from other coastal areas
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