Phytoplankton biomass and community structure at frontal zones in the surface waters of the Northern Gulf of Mexico
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Satellite images of chlorophyll concentration in the surface waters of the Gulf of Mexico suggest a high degree of heterogeneity in the phytoplankton biomass. The causes of this variability and the amount of variability in the phytoplankton community structure are not well understood. The physical and chemical conditions of a specific environment can influence phytoplankton community structure by selecting for those phytoplankton species able to survive within that environment. Varying salinity and temperature characteristics give water masses distinct surface water density signatures. This study examined the relationship between phytoplankton biomass, community structure, and different water mass properties by measuring chlorophyll a and algal group concentration across frontal zones. Continuous salinity and temperature measurements were used to calculate continuous density along transects during four cruises on the R/V Gyre between summer 2002 and spring 2004. Frontal zones were identified as areas of sharp density change where σt changed by 1.5 points over a distance of 1 km. Density fronts that coincided with visible temperature fronts (satellite AVHRR images) were selected for biomass and community structure analysis. Discrete water samples were analyzed using fluorometric analysis (total chlorophyll a concentration) and HPLC analysis (photosynthetic pigments). Community composition for discrete samples was determined using CHEMTAX and these values were used to interpolate community composition. Phytoplankton biomass and community structure were examined at a total of 21 density fronts. Unlike previous studies of frontal zones, phytoplankton biomass (measured as chl a concentration) was not significantly higher within frontal zones than in adjacent waters at any of the 21 fronts. Community composition (measured as algal group abundance and diversity) was significantly different between the front and at least one adjacent water mass at front 2 during summer 2002, at front 6 during summer 2003, at front 3 during fall 2003, and at front 3 during spring 2004. Both biomass and community composition were significantly different between fronts at all front pairs during summer 2002. The results of this study suggest that density fronts are not biologically important features in the northern Gulf of Mexico. Lack of high phytoplankton biomass at fronts in the Gulf of Mexico could indicate that unique physical, chemical, or biological processes are occurring.
Salazar, Alicia (2004). Phytoplankton biomass and community structure at frontal zones in the surface waters of the Northern Gulf of Mexico. Master's thesis, Texas A&M University. Texas A&M University. Available electronically from