Abstract
Phytoplankton was studied in two different seasons of 1998 (March-April and December), during two cruises along the western coasts of Baja California, in three zones. Two different protocols for obtaining and studying phytoplankton were followed. In the March-April season, phytoplankton had relatively low species richness and was dominated in cell density (up to 93%) by coccolithophorids (mainly Emiliania huxleyi), together with nanoplanktonic centric and pennate diatoms, with abundances ranging from 5.4 10 3 to 1.2 10 5 cells L -1 . In December, phytoplankton had higher species richness and was represented by larger, chain-forming diatom species, such as Pseudonitzschia delicatissima and P. pungens , which were widespread and numerically significant. There was a relative scarcity of coccolithophorids and thecate dinoflagellates, and densities were between 7 10 2 and 1.4 10 6 cells L -1 . Hydrographic and oceanographic conditions in March-April were influenced by the occurrence of El Nino and the phytoplankton structure was found to be modified accordingly, with nanoplanktonic coccolithophorids and diatoms being significant contributors to the total abundance. In contrast, post-upwelling conditions might have favoured relatively high densities of Pseudonitzschia and other diatoms in December, 1998. Coccolithophorids have not been previously regarded as important contributors to the phytoplankton abundances in Baja California.
Highlights
The western coastal area of Baja California, Mexico has traditionally been considered a very rich and productive system, in which the influence of the California Current is remarkable (Parrés Sierra et al, 1997) and upwellings play an important role in keeping this high productivity
Diatoms and “armoured” dinoflagellates have been studied in greater detail than other taxonomic groups and they are usually regarded as the dominant phytoplankton in the area (Allen, 1923, 1924, 1934; Balech, 1960), and more recently (Hernández-Becerril, 1988, 1989, 1995, 1998; Hernández-Becerril and Tapia, 1995)
Samples were taken basically following two distinct protocols: (1) In March-April, 1998, which is considered the decline of the El Niño 1997-1998 event, phytoplankton was collected with a Niskin bottle from three depths at 6 fixed stations and filtered through Millipore 0.45 μm meshed-filters with a vacuum pump
Summary
The western coastal area of Baja California, Mexico has traditionally been considered a very rich and productive system, in which the influence of the California Current is remarkable (Parrés Sierra et al, 1997) and upwellings play an important role in keeping this high productivity736 D.U. (Longhurst et al, 1967; Wang and Walsh, 1976; Parrés Sierra et al, 1997) The phytoplankton of this area have been relatively well-studied: there are studies on composition, distribution, primary productivity and variability of some of its features, as well as the main factors controlling the natural variation of phytoplankton (Smayda, 1975; Estrada and Blasco, 1979; Hernández-Becerril, 1985; Cardona et al, 1990; Millán-Núñez and Loya Salinas, 1993; Cohen Fernández, 2000). In the California current system, coccolithophorids and other flagellates have been identified and counted (in terms of cell numbers and cellular volume) (Reid et al, 1970), the relative importance of microplankton (netplankton) and nanoplankton has been stressed (Malone, 1971; Mullin, 1998), and the contribution to biomass and productivity of fractions of phytoplankton (smaller and larger than 8 μm) in a coastal area off Baja California has been studied (González Morales and GaxiolaCastro, 1991)
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