Nanoplankton fraction in the phytoplankton structure in the southern Gulf of Mexico (April 2000)

Abstract

Phytoplankton, especially the nanoplankton fraction, and its possible relationships with the hydrographic conditions in the southern Gulf of Mexico were studied during an oceanographic cruise in April 2000. Hydrographic data and phytoplankton were analyzed. Phytoplankton species composition showed heterogeneity among stations. A total of 180 species were found: 114 diatoms, 32 dinoflagellates, 32 coccolithophores, and 2 silicoflagellates. Diversity indices were variable (0.96–5.12 bits ind^–1). Phytoplankton was numerically dominated by coccolithophores or nanoplanktonic diatoms. Cell abundance was low to moderately high, ranging from 5.3 × 10³ to 1.4 × 10⁵ cells L^–1. The most abundant, dominant species were nanoplanktonic forms: two coccolithophores, Emiliania huxleyi and Gephyrocapsa oceanica; the pennate diatoms Nitzschia bicapitata, N. bifurcata and N. leehyi; and centric diatoms of the genus Thalassiosira. The high subsurface abundance of coccolithophores at two stations was related to a thermal front in the Grijalva-Usumacinta river system.