Distribution of chlorophyll and primary production by size classes along the Mexican Pacific coast

Abstract

From 15 January to 4 February, 1999, we carried out an oceanographic cruise along the west coast of Mexico, from the mouth of the Gulf of California to the Gulf of Tehuantepec. For the first time in this region, size-fractionated chlorophyll a and ¹⁴C assimilation experiments were carried out at seven stations. Nutrient concentrations (PO₄ and SiO₂) were also measured. Nutriclines were very shallow and developed within the euphotic zone depth. Sea surface temperature varied from 19.5ºC at the mouth of the Gulf of California to 28.3ºC in the Gulf of Tehuantepec. The integrated euphotic zone chlorophyll a ranged from 16.8 to 95.6 mg m^–2 d^–1 ( x = 41.4). Daily integrated primary production rates ranged from 0.16 to 1.15 g C m^–2 d^–1 (x  = 0.53). Nanophytoplankton (<20 µm) contributed most to both chlorophyll and primary production (60–100%). Microphytoplankton (>20 µm) was significant only at two southern stations (~30% and 44%). The contribution by the nanophytoplankton size fractions (<20 to >10 µm and <10 to >3 µm) and the picoplancton (<3 to >0.45 µm) was variable. Phytoplankton biomass and primary production rates showed a clear spatial pattern, increasing from north to south.