Alkenone abundance and its relationship to the coccolithophore assemblage in Gulf of California surface waters

Profiles of alkenone concentration, the abundance of the calcified alkenone-producing species Emiliania huxleyi and Gephyrocapsa oceanica as well as nutrient and phytoplankton pigment concentrations were measured through the euphotic zone in surface waters of the Gulf of California during cruises in two different summers (2004, 2005) and one winter (2005). As determined using a biomarker and pigment inventory approach, E. huxleyi and G. oceanica, always the most abundant coccolithophore species at all sampling sites and seasons, represented only a minor fraction of total haptophytes which themselves constituted only a minor fraction of the total phytoplankton community. In winter, alkenone concentration was highest at the surface and displayed relatively uniform values with depth throughout the nutrient-replete euphotic zone. In summer, it display a subsurface peak in nitrate-depleted waters situated just above the nitracline and the deep chlorophyll maximum layer (DCML), a feature found to be well-developed at most sites. Maximum biomarker abundance was comparable in the two seasons, although the putative alkenone-producing coccolithophore species were more abundant in winter than in summer. Estimates of cellular alkenone content for the combined abundance of E. huxleyi and G. oceanica at each sampling depth showed that cells residing in the nitrate-depleted upper euphotic zone in summer were typically more enriched than those residing in nutrient-replete waters found deeper in the euphotic zone or throughout the euphotic zone in winter. The former estimates were very similar to values documented in laboratory cultures of E. huxleyi harvested in a nutrient-depleted stationary growth phase and the latter to those harvested in an exponential growth phase, under the influence of incipient light limitation. Overall, our findings imply that calcifying forms of E. huxleyi and G. oceanica can account for the measured alkenone concentration at all sites and sampling times. Although the life history of these organisms is known to include non-calcifying stages, it is unnecessary to invoke such an explanation as the cause for the elevated alkenone content of cells from nutrient-depleted surface waters in summer.