Interannual summer variability in oceanic euphausiid communities off the Baja California western coast during 1998–2008

Abstract

Euphausiids are a major component of the zooplankton biomass due to their large size, contributing with high carbon content to other trophic levels in the pelagic ecosystem. We analyzed the summer interannual variability in euphausiid species composition based on carbon mass of the Baja California oceanic domain during 1998–2008. Selection of one exclusive season allowed the emphasis of interannual changes in order to research possible biological impacts. During the period 1998–2008 prevailed intense interannual activity, with four El Niño events, two of them (1997–1998 and 2006–2007) with SST anomalies propagating toward the eastern Pacific (EP-El Niño), while the other two (2002–2003 and 2004–2005) had SST anomalies limited to the central Pacific (CP-El Niño). There were also La Niña events in 1998–2000 and 2007–2008. The species with higher biomass contribution off Baja California were Nematoscelis difficilis, Euphausia gibboides, Thysanoessa gregaria, Euphausia eximia, Nyctiphanes simplex, and Euphausia pacifica, with a global geometric mean of 156, 66, 38, 30, 21, and 13 µg C m−3 respectively. N. difficilis and E. pacifica were dominant in the northern area (29.5-32°N), N. difficilis and E. gibboides in the central area (27–29.5°N), and E. eximia dominated in the southern area (24.5–27°N). 1998–2008 biomass anomalies showed a variety of patterns by species with the clearest footprint, in most of the species, during the strong EP-El Niño 1997–1998. CP-El Niño events also left a footprint in the biomass of some species but this was not always by anomalies of the same nature as EP-El Niño. The best examples were N. difficilis and N. simplex, which presented lightly positive anomalies during July 1998 but were strongly negative in the summer of 2003 and 2004. The opposite was observed in E. recurva, with a negative anomaly in July 1998 but positive in 2004 and 2005. The biophysical coupling between the species assemblage and environmental variables, using canonical correspondence analysis (CCA), explained 22% of the biomass variability. The first axis was responsible for thermal conditions in the upper layer (temperature at 10 m, 50 m, and the gradient between 10 and 100 m depth), while the second axis concentrated the oxygen gradient, oxygen and salinity at 50 m depth, and 200 m temperature. A large group of tropical-subtropical species showed covariance with axis-1, while E. pacifica and T. spinifera had an inverse covariance. The equatorial species E. distinguenda and E. lamelligera were close to axis-2, though the stations were limited to slope water where intense upwelling bring oxygen depleted deep water. Transition zone species (E. gibboides, N. difficilis, T. gregaria, and N. simplex) were relatively inert to both axes. Their response to climatic variability was less predictable and new variables should be explored, including bottom-up and top-down mechanisms.