Legs 1 & 7
Legs 1 and 7 will be conducted at the beginning and end of the expedition during transits of the R/V Western Flyer, southbound from San Diego to La Paz, and northbound from La Paz to San Diego. On these legs, the principal scientific activity will be standard hydrographic stations with a CTD (conductivity, temperature, depth) instrument and water sampling rosette deployed to depths of 1000 m. In addition, short plankton net tows will be made at each station. On the northbound leg, scuba divers will collect plankton at some stations. Between stations, the ship will tow an instrument designed to measure the iron content of the water that the ship passes through. The purpose of these activities is to gain a better understanding of the hydrography, chemistry, and biology of the California Undercurrent, which flows northward along the continental shelf of North America. The leg 1 and 7 coordinator is Dr. Francisco Chavez from MBARI. The Mexican collaborator for leg 1 is Gilberto Gaxiolla-Castro, and Bertha Lavaniegos will serve for leg 7– both are from CICESE.
The California Undercurrent (CU) is a poorly described
countercurrent that flows northward along the continental slope and shelf
of North America, transporting water and organisms from the Eastern
Tropical Pacific (ETP) to the California region (Figure 1). Its waters are
deficient in nitrate but may be enriched in iron due to interaction with
slope and shelf sediments. Off central California, the CU is typically
weak and subsurface during the first half of the year but strengthens and
shoals during the latter half of the year. Because the CU provides source
waters for coastal upwelling, its nutrient content can have significant
consequences to the overall biological productivity of the California
Figure 1. Schematic of the eastern tropical Pacific showing relation of North America, the California Current, coastal upwelling, and the California Undercurrent.
We plan to describe the
hydrography, nutrient chemistry, and phytoplankton of the CU and overlying
waters. We will trace the CU along a more than 1000 mile length of coast
off western North America, providing descriptions of (1) chemical
development of upwelling source waters as they flow north from the ETP,
(2) alongshore chemical and biological variability in coastal upwelling
and phytoplankton production, and (3) basin-scale north/south transects to
compare with the east/west transits of the Western Flyer to Hawaii (see
In addition, Steve
Haddock will be blue-water diving on the northbound transit to examine
the distribution of gelatinous organisms off the North American coast.
While many zooplankton species are reported to have biogeographic
boundaries at particular locations along the California/Mexican coast,
these are poorly documented for gelatinous species. During last year's
transit to Hawaii, blue-water divers documented changes in the assemblages
of gelatinous plankton. Population shifts such as these should be even
more striking along the north/south transect from Alta to Baja California.