Legs 1 & 7 

Leg Summary:

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.

History & Purpose: 

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 Current System. 

Figure 1. Schematic of the eastern tropical Pacific showing relation of North America, the California Current, coastal upwelling, and the California Undercurrent.

The Western Flyer transits to and from the Gulf of California (GOC) provide an ideal opportunity for tracing these waters from the ETP to central California. Scientists will make CTD (conductivity, temperature, depth) and underway measurements from Monterey Bay to 23^o N and then into the Gulf of California on both the south- and northbound transit legs of the cruise. 

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 Hawaii cruise). 

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.