New phytoplankton production as a tool to estimate the vertical component of water exchange between the Gulf of California and the Pacific

Abstract

Water exchange between the Gulf of California and the Pacific has a significant vertical component. Surface (0–200 m) gulf water flows out into the Pacific and deep (200–600 m) water flows into the gulf. A biogeochemical method is proposed to estimate this vertical component of water exchange assuming steady state for the concentration of nutrients in the gulf and using the net average annual input of nitrate needed to support new phytoplankton production in the whole Gulf of California (P_NEW). An annual average P_NEW of (2586.7 ± 131.7) × 10⁹ mol C yr^–1 was deduced from the literature for the whole gulf and for non-El Niño years. Using the Redfield N:C ratio (16:122), the nitrate needed to support P_NEW was estimated as (339 ± 17)× 10⁹ mol yr^–1. Annual representative averages of NO₃, for the mouth of the gulf and for the depth intervals 0–200 m and 200–600 m, were used to calculate the annual average vertical component of water exchange between the gulf and the Pacific to balance the nitrate needed to support P_NEW with the net input of nitrate from the Pacific, and the result was (0.67 ± 0.10) Sv in and out of the gulf. This relatively low value, possibly only ~7% of the whole water exchange, indicates that when considering a particular depth most of the time the inflow from the Pacific is equal or very similar to the outflow. Thus, most of the exchange between the gulf and the Pacific consists of the horizontal component.