Biological Production Rates in the Southern California Current System

Abstract

The isotopic composition of dissolved O₂ (<super>17</super>∆) and the biological O₂ saturation from O₂:Ar ratios were measured in the surface ocean during six cruises off the coast of Southern California from November 2005 to August 2008 to determine rates of gross oxygen production (GOP), net oxygen production (NOP), and the NOP:GOP ratio (a measure of potential export efficiency). <super>17</super>∆-GOP and O₂:Ar-NOP estimates were compared to on-deck incubation-based <super>14</super>C-Primary Production (<super>14</super>C-PP) and satellite-based PP algorithms for the CalCOFI region. <super>17</super>∆-GOP estimates were consistently greater than bottle <super>14</super>C-PP by a factor of 5.6±0.4 and greater than satellite PP estimates by a factor of 3.5±0.3 (mmol O₂:mmol C). The <super>17</super>∆-GOP to <super>14</super>C-PP factor was twice the expected factor of 2.7 determined from comparisons of incubation-based <super>18</super>O-GOP and <super>14</super>C-PP. The annual mean NOP:GOP ratio was 0.16±0.06 suggesting a potential export efficiency surprisingly similar to the open ocean using comparable methods. The consistent relationship between biological O₂ saturation derived from O₂:Ar dissolved gas ratio measurements and routinely measured O₂ saturation allows estimation of historical potential net community production (NCP) rates from the multi-decadal CalCOFI dataset. NCP rates were also estimated using a nitrate budget approach; historic f-ratio measurements for different regions of the CalCOFI grid coupled with <super>14</super>C-primary production rates; and the Vertically Generalized Productivity Model (VGPM) of Behrenfeld and Falkowski (1997) coupled with two commonly used export ratio algorithms. The time series of NCP estimates show lower NCP in response to strong El Niño events but an insignificant response to weak El Niño events. NCP estimates show no significant response to La Niña events. Since 2007, all estimates show increases in NCP which correspond to anomalously cool conditions in the northeast Pacific Ocean and stronger equatorward flow in the California Current.