Role of plankton in the carbon and nitrogen budgets of Santa Monica Basin, California

The photic zone portions of a particle budget, resolved as carbon and, through appropriate C:N ratios, as nitrogen, are reported for the Santa Monica Basin, a 900 m deep basin in Santa Monica Bay, within the Southern California Bight west of Los Angeles, California. Included are standing stocks of phytoplankton, bacteria and zooplankton, primary production, new production, and particle flux leaving the photic zone. These are compared with rates of fecal pellet production by macrozooplankton and consumption by bacteria and zooplankton within the photic zone using data from 3 October cruises and 2 spring cruises, 1985 to 1987. Because new production took place primarily within the nitracline, in the lower portion of the photic zone, the productivity and zooplankton measurements and some of the bacteria measurements were also made above and within the nitracline. Comparison of C and N fluxes out of the photic zone with standing stocks of particulate C (PC) and N (PN) indicated that PC had a shorter residence time than PN in the photic zone during both spring and fall. The carbon flux was also a greater fraction of daily primary production than was the product of the PN flux and the Redfield ratio. This conservation of nitrogen, relative to carbon, may be a general feature of oligotrophic surface waters, and probably is largely accomplished by preferential solubilization and assimilation of nitrogen by zooplankton and microheterotrophs. Such differential cycling of C and N suggests the possibility of incipient N-limitation of phytoplankton growth rates in spite of Redfield-like C:N ratios for the bulk suspended particulates. Increases in phytoplankton C:N ratios related to N-stress could be masked by relative increases in bacterial biomass with relatively low C:N composition ratios. A representative budget for particulate carbon in the photic zone is shown to illustrate rates, routes and reservoirs of PC and to point up the terms required to balance the budget. In addition to showing the greater flux (shorter recycling time) of PC relative to PN in the photic zone, the budget showed that most of the primary production (either as C or N) was recycled within the photic zone, which implied heavy grazing by organisms < 200 μm and/or by macrozooplankton that produced slowly settling fecal debris. Zooplankton food intake rate was always greater than the new production rate in the photic zone, and was usually > 20 % of the primary production rate. Estimates of bacter