Acceleration of Nutrient Uptake by Phytoplankton in a Coastal Upwelling Ecosystem: A Modeling Analysis

Studies of upwelling centers in the eastern Pacific suggest that maximum rates of nitrate uptake (light and nutrient saturated) increase, or shift-up, as newly upwelled water moves downstream. The rate of shift-up appears to be related to irradiance and the ambient concentration of limiting nutrient at the time of upwelling. A mathematical model was developed to evaluate effects of irradiance and initial nitrate concentration on temporal patterns of shift-up and subsequent time scales of nutrient utilization over a range of simulated upwelling conditions. When rates consistent with field studies were used, complete shift-up was possible only under certain conditions, and the time scale was on the order of 7-10 d. These results are consistent with field observations. Increased initial nitrate concentrations resulted in more rapid depletion of the nutrient supply. Making acceleration of V,,, constant and independent of the nitrate concentration reversed the qualitative pattern of nutrient utilization and predicted longer time scales in the region of optimal growth (1215 d) than have been observed in the field. Since changes in nitrogen-specific V,,,,, observed in situ may be due to downstream sinking of detrital nitrogen, a third hypothesis was evaluated, in which there was no shift-up in V,,,,,. This last scenario is untenable, predicting time scales of nutrient utilization two to three times longer than observed in the field. New primary production in upwelling systems results from the introduction of cold, nutrient-rich water into the illuminated surface layer (Dugdale and Goering 1967). Phytoplankton cells brought to the surface during upwelling are in a relatively inactive state, and changes occur in response to light and nutrient availability as the phytoplankton moves offshore with the upwelled water. Biomass-specific nitrate uptake rates increase downstream (MacIsaac et al. 1974; Dugdale 1976; Wilkerson and Dugdale 1987), and increases in rates of carbon-related processes appear to lag changes in nitrate uptake (MacIsaac et al. 198 5). Under certain conditions, upwelling systems can become highly productive, forming dense blooms (Barber and Smith 198 1). However, not all upwelling systems or events are as efficient at converting dissolved inorganic nutrients into biomass, as productivity ultimately depends on the total ’ This research was supported by NSF grants OCE 8215222 and OCE 85-05400 to the Organization of Persistent Upwelling Structures (OPUS) Program. 2 Contribution 419 from the Allan Hancock Foundation, University of Southern California. 3 Present address: Hopkins Marine Station, Pacific Grove, CA 93950. amount of nutrients added and the mixing depth of the system during each event (Codispoti et al. 1982). The transition from slow to fast growth in response to improved environmental conditions involves a complex series of biochemical steps, first described for Escherichia coli as shift-up (Schaechter 1968). The term shift-up has been extended to refer to increases in the biomass-specific nutrient uptake capacity of the phytoplankton as it responds to changes in the physical environmcnt associated with downstream transport in the upwelling plume (Dugdale 1976; MacIsaac et al. 1974, 1985). Completion of this transition in E. coli requires 2-3 doublings and a time scale on the order of 4 h. Slower doubling times characteristic of phytoplankton suggest that analogous processes occur at commensurately longer time scales. Shift-down can be induced by nutrient depletion, and the time required to return to a fully shifted-up state is related to the duration of nutrient depletion for short-term experiments (120 h or less: Collos 1980). However, studies of the effect of prolonged nutrient depletion and subsequent nutrient pulses of varying size on rates of shift-up have not been performed. The existence in newly upwelled water of