The use of large shipboard barrels and drifters to study the effects of coastal upwelling on phytoplankton dynamicsly

During the OPUS (Organization of Persistent Upwelling Structures) study of the upwelling center at Point Conception, California, large (380 liter) plastic shipboard enclosures (barrels) were tested as experimental tools to study phytoplankton processes in isolation from variability in circulation and grazing. Barrels were filled with water from the upwelling center during different stages of the upwelling cycle. Upwelling was simulated in one barrel series and the phytoplankton response to additional nutrients and shading was examined. Measurements of 15N uptake (VNO,-) and nitrate reductase activity (VNR) made in the barrels are reported in the context of a “conveyor-belt” hypothesis in which upwelled algae go through a “shift-up” in metabolism followed by a “shiftdown.” The results showed the same trends as those obtained with Lagrangian sampling along drifter tracks initiated at the same time and place as the barrels. Our hypothesis that the initial nutrient concentration has an effect on the rate of shift-up (defined as acceleration of VNO,or VNR) was substantiated by a positive correlation between acceleration of the rates, V/NO,and VNR, and nitrate concentration. At the upwelling center at 15’S, Peru, a similar observation was based both on VNO,acceleration from drogue and holdover experiments and on silicate concentration. In coastal upwelling areas subsurface waters are advected into the euphotic zone. The centers of such areas are three-dimensional point sources of nutrient-rich water that result in high productivity (Barber and Smith 198 1). A sequence of physiological changes by phytoplankton is thought to occur along the axis of the upwelling plume, the details of which are described elsewhere (Dugdale and Wilkerson 1985; MacIsaac et al. 198 5) and summarized as the “conveyor-belt” hypothesis in Fig. 1. Jones et al. ( 1983) characterized a series of idealized zones within the system. Newly upwelled phytoplankton near the upwelling center are “shifted-down” (Schaechter 1968; MacIsaac et al. 1974), growing and taking up nutrients slowly. As the algae adapt to the high light downstream, nitrogen rate processes initially “shift-up,” followed by carbon processes. Macromolecules are synthesized, leading to increased growth rate and biomass. Nutrient concentrations are rapidly reduced, and the cells respond by I This study was supported by NSF grants OCE 8215221 and OCE 85-05400 (OPUS program) and OCE 77-27006 (CUEA studies). 2 Contribution 4 18 of the Allan Hancock Foundation. undergoing rate decreases or shift-down. Throughout this cycle, algae will sink from the euphotic zone (Bienfang 198 1) and may be re-entrained. Observations of physiological responses of algae to upwelling were described by Barlow (1982a,b,c, 1984) working in the Benguela Current upwelling system; he investigated the relative proportions of algal-fixed 14C in small molecules and macromolecules in upwelled water of different ages. Both in water sampled followed upwelling and in recently upwelled water (type 1: Andrews and Hutchings 1980), most 14C was incorporated into low molecular weight precursors, whereas in more mature upwelled water (type 2: Andrews and Hutchings 1980) most of the label was found in protein (shift-up occurring), and in aged upwelled water (type 3) 14C occurred mainly in polysaccharides during bloom formation (balanced growth stage). The underlying mechanisms by which phytoplankton adapt to upwelling have not been studied in detail experimentally, but physiological data from free-living and cultured phytoplankton support many of the steps described here. For example, Syrett (198 1) listed a group of light-influenced biochemical systems involved with nitrate and