Richard C. Dugdale

Silicate regulation of new production in the equatorial Pacific upwelling

Surface waters of the eastern equatorial Pacific Ocean present the enigma of apparently high plant-nutrient concentrations but low phytoplankton biomass and productivity. One explanation for this ‘high-nitrate, low-chlorophyll’ (HNLC) phenomenon has been that growth is limited by iron availability,. Here we use field data and a simple silicon-cycle model to investigate the HNLC condition for the upwelling zone of this ocean region. Measured silicate concentrations in surface waters are low and largely invariant with time, and set the upper limit on the total possible biological utilization of dissolved inorganic carbon. Chemical and biological data from surface waters indicate that diatoms—silica-shelled phytoplankton—carry out all the ‘new production’ (nitrate uptake). Smaller phytoplankton (picoplankton) accomplish most of the total primary production, largely fuelled by nitrogen regenerated in reduced forms as a result of grazing by zooplankton. The model predicts values of new and export production (the production exported to below the euphotic zone) that compare well with measured values. New and export production are in balance for biogenic silica, whereas new production exceeds export for nitrogen. The HNLC condition in the upwelling zone can therefore be understood to be due to a chemostat-like regulation of nitrate uptake by upwelled silicate supply to diatoms: ‘low-silicate HNLC’. These results are not inconsistent with observations of iron-fertilized diatom growth during in situ experiments in ‘low-iron HNLC’ waters outside this upwelling zone,, but reflect the role of different supply rates of iron and silicate in determining the nature of the HNLC condition.

The role of a silicate pump in driving new production

Abstract In the past, the importance of silicate as a limiting nutrient for new production in the ocean, and in determining global productivity and carbon budgets, has been relegated to the lower ranks compared to the role of nitrogen and, more recently, iron. This paper describes a “silicate pump” that acts in diatom-dominated communities to enhance the loss of silicate from the euphotic zone to deep water compared to nitrogen, which is more readily recycled in the grazing loop, thus leading the system to silicate limitation. The impact of this silicate pump is described for the HNLC (High Nutrient-Low Chlorophyll) waters offshore from 15°S, Peru and reproduced in a simulation model of a diatom-dominated ecosystem. Silicate pumping to deep water results in low silicate, high nitrate conditions in the mixed layer, shown here to be a characteristic of many HNLC areas. These areas should more accurately be termed HNLSLC (High Nitrate-Low Silicate-Low Chlorophyll) areas. Silicate dynamics may control and dominate new production processes in these areas and consequently control the rate at which newly upwelled COZ in the surface regions is reduced by the phytoplankton. In such silicate-controlled systems, export production (i.e. production that is lost to deep water) of silicon and nitrogen are not equivalent, since export production of silicon is controlled by input of silicate, whereas export production of nitrogen is controlled by grazing rate and regeneration.

The kinetics of nitrate and ammonia uptake by natural populations of marine phytoplankton

Abstract The response of natural marine populations of phytoplankton to nitrate and ammonia concentrations has been investigated using nitrogen-15 tracer techniques. Experiments made in the Bering Sea, the waters of southeastern Alaska, and the northeastern tropical Pacific suggest that the uptake of both compounds follows the Michaelis-Menten expression for enzyme kinetics. A hyperbola, therefore, describes the relationship between the concentration of nitrate or ammonia and the uptake of that nutrient. Such a hyperbola can be obtained easily in experiments with ammonia and, under suitable conditions, with nitrate. The value of Kt, the nutrient concentration at which the maximum uptake rate is reduced by one-half, appears to be related to the nutrient and productivity regime of the region inhabited by the population. In the tropical oligotrophic area investigated, Kt(NO3−) ⩽ 0.2 μg-atom/l., while in corresponding eutrophic regions, Kt(NO3−) ⩾ 1.0 μg-atom/l. The values suggest that the phytoplankton populations of oligotrophic regions are adapted to the low ambient nutrient concentrations and are able to take up nutrients at a higher rate under these conditions than would phytoplankton species characteristics of eutrophic conditions and showing a higher value of Kt.

Interactions of light and inorganic nitrogen in controlling nitrogen uptake in the sea

The uptake of ammonium and nitrate in natural populations of marine phytoplankton is light dependent and often can be described adequately by the Michaelis-Menten equation. The half- saturation constant for light intensity in nitrate and ammonium uptake is found to be from 1 to 14 ~o of surface light intensity, an intensity range occurring near the bottom of the euphotic zone. Since the uptake of these compounds as a function of substrate concentration is also described by the same equation, many vertical profiles of nitrate and ammonium uptake obtained with the 1 ~N technique can be interpreted from the interaction of light and nutrient concentration at different depths. In oligotrophic waters, nutrient uptake is severely limited by nutrient concentrations; but in eutrophic areas, light intensity very often is the limiting factor. In the former, maximal uptake may occur deep in the euphotic zone, while in the latter, the maximum occurs near the surface. Ammonium is taken up more readily than nitrate by populations in all areas, and in oligotrophic regions may account for 80 % of the inorganic nitrogen uptake. In eutrophic areas, most of the inorganic nitrogen uptake is based on nitrate. Ratios of carbon to nitrogen taken up by phytoplankton vary over a wide range (12-76), with some indication that the lower values are associated with regions having a certain degree of environmental stability--either very rich or very poor waters.

Primary production cycle in an upwelling center

Abstract The cycle of nitrogen and carbon productivity of phytoplankton in an upwelling center at 15°S on the coast of Peru was studied during the JOINT-II expedition of the Coastal Upwelling Ecosystems Analysis program. The productivity cycle was characterized by repeated stations at various locations in the upwelling plume, a time series of stations in mid plume, and stations located along drogue tracks. Four zones of physiological condition were distinguished along the axis of the upwelling plume. In Zone I phytoplankton upwelled with nutrient-rich water were initially ‘shifted-down’; in Zone II they underwent light induced ‘shift-up’ to increased nutrient uptake, photosynthesis, and synthesis of macromolecules. In Zone III ambient nutrient concentrations were rapidly reduced, there was a rapid accumulation of phytoplankton biomass in the water column, and rate processes proceeded at maximal rates. In Zone IV ambient nutrient concentrations were significantly decreased, phytoplankton biomass remained high, and limitation of phytoplankton processes was beginning to be observed. Phytoplankton responded to the altered environment by undergoing ‘shift-down’ to lower rates of nutrient uptake, photosynthesis, and macromolecule synthesis. The time and space domain where this entire sequence occurs was relatively small; the cycle from initial upwelling to ‘shift-down’ was completed in 8 to 10 days within 30 to 60 km off the coast.

Estimating new production in the equatorial Pacific Ocean at 150°W

A major goal of the WEC88 cruise of the R/V Wecoma to the equatorial Pacific (made in February-March 1988), was to establish rates of new production along a meridional section at 150°W and to compare these measured rates with the relatively high values for the equatorial Pacific that had been reported previously using indirect methods and models. New production values were obtained from the traditional approach (in the sense of Dugdale and Goering (1967)) using 15N labelled nitrate uptake, and by using 14C fixation values multiplied by ƒ (proportion of new production) from various sources: from 15N data, from a 14C fixation versus ƒ relationship (Eppley and Peterson, 1979) or from a nitrate versus ƒ relationship (Platt and Harrison, 1985). The ratios of directly measured nitrate and carbon uptake and the ratios of nitrate to nitrate plus ammonium uptake, i.e., values of ƒ, agree well; values of ƒ calculated from carbon uptake or from nitrate concentration are overestimates for the equatorial upwelling region. Carbon to nitrogen uptake ratios measured with 14C and 15N, respectively, approximate the Redfield molar ratio, 6.6 C:N. The overall mean value of ƒ (0.17) helps confirm the view that the low primary production in the enriched eastern equatorial Pacific is due to failure of the nitrate uptake system.

New production in the upwelling center at Point Conception, California: temporal and spatial patterns

As part of the Organization of Persistent Upwelling Structures (OPUS) 1983 study of the Point Conception upwelling system, a station (G-1) at the cold center between Points Conception and Arguello was occupied almost daily, a section measuring biological variables was made regularly along a line (the C-line) extending south from Point Arguello, and a series of drifter-following experiments were made on the R.V. Velero IV. Water upwelled at 5-1 follows either a cyclonic or anticyclonic flow pattern and in both cases usually crosses the C-line to enter the Santa Barbara channel. As seen in previous upwelling studies, the ability to take up nitrate increased with time and along with the circulation pattern, this “shift-up” determined the pattern of new production. Weak upwelling was associated with the cyclonic path, longer travel time to the C-line, and higher new production rate at the south end of the C-line. High wind stress and strong upwelling was associated with the anticyclonic mode, short travel time, and low production rates at the north end of the C-line. Comparisons with the 15°S, Peru upwelling center and with the Cap Blanc (northwest Africa) upwelling system showed similarities with both. The highest new production rates at Point Conception and Cap Blanc were associated with relaxation events and the sequence of biological processes during upwelling at Point Conception closely followed the Peru pattern. New production rates in 1983 at Point Conception fell at the low end of the spectrum of these upwelling centers.

Climate anomalies generate an exceptional dinoflagellate bloom in San Francisco Bay

] We describe a large dinoflagellate bloom,unprecedented in nearly three decades of observation, thatdeveloped in San Francisco Bay (SFB) during September2004. SFB is highly enriched in nutrients but has lowsummer-autumn algal biomass because wind stress andtidally induced bottom stress produce a well mixed andlight-limited pelagic habitat. The bloom coincided withcalm winds and record high air temperatures that stratifiedthe water column and suppressed mixing long enough formotile dinoflagellates to grow and accumulate in surfacewaters. This event-scale climate pattern, produced by anupper-atmosphere high-pressure anomaly off the U.S. westcoast, followed a summer of weak coastal upwelling andhigh dinoflagellate biomass in coastal waters that apparentlyseeded the SFB bloom. This event suggests that some redtides are responses to changes in local physical dynamicsthat are driven by large-scale atmospheric processes andoperate over both the event scale of biomass growth and theantecedent seasonal scale that shapes the bloom community.

Interdecadal Variation of the Transition Zone Chlorophyll Front, A Physical-Biological Model Simulation between 1960 and 1990

The interdecadal climate variability affects marine ecosystems in both the subtropical and subarctic gyres, consequently the position of the Transition Zone Chlorophyll Front (TZCF). A three-dimensional physical-biological model has been used to study interdecadal variation of the TZCF using a retrospective analysis of a 30-year (1960–1990) model simulation. The physical-biological model is forced with the monthly mean heat flux and surface wind stress from the COADS. The modeled winter mixed layer depth (MLD) shows the largest increase between 30°N and 40°N in the central North Pacific, with a value of 40–60% higher during 1979–90 relative to 1964–75 values. The winter Ekman pumping velocity difference between 1979–90 and 1964–75 shows the largest increase located between 30°N and 45°N in the central and eastern North Pacific. The modeled winter surface nitrate difference between 1979–90 and 1964–75 shows increase in the latitudinal band between 30°N and 45°N from the west to the east (135°E–135°W), the modeled nitrate concentration is about 10 to 50% higher during the period of 1979–90 relative to 1964–75 values depending upon locations. The increase in the winter surface nitrate concentration during 1979-90 is caused by a combination of the winter MLD increase and the winter Ekman pumping enhancement. The modeled nitrate concentration increase after 1976–77 enhances primary productivity in the central North Pacific. Enhanced primary productivity after the 1976–77 climatic shift contributes higher phytoplankton biomass and therefore elevates chlorophyll level in the central North Pacific. Increase in the modeled chlorophyll expand the chlorophyll transitional zone and push the TZCF equatorward.

Chemical oceanography and primary productivity in upwelling regions

Abstract Features of the circulation, uptake, and regeneration of particularly nitrogen, phosphorus, and silica in upwelling regions are discussed in the light of recent contributions to nutrient uptake and regeneration theory. Emphasis is placed on the critical role of silica in the productivity of coastal upwelling areas and the resulting usefulness of this element as an indication of the recent history of circulation and production processes in an upwelling area. The significant contribution of herbivore regeneration to the nutrient regime of an upwelling area is described. Data from the Peru coastal upwelling system are discussed and interpreted in view of the physical and biological processes considered in the paper.