Frances P. Wilkerson

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.

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.

Nutrient Limitation of New Production in the Sea

Light and nutrients are the two well-known basic requirements for primary production in the sea and are usually supplied in opposing vertical gradients. When radiant energy of correct wavelengths is available, the vertical advection of nutrients from below the euphotic zone sets the maximum rate of absorption of these new nutrients and the ensuing primary production is considered to be new production (Dugdale and Goering, 1967) in contrast to the regenerated production based upon recirculating nutrients. When Sverdrup (1955) presented a map of world ocean primary production based upon his understanding of the vertical advective regimes of nutrients in various regions, he was actually providing the first global map of new production. Dinitrogen fixation also fuels new production, but this source of new nutrient (along with atmospheric and terrestrial inputs of nitrate and ammonium) is relatively minor compared to the advection of nitrate (e.g., Carpenter, 1983). The fractionation of nitrogen species from new into regenerated forms, through grazing and bacterial activity, and the existence of a practical tracer for nitrogen, the stable isotope 15N (e.g., Dugdale and Wilkerson, 1986), has made it possible to investigate the new and regenerated pathways in the marine ecosystem. The flow of nitrogen through the euphotic zone ecosystem has proved more complex than originally suggested (Dugdale and Goering, 1967) and is more realistically described by several possible schemes, including that of Michaels and Silver (1988), where the size fractions of the phytoplankton and bacteria are included explicitly along with other elements of the microbial loop. Their analysis showed that the microautotrophs provided the major source of new nitrogen for sinking particle formation, with the picoplankton participating primarily within the microbial grazing loop. However, both nanoplankton and picoplankton are capable of new production. For example, unicellular cyanobacteria (Glibert and Ray, 1990) and bacteria (Brown et al., 1975) may use nitrate and were shown to fix dinitrogen (e.g., Mitsui et al., 1986; Martinez et al., 1983).

Silicate versus nitrate limitation in the equatorial Pacific estimated from satellite-derived sea-surface temperatures

Abstract Productivity in the equatorial Pacific may be limited by a variety of factors including the availability of either nitrate (NO 3 ) or silicate (Si(OH) 4 ). The large size of this oceanic region makes it difficult to obtain a synoptic view of the nutrient field in traditional ways to assess NO 3 or Si(OH) 4 limitation. However, since concentrations of both nutrients are correlated with temperature, we have been able to employ remotely-sensed sea-surface temperatures (SST) from AVHRR to estimate the variability of surface NO 3 and Si(OH) 4 concentrations, and the ratio of one nutrient to the other which can be used to evaluate nutrient limitation in a Liebig sense. Using zonal transects along the equator, nutrient concentrations obtained from AVHRR-SST data from 1987 and 1988 show Si(OH) 4 values to be typically less than NO 3 and NO 3 :Si(OH) 4 > 1, indicating Si(OH) 4 to be the limiting nutrient and the equatorial Pacific to be a HNLSLC (High Nitrate, Low Silicate, Low Chlorophyll) area. However, in warmer climatic conditions, i.e. during 1987—an El Nino year and in the western part of the equatorial swath, NO 3 falls to undetectable levels and is most likely the more limiting macronutrient. Data from 1988 for 15°N to 15°S, 180°W to 90°W shows maximal concentrations of both nutrients to occur to the south of the equator with the ratio values indicating Si(OH) 4 limitation to the south and NO 3 limitation to the north of the equator.

Chapter 17 – Coastal Upwelling

Publisher Summary This chapter focuses on the nitrogen (N) in coastal upwelling. Views on how and which phytoplanktons in coastal upwelling areas respond to the high supply rates of nitrogen (mostly in the form of NO3) have progressed. Information included in this update include descriptions of cell size-fractionated nitrogen uptake, dissolved inorganic nitrogen (DIN) uptake kinetics, the interaction of other upwelled nutrients with DIN uptake, and how fluxes of nitrogen in upwelling systems can be modeled with this new information. However, many of the new data obtained since the original chapter, collected in coastal upwelling areas that were previously unstudied, confirm the earlier observations and serve to illustrate the common features of coastal upwelling systems. Since 1983, there have also been many technological advances that have been applied to the study of nitrogen flux and discussion reviews some preliminary studies pertinent to the knowledge of nitrogen in upwelling areas made using remote sensing and molecular genetics approaches. First the supply of nitrogen in coastal upwelling areas is reviewed and compared among different sites. Then the available data on the uptake of nitrogen and new production rates in different upwelling areas are summarized. The major phytoplankton groups contributing to these levels of new production are also discussed, followed by view of how these phytoplankton (typically diatoms) are physiologically adapted to injections of NO3 that accompany upwelling favorable winds. Some simulation models of new production in these ecosystems, along with some preliminary molecular studies appropriate for studying nitrogen flux in upwelling areas are outlined, concluding with a look to future studies and approaches.

Remote sensing of seasonal and annual variation of equatorial new production: A model for global estimates

Abstract Sea-surface temperatures (SSTs) are strongly correlated with surface nitrate concentrations in coastal upwelling regions. Upwelling also occurs in the equatorial Pacific; correlations between temperature and nitrate concentration are strong. The University of Miami weekly averaged Advanced Very High Resolution Radiometer (AVHRR) SST data for October 1986 through June 1989 have been used to compute surface nitrate concentrations from 90° – 180°W and from 15°S - 15°N. The surface areas with nitrate above detection limits are combined with existing nitrate uptake data to give weekly estimates of equatorial new production.