Barbara B. Prézelin

The linkage between Upper Circumpolar Deep Water (UCDW) and phytoplankton assemblages on the west Antarctic Peninsula continental shelf

Intrusion of Upper Circumpolar Deep Water (UCDW), which was derived from the Antarctic Circumpolar Current (ACC), onto the western Antarctic Peninsula (WAP) shelf region in January 1993 provided a reservoir of nutrient-rich, warmer water below 150 m that subsequently upwelled into the upper water column. Four sites, at which topographically-induced upwelling of UCDW occurred, were identified in a 50 km by 400 km band along the outer WAP continental shelf. One additional site at which wind-driven upwelling occurred was also identified. Diatom-dominated phytoplankton assemblages were always associated with a topographically-induced upwelling site. Such phytoplankton communities were not detected at any other shelf location, although diatoms were present everywhere in the 80,000 km 2 study area and UCDW covered about one-third the area below 150 m. Phytoplankton communities dominated by taxa other than diatoms were restricted to transition waters between the UCDW and shelf waters, the southerly flowing waters out of the Gerlache Strait, and/or the summertime glacial ice melt surface waters very near shore. We suggest that in the absence of episodic intrusion and upwelling of UCDW, the growth requirements for elevated silicate/nitrate ratios and/or other upwelled constituents (e.g. trace metals) are not sufficiently met for diatoms to achieve high abundance or community dominance. One consequence of this is that the ice-free regions of the outer WAP continental shelf will not experience predictable spring diatom blooms. Rather, this region will experience episodic diatom blooms that occur at variable intervals and during different seasonal conditions, if the physical structuring events are occurring. Preferential drawdown of silicate relative to nitrate was observed at each of the offshore upwelling sites and resulted in a reduction in the ambient silicate:nitrate ratio relative to the corresponding value for unmodified UCDW (1.5 versus 3.0 for UCDW). The magnitude of the nutrient drawdown in areas of topographically-induced upwelling suggested that diatom growth had been elevated in response to recent upwelling but that the resulting increased algal biomass was either dispersed by advective processes and/or consumed by the larger krill that were observed to be associated with each offshore upwelling site. Thus, diatom bloom conditions on the outer WAP shelf may not be recognized based on elevated biomass and/or rates of carbon fixation. It was likely that similar physical forcing of significant phytoplankton growth, especially diatoms, may occur but be undetected in regions where the southern boundary of the ACC nears the Antarctic continental shelf edge. Our analyses from the west Antarctic Peninsula demonstrate coupling of the structure of the physical environment with nutrient distributions and phytoplankton assemblages and through to the higher trophic levels, such as Antarctic krill. This environment-trophic coupling may also occur in other regions of the Antarctic, as suggested by correspondences between the distribution of Southern ACC boundary and regions of high concentrations of Antarctic krill. The many mechanisms underlying this coupling remain to be determined, but it was clear that the ecology and biology of the components of the marine food web of the Antarctic continental shelf cannot be studied in isolation from one another or in isolation from the physical environment.

Mechanisms of photoadaptation in three strains of the symbiotic dinoflagellate Symbiodinium microadriaticum

Mechanisms of photoadaptation of photosynthesis have been studied in three strains of the symbiotic dinoflagellate Symbiodinium microadriaticum. Algal strains isolated from the clam Tridacna maxima, the sea anemone Aiptasia pulchella, and the scleractinian coral Montipora verrucosa were maintained in the defined medium ASP-8A, and were grown at irradiances ranging from 22 to 248 μE m-2 s-1 on a 14 h:10 h (light:dark) photoperiod at 26°C. All algal cultures were analysed during log-phase of growth. At all light levels, rates of cell division and photosynthesis were determined, as were cell volumes, pigmentation (including chlorophyll a, chlorophyll c2, peridinin, β-carotene and xanthophylls), and carbon and nitrogen content. Low light-induced changes in pigmentation were evident to varying degrees in all three algal strains, although alterations in the photosynthesis-irradiance relations were distinctly different in each strain. The algae from T. maxima show the least photoadaptive capability, and seem to photoadapt by changing photosynthetic unit (PSU) size. Algae from A. pulchella appear to adapt by changing PSU number, while algae from M. verrucosa appear to photoadapt by changes in the activities of CO2-fixing enzymes or electron transport systems. These are the first observations that demonstrate functional differences in different strains of S. microadriaticum. The adaptive capabilities of the algae appear to correlate well with the ecological distribution of their respective hosts. The study was made from July 1981 through December 1982.

A nitrate-dependent Synechococcus bloom in surface Sargasso Sea water

Considerable debate exists concerning the magnitude of oceanic primary production, its rate of transfer to other trophic levels and turnover times of carbon and nitrogen1–5. In nitrogen-limited ocean systems, episodic increases in nitrate concentrations can support a significant fraction of annual phytoplankton production5. Yet little information is available regarding the distribution of nitrate in seasonally stratified oceanic surface waters, because concentrations are below the 0.03 μM detection limit of colorimetric methods6. We present the first evidence that high surface productivity in stratified Sargasso Sea water was supported by nanomolar changes in nitrate concentrations. This change was stoichiometrically consistent with the subsequent cellular production of a cyanobacterial (Synechococcus) bloom. Initially, cellular phycoerythrin and chlorophyll pigments increased, after which growth was enhanced to near maximum rates, and grazing was closely coupled to production. These observations suggest that Synechococcus occupies an important trophic position in the transfer of new nitrogen into the oceanic food web.

Diel periodicity in phytoplankton productivity

Daily variation in phytoplankton productivity influences the dynamics and linkages between several large scale processes in aquatic ecosystems. As part of an opening address to the 5th International workshop for the Group for Aquatic Productivity (GAP), the daily patterns of variability in photosynthesis for different algal classes was introduced and accompanied by a discussion of the sources of environmental and endogenous regulation of repeating biological oscillations that occur in phytoplankton on timescales of one day. It is suggested that one way to develop a database that serves to sort and predict phytoplankton variability over the day may be to encourage the creation of a ‘temporal library’. Such a library would be comprised of temporally fixed maps of circadian clock-controlled rhythms for individual species, as well as temporally variable maps of diel periodicities that only can be defined for a selected set of environmental conditions.

Physical Forcing of Phytoplankton Community Structure and Primary Production in Continental Shelf Waters of the Western Antarctic Peninsula

Analyses of a multidisciplinary data set, collected in continental shelf waters of the Western Antarctic Peninsula (WAP) during austral summer of January 1993, identified a previously unrecognized forcing mechanism that sets up a physical and chemical structure that supports and assures site-specific diatom-dominated communities and enhanced biological production (Prézelin et al., 2000). This forcing is active when the southern boundary of the Antarctic Circumpolar Current (ACC) flows along the shelf edge, thereby facilitating onshelf bottom intrusions of nutrient-rich Upper Circumpolar Deep Water (UCDW), which then is upwelled or mixed into the upper water column. At times or locations where UCDW is not introduced to the upper water column, diatoms no longer dominate phytoplankton assemblages over the midto outer WAP continental shelf. This analysis extends the area and seasons studied through similar analyses of multidisciplinary data sets collected on four additional cruises to the WAP that cover all seasons. Results show that onshelf intrusions of UCDW: (1) occur in other regions of the WAP continental shelf; (2) are episodic; (3) are forced by nonseasonal physical processes; and (4) produce areas of diatom-dominated phytoplankton assemblages and enhanced primary production. At times, multiple intrusions are observed on the WAP continental shelf, and each event may be in a different stage. Further, the occurrence of an intrusion event in one area does not necessarily imply that similar events are ongoing in other areas along the WAP shelf. The UCDW bottom intrusions originate along the outer shelf but they can extend into the inner shelf region because the deep troughs that transect the WAP shelf provide connections between the inner and outer shelf. The boundary between the intruded water and the shelf water is variable in location because of the episodic nature of the onshelf intrusions, and is moved farther inshore as an event occurs. These observations show clearly that the phytoplankton community structure on the WAP shelf is determined by physical forcing and that primary production is likely to be considerably greater than previously believed. Moreover, variability in this physical forcing, such as may occur via climate change, can potentially affect the overall biological production of the WAP continental shelf system. 1. Marine Science Institute and Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California, 93106, U.S.A. email: prezelin@lifesci.ucsb.edu 2. Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, Virginia 23529, U.S.A. 3. Biological Sciences Department, California Polytechnic State University, San Luis Obispo, California, 93407, U.S.A. Journal of Marine Research, 62, 419–460, 2004

Bio-Optical Models and the Problems of Scaling

Historically, the construction of global maps of ocean productivity has been a difficult task (Berger, 1989). Representative, precise, and accurate measurements of carbon fixation rates have been hampered by the errors associated with methodological problems and sampling limitations (Jahnke, 1990). The frustration of biological oceanographers in dealing with this issue was best summarized by Eppley (1980) during the first ‘Primary Productivity in the Sea’ symposium over a decade ago: “These disparate results beg for reconciliation as they suggest an order of magnitude uncertainty in the rate of primary production in the central oceans. Is it of the order 50–150 mg C m−2 d−1 as the standard 14C data have suggested for twenty years or is it 1–2 g C m−2 d−1 as the diel oxygen and POC changes and the PIT collections imply?” While this issue has yet to be completely resolved, considerable progress has been made during the last decade towards the reconciliation of differences in primary productivity estimates based on the standard 14C-labeling technique (Steeman Nielsen, 1952) and those based on geochemical tracer distributions (Jenkins and Goldman, 1985; Williams and Robertson, 1991). It appears that systematic errors in the different methodologies used to determine rates of oxygen production and carbon fixation contribute to observations of high photosynthetic quotients (mol O2 evolved per CO2 fixed) for phytoplankton communities (Laws, 1991; Prezelin and Glover, 1991; Williams and Robertson, 1991). In addition, when care is taken to minimize trace metal contamination (Fitzwater et al., 1982), avoid the toxic effects of latex and neoprene rubber closure mechanisms of Niskin® bottles (Price et al., 1986; Williams and Robertson, 1989), and incubate seawater samples under the appropriate spectral distribution of light (Laws et al., 1990), then 14C-measured rates of primary productivity for the central Pacific Ocean are several fold higher than historical values (i.e., 428 ± 249 mg C m−2 d−1, n = 11, 24 hour simulated in situ incubations, Station ALOHA, October 1988 to November 1989, HOT program, 1990).

Contribution ofSynechococcus spp. to size-fractioned primary productivity in three water masses in the Northwest Atlantic Ocean

The distribution of phycoerythrin-richSynechococcus spp. relative to eukaryotic algae and the contribution ofSynechococcus spp. toin situ primary production were compared at a neritic front, in warm-core eddy 84-E, and at Wilkinsons Basin, during a cruise to the Northwest Atlantic Ocean in July/August 1984. Immunofluorescence analyses ofSynechococcus strains demonstrated the restricted distribution of the tropical oceanic serogroup to the warm-core eddy, while strains of the neritic serogroup and those labelled by antiserum directed against a motile strain, were abundant in all three water masses. Although the majority ofSynechococcus spp. cells were observed in the 0.6 to 1 μm fraction, an increasing proportion of the totalSynechococcus spp. cells were found in the 1 to 5 μm fraction as nitrate concentrations increased near the base of the thermocline. From immunofluorescence analyses, we determined that the increasing proportion of largerSynechococcus spp. cells at depth was not the result of a change in strain composition, and may therefore be associated with increasing cell volume due to the enhanced nutrient supply. The contribution of the different size fractions to the total standing crop of chlorophyll and thein situ rate of photosynthesis was distincty different for the three water masses. At the neritic front, the larger photoautotrophs of the 1 to 5 μm and >5 μm fractions were the major contributors to chlorophyll concentrations and primary production.Synechococcus spp. appeared to provide only 6% of the dawn-to-duskin situ primary production at the neritic front. In modified Sargasso water in the warm-core eddy,Synechococcus spp. contributed 25% to thein situ rate of integrated primary production. In this warm-core eddy, the 0.2 to 0.6 μm fraction made a major contribution to the standing crop of chlorophyll and primary production that equalled or exceeded that of the larger sze categories. Furthermore, at the bottom of the euphotic layer, eukaryotes numerically dominated the 0.2 to 0.6 μm fraction, which contributed 61% of the primary productivity. At Wilkinsons Basin, theSynechococcus spp.-dominated 0.6 to 1.0 μm fraction made the greatest contribution to the standing crop of chlorophyll an primary production, while smaller photoautotrophs (0.2 to 0.6 μm) accounted for little of the chlorophyll or photosynthetic rates measured over the euphotic layer. Largest numbers ofSynechococcus spp. (2.9x108 cells l-1) occurred at the 18% isolume, coincident with a shoulder in the chlorophyll fluorescence profile and the site of maximumin situ primary productivity. At Wilkinsons Basin,Synechococcus spp. contributed 46% to thein situ photosynthesis integrated over the water-column.

Diel oscillations of the photosynthesis-irradiance (P-I) relationship in natural assemblages of phytoplankton

Diel oscillations in the photosynthesis-irradiance (P-I) relationship are described for marine phytoplankton assemblages at 6 stations in an upwelling area off the southern California coast (USA) between May and August 1980. The initial slope (α) and asymptote (Pmax) of P-I curves changed significantly over the day; both parameters were in phase and had similar changes in amplitude. The diel oscillations in photosynthesis appeared unrelated to changes in chlorophyll a concentrations. Amplitudes of daily variations in photosynthesis ranged from approximately 3 to 9, as measured by the maximum to minimum ratio for photosynthetic capacity (Pmax). Diatom-rich samples collected during an upwelling event and those dominated by dinoflagellates both had midday to early afternoon maxima in α and Pmax. Samples from other locations had peak photosynthetic activity later in the afternoon. The relationship between α and Pmax was consistent in all phytoplankton samples analyzed, with a surprisingly high correlation considering the spatial and temporal scales encompassed in this study. These results indicate that the photosynthesis-irradiance (P-I) relationship is time-dependent and, moreover, that changes in α and Pmax are closely coupled for a variety of natural phytoplankton assemblages.

Impact of ultraviolet-B radiation on photosystem II activity and its relationship to the inhibition of carbon fixation rates for Antarctic ice algae communities

One goal of the Icecolors 1993 study was to determine whether or not photosystem II (PSII) was a major target site for photoinhibition by ultraviolet‐B radiation (QUVB, 280–320 nm) in natural communities. Second, the degree to which QUVBinhibition of PSII could account for QUVBeffects on whole cell rates of carbon fixation in phytoplankton was assessed. On 1 October, 1993, at Palmer Station (Antarctica), dense samples of a frazil ice algal community were collected and maintained outdoors in the presence or a bsence of QUVBand l or ultraviolet‐A (QUVB, 320–400 nm) radiation. Samples were then collected at intervals over the day to track the time course of UV inhibition of primary production. The ice algae were assessed for changes in pigment composition and rates of carbon fixation. The quantum yield of PSII (ØIIc°) was measured by Pulse Amplitude Modulated fluorometry. Over the day, ØIIc° declined due to increasing time‐integrated dose exposure of QUVB. The QUVB‐driven inhibition of ØIIc° increased from 4% in the early morning hours to a maximum of 23% at the end of the day. The QUVB photoinhibition of PSII quantum yield did not recover by 6 h after sunset. In contrast, photoinhibition by QUVA and photosynthetically available radiation (QPAR, 400–700 nm) recovered during the late afternoon. Flourescence‐based estimates of carbon fixation rates were linearly correlated (P =0.002, r2=0.45) with measured carbon fixation. Fluorescence overestimated the observed QUVB inhibition in measured carbon fixation rates by 8% in the morning hours; however, the discrepancy increased during the afternoon. Therefore, researchers should be cautious in using fluorescence measurements to infer ultraviolet inhibition for rates of carbon fixation until there is a greater understanding of the coupling of carbon metabolism to PSII activity for natural populations. Despite these current limitations, fluorescence‐based technologies represent powerful tools for studying the impact of the ozone hole on natural populations on spatial/ temporal scales not possible using conventional productivity techniques.

The control of the production process of phytoplankton by the physical structure of the aquatic environment with special reference to its optical properties

This tutorial was designed for nonbiologists requiring an introduction to the nature and general timescales of phytoplankton responses to physical forcing in aquatic environments. As such, an effort was made to highlight biological markers which might assist in identifying, measuring and/or validating physical processes controlling the variability in the distribution, abundance, composition and activity of phytoplankton communities. Given the recent advances in environmental optics and remote sensing capabilities, a special emphasis was placed on the nature and utility of phytoplankton optical properties in current bio-optical modelling efforts to predict temporal and spatial variability in phytoplankton productivity and growth.