A phytoplankton absorption-based primary productivity model for remote sensing in the Southern Ocean

Recent global environmental changes such as an increase in sea surface temperature (SST) are likely to impact primary productivity of phytoplankton in the Southern Ocean. However, models to estimate net primary production using satellite data use SST and uncertain estimation of chlorophyll a (chl-a) concentration. A primary productivity model for satellite ocean color data from the Southern Ocean, which is based on the light absorption coefficient of phytoplankton to reduce uncertainties of sea surface chl-a estimations and bias in optimal values of chl-a normalized productivity derived from SST, has been developed. The new model was able to estimate net primary productivity in the water column (PP _eu) without dependency on temperature when in the range of −2 to 25°C, and it explained 51% of the observed variability in PP _eu with a root mean square error (RMSE) of 0.15. Application of the model revealed that the SST dependent model has overestimated PP _eu in warmer waters around the Subtropical Front, and underestimated PP _eu in colder waters poleward of the Sub-Antarctic Front. This absorption-based primary productivity model contributes to a study of the relationship among spatio-temporal variations in the physical environment, and biogeochemical cycles in the Southern Ocean.     

History and evolution of primary productivity studies of the Southern Ocean

The main objective of the article is to give an overview of the history and evolution of phytoplankton research in the Southern Ocean during the past century and a half. It traces the evolution of phytoplankton investigations as it went through several phases commencing with intensive collecting and cataloging and leading to the Discovery investigations with its extensive and detailed studies of Southern Ocean phytoplankton. Following, in the footsteps of the Discovery was the USNS Eltanin with its specialized cruises that centered around the study of the ecology of the primary producers and the dynamics of the lower trophic levels of the food chain. Spurred by the findings of the Eltanin cruises and with the growing concern over the impending exploitation of the Antarctic marine living resources, in particular krill, the BIOMASS program was initiated. The program was the first international collaborative effort to study the dynamic functioning of the Antarctic marine ecosystem. The success of the program has rekindled great international interest in the Southern Ocean which resulted in an explosion of programs such as SO-JGOFS, SO-GLOBEC, EPOS, and several others. In recent years, there has been a major shift in phytoplankton research in the Southern Ocean. This was in response to worldwide concern over the effects of global warming and stratospheric ozone depletion on species diversity, primary production and ecosystem function. This has led to process-oriented programs to study phenomena and processes of global significance in which Antarctica and its surrounding seas play key roles.In an effort to avoid any confusion in the strict usage of terminology, the author has opted to use the Antarctic Ocean and the Southern Ocean interchangeably in this review.     

Dependence of phytoplankton assimilation quotients on light and nitrogen source: implications for oceanic primary productivity

Photosynthetic production of oxygen by phytoplankton assemblagedominated by Peridinium in Lake Kinneret, Israel, generallyexceeds the molar equivalent rate of carbon assimilation. Carbonassimilation occurs only if oxygenic photosynthesis exceedsa light-dependent threshold. Assimilation quotients (mol C molO₂^–1) are a variable function of irradiance, and typicallyonly about one-half of the photoreductant produced during oxygenicphotosynthesis is used for reduction of carbon dioxide. Mostof the residual oxygenic photoreductant probably is used forlight-dependent reduction of nitrate, which competes with carbondioxide for oxygenic photoreductant. Nitrate is an importantsource of nitrogen for this algal assemblage, and light-dependentnitrate reduction probably is much larger than carbon dioxidereduction at lowest irradiances in the euphotic zone. Oxygenproduction also may be much larger than carbon assimilationat low light levels in other environments where oxidized formsof nitrogen are important nitrogenous nutrients for phytoplankton,as in the lower euphotic zone of the sea, where low rates ofcarbon assimilation by phytoplankton have been thought to beinconsistent with the amount of oxygen that accumulates duringsummer.     

Size-fractionated phytoplankton carboxylase activities in the Indian sector of the Southern Ocean

During the ANTARES 3 cruise in the Indian sector of the SouthernOcean in October–November 1995, the surface waters ofKerguelen Islands plume, and the surface and deeper waters (30–60m) along a transect on 62°E from 48°36'S to the iceedge (58°50'S), were sampled. The phytoplankton communitywas size-fractionated (2 µm) and cell numbers, chlorophyllbiomass and carbon assimilation, through Rubisco and ß-carboxylaseactivities, were characterized. The highest contribution of<2 µm cells to total biomass and total Rubisco activitywas reported in the waters of the Permanent Open Ocean Zone(POOZ) located between 52°S and 55°S along 62°E.In this zone, the picophytoplankton contributed from 26 to 50%of the total chlorophyll (a + b + c) with an average of 0.09± 0.02 µg Chl l^–1 for <2 µm cells.Picophytoplankton also contributed 36 to 64% of the total Rubiscoactivity, with an average of 0.80 ± 0.30 mg C mg Chla^–1 h^–1 for <2 µm cells. The picophytoplanktoncells had a higher ß-carboxylase activity than largercells >2 µm. The mixotrophic capacity of these smallcells is proposed. From sampling stations of the Kerguelen plume,a relationship was observed between the Rubisco activity perpicophytoplankton cell and apparent cell size, which variedwith the sampled water masses. Moreover, a depth-dependent photoperiodicityof Rubisco activity per cell for <2 µm phytoplanktonwas observed during the day/night cycle in the POOZ. In thenear ice zone, a physiological change in picophytoplankton cellsfavouring phosphoenolpyruvate carboxykinase (PEPCK) activitywas reported. A species succession, or an adaptation to unfavourableenvironmental conditions such as low temperature and/or availableirradiance levels, may have provoked this change. The high contributionof picophytoplankton to the total biomass, and its high CO₂fixation capacity via autotrophy and mixotrophy, emphasize thestrong regeneration of organic materials in the euphotic layerin the Southern Ocean.     

Spring phytoplankton assemblages in the Southern Ocean between Australia and Antarctica

Variations of phytoplankton assemblages were studied in November–December 2001, in surface waters of the Southern Ocean along a transect between the Sub-Antarctic Zone (SAZ) and the Seasonal Ice Zone (SIZ; 46.9°–64.9°S; 142°–143°E; CLIVAR-SR3 cruise). Two regions had characteristic but different phytoplankton assemblages. Nanoflagellates(<20 μm) and pico-plankton (∼2 μm) occurred in similar concentrations along the transect, but were dominant in the SAZ, Sub-Antarctic Front (SAF), Polar Front Zone (PFZ) and the Inter-Polar Front Zone (IPFZ), (46.9°–56.9°S). Along the entire transect their average cell numbers in the upper 70 m of water column, varied from 3 × 10⁵ to 1.1 × 10⁶ cells l⁻¹. Larger cells (>20 μm), diatoms and dinoflagellates, were more abundant in the Antarctic Zone-South (AZ-S) and the SIZ, (60.9°–64.9°S). In AZ-S and SIZ diatoms ranged between 2.7 × 10⁵ and 1.2 × 10⁶ cells l⁻¹, dinoflagellates from 3.1 × 10⁴ to 1.02 × 10⁵ cells l⁻¹. A diatom bloom was in progress in the AZ-S showing a peak of 1.8 × 10⁶ cells l⁻¹. Diatoms were dominated by Pseudo-nitzschia spp., Fragilariopsis spp., and Chaetoceros spp. Pseudo-nitzschia spp. outnumbered other diatoms in the AZ-S. Fragilaropsis spp. were most numerous in the SIZ. Dinoflagellates contained autotrophs (e.g. Prorocentrum) and heterotrophs (Gyrodinium/Gymnodinium, Protoperidinium). Diatoms and dinoflagellates contributed most to the cellular carbon: 11–25 and 17–124 μg C l⁻¹, respectively. Small cells dominated in the northern region characterized by the lowest N-uptake and new production of the transect. Larger diatom cells were prevalent in the southern area with higher values of N-uptake and new production. Diatom and nanoflagellate cellular carbon contents were highly correlated with one another, with primary production, and productivity related parameters. They contributed up to 75% to the total autotrophic C biomass. Diatom carbon content was significantly correlated to nitrate uptake and particle export, but not to ammonium uptake, while flagellate carbon was well correlated to ammonium uptake, but not to export. Diatoms have contributed highly to particle export along the latitudinal transect, while flagellates played a minor role in the export.     

Underwater light field and phytoplankton absorbance in different surface water masses of the Atlantic sector of the Southern Ocean

Spectral water transparency and phytoplankton light absorbance were studied in the Atlantic sector of the Southern Ocean during the Southern Ocean JGOFS ANT XIII/2 cruise in early austral summer 1995/1996. The study area comprised three zones, which differed markedly with respect to their hydrographic and planktological characteristics: the Antarctic Polar Frontal Zone with adiatom bloom, the Antarctic Circumpolar Current outside frontal systems with phytoplankton-poor water and a higher flagellate abundance than in the other two areas, and the marginal ice zone with a Phaeocystis bloom. The influence of phytoplankton on spectral water transparency was assessed by two independent procedures: the pigment-specific beam absorption coefficient, a_J*[5], at all stations, as estimated by spectroscopy of in vivo light absorption of plankton on glass fibre filters, and the pigment-specific light attenuation, (k_c[5]), as derived by regression analysis of spectral in situ vertical light attenuation coefficients in the sea against concomitant pigment concentrations. Values of a_J*[5] and vertical profiles of light attenuation by phytoplankton exhibited regional differences that corresponded with the three zones from which samples had been collected. These differences can be related to the specific characteristics of the three zones with respect to cell size distribution, pigment composition and biomass. The observed variations in a_J*[5] values should be considered when oceanic primary production is to be estimated by biooptical modelling.     

盐碱池塘浮游植物初级生产力日变化的研究

用黑白瓶测氧法对盐碱池塘浮游植物初级生产力的日变化进行了研究．结果表明,盐碱池塘浮游植物初级生产力存在着明显的日变化,其分布规律与光照、温度、浮游植物现存量等环境因子的变化是相关的．用黑白瓶法连续曝光24h测定初级生产力结果大大偏低于每次曝光2h的全天累积结果．对养鱼池而言,在10：00—14：00时曝光4h再推算全天的初级生产力较为适当．     

Viral lysis modifies seasonal phytoplankton dynamics and carbon flow in the Southern Ocean

Phytoplankton form the base of marine food webs and are a primary means for carbon export in the Southern Ocean, a key area for global pCO₂ drawdown. Viral lysis and grazing have very different effects on microbial community dynamics and carbon export, yet, very little is known about the relative magnitude and ecological impact of viral lysis on natural phytoplankton communities, especially in Antarctic waters. Here, we report on the temporal dynamics and relative importance of viral lysis rates, in comparison to grazing, for Antarctic nano- and pico-sized phytoplankton of varied taxonomy and size over a full productive season. Our results show that viral lysis was a major loss factor throughout the season, responsible for roughly half (58%) of seasonal phytoplankton carbon losses. Viral lysis appeared critically important for explaining temporal dynamics and for obtaining a complete seasonal mass balance of Antarctic phytoplankton. Group-specific responses indicated a negative correlation between grazing and viral losses in Phaeocystis and picoeukaryotes, while for other phytoplankton groups losses were more evenly spread throughout the season. Cryptophyte mortality was dominated by viral lysis, whereas small diatoms were mostly grazed. Larger diatoms dominated algal carbon flow and a single ‘lysis event’ directed >100% of daily carbon production away from higher trophic levels. This study highlights the need to consider viral lysis of key Antarctic phytoplankton for a better understanding of microbial community interactions and more accurate predictions of organic matter flux in this climate-sensitive region.Subject terms: Microbial ecology, Virus-host interactions     

The role of iron in phytoplankton photosynthesis,and the potential for iron-limitation of primary productivity in the sea

Iron supply has been suggested to influence phytoplankton biomass, growth rate and species composition, as well as primary productivity in both high and low NO₃ ^– surface waters. Recent investigations in the equatorial Pacific suggest that no single factor regulates primary productivity. Rather, an interplay of bottom-up (i.e., ecophysiological) and top-down (i.e., ecological) factors appear to control species composition and growth rates. One goal of biological oceanography is to isolate the effects of single factors from this multiplicity of interactions, and to identify the factors with a disproportionate impact. Unfortunately, our tools, with several notable exceptions, have been largely inadequate to the task. In particular, the standard technique of nutrient addition bioassays cannot be undertaken without introducing artifacts. These so-called bottle effects include reducing turbulence, isolating the enclosed sample from nutrient resupply and grazing, trapping the isolated sample at a fixed position within the water column and thus removing it from vertical movement through a light gradient, and exposing the sample to potentially stimulatory or inhibitory substances on the enclosure walls. The problem faced by all users of enrichment experiments is to separate the effects of controlled nutrient additions from uncontrolled changes in other environmental and ecological factors. To overcome these limitations, oceanographers have sought physiological or molecular indices to diagnose nutrient limitation in natural samples. These indices are often based on reductions in the abundance of photosynthetic and other catalysts, or on changes in the efficiency of these catalysts. Reductions in photosynthetic efficiency often accompany nutrient limitation either because of accumulation of damage, or impairment of the ability to synthesize fully functional macromolecular assemblages. Many catalysts involved in electron transfer and reductive biosyntheses contain iron, and the abundances of most of these catalysts decline under iron-limited conditions. Reductions of ferredoxin or cytochrome f content, nitrate assimilation rates, and dinitrogen fixation rates are amongst the diagnostics that have been used to infer iron limitation in some marine systems. An alternative approach to diagnosing iron-limitation uses molecules whose abundance increases in response to iron-limitation. These include cell surface iron-transport proteins, and the electron transfer protein flavodoxin which replaces the Fe-S protein ferredoxin in many Fe-deficient algae and cyanobacteria.     

Effect of cooling water chlorination on primary productivity of entrained phytoplankton at Kalpakkam,east coast of India

We studied the effect of various concentrations of chlorine, used in the cooling system of Madras Atomic Power Station (MAPS), on the primary productivity of entrained phytoplankton from October 1988 to December 1990. The rates of primary productivity (gross) were lower at the discharge point than in the intake area. The reduction in productivity ranged from 30 to 70% at low-dose chlorination (residual chlorine 0.05–0.20 mg l⁻¹ at the discharge point). During shock-dose chlorination, when residual chlorine at the discharge point ranged from 1.10 to 1.50 mg 1⁻¹ the reduction in productivity was 80–83%. A reduction of 16–17% was recorded without chlorination.     

Effects of bottle size in determinations of primary productivity by phytoplankton

Determinations of primary productivity were made by the radiocarbon method at regular intervals over a year in samples, taken from the eutrophic waters of the Menai Strait, North Wales, and contained in bottles of capacities from 135 ml up to 2290 ml. Fixation per unit volume in particulate matter retained by 0 · 45 m pore size membrane filters was independent of bottle size over most of the year. There was a consistent divergence, however, during the period of rapid phytoplankton growth in the spring, when small bottles gave up to three times higher values than large ones. Possible effects of bottle size on primary productivity determinations are discussed but the cause of this particular result is not apparent.     

The surface ocean productivity response of deeper water benthic foraminifera in the Atlantic Ocean

We test the relationship of deep sea benthic foraminiferal assemblage composition to the surface ocean productivity gradient in the low latitude Atlantic Ocean using 81 surface sediment samples from a water depth range between 2800 and 3500 m. The samples are selected so that the surface ocean productivity gradient, controlling the flux of organic carbon to the seabed, will be the most important environmental variable. The first two principal components of the assemblage data account for 73% of data variance and are clearly linked to the productivity gradient across the Atlantic. These components show that under higher productivity the assemblages contain a higher abundance of Uvigerina peregrina, Melonis barleeanum, Globobulimina spp. and other taxa with probable infaunal microhabitats. Alabaminella weddellensis, a species linked to episodic phytoplankton debris falls, is also important in these assemblages. As productivity decreases there is a regular shift in assemblage composition so that low productivity assemblages are dominated by Globocassidulina subglobosa and several Cassidulina species along with Epistominella exigua. We hypothesize that these taxa are epifaunal to very shallow infaunal since nearly all organic carbon oxidation occurs near the sediment-water interface in low productivity settings. Discriminant function analysis of the foraminiferal assemblages, with groups selected on the basis of surface ocean productivity, shows clear separation among five productivity levels we used. This analysis demonstrates that productivity variations have a strong influence on assemblage composition. Finally, we used two groups of samples from the Rio-Grande Rise representing water depths from 2007 to 2340 m and 2739 to 3454 m to test for effects produced by changing water depth. All these samples are from a low productivity region and represent nearly identical environmental conditions. Although the low productivity nature of all the Rio-Grande Rise samples is obvious, there are assemblage differences between our depth groups. We cannot account for the assemblage differences with changes in organic carbon flux, dissolution effects or other physical/chemical properties of the ocean. Thus there are as yet unidentified factors related to water depth which cause some assemblage variation in the low productivity setting we investigated.     

Photoprotective acclimation of micro- and nano-size phytoplankton assemblages in the Indian sector of the Southern Ocean

Phytoplankton in the mixed layer is exposed to increasing levels of light when transported to the surface layer of the ocean. The photoprotective response of natural assemblages of phytoplankton can differ among community structures. We investigated photoprotective acclimation and xanthophyll cycle pigments in size-fractionated natural phytoplankton assemblages during the austral summer in the Indian sector of the Southern Ocean. We estimated concentrations of phytoplankton pigments in the micro-size fractions (>20 μm) and nano-size fractions (2–20 μm) by subtracting concentrations in the <20 μm fractions from concentrations in the bulk samples, and by subtracting concentrations in the <2 μm fractions from concentrations in the <20 μm fractions, respectively. Changes in the ratios of the xanthophyll cycle pigments diadinoxanthin (DD) and diatoxanthin (DT) were determined at three optical depths in the mixed layer and during 48 h deck incubations under solar photosynthetically available radiation and ultraviolet radiation. Large variations in (DD + DT)/Chl a in the mixed layer (percent coefficient of variation >67 %) and in deck incubation bottles under variable light conditions (>75 % of the temporal variation) for the micro-size fractions suggest a higher potential for photoprotective acclimation than for the nano-size fractions. Decreases in DT/(DD + DT) with increases in the optical depth of the mixed layer (ζ _MLD) suggest that larger variations in light availability in the mixed layer might predict lower values of DT/(DD + DT) at the surface, regardless of cell size.     

Whales sustain fisheries: Blue whales stimulate primary production in the Southern Ocean

It has previously been asserted that baleen whales compete with fisheries by consuming potentially harvestable marine resources. The regularly applied “surplus‐yield model” suggests that whale prey becomes available to fisheries if whales are removed, and has been presented as a justification for whaling. However, recent findings indicate that whales enhance ecosystem productivity by defecating iron that stimulates primary productivity in iron‐limited waters. While juvenile whales and whales that are pregnant or lactating retain iron for growth and milk production, nonbreeding adult whales defecate most of the iron they consume. Here, we modify the surplus‐yield model to incorporate iron defecation. After modeling a simplistic trajectory of blue whale recovery to historical abundances, the traditional surplus‐yield model predicts that 10¹¹ kg of carbon yr^?1 would become unavailable to fisheries. However, this ignores the nutrient recycling role of whales. Our model suggests the population of blue whales would defecate 3 × 10⁶ kg of iron yr^?1, which would stimulate primary production equivalent to that required to support prey consumption by the blue whale population. Thus, modifying the surplus‐yield model to include iron defecation indicates that blue whales do not render marine resources unavailable to fisheries. By defecating iron‐rich feces, blue whales promote Southern Ocean productivity, rather than reducing fishery yields.     

Effect of phytoplankton size diversity on primary productivity in the North Pacific: trait distributions under environmental variability

While most biodiversity and ecosystem functioning (BEF) studies have found positive effects of species richness on productivity, it remain unclear whether similar patterns hold for marine phytoplankton with high local richness. We use the continuous trait‐based modelling approach, which assumes infinite richness and represents diversity in terms of the variance of the size distribution, to investigate the effects of phytoplankton size diversity on productivity in a three‐dimensional ocean circulation model driven by realistic physics forcing. We find a slightly negative effect of size diversity on primary production, which we attribute to several factors including functional trait‐environment interactions, flexible stoichiometry and the saturation of productivity at low diversity levels. The benefits of trait optimisation, whereby narrow size distributions enhance productivity under relatively stable conditions, tend to dominate over those of adaptive capacity, whereby greater diversity enhances the ability of the community to respond to environmental variability.     

Reproductive response of the copepod Rhincalanus gigas to an iron-induced phytoplankton bloom in the Southern Ocean

The reproductive response of Rhincalanus gigas to the build up of a phytoplankton bloom in the Southern Ocean was studied during the European iron fertilization experiment (EIFEX). Egg production experiments were conducted over a period of approximately 5 weeks during development of a diatom dominated bloom. R. gigas showed a clear response to increasing chlorophyll a concentrations and the total egg production of the R. gigas population was highest just after the peak of the bloom at day 29 after fertilization. The average peak production was 50 eggs female⁻¹ day⁻¹. The percentage of egg producing females increased from about 0 to 90% during the course of the experiment. Accordingly, the maturation of the gonads reflected the positive response towards enhanced chlorophyll a concentrations. The fast reproductive response indicate that R. gigas was food limited during the period of this study in the Antarctic Polar Front region (APF).     

Size-fractionated primary production in the open Southern Ocean in austral spring

Size-fractionated primary production was measured by carbon-14 uptake incubations on three transects between 47°S and 59°30S along 6°W in October/November 1992. Open Antarctic Circumpolar Current and ice-covered Weddell Gyre water showed comparable low productivity (0.3 gCm^–2 day^–1) and size distribution. Picoplankton (<2 m) was the dominant size fraction, contributing approximately half to the total water column production. The significance of larger (>20 m) phytoplankton was only minor. Productivity in the Polar Front Zone north of 50°S, with higher water column stability, was up to 10 times higher with microplankton (>20 m) being predominant. No ice-edge bloom occurred over the 2 months study period; this is explained by non-favourable hydrographic conditions for blooming and the lack of melt-water lenses upon ice retreat. Picoplankton tended to make higher contributions at lower water column stability, and microplankton to make higher contributions at higher stability. Mixing, together with light climate, are discussed as the driving forces for Antarctic primary production and for its size structure.     

Small phytoplankton contribute greatly to CO2-fixation after the diatom bloom in the Southern Ocean

Phytoplankton is composed of a broad-sized spectrum of phylogenetically diverse microorganisms. Assessing CO₂-fixation intra- and inter-group variability is crucial in understanding how the carbon pump functions, as each group of phytoplankton may be characterized by diverse efficiencies in carbon fixation and export to the deep ocean. We measured the CO₂-fixation of different groups of phytoplankton at the single-cell level around the naturally iron-fertilized Kerguelen plateau (Southern Ocean), known for intense diatoms blooms suspected to enhance CO₂ sequestration. After the bloom, small cells (<20 µm) composed of phylogenetically distant taxa (prymnesiophytes, prasinophytes, and small diatoms) were growing faster (0.37 ± 0.13 and 0.22 ± 0.09 division d⁻¹ on- and off-plateau, respectively) than larger diatoms (0.11 ± 0.14 and 0.09 ± 0.11 division d⁻¹ on- and off-plateau, respectively), which showed heterogeneous growth and a large proportion of inactive cells (19 ± 13%). As a result, small phytoplankton contributed to a large proportion of the CO₂ fixation (41–70%). The analysis of pigment vertical distribution indicated that grazing may be an important pathway of small phytoplankton export. Overall, this study highlights the need to further explore the role of small cells in CO₂-fixation and export in the Southern Ocean.Subject terms: Biogeochemistry, Biogeochemistry, Stable isotope analysis, Microbial ecology