Photoadaptation in marine phytoplankton : changes in spectral absorption and excitation of chlorophyll a fluorescence

The optical properties of marine phytoplankton were examined by measuring the absorption spectra and fluorescence excitation spectra of chlorophyll a for natural marine particles collected on glass fiber filters. Samples were collected at different depths from stations in temperate waters of the Southern California Bight and in polar waters of the Scotia and Ross Seas. At all stations, phytoplankton fluorescence excitation and absorption spectra changed systematically with depth and vertical stability of the water columns. In samples from deeper waters, both absorption and chlorophyll a fluorescence excitation spectra showed enhancement in the blue-to-green portion of the spectrum (470-560 nm) relative to that at 440 nm. Since similar changes in absorption and excitation were induced by incubating sea water samples at different light intensities, the changes in optical properties can be attributed to photoadaptation of the phytoplankton. The data indicate that in the natural populations studied, shade adaptation caused increases in the concentration of photosynthetic accessory pigments relative to chlorophyll a. These changes in cellular pigment composition were detectable within less than 1 day. Comparisons of absorption spectra with fluorescence excitation spectra indicate an apparent increase in the efficiency of sensitization of chlorophyll a fluorescence in the blue and green spectral regions for low light populations.     

Composition and distribution of the pelagic and sympagic algal assemblages in the Laptev Sea during autumnal freeze-up

The phytoplankton and ice algal assemblages in the SiberianLaptev Sea during the autumnal freeze-up period of 1995 aredescribed. The spatial distribution of algal taxa (diatoms,dinoflagellates, chrysophytes, chlorophytes) in the newly formedice and waters at the surface and at 5 m depth differed considerablybetween regions. This was also true for algal biomass measuredby in situ fluorescence, chlorophyll (Chl) a and taxon-specificcarbon content. Highest in situ fluorescence and Chl a concentrations(ranging from 0.1 to 3.2 µg l^–1) occurred in surfacewaters with maxima in Buor Khaya Bay east of Lena Delta. Thealgal standing stock on the shelf consisted mainly of diatoms,dinoflagellates, chrysophytes and chlorophytes with a totalabundance (excluding unidentified flagellates <10 µm)in surface waters of 351–33 660 cells l^–1. Highestalgal abundance occurred close to the Lena Delta. Phytoplanktonbiomass (phytoplankton carbon; PPC) ranged from 0.1 to 5.3 µgC l^–1 in surface waters and from 0.3 to 2.1 µg Cl^–1 at 5 m depth, and followed the distribution patternof abundances. However, the distribution of Chl a differed considerablyfrom the distribution pattern shown by PPC. The algal assemblagein the sea ice, which could not be quantified due to high sedimentload, was dominated by diatom species, accompanied by dinoflagellates.Thus, already during the early stage of autumnal freeze-up,incorporation processes, selective enrichment and subsequentgrowth lead to differences between surface water and sea icealgal assemblages.     

The distribution of chlorophyll a in the eastern equatorial Pacific Ocean in boreal autumn

The distribution and size fractions of chlorophyll a (Chl a) concentration in the eastern equatorial Pacific Ocean in boreal autumn were investigated during October and November, 2011. Environmental factors, including hydrology and nutrients, that might affect the distribution and size composition were analyzed. A total of 18 stations including 11 CTD stations and 7 navigation stations were selected which stretch from the northwest coast of South America to the area of the central Pacific Ocean south of the Hawaiian Islands (2.77°S–13.02°N, 84.11–154.02°W). The studied area can be divided into two transects: the 6°N transect (124–148°W) and the154°W transect (10–13°N). Results showed that the surface Chl a concentration was higher in the east near the northwest coast of South America (>0.200 mg/m³) and lower in the west (0.100–0.200 mg/m³), and it presented a highly significant negative correlation with sea surface temperature (p < 0.001). There were some differences between the sectional distribution of Chl a concentration between the 6°N and 154°W transects. The high values of Chl a concentration occurred near the surface along the 6°N transect (0–75 m), while they were relatively deeper along the 154°W transect (50–100 m). Iron might be the factor that limited the growth of phytoplankton in the eastern equatorial Pacific Ocean. Picophytoplankton (Pico) was the dominant taxa in the surveyed area, particularly in the waters along the two transects (>70% of total Chl a). The Pico to total Chl a ratio was higher in the upper layer (>70%) than in the deeper layer.     

Temperature-Correlated Changes in Phytoplankton Community Structure Are Restricted to Polar Waters

Globally distributed observations of size-fractionated chlorophyll a and temperature were used to incorporate temperature dependence into an existing semi-empirical model of phytoplankton community size structure. The additional temperature-dependent term significantly increased the model’s ability to both reproduce and predict observations of chlorophyll a size-fractionation at temperatures below 2°C. The most notable improvements were in the smallest (picoplankton) size-class, for which overall model fit was more than doubled, and predictive skill was increased by approximately 40%. The model was subsequently applied to generate global maps for three phytoplankton size classes, on the basis of satellite-derived estimates of surface chlorophyll a and sea surface temperature. Polar waters were associated with marked decline in the chlorophyll a biomass of the smallest cells, relative to lower latitude waters of equivalent total chlorophyll a. In the same regions a complementary increase was seen in the chlorophyll a biomass of larger size classes. These findings suggest that a warming and stratifying ocean will see a poleward expansion of the habitat range of the smallest phytoplankton, with the possible displacement of some larger groups that currently dominate. There was no evidence of a strong temperature dependence in tropical or sub-tropical regions, suggesting that future direct temperature effects on community structure at lower latitudes may be small.     

Physical-Biological Coupling in the Western South China Sea: The Response of Phytoplankton Community to a Mesoscale Cyclonic Eddy

It is widely recognized that the mesoscale eddies play an important part in the biogeochemical cycle in ocean ecosystem, especially in the oligotrophic tropical zones. So here a heterogeneous cyclonic eddy in its flourishing stage was detected using remote sensing and in situ biogeochemical observation in the western South China Sea (SCS) in early September, 2007. The high-performance liquid chromatography method was used to identify the photosynthetic pigments. And the CHEMical TAXonomy (CHEMTAX) was applied to calculate the contribution of nine phytoplankton groups to the total chlorophyll a (TChl a) biomass. The deep chlorophyll a maximum layer (DCML) was raised to form a dome structure in the eddy center while there was no distinct enhancement for TChl a biomass. The integrated TChl a concentration in the upper 100 m water column was also constant from the eddy center to the surrounding water outside the eddy. However the TChl a biomass in the surface layer (at 5 m) in the eddy center was promoted 2.6-fold compared to the biomass outside the eddy (p < 0.001). Thus, the slight enhancement of TChl a biomass of euphotic zone integration within the eddy was mainly from the phytoplankton in the upper mixed zone rather than the DCML. The phytoplankton community was primarily contributed by diatoms, prasinophytes, and Synechococcus at the DCML within the eddy, while less was contributed by haptophytes_8 and Prochlorococcus. The TChl a biomass for most of the phytoplankton groups increased at the surface layer in the eddy center under the effect of nutrient pumping. The doming isopycnal within the eddy supplied nutrients gently into the upper mixing layer, and there was remarkable enhancement in phytoplankton biomass at the surface layer with 10.5% TChl a biomass of water column in eddy center and 3.7% at reference stations. So the slight increasing in the water column integrated phytoplankton biomass might be attributed to the stimulated phytoplankton biomass at the surface layer.     

Fine-scale spatial patterns in the coastal epiplankton off southern California

Fine-scale vertical (5 – 40 m) and horizontal (50 –500 m) patterns of temperature, chlorophyll and abundance ofzooplankton species were sampled with a pump filtration systemin the surface waters offshore of San Diego in May and October,1978. Intense and consistent patterns were most apparent invertical profiles. Herbivorous zooplankton were more consistentlyassociated with the estimated primary productivity maximum thanwith the deeper chlorophyll maximum layer, which representeda phytoplankton biomass maximum. Predators were positively correlatedwith abundant potential prey species. Variations in body lengthwith depth suggest that these fine-scale patterns were sufficientlystable to influence zooplankton growth. Consequences for grazingand predator – prey interactions in pelagic ecosystemsare discussed. ¹Present address: NOAA/NMFS Southwest Fisheries Center, PO Box271, La Jolla, CA 92038, USA     

The presence of chlorophyll b and the estimation of phaeopigments in marine phytoplankton

A reverse-phase h.p.l.c. technique was used to estimate theconcentration of chlorophyll b in phytoplankton cultures, fecalpellets of Calanus pacificus, and suspended paniculate matterfrom the Central North Pacific, Oregon coastal waters, and DabobBay (a temperate fjord in Puget Sound, WA, USA). The purposewas to assess the distribution of this pigment in the euphoticzone and its effect on the fluorometnc estimation of phaeopigments.Analyses of natural waters confirm high chlorophyll b concentrations(median mass ratio of b:a > 0.3) at the depth of the chlorophylla maximum in tropical waters while values for temperate planktonare relatively low (median mass ratio of chl b:a = 0.05) andpatchy. Zooplankton fecal pellets showed a significant enrichmentin chlorophyll b, suggesting grazing as a mechanism to explainhigh concentrations of this pigment at the bottom of the euphoticzone. It is estimated that the presence of chlorophyll b couldcause an average overestimation of phaeopigment concentrationby the fluorometnc technique of 38% between 0 and 200 m in theCentral North Pacific. This effect is more pronounced at thelayer of chlorophyll b maximum (120–140 m). ¹Present address: Marine Biology Research Division, A-002, ScrippsInstitution of Oceanography, La Jolla, CA 92093, USA     

Phytoplankton productivity and its response to higher light levels in the Canada Basin

Phytoplankton productivity in the Canada Basin was measured in the late summer season, from mid-September to mid-October 2009, using a ¹³C–¹⁵N dual tracer technique. To understand potential production changes associated with sea ice melting in the Arctic Ocean, we examined the effects of light enhancement and nitrate enrichment on the carbon productivity of phytoplankton from the chlorophyll a maximum layer. The daily carbon productivity in the Canada Basin in 2009 was very low, with a mean of 4.1 mg C m⁻² (SD = 3.6 mg C m⁻²), compared with those reported in previous studies in the region. Among several explanations, the most plausible reason for the large difference in carbon productivity between this and the previous studies was strong seasonal variation in biomass and photosynthetic rate of the phytoplankton in the study region. Based on our results from light enhancement and nitrate enrichment experiments, we found that carbon productivity of phytoplankton in the chlorophyll a maximum layer could be stimulated by increased light condition rather than nitrate addition. Thus, potentially increasing light availability from current and ongoing decreases in the sea ice cover could increase the carbon production of the phytoplankton in the chlorophyll a maximum layer and produce a well-developed maximum layer at a deeper depth in the Canada Basin.     

Interannual variability in the distribution of the phytoplankton standing stock across the seasonal sea-ice zone west of the Antarctic Peninsula

The spatial distribution of phytoplankton cell abundance, carbon(C) biomass and chlorophyll a (Chl a) concentration was analysedduring three summers (1996, 1997 and 1999) in a seasonal sea-icearea, west of the Antarctic Peninsula. The objective of thestudy was to assess interannual variability in phytoplanktonspatial distribution and the mechanisms that regulate phytoplanktonaccumulation in the water column. Phytoplankton C biomass andChl a distributions were consistent from year to year, exhibitinga negative on/offshore gradient. The variations in C concentrationhad a close and non-linear relationship with the upper mixedlayer depth, suggesting that the vertical mixing of the watercolumn is the main factor regulating phytoplankton stock. Themagnitude of C gradients was 5-fold higher during 1996 thanduring 1997 and 1999. This was ascribed to interannual variationsin the concentration of diatom blooms in the region influencedby sea-ice melting. Vertical distribution of the phytoplankton,as estimated from Chl a profiles, also varied along an on/offshoregradient: Chl a was distributed homogeneously in the upper mixedlayer in coastal and mid-shelf stations and concentrated inthe deep layer (40–100 m) occupied by the winter waters(WW, remnants of the Antarctic surface waters during summer)in more offshore stations. The region with a deep Chl a maximumlayer (DCM layer) was dominated by a phytoplankton assemblagecharacterized by a relatively high concentration of diatoms.The extent of this region varied from year to year: it was restrictedto pelagic waters during 1996, extended to the shelf slope during1997 and occupied a major portion of the area during 1999. Itis hypothesized that iron depletion in near surface waters dueto phytoplankton consumption, and a higher concentration inWW, regulated this vertical phytoplankton distribution pattern.Furthermore, we postulate that year-to-year variations in thespatial distribution of the DCM layer were related to interannualvariations in the timing of the sea-ice retreat. The similaritybetween our results and those reported in literature for otherareas of the Southern Ocean allows us to suggest that the mechanismsproposed here as regulating phytoplankton stock in our areamay be applicable elsewhere.     

Studies on phytoplankton pigments in porto novo waters (India). II. Backwater

The annual variations of phytoplankton pigments were studied from January to December, 1971 at four stations in the backwater area (Porto Novo, India). At these stations, chlorophyll a varied from 2.91–65.56 μg/1; chlorophyll b from 0–1.90 μg/l; chlorophyll c from 0–38.60 μg/1; plant carotenoids from 0.30–22.34 MSPU/m³ and phaeopigments from 0–31.51 μg/l. The main (or primary) maximum of chlorophyll a appeared during June at Stations 3–5 and at Station 6, it was during May. Stations 3 and 4 had secondary chlorphyll a maxima during September and at Stations 5 and 6 during August. In general, all the stations showed an increasing trend of chlorphyll a values before the main peak. The chlorophyll a maximum was mainly due to the presence of phytoplankton species belonging to the genera such as Noctiluca, Coscinodiscus, Thalassiothrix, Biddulphia, Hemidiscus, Melosira and Rhizosolenia. Amongst all the backwater stations, Station 5 had the highest chlorophyll a (65.56 μg/1) and the lowest chlorophyll a was recorded at Station 3. Chlorophyll a concentration was normally affected by rainfall.     

Multiple subsurface phytoplankton blooms occurring simultaneously in the Skagerrak

The distribution and characteristics of phytoplankton in theSkagerrak in August–September 2000 were analysed in orderto evaluate the importance of subsurface phytoplankton peaksto water column ecology and primary production. In areas affectedby outflow from the Baltic, enhanced chlorophyll concentrationswere found in the warm surface waters (i.e. upper 10–20m). However, for the central Skagerrak, the major part (50–80%)of the chlorophyll in the water column was found below the warmsurface waters. The highest chlorophyll concentrations (up to>18 µg l^-1) in the study area were also found belowthe warm surface waters and up to 95% of total water columnprimary production was recorded below the warm surface waterlayer. Measurements of variable fluorescence (F_v/F_m) indicatedthe greatest potential capacity for electron flow in photosystemII in phytoplankton was located below the warm surface waters.Spectrophotometrically determined pigment ratios suggest thatthe enhanced capacity for photosynthesis in the deeper watersmay be related to greater nutrient availability here than insurface waters. Subsurface chlorophyll distributions seen inrelation to the different water masses identified in the area,as well as community analysis of the phytoplankton present inthe subsurface peaks, indicate the presence of at least threedistinct subsurface phytoplankton blooms in the Skagerrak duringthe study period. Local oxygen saturation maxima recorded immediatelyabove the subsurface peaks provide in situ evidence that thesepeaks are photosynthetically active. This suggests that newproduction is taking place in these peaks, although quantificationof this production is hampered due to a lack of informationconcerning the initial conditions in and lifetime of the subsurfacepeaks. The subsurface phytoplankton peaks were, generally, foundimmediately above an oxygen minimum that covered the entirestudy area. In the relatively cold deep Atlantic water foundbelow the oxygen minimum layer, no or very little chlorophyllwas recorded and oxygen concentrations increased. Thus, it isargued that the respiration of the organic material producedin the upper part of the water column during late summer mayprimarily occur in the intermediate layers of the water column.     

Résultats de la mission Hydratlante I du N. O. Jean Charcot (au large des côtes atlantiques françaises). 2. Pigments phytoplanctoniques

Results of Hydratlante I Cruise of R. V. Jean Charcot (off the Atlantic French Coast). 2. Phytoplankton Pignients Measurements of phytoplankton pigments have been made at all levels and stations prospected during Hydratlante I. In neritic as well as oceanic waters, the chlorophyll a content of seston was low: only 0.07–0.08 mg · m⁻³ at 5 and 20 m; maximum average value at 50 m: 0.14 mg · m⁻³. The chlorophyll a content of phytoplankton cells averages 6.2 pg but varies greatly with population density. The plant carotenoids, relatively more abundant than chlorophyll, average 0.7 mg · m⁻³ at 5 m; 0.9 at 20 m and 1.2 at 50 m. Pigment distributions seem to be largely dependent on thermal structure and nutrient concentration. They are also more or less strongly correlated with water transparency, rate of ¹⁴C fixation and organic seston content. Comparisons with previous results in the same area reveal a rather good agreement.     

Microzooplankton herbivory and bacterivory in Newfoundland coastal waters during spring, summer and winter

Grazing by microzooplankton on autotrophic and heterotrophicpicoplankton as well as >0.7 µm phytoplankton (as measuredby chlorophyll a) was quantified during July, August, October,January and April in the surface layer of Logy Bay, Newfoundland(47°38'14'N, 52°39'36'W). Rates of growth and grazingmortality of bacteria, Synechococcus and >0.7 µm phytoplanktonwere measured using the sea water dilution technique. Microzooplanktoningested 83–184, 96–366 and 64–118% of bacterial,Synechococcus and >0.7 µm phytoplankton daily potentialproduction, respectively and 34–111, 25–30 and 16–131%of bacterial, Synechococcus and >0.7 µm phytoplanktonstanding stocks, respectively. The trends in prey net growthrates followed the seasonal cycles of prey biomass, suggestingthat microzooplankton are important grazers in Newfoundlandcoastal waters. Ingestion was lowest during January and October(~2 µg C l^–1 day^–1) and highest in August(~20 µg C l^–1 day^–1). Aside from April when>0.7 µm phytoplankton represented the majority (~80%)of carbon ingested, bacterioplankton and <1 µm phytoplanktonrepresented most of the carbon ingested (~40–100%). Althoughmicrozooplankton have here-to-fore been unrecognized as an importantgrazer population in Newfoundland coastal waters, these resultssuggest that they play an important role in carbon flow withinthe pelagic food web, even at low temperatures in Logy Bay.     

Studies on the phytoplankton ecology of the trondheemsfjord. II. Chloroplast pigments in relation to abundance and physiological state of the phytoplankton

The quantitative composition of the chloroplast pigments of phytoplankton sampled weekly at one station in the Trondheimsfjord was studied by circular paper chromatography throughout 18 months. The concentrations of total chlorophyll a (T-chl a obtained by the trichromatic method) as well as of chromatographically purified chlorophyll a (chl a) followed the variations in phytoplankton concentration. Two spring blooms and a weak autumn flowering of phytoplankton were clearly reflected in the pigment contents found, namely 14–16 mg T-chl a/m³ for the spring maxima, corresponding to nearly 300 mg T-chl a/m² for the euphotic zone; and 3–4 mg/m³ or 32 mg/m² for the autumn peak. The concentrations between blooms amounted to ≈ 1 mg T-chl a/m³, while concentrations down to 0.03 mg/m³ were found for winter samples.The content of T-chl a was high in diatom cells prior to a bloom (20–40 × 10^?9 mg/cell). During rapid growth (a more or less exponential phase) the cell content of chloroplast pigments decreased (to 5–10 × 10^?9 mg). No degradation product of chlorophylls could be detected during this phase and the percentage of chl a (of T-chl a) was high (70–80 %). At the peak of the bloom, and especially when the nitrate content in the surrounding water had been exhausted, low values for T-chl a were found ($\text{0.3–0.5 × 10}{}^{\mathrm{<mtext>9</mtext><mtext>?</mtext>}}\text{mg/cell}$). As soon as the cell counts started to fall, or even before the decline could be clearly detected, the percentage of chl a dropped (to 40-20 %) and derived chlorophylls (not phaeophytin a) were present in the samples. Model studies with cultured algae showed a similar behaviour.It is concluded that the proportion of chl a to T-chl a and the occurrence of chlorophyll derivatives in phytoplankton samples can give valuable information on the stage of development of the algal populations involved.     

Changes in community structure and photosynthetic activities of total phytoplankton species during the growth,maintenance, and dissipation phases of a Prorocentrum donghaiense bloom

The potential interactions between the bloom-forming dinoflagellates and other phytoplankton during the algal bloom cycle are interesting, while the causes for the phytoplankton community changes were not fully understood. We hypothesized that phytoplankton community structure and photosynthetic activities of total phytoplankton have their special characteristics in different phases of the algal blooms. To test this hypothesis, a survey covering the process of a Prorocentrum donghaiense bloom in coastal waters between Dongtou and Nanji Islands was carried out from 9 to 20 May 2016, and the changes in the phytoplankton community and photosynthetic activities of total phytoplankton were determined. Surface seawater was sampled for microscopic analysis of phytoplankton composition and pulse amplitude modulated (PAM) chlorophyll fluorescence analysis of photosynthetic activities of the total phytoplankton species. A total of 25, 31, and 19 phytoplankton species were identified in its growth (9–12 May), maintenance (13–18 May) and dissipation phases (19–20 May), respectively. Diatoms were dominant in terms of species number while dinoflagellates were predominant at cell abundance. Dinoflagellates were the major dominant species during three phases of the bloom based on the dominance (Y) value, whereas the dominant species extended to dinoflagellates and diatoms including P. donghaiense, Coscinodiscus argus, Gonyaulax spinifera, Cyclotella sp. and Scrippsiella trochoidea in the dissipation phase. In the maintenance phase, the average cell abundances of the total phytoplankton and P. donghaiense were consistent with the chlorophyll a (Chla) concentration in the seawater; for the diversity indices of total phytoplankton species, Simpson index (C) was the highest while Shannon index (H′) and Pielou evenness index (J′) were the lowest. Furthermore, photosynthetic activities of the total phytoplankton species represented by the effective quantum yield (F_q'/F_m') and the maximum relative electron transport rate (rETR_max) in the maintenance phase were higher than those in the growth and dissipation phases. The results indicated that the characteristics of phytoplankton community structure and photosynthetic activities could be regarded as criteria in predicting the phases of algal blooms.     

Assessment of phytoplankton class abundance using in vivo synchronous fluorescence spectra

In this study the feasibility of using the in vivo synchronous fluorescence spectra (SFS) of phytoplankton samples for determining the relative abundance of specific classes of phytoplankton was investigated. In total, 405 SFS of nine phytoplankton species cultivated under different conditions were measured and evaluated. First, principal component analysis (PCA) was used to obtain nine representative spectra, and three feature spectrum bands at 200-235, 310-335, and 355-585 nm of SFS were found to have a better discriminatory capability. The nine phytoplankton species were spectrographically sorted into six classes. Second, a relationship between the chlorophyll a (chla) concentration of phytoplankton extracts and the in vivo SFS intensity was found; thus the corresponding phytoplankton class abundance can be expressed by the concentration of standard chla. For this data set, the detection limit ranged 1.02-6.89 μg/L chla for different classes. Finally, qualitative and quantitative analyses of 30 mixtures of phytoplankton were carried out using the nonnegative least square regression (NNLS) method. The dominant phytoplankton classes could be identified while the qualitative recognition correctness rate was 88% and the quantitative standard deviation with fluorometrically determined chla was −0.14-0.04. Hence, it was possible to estimate the abundance of dominant phytoplankton classes.     

Fertilisation de communautés phytoplanctoniques. ii. cas d'un milieu eutrophe: upwelling des côtes du sahara espagnol

Experiments with nutrient enrichment of phytoplankton communities have been carried out on surface waters from the upwelling area off Cap Blanc — Cap Corveiro (Spanish Sahara).None of the control flasks showed phytoplankton development despite the large quantities of nutrient available: this may be due to the presence of an inhibitor in the upwelling waters. On the other hand, most of the enrichments stimulated the autotrophic production. On average, the best growth rates were obtained when a full medium minus Fe-EDTA was added; marked phytoplankton development also took place when a medium without vitamins, or silicates, or phosphates, was added. Lack of metallic compounds (other than iron) and nitrates limit the algal growth.The maximum amounts of chlorophyll a (25–30 mg/m³) are close to the maxima usually observed in this area. The productivity rate (biomass produced/nutrients used) is very high.     

Statistical analysis of phytoplankton biomass in coastal waters: Case study of the Wadden Sea near Lauwersoog (The Netherlands) from 2000 to 2009

As the most important indicator of the coastal ecosystem, various studies on phytoplankton have frequently been proposed in coastal waters. In this study, the statistical analysis of phytoplankton biomass (in terms of chlorophyll a) is performed in the Wadden Sea near Lauwersoog (53.4167° N, 6.2000° E), The Netherlands, determined by the 10-year's historical dataset from 2000 through 2009. Boxplot analysis is introduced to give insight in the seasonal variations of phytoplankton biomass and physical–chemical conditions. Annually, a big difference is found in chlorophyll a, showing that the maximum values occur in the months of April and July, and the minimum values occur in winter (December, January, and February). The phytoplankton biomass is positively correlated with the physical conditions (salinity, light intensity, and water temperature), and is negatively correlated with the nutrients (nitrate, ammonium, and silicate). Factor analysis distinguishes the driving variables that represent most of the total variance using three extraction methods (principal component analysis, unweighted least squares, and maximum likelihood). Nitrate dominates the main activity in the first rotated component/factor, and ammonium contributes the most in the second rotated component/factor. The findings of this study are significant for understanding the roles of the environmental conditions upon the phytoplankton variability, and for reducing the overlapping information to characterize the driving variables.     

Variability of the microbial community in the western Antarctic Peninsula from late fall to spring during a low ice cover year

Although winter conditions play a major role in determining the productivity of the western Antarctic Peninsula (WAP) waters for the following spring and summer, a few studies have dealt with the seasonal variability of microorganisms in the WAP in winter. Moreover, because of regional warming, sea-ice retreat is happening earlier in spring, at the onset of the production season. In this context, this study describes the dynamics of the marine microbial community in the Melchior Archipelago (WAP) from fall to spring 2006. Samples were collected monthly to biweekly at four depths from the surface to the aphotic layer. The abundance and carbon content of bacteria, phytoplankton and microzooplankton were analyzed using flow cytometry and inverted microscopy, and bacterial richness was examined by PCR–DGGE. As expected, due to the extreme environmental conditions, the microbial community abundance and biomass were low in fall and winter. Bacterial abundance ranged from 1.2 to 2.8 × 10⁵ cells ml^?1 showing a slight increase in spring. Phytoplankton biomass was low and dominated by small cells (<2 μm) in fall and winter (average chlorophyll a concentration, Chl-a, of, respectively, 0.3 and 0.13 μg l^?1). Phytoplankton biomass increased in spring (Chl-a up to 1.13 μg l^?1), and, despite potentially adequate growth conditions, this rise was small and phytoplankton was still dominated by small cells (2–20 μm). In addition, the early disappearing of sea-ice in spring 2006 let the surface water exposed to ultraviolet B radiations (UVBR, 280–320 nm), which seemed to have a negative impact on the microbial community in surface waters.     

Dynamics in the size structure of Skeletonema costatum (Greville) Cleve under conditions of reduced photosynthetically available radiation in a dredged tropical estuary

This study investigates the size-fractionated productivity and chlorophyll a concentrations in Ponggol estuary, a heavily dredged, light-limited and eutrophic tropical estuary located on the northeastern coast of Singapore. A 90% reduction in the photosynthetically available radiation (PAR) was seen in the subsurface waters of the dredged stations, when compared to an average reduction of about 75% in the subsurface waters of the un-dredged station. High phytoplankton production rates and chlorophyll a concentrations were recorded in the surface waters, with a significant reduction in the subsurface waters, especially at the two dredged stations. Out of the four size-classes of phytoplankton (0.2-2 μm, 2-20 μm, 20-200 μm and >200 μm) investigated, the relative dominance of the smaller size class of 2-20 μm over the 20-200 μm size class shifted depending on the availability of PAR. The size class 2-20 μm was observed to contribute up to 60 and 57% of the total production, respectively, in the surface waters of the un-dredged and dredged stations of the estuary. A relatively major contribution of 49% came from the cells of the small size-fraction of 2-20 μm in the subsurface waters of the un-dredged station. On the contrary, cells of the 20-200 μm size-class contributed up to 58% in the subsurface waters of the dredged station. Microscopic examination of the phytoplankton cells sampled showed morphological differences in the cells of the phytoplankton species Skeletonema costatum with some cells being distinctly larger in size than others. The larger cells were predominant in the dredged subsurface waters. In situ mesocosms with treatments exposed to high light irradiation registered a significant contribution by the smaller size-class of 2-20 μm, in contrast to the larger 20-200 μm dominating in the two treatments subjected to low light conditions. Based on the observations in this study and a review of the literature, it is hypothesized that the cells of S. costatum may actually be of two genetically different strains, whose relative dominance in the environment may be controlled by the quantum of available light. Thus, this shift in the relative dominance of one size fraction over the other is a response to altered PAR levels as the result of dredging.