Journal of Plankton Research - recent issues

Diversity of Daphnia galeata life history traits in a vertically structured environment

Machacek, J., Seda, J. — 2008-04-15

We used laboratory experiments to investigate the life history traits of clonal lineages of Daphnia galeata isolated from two vertically segregated subpopulations in a deep dimictic reservoir: one from the epilimnion and the other from the deep hypolimnion. We collected clones twice for the experiments, first at the beginning of thermal stratification in May and then towards the end of the season in September. The results for May showed longer postembryonic development (PED), bigger eggs in the first clutch and slightly higher somatic increments in the hypolimnetic clones. In addition, a remarkable tendency to produce ephippia was recorded in the hypolimnetic clones, but not in the epilimnetic clones. In September, the results were reversed—hypolimnetic clones had shorter PED, lower somatic increments and a slightly higher number of eggs in the first clutch. Our results suggest a differentiation of life history traits in the two D. galeata populations inhabiting contrasting microhabitats of a reservoir as a result of (i) different depth preferences of certain clones in the beginning of thermal stratification in spring and (ii) divergent selection processes due to segregation in different environmental conditions. The ability of Daphnia to live in diverse environments, causing divergent selection processes, enhances the overall phenotypic and genotypic diversity of the whole reservoir population.

Redescription of Ditrichocorycaeus minimus indicus M. Dahl, 1912 (Copepoda: Cyclopoida, Corycaeidae) from the Adriatic Sea

Vidjak, O., Bojanic, N. — 2008-04-15

In 1999 the small corycaeid copepod Ditrichocorycaeus minimus indicus was recorded for the first time in the Adriatic Sea. Both sexes are characterized by almost equal length and width of the anal somite and presence of the medial ventral hooks on the genital somite. Ventral hooks are prominent in male and minute in female. Detailed description of the species is given on the basis of the material from the middle Adriatic Sea and compared to the existing descriptions.

Growth and grazing mortality rates of Prochlorococcus, Synechococcus and eukaryotic picophytoplankton in a bay of the Uwa Sea, Japan

Hirose, M., Katano, T., Nakano, S.-I. — 2008-04-15

Growth and grazing mortality rates of Synechococcus, Prochlorococcus and eukaryotic picophytoplankton were determined using dilution experiments in a bay of the Uwa Sea, Japan. Seasonal changes in the vertical distribution of each picophytoplankter were interpreted based on previous physico-chemical measurements and potential ecophysiological differences. Significant relationships between dilution and picophytoplankton growth rate were found only during the stratified period, suggesting that food linkage between the picophytoplankton and grazers was active during the stratified period. We found an almost 1:1 balance between growth and grazing mortality rates of Prochlorococcus and Synechococcus in the presence of both nanoflagellates and ciliates, suggesting that the abundance of the cyanobacteria is in a quasi-steady-state system with grazing losses compensated by growth. The relationship between growth and grazing mortality rate of eukaryotic picophytoplankton was unclear. We discuss the dynamics of each picophytoplankter abundance with special reference to the characteristics of the microbial food web in the bay, together with two physical events responsible for matter cycling within the bay.

Physical processes influencing the occurrence of the giant jellyfish Nemopilema nomurai (Scyphozoa: Rhizostomeae) around Jeju Island, Korea

Yoon, W. D., Yang, J.-Y., Shim, M. B., Kang, H.-K. — 2008-04-15

The physical and biological processes affecting the spatial distribution of the giant jellyfish Nemopilema nomurai were investigated in the northern East China Sea and southern Yellow Sea in July 2006, when large abundances of this jellyfish occurred in Korean waters. Well-characterized water masses were detected during the study period. These were the Tsushima Current water, Yellow Sea Bottom Cold Water and Changjiang Diluted Water (CDW). The spatial distribution of Chl a concentrations estimated by ocean colour imagery showed that the CDW flows to the northeast. The CDW was accompanied by high abundances of small and medium-sized zooplankton (0.2–0.5 mm and 0.5–1.0 mm, respectively) and N. nomurai, in comparison with the other water masses. Large (1.0–2.0 mm) and macro-sized zooplankton (2.0–5.0 mm) did not show a relationship with the CDW. The distribution of N. nomurai was very restricted and intimately related with the CDW. In fact, large abundances only occurred in the western boundary region of the CDW. The results were supported by Pearson’s correlation analysis, which showed a positive relationship between the CDW, small- and medium-sized zooplankton and the giant jellyfish. We conclude that the CDW had entrained N. nomurai into the waters off the southwest coast of the Korean peninsula and it was then transported into the Yellow and East/Japan seas.

Microbial dynamics during the decline of a spring diatom bloom in the Northeast Atlantic

2008-04-15

The microbial dynamics during a spring diatom bloom decline was monitored in the Northeast Atlantic during a 5-day Lagrangian study (8–12 April 2002). Phytoplankton abundance, composition and health status were related to viral and bacterial abundance, zooplankton abundance and grazing rates, as well as bacterial production. Phytoplankton reached maximum concentration on Day 3 (Chl a >5 µg L^–1) and declined on Day 5 (Chl a ~2 µg L^–1) and was dominated (70% of Chl a) by diatoms. Bacterial production increased substantially to >20 µg C L^–1 day^–1 on Day 3 and concomitantly large viruses decreased in number by half to <10 x 10³ mL^–1. This was followed by a 5-fold increase in large viruses on Day 5, indicating infection and subsequent lysis on Days 3 and 5, respectively. Micro- and mesozooplankton grazing were not the principal cause for the decline of the bloom and pheophorbide-a showing little variation in concentration from Days 1–4 (~100 ng L^–1) although doubled on Day 5. The poor physiological status of the diatoms, indicated by the high chlorophyllide-a concentrations (50–480 ng L^–1), likely promoted a series of closely interrelated events involving bacteria and viruses leading to the demise of the diatom bloom.

A rapid analysis of copepod feeding using FlowCAM

Ide, K., Takahashi, K., Kuwata, A., Nakamachi, M., Saito, H. — 2008-04-15

This study addressed the usefulness and reliability of using a new plankton image analyzer, FlowCAM, for rapid analysis of copepod feeding by comparison with the conventional microscopic analysis. We carried out bottle incubation experiments with two copepod species in the Oyashio region and analyzed the prey abundance prior to and after the incubation with a FlowCAM. From the volume-specific fluorescence intensity of particles, the FlowCAM successfully distinguished between zooplankton and phytoplankton and allowed an adequate evaluation of the copepod feeding on zooplankton and phytoplankton. The analysis time for one plankton sample was about 10 min, which was less than one-tenth of the time required for microscopic enumeration. The FlowCAM is considered to be an efficient tool for rapid analysis of copepod feeding particularly in studies of omnivory.

Temporal patterns of species composition of siliceous phytoplankton flux in the Santa Barbara Basin

Venrick, E. L., Lange, C. B., Reid, F. M. H., Dever, E. P. — 2008-04-15

We have examined the species composition of 127 sediment trap samples collected sequentially from the Santa Barbara Basin, USA, during a 7-year period. The sampling period included two warm-water periods (El Niños) and two cold-water periods (one La Niña and one more local). We examined changes in total species composition and changes in subsets of species accompanying extreme environmental conditions, as well as changes in flux composition associated with periods of anomalous flux. Our goal was to improve the scale and precision of hindcasting past conditions from the sedimentary record and, hence, prediction of the biological consequences of environmental change. Although large-scale climate events were accompanied by changes in the flux composition, there was little similarity in composition during environmentally similar periods, precluding generalization. Eighty-four percent of the flux (number of cells m^–2 d^–1) and 89% of its variability were due to changes in the flux of four dominant species. Anomalous flux events were due to changes in flux of these species rather than introduction of new flora. Rare species showed the same patterns as the flora as a whole: there were changes in composition associated with extreme environmental temperatures but little redundancy of species between similar periods. Our data indicate that, at present, we can predict only that the specific composition of flux will change in response to extreme environmental conditions. It is premature to generalize about the specific composition of that response. Knowledge about mechanisms linking flux with ocean environment is currently insufficient to permit the precision of hindcast and prediction that we were seeking from our data. Nevertheless, species composition remains a potentially important tool for interpreting past environmental conditions on both the regional and the local scales.

Productivity and grazing impact of Oikopleura dioica (Tunicata, Appendicularia) in Tokyo Bay

Sato, R., Ishibashi, Y., Tanaka, Y., Ishimaru, T., Dagg, M. J. — 2008-04-15

To estimate the productivity and grazing impact of a common appendicularian species Oikopleura dioica in Tokyo Bay, monthly observations on its abundance and vertical distribution were conducted during 2000. The abundance peaked in February and October, but was low during summer. Seasonal fluctuations in productivity were similar to those of the abundance, with maximum values of 92, 134 and 508 mg C m^–2 day^–1 for somatic, new house and discarded house productivity in October and 206 mg C m^–2 day^–1 for fecal pellet productivity in February, respectively. The averaged biomass of O. dioica was only 2.5% of that of copepods; however, the secondary productivity of the former corresponded to 12.4% of the latter. Daily grazing impact on particulate organic carbon ranged from 0.05% to 5%, which is close to the impact by copepods. These results imply that in Tokyo Bay, where small copepods and jellyfish are abundant, O. dioica is an important component of the ecosystem because it bridges between small primary producers and higher consumers.

Magnitude of mesozooplankton variability: a case study from the Marginal Ice Zone of the Barents Sea in spring

Blachowiak-Samolyk, K., Kwasniewski, S., Hop, H., Falk-Petersen, S. — 2008-04-15

Zooplankton was studied on eight stations in the marginal ice zone (MIZ) of the Barents Sea, in May 1999, along two transects across the ice edge. On each station, physical background measurements and zooplankton samples were taken every 6 h over a 24 h period at five discrete depth intervals. Cluster analysis revealed separation of open water stations from all ice stations as well as high similarity level among replicates belonging to particular station. Based on five replicates per station, analysis of variance (ANOVA) confirmed significant differences (P < 0.05) in abundances of the main mesozooplankton taxa among stations. Relations between the zooplankton community and environmental parameters were established using redundancy analysis (CANOCO). In total, 55% of mesozooplankton variability within studied area was explained by eight variables with significant conditional effects: depth stratum, fluorescence, temperature, salinity, bottom depth, latitude, bloom situation, and ice concentration. GLM models supported supposition about clear and negative relationship between concentration of Oithona similis, and overall mesozooplankton diversity. The analyses showed a dynamic relationship between mesozooplankton distribution and hydrological conditions on short-term scale. Furthermore, our study demonstrated that variability in the physical environment of dynamic MIZ of the Barents Sea has measurable effect on the Arctic pelagic ecosystem.

Diving behaviour of jellyfish equipped with electronic tags

2008-04-15

Jellyfish are one of the most abundant and conspicuous members of our coastal marine fauna and are now known to play major trophic roles in marine systems. However, little is known about the movements and behaviour of individuals. We equipped individual compass jellyfish (Chrysaora hysoscella) (n = 15) off the Dingle coast, Ireland, with miniature time-depth recorders to log their depth over periods of a few hours. Vertical movements were extensive, with all jellyfish changing their depth during tracking. A range of vertical movements were seen including initial diving from the surface down to a maximum of 29.6 m after device attachment, some jellyfish remaining near the bottom, some moving up and down in mid-water and some moving back near the surface. These results show that jellyfish actively reposition themselves in the water column over small time-scales and open the way for more extensive studies equipping jellyfish with electronic tags.

The emergence of automated high-frequency flow cytometry: revealing temporal and spatial phytoplankton variability

2008-04-15

Phytoplankton observation is the product of a number of trade-offs related to sampling processes, required level of diversity and size spectrum analysis capabilities of the techniques involved. Instruments combining the morphological and high-frequency analysis for phytoplankton cells are now available. This paper presents an application of the automated high-resolution flow cytometer Cytosub as a tool for analysing phytoplanktonic cells in their natural environment. High resolution data from a temporal study in the Bay of Marseille (analysis every 30 min over 1 month) and a spatial study in the Southern Indian Ocean (analysis every 5 min at 10 knots over 5 days) are presented to illustrate the capabilities and limitations of the instrument. Automated high-frequency flow cytometry revealed the spatial and temporal variability of phytoplankton in the size range 1–~50 µm that could not be resolved otherwise. Due to some limitations (instrumental memory, volume analysed per sample), recorded counts could be statistically too low. By combining high-frequency consecutive samples, it is possible to decrease the counting error, following Poisson’s law, and to retain the main features of phytoplankton variability. With this technique, the analysis of phytoplankton variability combines adequate sampling frequency and effective monitoring of community changes.

Patrick Michael Holligan: a short biography

Robinson, C., Owens, N. J. P. — 2008-03-17

Single cell analysis in biological oceanography and its evolutionary implications

Yentsch, C. S., Yentsch, C. M. — 2008-03-17

Historically single cell analysis techniques have been used to supplement more traditional studies of primary production. The techniques have often been used as a surrogate for microscopic analysis and to close a gap in sea truth coverage for remote sensing and other mapping activities. In the course of development from Coulter counting to flow cytometry/cell sorting, the instruments and techniques have become powerful tools for allometric and ataxonomic analysis as well as the quantification of pigments and added metabolic stains and tagged reagents. The specific questions we ask here are: Can flow cytometry-derived data be raised to a level to discern evolutionary direction and diversity/complexity? Can we account for changes in community structure based on allometric and ataxonomic relationships across major ocean boundaries? We present evidence from different approaches and use examples from flow cytometry/cell sorting that address the causes of variation in cell size and chlorophyll fluorescence in phytoplankton. The horizon is rapidly expanding yet questions and limitations of ocean study persist. We believe that a road of commonality among oceanographers, ecologists, modelers, microbiologists, molecular biologists, physiologists and paleontologists is needed.

Space-time variability of carbon standing stocks and fixation rates in the Gulf of Maine, along the GNATS transect between Portland, ME, USA, and Yarmouth, Nova Scotia, Canada

2008-03-17

The Gulf of Maine North Atlantic Time Series has been run since 1998 and is the longest transect time series in the Gulf of Maine (GoM), USA. Here we use this coastal time series to document the space–time variability of hydrography, nutrients, phytoplankton standing stocks and carbon fixation in the GoM, in response to several years of extreme river discharge. We hypothesize that, during wet years, fresh water input capped the surface euphotic layer, impeding the upward diffusion of nutrients, thus lowering the phytoplankton biomass and carbon fixation rates. Regional algorithms were derived to estimate particulate organic carbon and carbon fixation. The Howard–Yoder algorithm was implemented to predict integral primary production using satellite ocean color data. Calcification was significantly correlated to primary production, thus allowing regional, satellite-derived calcification estimates. Total GoM and Georges Bank phytoplankton photosynthesis was 38.12 Tg C year^–1 and total calcification was 0.55 Tg C year^–1, yielding an overall ratio of calcification to photosynthesis of 1.44%. Carbon fixation in GoM coastal water (<60 m bottom depth), GoM deep water (>60 m) and Georges Bank waters (<60 m) averaged 33, 56 and 11% of the total primary production of the combined GoM and Georges Bank study area, respectively, and 22, 67 and 11% of the total calcification of the study area, respectively.

Calcium carbonate cycling in future oceans and its influence on future climates

Tyrrell, T. — 2008-03-17

In the last few years, evidence has accumulated that calcifying organisms are likely to be affected by ocean acidification. Therefore, the production of calcium carbonate will probably decline, although conversely global warming, increasing stratification and sea level rise may also stimulate increases in global calcification. As acidification reaches the deep ocean, it will cause pronounced shallowing of the lysocline depths for calcite and aragonite, leading most probably to an almost complete cessation of deep-sea calcium carbonate burial for some centuries. Here, I briefly review the consequences of these and other changes on future ocean calcium carbonate cycling, and the consequences of this for future climate. Associated climate impacts are not likely to be significant over the next few centuries, but will become increasingly important thereafter. After the carbonate compensation response to acidification has run its course, extra CO₂ is expected to be left behind in the atmosphere, protecting against future ice ages.

Inter-specific scaling of phytoplankton production and cell size in the field

Maranon, E. — 2008-03-17

This study tests the hypothesis that the interspecific scaling of phytoplankton production and cell size in the field follows the 3/4-power scaling law. Published data of cell size and in situ, cell-specific carbon production rates by single phytoplankton species, collected in surface waters of lakes, rivers, estuaries and oceans, are reviewed. Across more than nine orders of magnitude in cell volume, 98% of the variability in carbon production rates was explained by cell size. The slope (b) in the log–log relationship between carbon production rate and cell volume did not differ significantly from 1, either for diatoms (b = 1.01) or for dinoflagellates (b = 0.89). For all phytoplankton species considered together, which included also cyanobacteria and haptophytes, b took a value of 0.91, which is significantly higher than 3/4. The observed nearly isometric scaling relationships between production rate and cell volume suggest that there is no relationship between phytoplankton growth rate and cell size. The present analysis confirms recent evidence showing that phytoplankton metabolism in natural conditions does not follow the 3/4-power scaling rule. It is argued that allometric models of plankton growth and metabolism should incorporate scaling parameters measured in situ on natural phytoplankton assemblages, rather than those obtained in the laboratory with monospecific cultures.

Functional links between bioenergetics and bio-optical traits of phytoplankton taxonomic groups: an overarching hypothesis with applications for ocean colour remote sensing

2008-03-17

We review the concept of phytoplankton functional types (PFTs) in marine ecosystems as a means of advancing bio-mechanistic models that can be coupled to the global carbon cycle and the Earth's climate system. Conventional classification of phytoplankton by size may seem arbitrary, but there appears clear links between size and environmental characteristics (availability of essential nutrients and light) that regulate photosynthesis, phytoplankton selection and succession. Taking a minimalist approach, small phytoplankton (picoplankton) survive in permanently stratified systems with low nutrients, high surface light and low light in deep clines, whereas large phytoplankton (microplankton) thrive in high nutrient, turbulent, high light, near surface systems. Nutrient-light environmental conditions are characteristic properties of globally, latitudinal-dispersed biogeochemical provinces. These contrasting nutrient-light regimes define the extreme ends of the bio-energetic scale of photosynthesis and set the end points of the primary range of phytoplankton functional processes. To determine PFTs from remotely sensed ocean colour data, there must be a specific bio-optical trait (BOT) that can be associated with the phytoplankton species or taxa. We investigate the connection of the bio-energetic scale to phytoplankton types and their BOTs, which is the first, but crucial step for classifying PFTs on the basis of functional processes, from which refinements and further partitioning can be developed.

Potential impacts of the spring-neap tidal cycle on shelf sea primary production

Sharples, J. — 2008-03-17

Spring-neap modulation of tidal mixing could potentially have significant effects on the timing and magnitude of primary production in stratified shelf seas. A 1D turbulence model, coupled to a simple model of primary production, is used to identify potential spring-neap impacts. According to this model, changes in the timing of the spring-neap cycle could contribute 10% of inter-annual variability of bloom timing in weak tidal regimes and 25% in areas with stronger tidal currents. In stratified regions away from the tidal mixing fronts, the spring-neap cycle is predicted to result in periodicity in the biological rates within the thermocline, and the turbulent flux of organic carbon into the bottom water. The strongest impacts are predicted within 15–50 km of the tidal mixing fronts, with increases in sub-surface primary production and carbon export. At the fronts, there is substantial extra primary production driven by the spring-neap cycle, contributing an extra 70% annually compared to fronts forced by the M₂ tide only. This impact is reflected in the organic carbon mixed downward into the bottom waters near the front. The results have important implications for the interpretation of observations of primary production, and for the resolution required by shelf-wide models of the marine ecosystem.

Effect of short-term exposure to UVA and UVB on potential phytoplankton production in UK coastal waters

Joint, I., Jordan, M. B. — 2008-03-17

The influence of vertical mixing on phytoplankton sensitivity to UV light has been assessed over an annual cycle. Photosynthesis rates of natural assemblages were compared in samples that were incubated at fixed position in a light gradient and with duplicate samples that simulated vertical mixing by movement in the same gradient with a periodicity of 4 h. This is the typical time-scale of vertical mixing in coastal waters in the English Channel. There were clear seasonal differences in the short-term response of phytoplankton to enhanced UVA+UVB. For most of the year, there was no detectable effect of UV on photosynthetic carbon fixation. But natural assemblages in late winter/early spring, when high UV light may sporadically occur at this latitude, were sensitive to UVA+UVB. In some samples, primary production was 40% of that measured in the absence of UV light. At the time of maximum sensitivity to UV, the phytoplankton assemblage was dominated by diatoms. Simulated vertical mixing resulted in more inhibition of photosynthesis by UVA+UVB light than when samples were at constant light with the same time-integrated irradiance. Transient increases in UVA+UVB due to ozone depletion, such as have been observed over Northern Europe, could have a serious impact on coastal phytoplankton production in late winter/early spring.

Differential microbial uptake of dissolved amino acids and amino sugars in surface waters of the Atlantic Ocean

Zubkov, M. V., Tarran, G. A., Mary, I., Fuchs, B. M. — 2008-03-17

Nitrogen bioavailability is considered to limit the productivity of oceanic oligotrophic gyres, the largest biomes on Earth. In order to assess the microbial requirement for small organic nitrogen molecules in these and other waters, the microbial uptake rates of amino acids (leucine, methionine and tyrosine) and amino sugars (glucosamine and N-acetyl-glucosamine) as well as glucose were compared using a bioassay technique of radiotracer dilution. The bioassays were carried out on four mid-Atlantic meridional transects spanning a latitudinal range from 60°N to 42°S. The mean concentrations of both bioavailable N-acetyl-glucosamine and glucose in the gyres were 1 nM, four times higher than the mean leucine concentration. Despite its lower concentration, the mean turnover time of leucine in the gyres of 15 h was 90 and 9 times shorter than the turnover time of N-acetyl-glucosamine and glucose, respectively. In addition, among amino acids, leucine was taken up in the gyres at a rate of 1.5 times faster than methionine and 2.5 times faster than tyrosine. Hence, oceanic bacterioplankton as a community showed a clear preference for amino acids, particularly leucine, compared with amino sugars. The preferential uptake of amino acids to sugars challenges the concept of microbial nitrogen or carbon limitation in the open ocean.

How best to include the effects of climate-driven forcing on prey fields in larval fish individual-based models

Daewel, U., Peck, M. A., Schrum, C., St John, M. A. — 2008-02-14

If we intend to examine the indirect effects of climate variability on the vital rates of key marine species, climate-induced changes in the spatial-temporal dynamics of prey must be resolved. Recently, structured population simulations have been coupled to ecosystem (nutrient-phytoplankton-zooplankton-detritus, NPZD) models to derive prey fields. Model-derived prey fields offer advantages (e.g. increased spatial-temporal coverage, direct links to climate forcing). In contrast, employing structured population simulations (e.g. stage-based copepod models) has several disadvantages, including the lack of realistic utilization of phytoplankton production, the absence of boundary condition data and a vastly increased coupled model complexity. To avoid the pitfalls limiting the utility of structured population models, we argue for a more simple approach for obtaining a size-structured prey field using NPZD model estimates of bulk zooplankton carbon and in situ zooplankton abundance-at-size data. The approach was developed to obtain prey fields for a larval fish individual-based model (IBM), but the method may offer wide applicability. Moreover, our approach greatly simplifies the coupling of NPZD models and larval fish IBMs and is an example of the reduction in model complexity that will be critical to the development of end-to-end ecosystem models that use, for example, a rhomboid approach to examine trophodynamic climate impacts at basin scales.

Depth distribution during the life history of Neocalanus plumchrus in the Strait of Georgia

Campbell, R. W., Dower, J. F. — 2008-02-14

Neocalanus plumchrus is the most common large copepod in the Northeast Pacific, and plays an important role in the ecosystems of that area; it is particularly common in the Strait of Georgia. In this study, seasonal changes in the vertical distribution of late-stage N. plumchrus at a single site in the Strait of Georgia were measured with an optical plankton counter (OPC) and depth-stratified net samples. Particles measured in situ by the OPC were compared to an empirically measured size distribution for live N. plumchrus. Below 200 m (primarily overwintering individuals), OPC estimated abundance usually corresponded to abundance estimates from net samples. Abundance in the top 100 m was consistently overestimated by the OPC, relative to results from net samples. During overwintering, the vertical distribution shifted downward over time, coinciding with deepwater renewal events in the Strait, and was followed by a partial upward spawning migration by newly moulted adults. Spatial structure at several scales was inconsistent, but significantly different from random. A simple simulation model of depth as a function of time showed that both hydrographic changes and endogenous changes within the animals may influence depth distributions, and suggests that maintenance of position in the water column during diapause is not a passive process.

Carbon isotopic composition of Trichodesmium spp. colonies off Bermuda: effects of colony mass and season

Tchernov, D., Lipschultz, F. — 2008-02-14

Colonies of Trichodesmium spp. are conspicuous, macroscopic components of the life in tropical and subtropical oceans. The large size and the morphology of the colony raise questions regarding the mechanism of carbon supply for photosynthesis. Constraints on these mechanisms may be indicated by the stable carbon isotopic composition (¹³C) that reflects the balance between carbon supply and speciation, as well as the growth rate and colony size. The ¹³C of Trichodesmium off Bermuda measured here revealed a strong correlation between size of individual colonies and season. The smallest colonies, 2–7 µg C colony^–1, showed the lightest ¹³C composition (~–19), increasing to asymptotic values of ~–12 above 7 µg C colony^–1. The average ¹³C of the colonies was lightest immediately after the onset of stratification in the Sargasso Sea, gradually increasing by ~4 to heavier values during the summer. We propose that the mass effect is due to increased use of HCO₃^– by the larger colonies, whereas the seasonal influence may be related to changes in irradiance and pCO₂ affecting the internal carbon cycling.

Interactions between phytoplankton and zooplankton in the hypertrophic Swarzedzkie Lake in western Poland

Goldyn, R., Kowalczewska-Madura, K. — 2008-02-14

The hypertrophic Swarzedzkie Lake, Poland, is characterized by high species diversity, abundance and biomass of both phytoplankton and zooplankton (up to 99.5 mg WW L^–1 and 817.75 µg DW L^–1, respectively). The community grazing rate calculated with the use of two empirical models, and based on herbivorous crustaceans, peaked in spring and early autumn up to 150.6% of water filtered per day, and was the lowest during winter. Simple statistics revealed a positive correlation between zooplankton biomass and chlorophyll a concentration (r = 0.404, P = 0.033) and between zooplankton abundance and phytoplankton biomass (r = 0.42, P = 0.028). Canonical statistics indicated, however, that the relationship exists only with size groups and/or living forms of a few taxonomical groups of phytoplankton. Redundancy analysis (RDA) confirmed a positive influence of the community grazing rate on micro- and nanoplanktonic Cryptophyceae, but not on the microplanktonic Cyanobacteria, as was suggested by canonical correlation analysis. RDA also indicated a weak negative influence on nanoplanktonic Euglenophyceae and Chlorophyceae exerted by filtering crustaceans. Some taxonomically diverse flagellated nanoplanktonic algae were grazing sensitive, whereas microplanktonic cryptophytes and coenobial green algae were significantly grazing resistant.

On the relationship between carbon fixation efficiency and bio-optical characteristics of phytoplankton

Prieto, L., Vaillancourt, R. D., Hales, B., Marra, J. — 2008-02-14

Bio-optical characteristics of phytoplankton and carbon assimilation were measured in the NW Atlantic Ocean near the continental shelf break South of New England (40°N/70°W) in August 2002. Discrete samples included: chlorophyll a (Chl a), phytoplankton absorption (a_ph), the photosynthetic versus irradiance (P versus E) response using carbon fixation, the maximum quantum yield for charge separation at Photosystem II (PSII) (F_v/F_m) and its recovery kinetics and the functional absorption cross-section (_PSII). In this study, we present a robust empirical relationship that illustrates the possibility of estimating photosynthetic carbon uptake using independent measures of absorption and variable fluorescence. Covariations between the initial slope of the photosynthesis-irradiance curve normalized per Chl a, the a_ph index of the ratio of photoprotective to total photosynthetic carotenoids and the dark recovery of the maximum quantum yield efficiency of PSII suggest a new pathway to estimate primary production in the marine environment, including the effect of the recent light history. Recovery kinetics in laboratory culture confirmed the effects of light history in the derived parameter from the dark incubations from natural populations.

Concentration of fixed plankton samples via settling: how long is long enough?

Claessens, M., Prast, M. — 2008-02-14

Enumerating plankton cells, especially ciliates, usually requires settling of samples in order to concentrate the cells. Poorly settled samples could introduce large errors into plankton counts. The time sufficient to settle all ciliates has, however, never been established in the literature. Here, using both theoretical and empirical studies, we suggest improvements of the current method, which mostly relies on experience to determine settling times. Ciliate density was used to calculate the theoretical settling time of fixed ciliates with the Stokes equation. To determine ciliate density (g mL^–1), we modified and established a density gradient centrifugation method. We found that ciliate density was in the range 1.02–1.08 g mL^–1. Additionally, empirical sinking rates were gathered semi-automatically with a digital camera system. The theoretical and experimental settling times were in the same range, though there were differences for some species. From this, we recommend working with the empirical sinking rates that are more reliable: 0.5 and 1.7 mm min^–1 for fixed marine (at salinities of 16 and 40, respectively) samples and 2.4 mm min^–1 for fixed freshwater samples. Using these rates potentially saves up to 95% of the time for settling compared to old, experience derived times. Although ciliate density was significantly correlated with settling rates, there was no correlation with particle size and shape.

Diel cycles of reproduction and vertical migration in the marine cladocerans Pseudevadne tergestina and Penilia avirostris

Wong, C. K., Li, V. C. Y., Chan, A. — 2008-02-14

The marine cladocerans Pseudevadne tergestina and Penilia avirostris use different strategies to avoid visual predators. Pseudevadne tergestina exhibited a pronounced diel cycle in reproduction, but did not perform diel vertical migration. Parthenogenetic females with fully developed embryos were absent during the day in both fishless enclosures and in the natural environment. This observation suggests that the diel cycle was due to nocturnal release of neonates and not the result of selective predation by fish. The need to use the compound eye to locate food would force P. tergestina to remain in the illuminated surface water during the day and release their offspring in darkness thus decreasing their visibility to fish. The diel cycle in reproduction in P. avirostris was not as pronounced as that in P. tergestina, but females with mature embryos were still more common at night than during the day. As a grazer of phytoplankton, P. avirostris performed diel vertical migration and retreated to dark, deeper waters during the day, avoiding visual predators.

Analysis and modelling of the interactive effects of temperature and light on phytoplankton growth and relevance for the spring bloom

Nicklisch, A., Shatwell, T., Kohler, J. — 2008-02-14

Global climate change alters the relationship between temperature and light in aquatic ecosystems, which is expected to affect the success of different phytoplankton species. To examine this, the interactions between temperature, photoperiod and light exposure (LE) (integral daily light supply) on specific growth rates were analysed for Limnothrix redekei, Planktothrix agardhii (cyanobacteria), Nitzschia acicularis and Stephanodiscus minutulus (diatoms). A model of factor interactions was developed based on new (P. agardhii and St. minutulus) and previously published laboratory studies. It describes the measured data with high precision. Temperature and photoperiod affect the parameters of the light-growth response curve differently, but these effects are the same for all species. The link between functions for temperature and photoperiod is more species-specific. Using meteorological data, the model developed here was used to study the interplay of these factors during a spring bloom in Lake Müggelsee (Berlin). It was found that while all three factors influenced phytoplankton growth, temperature and photoperiod were more important than LE. Both the intensities of the factors and the interactions between them influenced each species to a different degree. The results may help improve our understanding and ability to predict shifts in phytoplankton communities caused by weather patterns and climate change.

Contribution of Crenarchaeota and Euryarchaeota to the prokaryotic plankton in the coastal northwestern Black Sea

Stoica, E., Herndl, G. J. — 2008-02-14