Discrimination of marine algal taxonomic groups using delayed fluorescence spectroscopy

We present a method for in situ monitoring of phytoplankton composition changes in a marine environment. The method is based on delayed fluorescence excitation spectra analyzed with CHEMTAX software, which is generally used for determination of phytoplankton communities with HPLC pigment data. Delayed fluorescence (DF) is a photosynthetic parameter that can only be measured in living cells. Algal DF excitation spectra are group-specific, based on their composition of photosynthetic pigments.DF excitation spectra of 14 marine algal species from different families were measured with a delayed fluorescence spectrometer. Mixtures were prepared from northern Adriatic algal species representing six taxonomic groups: dinoflagellates (Prorocentrum minimum), diatoms (Skeletonema costatum), cyanobacteria (Synechococcus sp.), prasinophytes (Micromonas sp.), cryptophytes (Teleaulax sp.), and prymnesiophytes (Isochrysis galbana). The DF excitation spectra (DFS) and HPLC pigment compositions of the mixtures were analyzed with CHEMTAX software. The prediction power of DFS–CHEMTAX method was comparable to HPLC–CHEMTAX.     

Grazing impact of microzooplankton on a diatom bloom in a mesocosm as estimated by pigment-specific dilution technique

To investigate the impact of microzooplankton grazing on phytoplankton bloom in coastal waters, an enclosure experiment was conducted in Saanich Inlet, Canada during the summer of 1996. Daily changes in the microzooplankton grazing rate on each phytoplankton group were investigated with the growth rates of each phytoplankton group from the beginning toward the end of bloom using the dilution technique with high-performance liquid chromatography (HPLC). On Day 1 when nitrate and iron were artificially added, chlorophyll a concentration was relatively low (4.3 μg l⁻¹) and 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes were predominant in the chlorophyll biomass. However, both the synthetic rates and concentrations of 19′-hexanoyloxyfucoxanthin declined before bloom, suggesting that 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes weakened. Chlorophyll a concentration peaked at 23 μg l⁻¹ on Day 4 and the bloom consisted of the small chain-forming diatoms Chaetoceros spp. (4 μm in cell diameter). Diatoms were secondary constituents in the chlorophyll biomass at the beginning of the experiment, and the growth rates of diatoms (fucoxanthin) were consistently high (>0.5 d⁻¹) until Day 3. Microzooplankton grazing rates on each phytoplankton group remarkably increased except on alloxanthin-containing cryptophytes after the nutrient enrichments, and peaked with >0.6 d⁻¹ on Day 3, indicating that >45% of the standing stock of each phytoplankton group was removed per day. Both the growth and mortality rates of alloxanthin-containing cryptophytes were relatively high (>1 and >0.5 d⁻¹, respectively) until the bloom, suggesting that a homeostatic mechanism might exist between predators and their prey. Overall, microzooplankton grazing showed a rapid response to the increase in phytoplankton abundance after the nutrient enrichments, and affected the magnitude of the bloom significantly. High grazing activity of microzooplankton contributed to an increase in the abundance of heterotrophic dinoflagellates with 7-24 μm in cell size, the fraction of large-sized (>10 μm) chlorophyll a, and stimulated the growth of larger-sized ciliates after the bloom.     

Pigment-Based Chemotaxonomy - A Quick Alternative to Determine Algal Assemblages in Large Shallow Eutrophic Lake?

Pigment-based chemotaxonomy and CHEMTAX software have proven to be a valuable phytoplankton monitoring tool in marine environments, but are yet underdeveloped to determine algal assemblages in freshwater ecosystems. The main objectives of this study were (1) to compare the results of direct microscopy and CHEMTAX in describing phytoplankton community composition dynamics in a large, shallow and eutrophic lake; (2) to analyze the efficiency of the pigment-based method to detect changes in phytoplankton seasonal dynamics and during rapid bloom periods; (3) to assess the suitability of specific marker pigments and available marker pigment:chlorophyll a ratios to follow seasonal changes in eutrophic freshwater environment. A 5-year (2009-2013) parallel phytoplankton assessment by direct microscopy and by CHEMTAX was conducted using published marker pigment:chlorophyll a ratios. Despite displaying some differences from microscopy results, the pigment-based method successfully described the overall pattern of phytoplankton community dynamics during seasonal cycle in a eutrophic lake. Good agreement between the methods was achieved for most phytoplankton groups - cyanobacteria, chlorophytes, diatoms and cryptophytes. The agreement was poor in case of chrysophytes and dinoflagellates. Our study shows clearly that published marker pigment:chlorophyll a ratios can be used to describe algal class abundances, but they need to be calibrated for specific freshwater environment. Broader use of this method would enable to expand monitoring networks and increase measurement frequencies of freshwater ecosystems to meet the goals of the Water Framework Directive.     

Study of phytoplankton characteristics in Tolo Harbour,Hong Kong,by HPLC analysis of chemotaxonomic pigments

Spatial and seasonal characteristics of phytoplankton in Tolo Harbour, Hong Kong, were studied by microscopic observation of phytoplankton samples and HPLC analysis of chemotaxonomic pigments. Diatoms dominated the phytoplankton. Common diatoms included Skeletonema costatum and species of Cerataulina, Leptocylindrus, Pseudo-nitzschia and Thalassiosira. Dinoflagellates occurred sporadically and mainly in the inner part of the harbour. The dinoflagellate Scrippsiella trochoidea was the causative organism for the red tide occurrences in March, April and September 2001. Significant positive correlations between fucoxanthin and diatoms and between peridinin and dinoflagellates suggested that fucoxanthin and peridinin were valuable chemotaxonomic markers for diatoms and dinoflagellates, respectively. Analysis of pigment ratios revealed that red tide events caused by dinoflagellates were marked by increase in the value of PERI:chl a and decrease in the value of FUCO:chl a. Increase in the value of FUCO:chl a also revealed the presence of a dense population of Pseudo-nitzschia that was not indicated by increase in chlorophyll a and fucoxanthin concentrations. Pigment analysis also revealed the presence of cyanobacteria, silicoflagellates, cryptophytes and green algae in the surface waters of Tolo Harbour.     

Applicability of Hydrogen Peroxide in Brown Tide Control – Culture and Microcosm Studies

Brown tide algal blooms, caused by the excessive growth of Aureococcus anophagefferens, recur in several northeastern US coastal bays. Direct bloom control could alleviate the ecological and economic damage associated with bloom outbreak. This paper explored the effectiveness and safety of natural chemical biocide hydrogen peroxide (H₂O₂) for brown tide bloom control. Culture studies showed that H₂O₂ at 1.6 mg L⁻¹ effectively eradicated high density A. anophagefferens within 24-hr, but caused no significant growth inhibition in the diatoms, prymnesiophytes, green algae and dinoflagellates of >2–3 μm cell sizes among 12 phytoplankton species tested over 1-week observation. When applied to brown tide bloom prone natural seawater in a microcosm study, this treatment effectively removed the developing brown tide bloom, while the rest of phytoplankton assemblage (quantified via HPLC based marker pigment analyses), particularly the diatoms and green algae, experienced only transient suppression then recovered with total chlorophyll a exceeding that in the controls within 72-hr; cyanobacteria was not eradicated but was still reduced about 50% at 72-hr, as compared to the controls. The action of H₂O₂ against phytoplankton as a function of cell size and cell wall structure, and a realistic scenario of H₂O₂ application were discussed.     

Phytoplankton Assemblages and Their Dominant Pigments in the Nervion River Estuary

In the Nervion River estuary surface samples were taken from March to September 2003 at six sites covering most of the salinity range with the aim to know the biomass and taxonomic composition of phytoplankton assemblages in the different segments. Nine groups of algae including cyanobacteria, diatoms, dinoflagellates, chlorophytes, prasinophytes, euglenophytes, chrysophytes, haptophytes, raphidophytes and cryptophytes were identified by means of a combination of pigment analysis by high-performance liquid chromatography (HPLC) and microscopic observations of live and preserved cells. Diatoms, chlorophytes and cryptophytes were the most abundant algae in terms of cells number, whereas fucoxanthin, peridinin, chlorophyll b (Chl b) and alloxanthin were the most abundant auxiliary pigments. Based on multiple regression analysis, in the outer estuary (stations 0, 1, 2 and 3) about 93% of the chlorophyll a (Chl a) could be explained by algae containing fucoxanthin and by algae containing Chl b, whereas in the rest of the estuary most of the Chl a (about 98%) was accounted for by fucoxanthin, Chl b and alloxanthin containing algae. The study period coincided with that of most active phytoplankton growth in the estuary and fucoxanthin was by far the dominant among those signature pigments. Several diatoms, chrysophytes, haptophytes and raphydophytes were responsible for fucoxanthin among identified species. Besides, dinoflagellates with a pigment pattern corresponding to chrysophytes and type 4 haptophytes were identified among fucoxanthin-bearing algae. Cryptophytes were the most abundant species among those containing alloxanthin. The maximum of Chl b registered at the seaward end in April coincided with a bloom of the prasinophytes Cymbomonas tetramitiformis, whereas the Chl b maxima in late spring and summer were accounted for by prasinophytes in the middle and outer estuary and by several species of chlorophytes in the middle and inner estuary. Other Chl b containing algae were euglenophytes and the dinoflagellate Peridinium chlorophorum. Dinoflagellates constituted generally a minor component of the phytoplankton.     

Phytoplankton and physical-chemical features of Tavropos Reservoir,Greece

Phytoplankton biomass values in Tavropos Reservoir, ranging from 92 to 1071 mg m^–3, are within a range characteristic of oligotrophic waters. The seasonal sequence of biomass shows three annual peaks, differing from the monoacmic pattern seen in oligotrophic lakes. This sequence was profoundly affected by changes in water withdrawal and inflow rates. Diatoms, cryptophytes, chrysophytes and dinoflagellates, in that order, were the major constituents of the reservoir phytoplankton. The succession, from diatoms and chrysophytes in late winter-spring, to centric diatoms in late spring-summer and again to diatom-chrysophytes in late autumn was similar to that in oligotrophic lakes.     

Development of phytoplankton in Lake Pääjärvi (Finland) during under-ice convective mixing period

The development of winter phytoplankton communities was studied in both shallow and deep areas of Lake Pääjärvi, southern Finland, during the final 2 weeks of winter ice cover. Phytoplankton was mainly composed of diatoms, cryptophytes and chrysophytes. The diatoms Aulacoseira and Rhizosolenia were always uniformly distributed with depth, initially probably due to mixing induced by heat flux from the sediment and later due to thermal convection. Motile Rhodomonas cryptophytes and Chrysococcus chrysophytes were most abundant near the ice showing that, despite their small size, they were partly able to resist mixing by convection. Their ability to stay in more illuminated water layers was reflected in net rates of increase about an order of magnitude higher than those of diatoms in the middle of the lake. Given the low temperatures and convection, the observed net rates of increase of motile taxa were very high compared to growth rates reported in the literature. The gradual increase in light availability following melting of ice led to a consistent increase in the abundances of major phytoplankton taxa irrespective of deep convective circulation. It is suggested that those algae most abundant at the time of ice break have a competitive advantage in the following open water conditions when nutrients are abundant but deep water circulation limits light availability. The results emphasize that in lakes which cool below the maximum density of water before freezing, apparently small differences in temperature and light conditions can cause important changes in the circulation patterns that impact on phytoplankton development.     

Lipid biomarkers and trophic linkages between phytoplankton, zooplankton and anchovy (Engraulis encrasicolus) larvae in the NW Mediterranean

Identification and quantification of the main fatty acids inphytoplankton, zooplankton and Engraulis encrasicolus larvaewere used to study the relationship among the different trophiclevels. Three northwestern Mediterranean sea areas were studiedas representative of potentially heterogeneous environments.HPLC pigment analysis was performed to assess the contributionof different phytoplankton groups to total chlorophyll biomass.In spite of the physical variability, a homogeneous patternin phytoplankton composition among areas was found, with thePrymnesiophyceae representing more than half of the phytoplanktonchlorophyll biomass. This homogeneous spatial pattern was alsoreflected in the phytoplankton fatty acid proportions. Thus,significant amount of 14:0, 18:1(n-9) and 18:4(n-3) could berelated to the dominance of the prymnesiophytes; the low proportionof fatty acid markers of diatoms and dinoflagellates agreedwith the low abundances of those phytoplankton groups. Zooplanktonfatty acids also showed homogeneous proportions in the studiedareas, with a preferential accumulation of 16:1(n-7) and 20:5(n-3),and 22:6(n-3). This observation may indicate a preferentialgrazing on diatoms and dinoflagellates. Anchovy larvae displayeddiffering fatty acids contributions related to size. Large larvaecontained a higher proportion of polyunsaturated fatty acid(PUFA). Small larvae showed a high percentage of 18:1(n-9) and18:4(n-3) fatty acids, considered Prymnesiophyceae biomarkers.These results suggest an additional food resource for the anchovythat is complementary to a zooplankton diet, but probably ofprotozoan origin (i.e. microzooplankton). Significant logarithmicrelationships were found between dry mass and 20:5(n-3) and22:6(n-3) fatty acid proportions in E. encrasicolus larvae.These PUFA are considered essential for the fish-larvae development.     

Shifts in the dominance between diatoms and cryptophytes during three late summers in the Bransfield Strait (Antarctic Peninsula)

Recent global warming reduces surface water salinity around the Antarctic Peninsula as a result of the glacial meltwater runoff, which increases the occurrence and abundance of certain phytoplankton groups, such as cryptophytes. The dominance of this particular group over diatoms affects grazers, such as Antarctic krill, which preferentially feed on diatoms. Using three late summer data sets from the Bransfield Strait (2008–2010), we observed variations in the dominant phytoplankton groups determined by HPLC/CHEMTAX pigment analysis and confirmed by microscopy. Results indicate that the dominance of diatoms, particularly in 2008 and 2009, was associated with a deeper upper mixed layer (UML), higher salinity and warmer sea surface temperature. In contrast, cryptophytes, which were dominant in 2010, were associated with a shallower UML, lower salinity and colder sea surface temperatures. The low diatom biomass observed in the summer of 2010 was associated with high nutrient concentration, particularly silicate, and low chlorophyll a (summer monthly average calculated from satellite images). The interannual variability here observed suggests a delayed seasonal succession cycle of phytoplankton in the summer of 2010 associated with a cold summer and a late ice retreat process in the region. This successional delay resulted in a notable decrease of primary producers’ biomass, which is likely to have impacted regional food web interactions. This study demonstrates the susceptibility of the Antarctic phytoplankton community structure to air temperature, which directly influences the timing of ice melting and consequently the magnitude of primary production and succession pattern of phytoplankton groups.     

Confirmation of the wide host range of Parvilucifera corolla (Alveolata,Perkinsozoa)

Marine parasites of the genus Parvilucifera have been described as endoparasitoids of dinoflagellates. Recently, the species Parvilucifera corolla was described, but its host range was not examined. Here, the host selectivity of P. corolla was screened, including 110 strains of dinoflagellates (24 genera) and other microalgal groups as potential hosts. Infections and the full life cycle of the parasitoid were observed in 73 strains (16 genera) of dinoflagellates. Parvilucifera corolla did not infect most chlorophytes, cryptophytes, chrysophytes, diatoms, haptophytes and raphidophytes but one strain of Pyramimonas (chlorophyte) was infected, although without viable sporangia. In Symbiodinium natans, a transition to the coccoid stage was induced above a certain parasite:host ratio. These results confirm P. corolla as a generalist parasitoid of dinoflagellates, with important differences in host range regarding other species of the genus.     

The evolution and phytoplankton composition of mucilaginous aggregates in the northern Adriatic Sea

Recurrent occurrences of visible mucilage “clouds” that cover areas up to several hundred kilometres with vertical dimensions of 20–30 m have been recorded in the stratified water column in the northern Adriatic. In the past this was described as “mare sporco” phenomenon. Past studies of the phenomenon indicated that phytoplankton is an important component of mucilage. Our research revealed the composition of phytoplankton assemblages in different types of mucilaginous aggregates collected during the summers of 1997 and 2000 using pigment biomarkers (HPLC). Phytoplankton biomass in the mucilage samples was very high, ranging from 7.9 μg g⁻¹ to 390.8 μg g⁻¹ of chlorophyll a per unit of dry mass of mucilage. The phytoplankton community in the early, loose stage of mucilaginous aggregates was heterogeneous, as indicated by the diversity of detected pigments. The number of phytoplankton groups decreased as the aggregates aged and diatoms increased in relative biomass (up to 92.7%). Phytoplankton biomass in seawater was similar in years with and without mucilage; however, significantly higher contributions to the total biomass of 19′-hexanoyloxyfucoxanthin-containing phytoplankton (prymnesiophytes) were found in the upper 10 m in spring of the “mucilaginous years” (1997 and 2000) followed by prevalence of diatoms in summer. The F_p pigment index used to assess seawater trophic conditions reached lower values in April–May in mucilaginous years (1997 and 2000) compared to non-mucilaginous years (1998 and 1999). We conclude that the role of prymnesiophytes and other small flagellates is crucial for the initial phases of mucilage appearance. Aggregates represent a favourable environment for the secondary development of opportunistic diatoms that foster mucilage formation.     

Different photochemical responses of phytoplankters from the large shallow Taihu Lake of subtropical China in relation to light and mixing

The maximum quantum yield of photosystem II was estimated from variable chlorophyll a fluorescence in samples of phytoplankton collected from the Taihu Lake in China to determine the responses of different phytoplankters to irradiance and vertical mixing. Meteorological and environmental variables were also monitored synchronously. The maximum quantum yield of three phytoplankton groups: cyanobacteria, chlorophytes, and diatoms/dinoflagellates, showed a similar diurnal change pattern. F _v/F _m decreased with a significant depth-dependent variation as irradiance increased during the morning and increased as irradiance declined in the afternoon. Furthermore, the rates of F _v/F _m depression were dependent upon the photon flux density, whereas the rates of recovery of F _v/F _m were dependent upon the historical photon density. Moreover, photoinhibition affected the instantaneous growth rates of phytoplankton. Although at noon cyanobacteria had a higher photoinhibition value (up to 41%) than chlorophytes (32%) and diatoms/dinoflagellates (34%) at the surface, no significant difference in diurnal growth rates among the three phytoplankton groups were observed indicating that cyanobacteria could photoacclimate better than chlorophytes and diatoms/dinoflagellates. In addition, cyanobacteria had a higher nonphotochemical quenching value than chlorophytes and diatoms/dinoflagellates at the surface at noon, which indicated that cyanobacteria were better at dissipating excess energy. The ratios of enclosed bottle samples F _v/F _m to free lake samples F _v/F _m showed different responses for the three phytoplankton groups to irradiance and vertical mixing when wind speed was approximately constant at about 3.0 m s⁻¹. When wind speed was lower than 3.0 m s⁻¹, cyanobacteria accumulated mainly at the surface and 0.3 m, because of their positive buoyancy, where diurnal growth rates of phytoplankton were relatively higher than those at 0.6 m and 0.9 m. Chlorophytes were homogenized completely by vertical mixing, while diatoms/dinoflagellates avoided active high irradiance by moving downward at noon, and then upward again when irradiance decreased. These results explain the dominance of cyanobacteria in Taihu Lake. Handling editor: L. Naselli-Flores     

Temporal and spatial dynamics of phytoplankton diversity in the East China Sea near Jeju Island (Korea): A pyrosequencing-based study

The East China Sea (ECS) has long been considered an important monitoring site for oceanic ecosystem changes because many water currents and river discharges constantly influence this area. In this study, the community structure and diversity of phytoplankton in the northern part of the ECS adjacent to Jeju Island were explored using small subunit ribosomal RNA (SSU) pyrosequencing. We analysed samples collected from four stations from the surface and at 30-m and 50-m depths during April and September 2011. We observed spatial and temporal variations in the phytoplankton community. Among phytoplankton, diatoms and dinoflagellates constituted a major portion at all stations (60–90%). However, comparison of the April and September samples showed seasonal variation and shifts in the dispersion of diatom and dinoflagellates among stations. Among stations, diatoms dominated St. 1 and others were dominated by dinoflagellates. Furthermore, phylotypes of potentially toxin-producing genera such as Karlodinium, Heterocapsa, Gymnodinium, Gyrodinium, and Pseudo-nitzschia were dominant in this area.     

Composition of inorganic and organic nutrient sources influences phytoplankton community structure in the New River Estuary, North Carolina

The New River Estuary, NC, is a nutrient-sensitive, eutrophic water body that is prone to harmful algal blooms. High annual loading from the watershed of varying nutrient forms, including inorganic phosphorus and inorganic and organic nitrogen, may be linked to the persistence of algal blooms in the estuary. In order to evaluate phytoplankton response to nutrient inputs, a series of in situ nutrient addition experiments were carried out during June 2010 to July 2011 on water from an estuarine site known to support algal blooms. Estuarine water was enriched with nutrients consisting of individual and combined sources of dissolved inorganic nitrogen, orthophosphate, urea, and a natural dissolved organic nitrogen (DON) addition derived from upstream New River water. The combined inorganic N and P addition most frequently stimulated phytoplankton biomass production as total chlorophyll a. The responses of diagnostic (of major algal groups) photopigments were also evaluated. Significant increases in peridinin (dinoflagellates), chlorophyll b (chlorophytes), and myxoxanthophyll (cyanobacteria) were most frequently promoted by additions containing riverine DON. Significant increases in zeaxanthin (cyanobacteria) were more frequently promoted by inorganic nitrogen additions, while increases in fucoxanthin (diatoms) and alloxanthin (cryptophytes) were not promoted consistently by any one nutrient treatment. Evaluating the impact of varying nutrient forms on phytoplankton community dynamics is necessary in order to develop strategies to avoid long-term changes in community structure and larger-scale changes in ecosystem condition.     

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.     

Responses of Phyto- and Zooplankton Communities to Prymnesium polylepis (Prymnesiales) Bloom in the Baltic Sea

A large bloom of Prymnesium polylepis occurred in the Baltic Sea during the winter 2007 – spring 2008. Based on numerous reports of strong allelopathic effects on phytoplankton exerted by P. polylepis and its toxicity to grazers, we hypothesized that during this period negative correlations will be observed between P. polylepis and (1) main phytoplankton groups contributing to the spring bloom (i.e., diatoms and dinoflagellates), and (2) zooplankton growth and abundance. To test these hypotheses, we analyzed inter-annual variability in phytoplankton and zooplankton dynamics as well as growth indices (RNA∶DNA ratio) in dominant zooplankton in relation to the Prymnesium abundance and biomass. Contrary to the hypothesized relationships, no measurable negative responses to P. polylepis were observed for either the total phytoplankton stocks or the zooplankton community. The only negative response, possibly associated with P. polylepis occurrence, was significantly lower abundance of dinoflagellates both during and after the bloom in 2008. Moreover, contrary to the expected negative effects, there were significantly higher total phytoplankton abundance as well as significantly higher winter abundance and winter-spring RNA∶DNA ratio in dominant zooplankton species in 2008, indicating that P. polylepis bloom coincided with favourable feeding conditions for zooplankton. Thus, primary consumers, and consequently also zooplanktivores (e.g., larval fish and mysids), may benefit from haptophyte blooms, particularly in winter, when phytoplankton is scarce.     

Short-term interactive effects of ultraviolet radiation,carbon dioxide and nutrient enrichment on phytoplankton in a shallow coastal lagoon

The main goal of this study was to evaluate short-term interactions between increased CO₂, UVR and inorganic macronutrients (N, P and Si) on summer phytoplankton assemblages in the Ria Formosa coastal lagoon (SW Iberia), subjected to intense anthropogenic pressures and highly vulnerable to climate change. A multifactorial experiment using 20 different nutrient-enriched microcosms exposed to different spectral and CO₂ conditions was designed. Before and after a 24-h in situ incubation, phytoplankton abundance and composition were analysed. Impacts and interactive effects of high CO₂, UVR and nutrients varied among different functional groups. Increased UVR had negative effects on diatoms and cyanobacteria and positive effects on cryptophytes, whereas increased CO₂ inhibited cyanobacteria but increased cryptophyte growth. A positive synergistic interaction between CO₂ and UVR was observed for diatoms; high CO₂ counteracted the negative effects of UVR under ambient nutrient concentrations. Nutrient enrichments suppressed the negative effects of high CO₂ and UVR on cyanobacteria and diatoms, respectively. Beneficial effects of CO₂ were observed for diatoms and cryptophytes under combined additions of nitrate and ammonium, suggesting that growth may be limited by DIC availability when the primary limitation by nitrogen is alleviated. Beneficial effects of high CO₂ and UVR in diatoms were also induced or intensified by ammonium additions.     

Distribution and development of under-ice phytoplankton in 90-m deep water column of Lake Päijänne (Finland) during spring convection

Distribution and development of phytoplankton were studied in the deep and large Lake Päijänne from mid-winter until the disappearance of ice. Diatoms were an important part of the phytoplankton assemblage and, with cryptophytes and chrysophytes, made up 50–80% of the phytoplankton biomass. In mid-winter, chlorophyll a and phytoplankton biomass were uniformly distributed over the whole water column down to a depth of 90 m. Thus, most of the phytoplankton was in virtual darkness and there was negligible growth. Only motile cryptophytes were concentrated in the layers below the ice and were rare in deep water. After the disappearance of snow, convection developed, but at first cryptophytes were able to resist mixing. When convection turned from penetrative to predominantly horizontal, all phytoplankton were generally uniformly distributed in the water column. In spite of the full under-ice overturn with low average availability of light, the phytoplankton biomass doubled in April. The growth of cryptophytes was higher than that of diatoms, suggesting that motile species gained an advantage by being able to maintain themselves in the upper, illuminated layers. The results show that knowledge of the basic physical framework is essential for interpretation of under-ice phytoplankton results.     

Algal Diet of Small-Bodied Crustacean Zooplankton in a Cyanobacteria-Dominated Eutrophic Lake

Small-bodied cladocerans and cyclopoid copepods are becoming increasingly dominant over large crustacean zooplankton in eutrophic waters where they often coexist with cyanobacterial blooms. However, relatively little is known about their algal diet preferences. We studied grazing selectivity of small crustaceans (the cyclopoid copepods Mesocyclops leuckarti, Thermocyclops oithonoides, Cyclops kolensis, and the cladocerans Daphnia cucullata, Chydorus sphaericus, Bosmina spp.) by liquid chromatographic analyses of phytoplankton marker pigments in the shallow, highly eutrophic Lake Võrtsjärv (Estonia) during a seasonal cycle. Copepods (mainly C. kolensis) preferably consumed cryptophytes (identified by the marker pigment alloxanthin in gut contents) during colder periods, while they preferred small non-filamentous diatoms and green algae (identified mainly by diatoxanthin and lutein, respectively) from May to September. All studied cladoceran species showed highest selectivity towards colonial cyanobacteria (identified by canthaxanthin). For small C. sphaericus, commonly occuring in the pelagic zone of eutrophic lakes, colonial cyanobacteria can be their major food source, supporting their coexistence with cyanobacterial blooms. Pigments characteristic of filamentous cyanobacteria and diatoms (zeaxanthin and fucoxanthin, respectively), algae dominating in Võrtsjärv, were also found in the grazers’ diet but were generally avoided by the crustaceans commonly dominating the zooplankton assemblage. Together these results suggest that the co-occurring small-bodied cyclopoid and cladoceran species have markedly different algal diets and that the cladocera represent the main trophic link transferring cyanobacterial carbon to the food web in a highly eutrophic lake.