Microphytoplankton Assemblages Research Articles

Dinoflagellate cyst and nannoplankton assemblages from the Middle Eocene Kuma Formation of Crimean Peninsula — biostratigraphy and palynofacies

The stratigraphic correlation of the Kuma Formation from the south of the Russian Platform and the Crimea-Caucasian region has been a matter of debate for decades. A bed-by-beds study of dinocysts and nannoplankton from the the Kuma Formation made it possible to recognize a sequence of biotic events, important for defining and correlating the biozones. At the Bakhchisarai limestone quarry, six dinoflagellate zones and six nannoplankton zonal units were recognized, assignable to the Middle Eocene Lutetian and Bartonian stages. Our paleoecological analyses of the organic-walled microphytoplankton assemblages and of the palynofacies indicate that the the Kuma Formation deposited in the inner shelf zone, probably of anoxic and eutrophic settings within the Pontic-Transcaucasian magmatic belt. Assemblages of organic-walled microphytoplankton comprise potentially toxic dinoflagellate species of Alexandrium. The Middle Eocene Climatic Optimum event is recorded in the upper part of the the Kuma Formation, presumably indicated by the Dracodinium rhomboideum dinoflagellate zone, which is characterized by an acme of Dracodinium laszczynskii Gedl and the lowest occurrence of nannofossils Reticulofenestra bisecta (Hay, Mohler and Wade) Roth. Two new species are described: Pentadinium rugosum Vasilyeva, n. sp. (dinocyst) and Corannulus tauricum Musatov, n. sp. (nannofossil). The bioevents and biozones established herein are significant for correlations of the Middle Eocene of the Crimean Peninsula and the south of the Russian Platform.

Spatial-temporal variability of microphytoplankton assemblages including harmful microalgae in a tropical semi-enclosed strait (Johor Strait, Malaysia)

Harmful algal blooms (HABs) were not new to the tropical semi-enclosed Johor Strait, with incident records that could trace back to the 1980s. HAB monitoring in the area, often, is reactive, focusing only on HAB taxa previously causing problems but neglecting potential emerging HABs. To develop datasets on HABs that can better inform and improve management practices, monitoring should expand to sample whole microphytoplankton communities. In this study, microphytoplankton community structure across the Strait was investigated. Abundances of microphytoplankton and a suite of in situ water parameters of temperatures, salinity, pH, dissolved oxygen levels, macronutrients, and chlorophyll-a contents were collected at ten sites across the Strait at monthly intervals from January 2017 to December 2018. A total of 48 genera (51 taxa) microphytoplankton were identified microscopically. Diatom was the most diverse group (32 genera), followed by dinophyte (15 genera). Bloom-forming species included diatoms Chaetoceros, Coscinodiscus, Eucampia, Pseudo-nitzschia, Rhizosolenia, Skeletonema, Thalassiosira, and dinophytes Blixaea quinquecornis and Scrippsiella. Diatom taxa that exhibit high in situ growth rates were predominant in the low-nutrient marine-influenced environment. Bloom-forming taxa including HAB taxa were found dominant in the environment with high nutrient levels and mesohaline, salinity-stratified conditions. This study provides valuable baseline data that could assist in monitoring and prediction of HABs in the Johor Strait and could be of reference to other similar tropical coastal systems.

Microphytoplankton from middle palaeolatitudes of the Southern Hemisphere – a record from climate change strata of Baltica’s O/S boundary

In the present study, data on the diversity of acritarch and prasinophyte microphytoplankton were collected from Ordovician–Silurian transitional strata at three different geological localities in Poland: southern part of the Holy Cross Mountains (Malopolska Block, southern Poland), East European Platform (Baltica), and Koszalin–Chojnice Zone (NW Poland). The material represents the mucronata (trilobite) to vesiculosus (graptolite) biozones. The Ordovician microphytoplankton assemblages are characterized by low frequency and low diversity: up to 110 specimens and 12 genera per slide, in contrast to Silurian ones that are more frequent and more diverse – >3000 specimens and 16 genera per slide (diversity at the species level is also higher). Throughout the Ordovician part of the succession, typical Ordovician genera occur, such as Acanthodiacrodium , Ordovicidium , Orthosphaeridium and large Baltisphaeridium , together with isolated occurrences of typical Silurian genera, especially in the upper part of the Hirnantian (e.g., Diexallophasis ). The Silurian assemblages are typified by high frequency of prasinophytes followed by typical Silurian acritarchs: Tylotopalla , Ammonidium , Domasia and Oppilatala . The palynological material from Poland is compared with material known from other localities: the Rapla and Valga boreholes (Estonia) and Anticosti Island (Canada), as well as others placed in different palaeocontinents and different bathymetric zones. Palynological assemblages obtained from Polish material confirm that bathymetry played the key role in taxonomic diversification of the microphytoplankton assemblage. This supports the existing models of distribution: dominance of prasinophytes (leiospheres) and cryptospores is characteristic for shallow-water environments. In deeper water, dominance of acanthomorphs is observed. In the deepest-water zones – mixed assemblages occur. For the Holy Cross Mountains, microphytoplankton frequency is compared with a TOC curve in the same interval.

An endemic microphytoplankton assemblage from Middle Devonian Iberia and its paleogeographical significance

Diverse microphytoplankton assemblages, including 72 taxa belonging to 27 genera of acritarchs and prasinophyte phycomata, have been recovered from Middle Devonian rocks in northern Spain, revealing an endemic flora dissimilar to coeval assemblages. These deposits are of early Givetian age and consist of the laterally equivalent Naranco, Huergas and Gustalapiedra formations of Asturias, León and Palencia provinces. At the time, Iberia was part of the Armorican Terrane Assemblage, a comparatively isolated island chain positioned between Laurussia and Gondwana. The studied formations represent a marine transect across a nearshore–offshore gradient and consist of a large clastic unit sandwiched between extensive carbonate deposits. This clastic unit incorporates the Kačák Event, an important global extinction event associated with marine anoxia. Herein, the suite of generally well-preserved microphytoplankton assemblages is described and their stratigraphical and biogeographical importance are considered. The microphytoplankton represents a single assemblage deposited in a short interval and is interpreted as being endemic. The assemblage is only moderately similar to contemporary assemblages from Laurussia and Gondwana and, although certain characteristic Middle Devonian taxa are present, other common species such as Arkonites bilixus and Tyligmasoma alargada are absent. While no unique taxa are found here, the taxa which are present represent a particular combination of species not seen elsewhere. Certain taxa appear which may have discordant temporal ranges, though no major inferences can be made from them as only two taxa both occur in significant numbers and have a confident identification. This assemblage adds to our knowledge of phytoplankton palaeobiogeography, representing a significantly endemic assemblage within the generally cosmopolitan microphytoplankton flora of the Middle Devonian.

Spatial patterns in abundance, taxonomic composition and carbon biomass of nano- and microphytoplankton in Subarctic and Arctic Seas

In the summers of 2007 and 2008, we studied assemblages of nano- and microphytoplankton from the subsurface chlorophyll maximum (SCM) across five broad oceanographic domains in the seas surrounding northern North America. These domains are the eastern Subarctic North Pacific (ESNP), Bering and Chukchi Seas (BE-CH), Beaufort Sea and Canada Basin (BS-CB), Canadian Arctic Archipelago (CAA), and Baffin Bay and Labrador Sea (BB-LS). Average abundance and total carbon biomass (C) of phytoplankton (>2 µm) varied ∼10-fold and ∼20-fold, respectively, across the five domains. In the BE-CH, CAA and BB-LS, diatoms averaged 35–70% and dinoflagellates 11–45% of total phytoplankton C (>2 µm), whereas in the ESNP and BS-CB, unidentified flagellates/coccoids (2–8 µm) represented a greater proportion of total C (27% and 39% respectively) than in the other domains.In the BE-CH and BB-LS, phytoplankton C (>2 µm) was dominated by dinoflagellates of the genus Gymnodinium, centric diatoms including Thalassiosira spp. and Chaetoceros spp., unidentified flagellates/coccoids (2–8 µm), and cryptomonads. In contrast, diatoms such as Thalassiosira spp. and its resting spores dominated C in the CAA, with dinoflagellates being less significant than in the BE-CH and BB-LS. Unidentified flagellates/coccoids (2–8 µm), Gymnodinium spp., and cryptomonads dominated in the ESNP, and particularly in the BS-CB, where diatoms contributed only 18% of the very low levels of total phytoplankton C (>2 µm).Phytoplankton C (>2 µm) to chlorophyll a ratios (phyto C:chl a) averaged only 31 g C g chl a−1 in the oligotrophic BS-CB domain, and 51–150 g C g chl a−1 in the other domains, whereas ratios of biogenic silica to phytoplankton C (>2 µm) (bSi:phyto C) were lowest in the eastern domains. Estimates of phytoplankton C were highly sensitive to the choice of C to cell volume equations (C:vol) adopted in the calculations, particularly in diatom-rich areas.This study highlights how diatoms and dinoflagellates are the main drivers of large-scale variations in C biomass for phytoplankton (> 2 µm), whereas unidentified flagellates/coccoids (2–8 µm) make a significant contribution to C biomass in oligotrophic domains, such as BS-CB, where diatoms and dinoflagellates are less abundant. Reduced surface water density (σT) was associated with deeper SCM layers, and with decreased C biomass of unidentified flagellates/coccoids (2–8 µm). These observations confirm recent studies highlighting the role of surface water stratification caused by melting sea ice in shaping nano- and microphytoplankton assemblages.

A study of microphytoplankton and cyanobacteria consortia in four oligotrophic regimes in the western part of the North Pacific subtropical gyre and in the warm pool

The spatial distribution of nanocyanobacteria-like cells and microphytoplankton was investigated along a ~5000 km transect in the North Western Pacific Subtropical Gyre and the Warm Pool. Three different consortia: Richelia intracellularis/ Calothrix rhizosoleniae, Katagnymene spp. and Trichodesmium spp. and the associated diatoms were studied. Although colonies of Trichodesmium spp. were identified in the Warm Pool, we found that up to 94% of diatoms were associated with the individual trichomes identified as Katagnymene spp. In the microphytoplankton assemblages, pennate diatoms lacking cyanobionts were embedded in large quantities of mucilaginous matrix and were characterized by a particular diatom assemblage not seen at other stations. This assemblage was observed in the Warm Pool when numerous dinoflagellates were present as well. As a consequence, we suggest that this high percentage of small pennate diatoms embedded in mucus is the result of an efficient anti-grazing strategy. When large diatoms such as Rhizosolenia spp. and chain-forming Chaetoceros were outside the mucus matrix, they were mainly associated with cyanobiont R. intracellularis/ C. rhizosoleniae in all the areas covered during the cruise. A nanocyanobacteria-like cell group was also identified and seems to have been confined to the Transition Zone and the periphery of the Trichodesmium spp. bloom, suggesting that cyanobacteria species and consortia could be distributed along an oligotrophic gradient.

Water flux management and phytoplankton communities in a Mediterranean coastal lagoon. Part II: Mixotrophy of dinoflagellates as an adaptive strategy?

Dinoflagellate proliferation is common in coastal waters, and trophic strategies are often advanced to explain the success of these organisms. The Biguglia lagoon is a Mediterranean brackish ecosystem where eutrophication has long been an issue, and where dominance of dinoflagellates has persisted for several years. Monthly monitoring of fluorescence-based properties of phytoplankton communities carried out in 2010 suggested that photosynthesis alone could not support the observed situation all year round. Contrasting food webs developed depending on the hydrological season, with a gradual shift from autotrophy to heterotrophy. Progressively, microphytoplankton assemblages became unequivocally dominated by a Prorocentrum minimum bloom, which exhibited very weak effective photosynthetic performance, whereas paradoxically its theoretical capacities remained fully operational. Different environmental hypotheses explaining this discrepancy were examined, but rejected. We conclude that P. minimum bloom persistence is sustained by mixotrophic strategies, with complex compromises between phototrophy and phagotrophy, as evidenced by fluorescence-based observations.

Microphytoplankton variations during coral spawning at Los Roques, Southern Caribbean.

Phytoplankton drives primary productivity in marine pelagic systems. This is also true for the oligotrophic waters in coral reefs, where natural and anthropogenic sources of nutrients can alter pelagic trophic webs. In this study, microphytoplankton assemblages were characterized for the first time in relation to expected coral spawning dates in the Caribbean. A hierarchical experimental design was used to examine these assemblages in Los Roques archipelago, Venezuela, at various temporal and spatial scales for spawning events in both 2007 and 2008. At four reefs, superficial water samples were taken daily for 9 days after the full moon of August, including days before, during and after the expected days of coral spawning. Microphytoplankton assemblages comprised 100 microalgae taxa at up to 50 cells per mL (mean ± 8 SD) and showed temporal and spatial variations related to the coral spawning only in 2007. However, chlorophyll a concentrations increased during and after the spawning events in both years, and this was better matched with analyses of higher taxonomical groups (diatoms, cyanophytes and dinoflagellates), that also varied in relation to spawning times in 2007 and 2008, but asynchronously among reefs. Heterotrophic and mixotrophic dinoflagellates increased in abundance, correlating with a decrease of the diatom Cerataulina pelagica and an increase of the diatom Rhizosolenia imbricata. These variations occurred during and after the coral spawning event for some reefs in 2007. For the first time, a fresh-water cyanobacteria species of Anabaena was ephemerally found (only 3 days) in the archipelago, at reefs closest to human settlements. Variability among reefs in relation to spawning times indicated that reef-specific processes such as water residence time, re-mineralization rates, and benthic-pelagic coupling can be relevant to the observed patterns. These results suggest an important role of microheterotrophic grazers in re-mineralization of organic matter in coral reef waters and highlight the importance of assessing compositional changes of larger size fractions of the phytoplankton when evaluating primary productivity and nutrient fluxes.

Diatoms and dinoflagellates macroscopic regularities shaped by intrinsic physical forcing variability in Patagonian and Fuegian fjords and channels (48°–56°S)

The distribution of planktonic organisms remains a central focus of research due to the fact that physical and biological interactions affect the marine pelagic ecosystem in different ways and at different scales. Therefore, different patterns in the structure of phytoplankton community emerge according to the observation scale. In the Chilean fjords systems, suggested to be autotrophic and showing neither a nitrate nor a phosphate deficit, we studied the phytoplankton structures from data sets obtained in four seasonal cruises across a large geographical region in Patagonia and Tierra del Fuego, including the biogeographic Magellan District and a portion of the Fuegian District (48–55°S) separated in three sectors. We also processed a database from a spring cruise at a sub-regional spatial scale, which covers a portion of the central and southern Magellan sectors (52–55° S) separated in seven areas with distinctive geographic characteristics. More defined patterns of microphytoplankton at large spatial scales were predicted, with greater heterogeneity and differences between assemblages at a smaller scale. Therefore, at a smaller scale particular local zones with characteristic phytoplankton assemblage are expected as a result of particular environmental conditions of the different geographical areas. In order to evaluate the phytoplankton community structure in both data bases, statistical tools such as multidimensional scaling (MDS), analysis of similarities (ANOSIM), Similarity Percentages (SIMPER) and Cluster Analysis were used when required. At a sub-regional mesoscale, principal component (PCA) and factor analysis were used for environmental data in order to determine if their distribution patterns influences the formation of local zones with similar microphytoplankton assemblages. A Canonical Correspondence Analysis (CCA) was performed to find the relationship between environmental variables and diatom and dinoflagellate grouped by genera. At the regional scale, clearly differentiated groups during the different seasons were found. Of these groups, the northern sector of the Magellan region (48–52°S) forms a distinct group in all four seasons in agreement with a priori well known areas of higher probability of toxic shellfish detection, which have been defined as geographical Paralytic Shellfish Poison (PSP) toxicity cores, produced by a single toxic dinoflagellate species. These larger scale patterns respond to environmental forcing, probably temperature, winds, solar irradiance and circulation pattern. Results at a sub-regional scale showed no distinct microphytoplankton groups (Global R=0.26), while significant differences between stations (p<0.05) indicated a high heterogeneity within the same area. Temperature and nitrate were the most important variables correlated with the microphytoplankton genera groups. In conclusion, spatial regularities in microphytoplankton structure are found at the macroscopic scale (regional scale) and the PSP toxicity cores may be associated with changes in the composition and abundance of the entire microphytoplankton assemblage. Jointly, despite at small geographic scales, there is increased variability, several microphytoplankton local zones were detected and can be explained by exogenous influences, where nutrient input and temperature play the most important role.

Topographical and hydrographical impacts on the structure of microphytoplankton assemblages on the Abrolhos Bank region, Brazil

This study was conducted at the Abrolhos Bank (15°60′–21°30′S; 37°00′–40°30′W), Brazil, in July and August 2007, to evaluate the topographic and hydrographic influences on microphytoplankton composition and relative abundance. Net phytoplankton was collected from the top 200m of the water column to assess diversity proxies (species richness, Shannon index, dominance and equitability) and compared with thermohaline, nutrient and chlorophyll profiles. A total of 326 taxa occurred in the area. Patterns in spatial distribution of microphytoplankton assemblages were two-fold: a north–south gradient linked to variations in temperature and nitrite, and a coast-offshore gradient associated with the depth of the mixed layer and the Brunt–Väisälä maximum frequency. Microphytoplankton assemblages were typical of tropical oligotrophic environments. However, the inshore community found on the Abrolhos Bank was enriched by bottom dwelling, large-sized cells ressuspended from local sediments as a result of the highly dynamic coastal circulation. Species diversity was high in oceanic sites where water column stability as measured by the Brunt–Väisälä frequency achieved its maxima, but high values of ecological indexes were also found in the southern part of the study area influenced by bottom intrusions of nutrient-rich oceanic waters, giving support to the notion that phytoplankton diversity increases at intermediate levels of environmental disturbance.

Scaling of growth rate and mortality with size and its consequence on size spectra of natural microphytoplankton assemblages in the East China Sea

Abstract. Allometric scaling of body size versus growth rate and mortality has been suggested to be a universal macroecological pattern, as described by the metabolic theory of ecology (MTE). However, whether such scaling generally holds in natural assemblages remains debated. Here, we test the hypothesis that the size-specific growth rate and grazing mortality scale with the body size with an exponent of −1/4 after temperature correction, as MTE predicts. To do so, we couple a dilution experiment with the FlowCAM imaging system to obtain size-specific growth rates and grazing mortality of natural microphytoplankton assemblages in the East China Sea. This novel approach allows us to achieve highly resolved size-specific measurements that would be very difficult to obtain in traditional size-fractionated measurements using filters. Our results do not support the MTE prediction. On average, the size-specific growth rates and grazing mortality scale almost isometrically with body size (with scaling exponent ∼0.1). However, this finding contains high uncertainty, as the size-scaling exponent varies substantially among assemblages. The fact that size-scaling exponent varies among assemblages prompts us to further investigate how the variation of size-specific growth rate and grazing mortality can interact to determine the microphytoplankton size structure, described by normalized biomass size spectrum (NBSS), among assemblages. We test whether the variation of microphytoplankton NBSS slopes is determined by (1) differential grazing mortality of small versus large individuals, (2) differential growth rate of small versus large individuals, or (3) combinations of these scenarios. Our results indicate that the ratio of the grazing mortality of the large size category to that of the small size category best explains the variation of NBSS slopes across environments, suggesting that higher grazing mortality of large microphytoplankton may release the small phytoplankton from grazing, which in turn leads to a steeper NBSS slope. This study contributes to understanding the relative importance of bottom-up versus top-down control in shaping microphytoplankton size structure.

Seasonal and year-on-year variability in chlorophyll a and microphytoplankton assemblages for 9 years (2001^|^ndash;2009) in the neritic area of Sagami Bay, Japan

Seasonal and year-on-year variability in chlorophyll a and microphytoplankton assemblages for 9 years (2001^|^ndash;2009) in the neritic area of Sagami Bay, Japan

Temporal dynamics of phytoplankton communities in a semi-enclosed mariculture pond and their responses to environmental factors

Variations in physical-chemical factors, species composition, abundance and biomass of nano- and micro-phytoplankton assemblages, as well as their responses to environmental factors, were investigated over a complete cycle (6 months) in a semi-enclosed shrimp-farming pond near Qingdao, northern China. The aim was to establish the temporal patterns of phytoplankton communities and to evaluate protists as suitable bioindicators to water quality in mariculture systems. A total of 34 taxa with nine dominant species were identified, belonging to six taxonomic groups (dinoflagellates, diatoms, cryptophyceans, chlorophyceans, euglenophyceans and chrysophyceans). A single peak of protist abundance occurred in October, mainly due to chlorophyceans, diatoms and chrysophyceans. Two biomass peaks in July and October were primarily due to dinoflagellates and diatoms. Temporal patterns of the phytoplankton communities significantly correlated with the changes in nutrients, temperature and pH, especially phosphate, either alone or in combination with NO3-N and NH3-N. Species diversity, evenness and richness indices were clearly correlated with water temperature and/or salinity, whereas the biomass/abundance ratio showed a significant correlation with NO3-N. The results suggest that phytoplankton are potentially useful bioindicators to water quality in semi-enclosed mariculture systems.

Seasonal variations of microphytoplankton assemblages and environmental variables in the coastal zone of Bozcaada Island in the Aegean Sea (NE Mediterranean Sea)

Seasonal changes in the microphytoplankton assemblages were examined in the coastal zone of Bozcaada Island with regard to some major physical and chemical variables. Samples were collected from May 2000 to December 2001 at four stations. A total of 108 dinoflagellates, 102 diatoms, 1 chrysophycean, 3 dictyochophycean, and 1 prasinophycean species were identified and quantified during the study period. Diatoms and dinoflagellates were the most important in terms of species number and abundance. The maximum values of total microphytoplankton were observed at 0.5 m depth (46.2 × 103 cells l−1 at st. 3) in May as this was the month when the diatom Pseudo-nitzschia pungens bloomed. Chlorophyll (chl) a concentration ranged between 0.08 (August) and 0.78 μg l−1 (February). May was another important month in which chlorophyll a increased (0.41–0.47 μg l−1). Species diversity values (H′log2) ranged from 1.66 bits (June, 20 m) to 4.11 bits (November, 0.5 m). The increase was attributed to a more balanced distribution of abundance among species. The amounts of nitrate + nitrite (0.6−3.7 μg-at N l−1), phosphate (0.2−0.6 μg-at P l−1) and silicate (0.7−2.5 μg-at Si l−1) were recorded on each sampling occasion. Nutrient concentrations and chl a values of the research area were found to be poorer than those of the many other coastal areas in the northeastern Mediterranean. The mean atomic ratio of nitrogen to phosphorus varied from 1.3 (June) to 12.9 (February). This ratio was lower than the Redfield ratio of 16 for ocean phytoplankton, and phytoplankton was potentially limited by nitrogen for most of the months. The result of this study confirms and emphasizes the oligotrophic nature of the eastern Mediterranean.

Microphytoplankton assemblages in shallow waters at Admiralty Bay (King George Island, Antarctica) during the summer 2002–2003

Microphytoplankton populations were studied in shallow coastal water ( 104 cells l−1) were caused by pennate benthic diatoms (Fragilaria striatula Lyngbye) that occurred mainly in early summer, associated with the presence of ice. In late summer, when the water temperature (−0.4 to 1.5°C), salinity (34 to 35), and phosphate (2.6 to 4.5 μmol l−1) were highest and the dissolved oxygen was lowest (6.4 to 2.9 ml l−1), centric diatoms (Thalassiosira spp.) were more abundant, suggesting an influence of oceanic waters. Phytoplankton abundance (≤102 cells l−1) and chlorophyll a concentrations (0.22 μg l−1) were lowest close to EACF. Pennate diatoms were dominant close to shore and in surface waters elsewhere, probably because of ice melting or sediment resuspension caused by water mixing.

Preponderance of a few diatom species among the highly diverse microphytoplankton assemblages in the Bay of Bengal

The microphytoplankton assemblages were studied from water samples collected at eight discrete depths in the top 120 m at five central (open ocean) and four western (shelf/slope region) locations in the Bay of Bengal. The Bay is a low-productive warm pool regime with poor inorganic nutrient inputs to its intensely stratified surface layer despite the very large riverine influx. In addition, the prolonged cloud cover has an adverse effect on the top 25–40 m, on primary production, chlorophyll concentration and phytoplankton assemblages. Microphytoplankton were the most abundant in the northern area of the Bay. A total of 153 phytoplankton species were identified during this study. The most abundant species (at least 1,800 individual cells belonging to a given species or ≥2% of the total counts of identified specimens) during this study were: Thalassiothrix longissima, Thalassiothrix fauenfeldii, Rhizosolenia styliformis, Nitzschia angularis, Thalassionema nitzschioides, Coscinodiscus radiatus, Chaetoceros eibenii, Skeletonema costatum, Coscinodiscus concinnus and Chaetoceros coarctatus. Similarly, there were 20 moderately abundant (≥0.5 but <2%) species. Thirty-three of the least abundant species (<0.5%) occurred at one station. Diatom species, T. fauenfeldii and T. longissima were ubiquitous in the study area. The exclusive occurrence of S. costatum in the northernmost stations suggests that it proliferates only in the low-salinity regions with adequate silica from the land inputs. Abundance of pennate diatoms was higher in the open Bay compared to that of centric diatoms in the more productive northern locations and the western Bay. There appears to be a basic difference between near shore flora and offshore flora. This first analysis of phytoplankton assemblages from the offshore Bay suggests that while there is predominance of only a few species, the Bay harbors very diverse diatom communities that seem to be syntrophic, non-competitive and co-habiting in the generally low nutrient, stratified surface waters.

The massive sulphide event in the Iberian Pyrite Belt: confirmatory evidence from the Sotiel-Coronada Mine

Well-preserved miospore and organic-walled microphytoplankton assemblages have been recovered from the black shaly series hosting the massive sulphide deposits of Sotiel-Coronada Mine (Iberian Pyrite Belt). The productive samples yielded miospore assemblages representing the uppermost Famennian Retispora lepidophyta–Verrucosisporites nitidus (LN) miospore Biozone of Western Europe. This palynological evidence has important implications for the local geology, constraining the commencement of the volcanic activity and corroborating the previously-defined local tectonic style. Moreover, at regional scale, the new data permit correlation of the black shaly series (which hosts the mineralization in all the sulphide deposits hitherto dated in the region), reinforcing the hypothesis of an anoxic event occurring in the Iberian Pyrite Belt in the latest Devonian times.

Late Devonian and early Carboniferous microfloras from the Hakkarı Province of southeastern Turkey

Late Devonian and early Carboniferous miospore and microphytoplankton assemblages are described for the first time from southeastern Turkey. The preliminary data show that assemblages recorded from the upper part of the Yiginli Formation are late Famennian in age and can be correlated with the VH, LL, LE Miospore biozones of western Europe, whilst assemblages from the overlying Köprülü Formation are considered to be middle to late Tournaisian in age and are tentatively assigned to the PC Miospore biozone of western Europe. Regional palynological correlations with other sections in North Africa and the Middle East are proposed. A new species, Verruciretusispora loboziakii is described.

Microphytoplankton assemblages in the waters surrounding South Georgia, Antarctica during austral summer 1994

The composition and distribution of microphytoplankton assemblages in the vicinity of South Georgia were examined during a krill survey conducted during voyage 119 the RV Africana in austral summer (January/February) 1994. Microphytoplankton distribution was studied at 83 stations along a zig-zag transect in the waters surrounding South Georgia. Oceanographic and zooplankton data suggest that conditions were anomalous during the investigation, in that sub-Antarctic surface waters had probably been advected southwards and were present around the island. The two most widely distributed species were Corethron criophilum and Eucampia antarctica, which were recorded at all stations sampled. Also well-represented amongst the microphytoplankton assemblages were temperate, neritic species such as Chaetoceros atlanticus, Proboscia alata and Odontella weissflogii. This can probably be related to the intrusion of sub-Antarctic surface waters unusually far south. Cluster and ordination analysis identified three distinct groupings of stations in the waters surrounding the islands. These were found to the east, west and north of the islands. The diatom groupings identified during this investigation broadly correspond to three water masses previously described in the vicinity of South Georgia. Despite the anomalous hydrographical conditions that existed during the investigation, the differences in phytoplankton were negligible when compared to normal years. Small mesoscale features are, however, important in imposing biogeographic patterns on microphytoplankton.

Biogeographical structure of the microphytoplankton assemblages in the region of the Subtropical Convergence and across a warm-core eddy during austral winter

The distribution of microphytopla nkton in the region of the Subtropical Convergence (STC) and across a warm-core eddy shed from the Agulhas Return Current was investigated along two transects in late austral winter (June-July) 1993, during the South African Antarctic Marine Eco- system Study (S AAMES) III cruise. Sampling was undertaken for the analysis of nutrients, and for the enumeration and identification of microphytoplankton species. Along both transects, chlorophyll con- centrations were highest at stations at the southern boundary of the STC and at the periphery of the warm-core eddy. Of the variance associated with chlorophyll concentration, temperature accounted for 65% of the total. Along both transects, a decrease in species richness from north to south was observed. The spatial distribution of the numerically dominant diatom species was similar in both tran- sects. The microphytoplankton assemblage was dominated by the subtropical diatom species Chactoccros constnctus north of the STC, and by Pseudoeunotia doliolus within the eddy and south of the STC. Using cluster and ordination analyses, three significantly different groupings of stations were identified along the combined transects. These coincided with stations located north and south of the STC and with the warm-core eddy proper, confirming that the STC represents a strong biogeograph- ical boundary. The predominance of the warm-water species P.doliolus and Planktoniella sol in and around the warm-core eddy south of the STC suggests that eddies are important in the transfer of micTophytoplankton across this strong biogeographical boundary.