Rhabdosphaera Clavigera Research Articles

Two-year seasonality (2017, 2018), export and long-term changes in coccolithophore communities in the subtropical ecosystem of the Gulf of Aqaba, Red Sea

A two-year (2017, 2018) study of coccolithophores was undertaken in the Gulf of Aqaba in the northern Red Sea to 1) determine the local diversity and community succession patterns with high temporal and taxonomic resolution, 2) determine the contribution to exported inorganic carbon and 3) evaluate changes in communities relative to those in the mid-1970's reported in Winter et al. (1979). As typical, the oceanographic conditions in the Gulf alternated between stratified, oligotrophic through spring/summer, and mesotrophic during winter due to deep convective mixing. Noticeably, the second summer of 2018 was warmer resulting in a more pronounced thermocline. Overall, the coccolithophore dynamics followed the seasonal changes in oceanographic conditions. During the mixing period, coccolithophores were denser (>25 coccospheres mL−1) and were estimated to account on average for 3.46% of the total Chl-a. The communities were fairly homogeneous through depths and over time with a marked dominance of Emiliania huxleyi and Gephyrocapsa ericsonii. During the stratified season, coccolithophore densities were lower, particularly at the surface, such that coccolithophores were estimated to represent on average 0.72% of the total Chl-a. However, the diversity was higher and a clear vertical zonation developed with well-defined upper, intermediate and deep-dwelling communities. These included Umbellosphaera spp., Rhabdosphaera clavigera, a variety of Syracosphaera spp., holococcolithophores and Florisphaera profunda, respectively, as typical in oligotrophic systems. This further underscores the pelagic nature of the Gulf of Aqaba. Noticeably, however, coccolithophores declined in abundances and diversity during 2018, possibly due to the warmer conditions. The relative export of inorganic carbon by coccolithophores largely correlated with the density of living communities, representing <4% and <10% of total inorganic carbon exports during the stratified and mixing periods, respectively. A marked differentiation between different coccolithophore life phases was also detected. Broadly, holococcolithophores inhabited preferentially shallow, oligotrophic water layers in the stratified season, while heterococcolithophores prevailed in deeper water layers or in winter. Yet, species-specific deviations to this pattern were also detected. Finally, comparisons with the 1970's revealed marked changes compared to current communities. G. ericsonii appeared less prevalent and Umbellosphaera spp. displayed a wider temporal residence through spring/summer. Moreover, F. profunda and other deep dwelling species absent in the past are now common during the peak of summer. These observations are indicative of changes in local microbial communities, possibly linked to the ongoing rise in sea temperatures. This study provides a detailed baseline information on coccolithophores for future reassessments of community changes in the GoA.

Coccolithophore assemblages and a new species of Alisphaera from the Faafu Atoll, Maldives, Indian Ocean

Coccolithophore assemblages were investigated from the Faafu Atoll in November 2018. The surface waters were sampled around the islands of Magoodhoo and Adanga, from the coast to the open water towards both the lagoon and the ocean, as well as waters up to 40 m depth at selected stations. Coccolithophores were scarce close to the coast (0.1–4 × 103 coccospheres/l), increased in abundance towards both the lagoon and the open ocean (20–44 × 103 coccospheres/l) and showed no clear depth trend. The assemblages were dominated by Gephyrocapsa oceanica with a large contribution by species that are typically reported from the mid- to lower-photic zone, i.e., primarily Oolithotus antillarum, followed by Calciosolenia spp., Michaelsarsia adriaticus, Calciopappus rigidus, Ophiaster spp. and Algirosphaera robusta. As for other tropical settings, Emiliania huxleyi made a minor contribution. Taxa that are common in the upper layers of oligotrophic stratified settings like Umbellosphaera spp., Discosphaera tubifera and Rhabdosphaera clavigera were scarce. Overall, such assemblages displayed the characteristics of neritic settings, with high trophic resources and elevated water turbidity, even though the measured chlorophyll-a concentration was low throughout the investigated area.

Minor Contribution by Biomineralizing Phytoplankton to Surface Ocean Biomineral Pools in the Late Stratified Period

Vertical distributions of biogenic silica (bSi), particulate inorganic carbon (PIC) and key biomineral-forming phytoplankton indicate vertical zoning, or partitioning, during the late summer stratified period in the northeast Atlantic. Coccolithophores were generally more numerous in the surface mixed layer, whilst PIC concentrations were more homogenous with depth throughout the euphotic zone. Diatoms were notably more abundant and more diverse in the lower euphotic zone beneath the mixed layer in association with subsurface maxima in chlorophyll-a, bSi and oxygen concentrations. The four dominant coccolithophore species (Emiliania huxleyi, Gephyrocapsa muellerae, Syracosphera spp., and Rhabdosphaera clavigera) represented 78 ± 20% (range 31–100%) of the observed community across all sampled depths yet simultaneously contributed an average of only 13% to measured PIC pools. The diatom community, which was dominated by Pseudo-nitzschia spp. and by a species tentatively identified as Nanoneis longta, represented only ~1% of the bSi pool on average, with contributions increasing within the chlorophyll maximum. Despite a slow gradual deepening of the surface mixed layer in the period prior to observation, and adequate nutrient availability beneath the mixed layer, biomineral pools at this time consisted largely of detrital rather than cellular material.

The last 1 million years of the extinct genus Discoaster: Plio–Pleistocene environment and productivity at Site U1476 (Mozambique Channel)

A detailed paleoenvironment reconstruction from the Mozambique Channel, western Indian Ocean, based on the calcareous nannoplankton assemblages was conducted for the interval between 2.85 and 1.85 Myr. This study covers the period during which the successive extinction of the last five species of discoasters occurred. New productivity data obtained from the abundances of the Discoaster species (Discoaster brouweri, D. triradiatus, D. pentaradiatus, D. surculus, and D. tamalis) and other indicative calcareous nannoplankton taxa showed abundance variations, which were at paced with the 100, 41, and 23 kyr astronomical periodicities. A shift in the productivity and water-column stratification proxies occurred at ~2.4 Ma, after the onset of the Northern Hemisphere glaciation. Here we propose that the variability recorded at International Ocean Discovery Program Site U1476 reflects the interplay between forcing associated with warm tropical Pacific and cold southern ocean influences. The former is shown by consistent occurrence of warm water taxa (Calcidiscus leptoporus, Oolithotus spp., Rhabdosphaera clavigera, Syracosphaera spp., Umbellosphaera spp.), typical of Indonesian Throughflow surface waters. On the other hand, the occurrence of Coccolithus pelagicus indicates the influence of cold, nutrient-rich sub-Antarctic surface waters. A more mixed water column initiated at ~2.4 Ma, and a consequent productivity increase led to the gradual reduction of the Discoaster species, until their extinction at 1.91 Ma. This period was characterized by the low values of the Florisphaera profunda index and high abundances of upper photic zone flora, indicative of nutrient-rich surface water conditions. High productivity at the location during this period could have also been amplified by localized upwelling events driven by the Mozambique Channel eddies.

COMPARING LIVING AND HOLOCENE COCCOLITHOPHORE ASSEMBLAGES IN THE AEGEAN MARINE ENVIRONMENTS

Detailed quantitative analyses of coccolithophores performed on the shallow deposits of the southeastern Aegean region (core NS-14, 505 m depth), evidenced that the distribution of calcare-ous nannoplankton assemblages during the last 13 ka BP reflects paleoenvironmental changes which are directly related to parameters such as temperature, salinity, productivity and nutrient flux in the water column. Analysis enabled the separation of the assemblages in four groups. Group A consists of Emiliania huxleyi and the subtropical species Syracosphaera spp. and Rhabdosphaera clavigera, Group B is composed of Helicospaera spp. and Florisphaera profunda, typical species for high productivity conditions in the middle-lower photic zone, Group C consists of Gephyrocapsa oceanica and Braarudosphaera bigelowii, that characterise low salinity conditions and Group D includes Umbilicosphaera spp. and Calcidiscus spp. which are described as relatively eutrophic species. The Holocene assemblages differ distinctly from the living coccolithophore communities in the coastal ecosystems of the Aegean Sea, where only Group A coccolithophores are thriving in the seasonally controlled marine environment.

Seasonal and interannual variations in coccolithophore abundance off Terceira Island, Azores (Central North Atlantic)

In order to characterize the natural coccolithophore community occurring offshore Azores and to determine their annual and interannual patterns, monthly samples were collected, from September 2010 to December 2014, in the photic zone off Terceira Island.The present study revealed a clear seasonal distribution and a considerable interannual variability of the living coccolithophore community. The highest coccolithophore abundances were observed during spring and winter months, especially due to the smaller species Emiliania huxleyi and Gephyrocapsa ericsonii. In fact, the highest biomass period was registered during April 2011, associated with enhanced abundance of the overcalcified morphotype of E. huxleyi, which was possibly influenced by subpolar waters and subsequent upwelling conditions. The highest abundances of Gephyrocapsa muellerae were recorded during June 2011 and 2014, indicating that this species characterizes the transition between the period of maximum productivity and the subsequent smoother environmental conditions, the first and the later stages of the phytoplankton succession described by Margalef, respectively. During summer to early fall, a gradual decrease of the overall coccolithophore abundance was observed, while the species richness (Margalef diversity index) increased. A subtropical coccolithophore assemblage mainly composed by Umbellosphaera tenuis, Syracosphaera spp., Discosphaera tubifera, Rhabdosphaera clavigera and Coronosphaera mediterranea indicated the presence of surface warmer waters accompanied by reduced mixing and low nutrients concentration. During late fall to winter, the coccolithophore abundance increased again with a concomitant reduction in species diversity. This is potentially linked to low sea surface temperatures, moderate nutrients concentration and surface mixed layer deepening. During 2011, colder and productive waters led to an increase in the total coccolithophore abundances. On contrary, during 2012, characterized by milder environmental conditions, an increase in the diversity of the community was prevalent. The noticeably coccolithophore response to variations in the main physical variables and on main traditional nutrients corroborates the importance of this group as proxy of marine environmental conditions in the past.

The composition and distribution of living coccolithophores in the Aegean Sea (NE Mediterranean)

This study presents the species composition of living coccolithophore communities in the Aegean Sea (northeastern Mediterranean), investigating their spatial and temporal variations in various environmental conditions from mesotrophic to ultra- oligotrophic regions. Coccolithophores of the photic zone in the Aegean Sea are relatively diverse (65 heterococcolithophores and 34 holococcolithophores) and dominated mostly by Emiliania huxleyi, Syracosphaera spp., Rhabdosphaeraceae and holococcolithophores. Hierarchical classification using R-mode cluster analysis distinguished five coccolithophore groups: Group Ia (Emiliania huxleyi, Syracosphaera molischii and Syracosphaera ossa) prevails in the high cell density and low diversity assemblages during the winter and early spring, when low temperatures and high nutrient concentrations are prevailing. Particularly in the north Aegean, E. huxleyi is dominating the upper photic zone being affected by the Black Sea Water inflow and the associated control on the water column stratification. Group Ib (Florisphaera profunda, Algirosphaera robusta, Syracosphaera anthos and Syracosphaera lamina) becomes important in the lower photic zone, making up the typical deep assemblages, whereas Group Ic (mainly Helicosphaera carteri and Gephyrocapsa oceanica) implies an opportunistic behavior in distinctly polluted neritic regions. Group IIa (Rhabdosphaera clavigera, Syracosphaera protrudens, Syracosphaera halldalii and numerous holococcolithophores) dominates the late spring-early autumn low cell density and high diversity assemblages, mainly in the thermally-stratified south Aegean and/or shallow, coastal environments with normal/ oligotrophic conditions, while Group IIb (Umbellosphaera tenuis and Syracosphaera pulchra) dominates the coccolithophore assemblages mainly during the early autumn in the north Aegean, thus reflecting the influence of Levantine Intermediate Water masses in the middle-lower photic zone. Our results suggest that abundance and variability in Aegean Sea coccolithophore assemblages are primarily controlled by surface water circulation and the associated water column stratification,with the sea temperature gradient affecting species composition.

Ultrastructure and Crystallography of Nanoscale Calcite Building Blocks in Rhabdosphaera clavigera Coccolith Spines

Coccolithophores create an intricate exoskeleton from nanoscale calcite platelets. Shape, size, and crystal orientation are controlled to a remarkable degree. In this study, the structure of Rhabdosphaera clavigera is described in detail for the first time through a combination of electron microscopy techniques, including three-dimensional electron tomography. The coccolithophore exhibits several micrometer long 5-fold symmetric spines with diameters of approximately 0.5 μm. The nanorystals constituting the spine are arranged radially along the longitudinal axis, protruding from the almost flat disks that form the coccosphere. The stem of the spine is shown to consist of {104} calcite rhombohedra single crystalline platelets, arranged in five separate spiral “staircases”. The spine tip shows 15 structural elements: five large “panels” protruding outward along the lateral plane and five leaf-shaped smaller units which form the topmost steps of the staircases. The outer tip consists of five long thin platel...

Spatial distribution of living coccolithophores in the southwestern Gulf of Mexico

Abstract. The present study was conducted to provide information about the upper water column distribution of living coccolithophores in the subtropical Gulf of Mexico. In total, 52 plankton samples from 6 stations collected in March 2006 were analysed. Coccolithophore standing crops range from zero in deep-water samples (200 m) to about 23 000 to 46 500 coccospheres per litre at intermediate water depths. From 39 identified taxa, only nine species contribute significant cell numbers of more than 1000 cells 1–1 and comprise more than 5 % of the communities in at least one sample. Emiliania huxleyi was the most abundant species throughout the stations with concentrations of up to 22 700 cells l–1. At all stations, a vertical succession of coccolithophore species was found. Umbellosphaera tenuis (type IV), Discosphaera tubifera, Rhabdosphaera clavigera and, as a minor component, species of the genus Syracosphaera, were mainly restricted to surface waters (≤50 m), all adapted to lower nutrient levels. The assemblage in the deep photic zone (50–150 m water depth) was composed mainly of abundant Florisphaera profunda, Gladiolithus flabellatus and Algirosphaera robusta indicating a tolerance of lower light availability. Thus, the vertical distribution of all coccolithophorid taxa, except the placolith-bearing species, in particular E. huxleyi, was probably controlled by upper photic-zone water temperature and stratification of the water column.

Seasonal phytoplankton dynamics in the coastal waters of the north-eastern Adriatic Sea

This study describes the dynamics of phytoplankton in relation to environmental factors in coastal waters of the north-eastern Adriatic Sea. The distant Po River's influence is rarely noted on the eastern coastal part of the northern Adriatic, but it does trigger the crucial alternations in the community, e.g. mass development of diatoms in July 2009. A highly variable ecosystem, like the northern Adriatic, sustains high species diversity. Even though quite a few species were present for more than several months, we identified the seminal species of the seasonal succession. During spring Prorocentrum micans, Leptocylindrus minimus, Chaetoceros throndsenii, Ceratium furca and Ceratium fusus were most characteristic. These species thrive in low salinity and mixed waters. Typical summer diatom species were Chaetoceros vixvisibilis and Proboscia alata, while Rhabdosphaera clavigera and Syracosphaera pulchra were identified as distinctive summer coccolithophorids. All the summer species preferred warmer waters with low nitrate and low silica content. The autumn community was characterised by species such as Calciosolenia murrayi, Chaetoceros socialis, Asterionellopsis glacialis, and Lioloma pacificum. These species were related not only to high nutrient conditions, but also to low Po River influence. In winter a prevalence of large diatoms Thalassiosira rotula, Neocalyptrella robusta and Pseudosolenia calcar-avis was recorded. The winter assemblage was characterised by species of cold water preference able to grow in well mixed conditions.

The coccolithophore summer–autumn assemblage in the upwelling waters of Portugal: Patterns of mesoscale distribution (1985–2005)

A study of the mesoscale distribution of phytoplankton communities observed along the Portuguese upwelling coast in 1985–1986 made it possible to identify an assemblage composed by the coccolithophores Helicosphaera carteri, Syracosphaera pulchra and Coronosphaera mediterranea. The assemblage was consistently present in the coast in the following years (1991, 1992, 1994, 2003 and 2005) and from 1992 onwards, Rhabdosphaera clavigera was recognised as being part of it, although in low concentrations During late summer, this group of species showed higher affinities with upwelling waters of subtropical origin (ENACWst) on the SW coast. At the end of the upwelling season, in autumn, the assemblage was advected to the NW shelf due to the intensification of the poleward surface circulation or meridional seasonal changes of environmental conditions. The species maximum abundances were not coincident in space: H. carteri developed at the central part of the western coast while blooms of S. pulchra and C. mediterranea generally occurred further south, such as at the upwelling centre of cape S. Vicente. Syracosphaera pulchra blooms were characterised by lower concentrations of C. mediterranea and vice-versa. The above patterns were recurrent along the years but the relative abundance of each species appeared related to the interannual variability of physical conditions as the upwelling.

Coccolithophores in the upwelling waters of Portugal: Four years of weekly distribution in Lisbon bay

From July 2001 to May 2005, seawater samples were collected once a week at a fixed station in Lisbon bay (38°41′N, 09°24′W) in order to describe the ecological dynamics of the coccolithophore community. The seasonal and interannual distribution patterns of the different species and their relationships with environmental parameters are addressed. The present work aimed to identify potential proxies for different local water bodies and environmental conditions. Throughout the period of study, the upwelling events were weak and progressively more persistent. High sea surface temperatures (SST) were observed earlier in the year; summers and winters were gradually warmer and colder, respectively. Salinity variations reflected the different weather conditions as they are strongly influenced by rainfall and thus by the Tagus river flow. The extended periods of weak upwelling and the overall increase in SST resulted in the development of phytoplankton populations as measured by chlorophyll a. However, the persistence of the upwelling, and thus shorter convergence periods, favoured other phytoplankton groups than coccolithophore populations as these decreased towards the end of the sampling period. The annual structure of the coccolithophore assemblage showed a pronounced and recurrent seasonal variability, mainly related with the intensity and persistence of upwelling. The highest cell densities were recorded from spring to autumn. An overall preference by most species for mature upwelled waters and low turbulent conditions was observed associated with high temperatures and salinities, although the species develop in different windows with mismatching maxima. The coccolithophores observed were capable of withstanding coastal processes such as turbulence and were well adapted to an environment rich in nutrients provided by both continental runoff and upwelling. The consistency of the results enabled local oceanographic tracers to be defined. Emiliania huxleyi and Gephyrocapsa species can be used as proxies of surface productivity waters during spring and summer while Coccolithus pelagicus indicates the presence of upwelling fronts. Calcidiscus leptoporus is a tracer of the convergence of subtropical oceanic waters onto the shelf, during winter while Coronosphaera mediterranea, Syracosphaera pulchra, Helicosphaera carteri and Rhabdosphaera clavigera revealed the presence of those waters during the short period that characterized the transition from upwelling to downwelling seasons.

Vertical and lateral variations in coccolithophore community structure across the subtropical frontal zone in the South Atlantic Ocean

In this study the coccolithophore compositions of 11 plankton depth stations along a N–S transect from the South Atlantic Subtropical Gyre to the Subantarctic Zone were examined qualitatively and quantitatively. The lateral and vertical distribution patterns of not only the most abundant taxa but also of the morphotypes of distinct species complexes, such as Calcidiscus leptoporus, Emiliania huxleyi, and Umbellosphaera tenuis were the focus. Geographic variation among morphotypes mirrors different ecological affinities of the members of a species complex. Multivariate statistics were used to infer the relationship between a set of known environmental data and species concentrations. The results of the detrended Canonical Correspondence Analysis (CCA) revealed the presence of distinct species assemblages. The Subtropical Gyre assemblage within the upper 50 m of the photic zone is mainly composed of Umbellosphaera irregularis, U. tenuis types III and IV, Discosphaera tubifera, Rhabdosphaera clavigera, S. pulchra and E. huxleyi var. corona, adapted to warm and oligotrophic conditions. In the deeper photic zone abundant Florisphaera profunda, Gephyrocapsa ericsonii and Oolithotus spp. are encountered, benefiting from higher nutrient concentrations in the vicinity of the nutricline. A well-defined Subtropical Frontal Zone (STFZ) association is clearly dominated by E. huxleyi types A and C throughout the upper 100 m of the water column. Secondary contributors in the upper photic zone are Syracosphaera spp. (mainly S. histrica, S. molischii), Michaelsarsia elegans, Ophiaster spp. and U. tenuis type II. This assemblage is associated with cooler, nutrient-rich waters. E. huxleyi type B is found deeper in the water column. Here it is accompanied by Algirosphaera robusta, G. muellerae, and S. anthos indicating a tolerance of lower light availability in environments with elevated productivity. C. leptoporus spp. leptoporus shows relatively high cell numbers in all sampled water levels throughout the STFZ. Interestingly, its coccoliths are often smaller 5 μm in lith diameter. The mean coccolithophore assemblages of a station were compared to the underlying surface sediment assemblages. For the most part, the distribution of the morphotypes is reflected in the sedimentary archive, thus proving their potential as paleoecological proxies.

Seasonality and ecology of living coccolithophores in Eastern Mediterranean coastal environments (Andros Island, Middle Aegean Sea)

This study concerns the species composition and seasonality of extant coccolithophore communities in Eastern Mediterranean coastal environments and investigates their significance as environmental indicators. Water samples were collected during six one-day cruises carried out between 2001 and 2004, in two separate marine ecosystems (Gulfs of Korthi and Kastro, southeastern coast of Andros Island). High species diversities (65 species of heterococcolithophores and 45 species of holococcolithophores), associated with the dominant K-selected taxa, Rhabdosphaeraceae, and holococcolithophores, were recorded during summer whereas low diversities coupled with maximum concentration of cells and the predominance of r-selected species (Emiliania huxleyi) were observed during late autumn-early spring. The coccolithophores of the upper photic zone have been grouped into four distinct communities (Emiliania huxleyi group, Palusphaera vandelii group, Rhabdosphaera clavigera group and Helladosphaera cornifera group). A taxonomic study of the modern coccolithophores of Aegean Sea is presented. Several combination coccospheres of Algirosphaera robusta with Sphaerocalyptra quadridentata have been well documented, therefore their synonymy is proposed and the remaining species of Sphaerocalyptra have been transferred to the genus Holococcolithophora. One more holococcolith-heterococcolith association between Corisphaera sp. type A, Zygosphaera bannockii, Zygosphaera amoena and Syracosphaera bannockii is presented, recombining all taxa into Syracosphaera amoena comb. nov. A new species, Holococcolithophora kastriensis sp. nov. is described.

Intraspecific morphotypic variability in the Family Rhabdosphaeraceae

Detailed inventory of filtered samples taken across the southern Indian Ocean in May-June 2003 yields quantifiable morphological groundwork for species analysis. Two samples were remarkable by their high content in Discosphaera tubifera , Rhabdosphaera clavigera , R. stylifera and Palusphaera vandelii , providing us with the opportunity to quantify intraspecific variability in these taxa. Despite its extreme geographic and ecologic restriction, this study is the first to provide an integrated description of rhabdoliths and rhabdospheres based on a large number of specimens. We show that the amplitude of variations varies broadly among Rhabdosphaeraceae species and that R. clavigera and R. stylifera are separate taxa. We discuss the implications of our results with regard to species definition, characterization of dimorphism, and inferences of living communities from fossil assemblages. We also report for the first time on malformations in these taxa.

Coccolithophore export production and response to seasonal surface water variability in the oligotrophic Cretan Sea (NE Mediterranean)

A single mooring with two sediment traps (trap A at 500 and trap B at 1700 m below sea surface) was deployed at the southern margin of the Cretan Sea at a depth of 1750m. The duration of the deployment was 12 months (from January 2001 to February 2002) with sampling intervals of approximately 15 days. The results obtained from trap A at 500 m depth show that the total coccolithophore export production was generally low. The highest productivity season was recorded from January till late September with the highest flux values observed between late March to late June (maximum flux: 9 x 10 8 coccoliths m -2 day -1 and 9.4 x10 5 coccospheres m -2 day -1 ). During this period the pattern was charac- terized by three main phases that correlate well with the gradual increase in SST from January till September. The lowest fluxes were re- corded between October 2001 and January 2002. The total calcareous dinocysts flux also followed the same pattern. The annual coccolith flux of 9.6 x 10 10 m -2 year -1 at 500 m was one order of magnitude higher compared to that previously recorded in Bannock Ba- sin at 3000 m water depth. The maximum coccosphere flux from trap B at 1700 m depth (3,9x10 5 m -2 day -1 ) occurred in late April, a month later compared to trap A. Apparently the magnitude of the coccosphere flux decreased with depth, but the overall seasonal pattern was preserved in the deeper environments. The annual coccosphere flux at 1700 m was 3,4x10 7 m -2 year -1 and calcareous dinocysts displayed a similar trend with highest values occurring at mid April 2001. Both coccolith and coccosphere sinking assemblages at 500m were characterized by high abundances of Emiliania huxleyi, (average 65% and 46% respectively). Secondary species in coccolith fluxes were Florisphaera profunda (average 9%), Rhabdosphaera clavigera (average 6%), Syracosphaera spp. (average 5%). The calculated total coccolithophore and calcareous dinocyst carbonate flux is relatively low (between 0-26% of the total carbonate flux) attaining higher values during the high productivity period. It is evident that the coccolithophore productivity pattern in the Cretan Sea (highest values at late March - late June) is the result of fertilization in the upper photic zone from the intrusion of the Transition Mediterranean Water (TMW) water masses from the east. Sea surface temperature (SST) and precipitation are environmental parameters directly associated to seasonality and therefore influence the observed coccolithophore productivity patterns.

Rhabdosphaera clavigera (calcareous nannofossils) as paleotemperature indicators: a comparison with isotopic data

Rhabdosphaera clavigera (calcareous nannofossils) as paleotemperature indicators: a comparison with isotopic data

Coccolithophore dynamics off Bermuda (N. Atlantic)

Abstract Standing stocks of coccolithophores in the photic zone and their taxonomic composition were investigated at Bermuda Hydrostation “S” from January 1991 to January 1994 (217 samples from 32 vertical profiles, 0–200 m water depth). Their variability is compared with environmental parameters measured by the JGOFS Group at nearby BATS station during the same time period. Coccolithophores showed a pronounced seasonal variability, with the highest standing stocks (106×103 cells l−1) during late winter to spring and lowest during summer. Average cell density was 20×103 cells l−1. Emiliania huxleyi dominated in spring (up to 93×103 cells l−1) and Florisphaera profunda accounted for the regional subsurface cell density peak in fall (up to 68×103 cells l−1). In summer and fall, during maximum stratification, Umbellosphaera irregularis dominated near the surface and U. tenuis at 50–75 m water depth. Both species had much lower cell densities than E. huxleyi or F. profunda. The interannual variability was large, with average cell densities in 1993 only about half those of 1992. Cell densities decreased rapidly below 100 m water depth. Seasonal relative abundance changes were rather stable from year to year. Total coccolithophore (mostly E. huxleyi) cell densities were highest where light was available (upper 50 m) and increased within a few weeks after the seasonal advection of nitrate (at temperatures of 18–23°C). They remained relatively low when only phosphate increased in the upper photic zone. F. profunda peaks occurred at slightly elevated nitrate and phosphate concentrations and at moderate light intensities. U. irregularis and U. tenuis developed in nitrate-depleted waters. The optimum temperature and light ranges of U. irregularis were higher than those of U. tenuis. Among the minor taxa, Gephyrocapsa oceanica, small coccospheres, Calciosolenia spp., Ophiaster hydroideus and Syracosphaera molischii appeared to have ecological requirements similar to those of E. huxleyi. Holococcolithophores, Calcidiscus leptoporus, Ceratolithus cristatus and Helicosphaera spp. increased at lower nitrate concentrations and higher temperatures and light intensities than E. huxleyi. Population changes of Thorosphaera flabellata were similar to those of F. profunda. Discosphaera tubifera and Rhabdosphaera clavigera increased in warm waters depleted in nitrate and showed a preference for very high light intensities. Most species seem to survive at a wide range of local environmental conditions but usually at very low cell densities. Individual taxa flourished under a relatively restricted range of favourable environmental conditions. Correlations between coccolithophores and abiotic parameter variability suggest that up to 50% of the dynamics of the most abundant coccolithophore species may be driven by variability in the physical parameters, i.e. by “bottom-up” controls.

Assemblages of coccolithophorids and other living microplankton off the coast of Puerto Rico during January–May 1995

Much effort is being spent on estimating phytoplankton productivity and their role in sequestering CO2. Most of the time these estimates are based on chlorophyll a determination either from satellite spectral data or from ground truthing. No appreciable increase in microplankton standing crops occurred at stations which have high chlorophyll a contents, indicating that chlorophyll a peaks may have been associated with another group of organisms such as picoplankton which do not produce hard parts.Microplankton samples were collected biweekly from a number of depths at a fixed station off the coast of Puerto Rico between 30 January and 10 May 1995. The microplankton composition was counted and the coccolithophorid species assemblages identified. The total microplankton cell concentrations, usually ranged between 103 and 104cells/l, and were found to be consistent with previous findings from the Caribbean Sea and other ‘blue water’ subtropical regions. A spring diatom bloom did not occur at our station, although relatively higher numbers of diatoms were present in late January. Apart from a small ‘event’ in late February–early March, the diatoms (especially the centric diatoms) decreased in relative abundance and sank into deeper photic waters as spring progressed, while the relative abundance of coccolithophorids increased. During the study period, the coccolithophorid community was distributed vertically into the three classical sections of the photic zone, although the lower photic zone was not always encountered. The upper photic zone was dominated either by Emiliania huxleyi or Gephyrocapsa oceanica from January to early March, after which they declined in numbers, and Umbellosphaera irregularis, Umbellosphaera tenuis, Discosphaera tubifera, Rhabdosphaera clavigera, holococcolithophorids, and syracosphaerids then co-dominated throughout the spring months, showing monthly fluctuations in relative abundance. In the lower photic zone Florisphaera profunda was always the dominant species. However, the occasional absence or low percentage of this species and other lower photic species, probably indicates that a deepening of the nutricline occurred. The middle photic zone was characterized in late spring by peaks of E. huxleyi and centric diatoms, both of which prefer to avoid unfavourable conditions at the surface and thus their presence in this zone appears to signify a survival strategy.These results show that the Puerto Rican slope waters still exhibit seasonality even though they are considered oligotrophic and permanently stratified. There is also some evidence that material (e.g. neritic diatoms, chrysophyte statospores, pollen) from the Puerto Rican coast and mainland can be transported to the slope area at particular times of the year, probably as a result of shelf water movement in conjunction with periods of heavy monsoonal rainfall. There is also a possibility that waters originating from the Orinoco River, which reach the Puerto Rican coast in September and October each year, play some role in determining the phytoplankton composition during the winter months. It is hoped that future sampling will shed further light on the seasonality and annual cyclicity of the phytoplankton from this subtropical area.

Environmental control on the biogeography of modern coccolithophores in the southeastern Indian Ocean offshore of Western Australia

The biogeography of living calcareous nannoplankton for the southeastern Indian Ocean offshore of Western Australia was studied by analyzing 192 filter samples collected within the upper 200m of the ocean at 33 stations.The cell density of total nannoplankton decreased significantly in some nearshore stations along the Western Australian coast where the Leeuwin Current flows.A total of 92 taxa were identified by SEM observation from 118 of the samples, i.e. those which had common nannoplankton and allowed the distinction between upper- and lower-photic zone floras.The most abundant taxa were classified into three main groups and two independent species based on their distribution pattern. Of the latter, Umbellosphaera irregularis is common in the shallower samples in the tropics, whilst Gephyrocapsa oceanica occurs in isolated patches. Group U taxa (Discosphaera tubifera, Rhabdosphaera clavigera var. stylifera and Umbellosphaera tenuis) occur in the shallower depths of the subtropical water. Group UL taxa, which include Emiliania huxleyi, Gephyrocapsa ericasonii, Syracosphaera spp., Umbilicosphaera hulburtiana and various other taxa whose coccospheres are covered by holococcoliths (holococcolithophores), are common in the subtropics. Group L taxa, which are composed of Florisphaera profunda, Oolithotus antillarum, Gladiolithus flabellatus and Algirosphaera oryza, are most common in the deep water.In order to clarify the mega-trends of floral variations in the upper- and lower-photic zone communities, we obtained the following two rations. The ratio of U. irregularis to groups U+UL in the upper-photic community is positively related to temperature, whereas its relationship with salinity and water density is negative. In the lower-photic community, the ratio of group L to UL varies in the nutrient-poor environments, but is consistently high in eutrophic environments. The occurrence of G. oceanica is characterized by higher temperature, lower salinity and lower water density in the upper-photic community.