Fluctuations In Relative Abundance Research Articles

Biotope preferences in groups of closely related species: a case study of Sylvaemus (Mammalia)

The trapping of small mammals was carried out in 2017 to 2023 in the territory of the Kharkiv Oblast, Ukraine. In total, 19 biotopes were studied: different types of oak forest, dry and floodplain meadows, riparian vegetation, areas of the steppe, field protection forest strips, fields, as well as various ecotones. The trapping of small mammals was carried out using the standard trap-line method on 93 trap lines. With a trapping effort of 17 310 trap-nights, 1295 individuals of small mammals were caught, 689 of which belonged to the genus Sylvaemus. Three species of mice of the genus Sylvaemus were found in the territory of Kharkiv Oblast during the study period: Sylvaemus uralensis, Sylvaemus sylvaticus, and Sylvaemus tauricus. According to the relative abundance, wood mice can be arranged in the following order: uralensis > sylvaticus > tauricus. The cyclicity of annual fluctuations in relative abundance is not noticeable. The similarity of the abundance fluctuation in S. sylvaticus and S. tauricus was revealed, taking into account that these are competing species. S. uralensis and S. sylvaticus lead the top three dominant species in the overall dominance structure of small mammals. The highest relative abundance of wood mice during the study period was recorded on dry meadows, on the border of riparian vegetation with fields and floodplain meadows, on the border of field protective forest strips with fields, and in dry maple-linden-oak forest. S. uralensis belongs to the eurytopic species, is found in almost all studied biotopes, avoids oak forests, occurs on the edges of forests, dominates in abundance in open biotopes, in particular in the steppe, on dry meadows, riparian vegetation, and field protection forest strips. S. sylvaticus is also prone to eurytopy, it is recorded in significant abundance in both open and forest biotopes, mostly prefers riparian vegetation, field protection forest strips, dry coniferous forest, fresh maple-linden-oak forest and forest edges. S. tauricus is found mostly in forest biotopes, in particular in various types of oak forest and on forest edges, it prefers field protection forest strips with an old stand of oak, linden, and robinia. All three species of mice of the genus Sylvaemus are common species.

Protist Interactions and Community Structure During Early Autumn in the Kerguelen Region (Southern Ocean)

This study investigated protist community composition and biotic interactions focusing on microplankton at four distinct sites around the Kerguelen Islands (Southern Ocean) after the summer phytoplankton bloom. Protist diversity in different size fractions, sampled with Niskin bottles and plankton nets, was assessed by sequencing of the V4 18S rDNA region. Combining different approaches, i.e. sequencing of different plankton size fractions, and isolation and sequencing of single cells, provided new insights into microbial interactions in protist communities. The communities displayed high variability, including short-term fluctuations in relative abundance of large protists (>35μm) highlighted by the plankton net samples. Size fractionation of protist communities showed high concentrations of free Syndiniales spores but relatively few Syndiniales associated with microplankton, suggesting low parasitic infection in early autumn. Co-variance network analyses and sequencing of individually isolated single cells highlighted the important role of Rhizaria as consumers of a wide range of different diatom taxa. The data also raised the hypothesis that different Syndiniales clades might be directly or indirectly associated with some diatom genera, thus suggesting a potentially wider host range of these parasites than has been previously reported. These associations and the potential impact on carbon fluxes are discussed.

Distinct nasal airway bacterial microbiotas differentially relate to exacerbation in pediatric patients with asthma

In infants, distinct nasopharyngeal bacterial microbiotas differentially associate with the incidence and severity of acute respiratory tract infection and childhood asthma development. We hypothesized that distinct nasal airway microbiota structures also exist in children with asthma and relate to clinical outcomes. Nasal secretion samples (n=3122) collected after randomization during the fall season from children with asthma (6-17years, n=413) enrolled in a trial of omalizumab (anti-IgE) underwent 16S rRNA profiling. Statistical analyses with exacerbation as the primary outcome and rhinovirus infection and respiratory illnesses as secondary outcomes were performed. Using A549 epithelial cells, we assessed nasal isolates of Moraxella, Staphylococcus, and Corynebacterium species for their capacity to induce epithelial damage and inflammatory responses. Six nasal airway microbiota assemblages, each dominated by Moraxella, Staphylococcus, Corynebacterium, Streptococcus, Alloiococcus, or Haemophilus species, were observed. Moraxella and Staphylococcus species-dominated microbiotas were most frequently detected and exhibited temporal stability. Nasal microbiotas dominated by Moraxella species were associated with increased exacerbation risk and eosinophil activation. Staphylococcus or Corynebacterium species-dominated microbiotas were associated with reduced respiratory illness and exacerbation events, whereas Streptococcus species-dominated assemblages increased the risk of rhinovirus infection. Nasal microbiota composition remained relatively stable despite viral infection or exacerbation; only a few taxa belonging to the dominant genera exhibited relative abundance fluctuations during these events. Invitro, Moraxella catarrhalis induced significantly greater epithelial damage and inflammatory cytokine expression (IL-33 and IL-8) compared with other dominant nasal bacterial isolates tested. Distinct nasal airway microbiotas of children with asthma relate to the likelihood of exacerbation, rhinovirus infection, and respiratory illnesses during the fall season.

Complexity in the Biological Recovery of Tatra Mountain Lakes from Acidification

Alpine lakes of the Tatra Mountains were severely affected by acidification, with minimum recorded values of pH ∼4.5 in the mid-1980s. Since the 1990s, a dramatic decrease in the deposition of acidifying compounds has led to a considerable reversal in lake water chemistry (to pH∼5 in the most severely affected lakes). We studied changes of planktonic crustaceans and chironomid occurrence during the acidification period and the following period of recovery from acidification in three categories of 50 Tatra Mountain lakes (non-acidified, acidified and strongly acidified, according to their status at the beginning of the 1980s). In acidified and strongly acidified lakes, the planktonic crustaceans completely disappeared already by about 1976 except for a few individuals of ubiquitous species in littoral zone due to acidification-induced oligotrophication. In strongly acidified lakes, the original planktonic crustaceans disappeared and littoral species became more abundant already before 1976 due to acidification-induced eutrophication and aluminium toxicity. These processes were quickly reversed following the increase in lake water pH. Extinct species started to return to several acidified and strongly acidified lakes already in the beginning of 1990s. The process of recovery was delayed in many other lakes of the same categories, however, or it did not even start before 2008 despite the improved water chemistry and feeding resources (concentration of chlorophyll-a). Compared to planktonic crustaceans, the reaction of chironomids to acidification and recent recovery has been less pronounced. An analysis of sediment records showed that fluctuations in relative abundance of the dominant chironomid taxa and a decrease of their density occurred. In spite of the fact that chironomid fauna exhibited clear signs of recovery in the last two decades, the recovered assemblage does not exactly reflect the pre-acidification status in the lake. The occurrence and higher proportion of more thermophilous chironomid species in some alpine lakes of all categories could be correlated with increasing air temperature. The considerable effect of climatic factors may thus prevent the full re-establishment of the original status even when the acidification stress completely ceases. The delayed return of planktonic crustaceans to some recovered lakes could be a consequence of the short water residence time of these lakes. In addition, a shortening of the water residence time in recent decades, probably related to recent climate change, in interaction with the ecology of planktonic crustaceans, may possibly be causing further delays in their return.

Amphibian skin microbiota exhibits temporal variation in community structure but stability of predicted Bd-inhibitory function.

Host-associated microbiomes are increasingly recognized to contribute to host disease resistance; the temporal dynamics of their community structure and function, however, are poorly understood. We investigated the cutaneous bacterial communities of three newt species, Ichthyosaura alpestris, Lissotriton vulgaris and Triturus cristatus, at approximately weekly intervals for 3 months using 16S ribosomal RNA amplicon sequencing. We hypothesized cutaneous microbiota would vary across time, and that such variation would be linked to changes in predicted fungal-inhibitory function. We observed significant temporal variation within the aquatic phase, and also between aquatic and terrestrial phase newts. By keeping T. cristatus in mesocosms, we demonstrated that structural changes occurred similarly across individuals, highlighting the non-stochastic nature of the bacterial community succession. Temporal changes were mainly associated with fluctuations in relative abundance rather than full turnover of bacterial operational taxonomic units (OTUs). Newt skin microbe fluctuations were not correlated with that of pond microbiota; however, a portion of community variation was explained by environmental temperature. Using a database of amphibian skin bacteria that inhibit the pathogen Batrachochytrium dendrobatidis (Bd), we found that the proportion of reads associated with 'potentially' Bd-inhibitory OTUs did not vary temporally for two of three newt species, suggesting that protective function may be maintained despite temporal variation in community structure.

Variación intraestacional en los visitantes florales de Erysimum mediohispanicum en Sierra Nevada

Valverde, J., Calatayud, J., Gomez, J.M., Perfectti, F. 2014. Intra-seasonal variation of Erysimum mediohispanicum flower visitors in Sierra Nevada (Spain). Ecosistemas 23(3): 83-92. Doi.: 10.7818/ECOS.2014.23-3.11 Plant-pollinator interactions have traditionally been a hot topic in ecology and evolutionary biology due to both the ecological services it offers and its complexity as a system. Plants generally show a high diversity of floral visitors, making generalism a more common condition than expected. Furthermore floral visitor community seems to have spatio-temporal fluctuations in diversity and composition. Those variations respond to plant intrinsic and extrinsic factors and can affect to evolutionary and ecological aspects of plant populations. Here we assess the temporal variation in the floral visitor assemblage of the generalist plant Erysimum mediohispanicum Polatschek (Brassicaceae). We set up two experimental plots homogenizing for spatial and microenvironmental conditions to ascertain only the temporal variations in the floral visitors assemblage. During the flowering season we did daily censuses of the floral visitors, determining them to morpho-species level and clumped them in 16 functional groups based on morphological and behavioral characteristics. We found a high diversity for the whole assemblage in both experimental plots with high temporal fluctuations during the flowering season. Our results suggest an important species turnover with high fluctuations in relative abundance for some functional groups, which may turn into important ecological and evolutionary consequences.

A comparison of the wet and dry season DNA-based soil invertebrate community characteristics in large patches of the bromeliad Bromelia pinguin in a primary forest in Costa Rica

In Costa Rica, the Maquenque National Wildlife Refuge (MNWLR) contains a unique habitat gradient ranging from primary old growth forests, grasslands, pastures, to various ages of secondary forests. Within these primary old growth forests are extremely dense naturally occurring Bromelia pinguin (Bromeliaceae) patches that often grow with densities up to 2 plants per square meter. A previous study found that anti-fungal activity of this particular plant appears to be altering the fungal community in soils adjacent to these plants. No work has been previously conducted on the possible effects of this plant community on soil faunal communities and if seasonality contributes to changes in soil invertebrate populations along a moisture gradient. Thus, a study was conducted to assess the effects of this specialized plant community on soil invertebrates with respect to season, and if these changes in soil fauna guild structure could prove to be valid candidates as indicators of ecosystem condition with changes in precipitation. In addition, a meta-analysis was done to determine how the bromeliad-associated soil invertebrate communities differ from those in adjacent primary forest soils. Therefore, comparisons were determined from previous primary forest soil invertebrate environmental DNA (eDNA) to the current wet season bromeliad soil invertebrate eDNA. Roche 454 pyro-sequencing was conducted on the 650bp fragment of the cytochrome oxidase subunit I (COI) gene of invertebrates to obtain and characterize soil invertebrate sequence composition. To determine relationships among soil fauna guilds across seasons, relative abundance of the sequences were calculated, and used in conjunction with EcoSim niche overlap and co-occurrence values. From the bromeliad seasonal soil fauna eDNA study, it appears certain invertebrate guilds are driven by moisture as indicated by fluctuations in relative abundance of each invertebrate guild across seasons in bromeliad patch soils, as well as indicated by EcoSim niche overlap values. In particular, Guilds 1, 4, and 5, should warrant further investigation as indicators of habitat condition. The meta-analysis showed that a naturally occurring modified environment (the bromeliad patches), can result in differences in relative abundance and partitioning of a limited resource between invertebrate guild structure. Those guilds associated with microbivorous and complex decomposition activities (i.e. Guilds 3 and 4), are more abundant in primary forest soils than bromeliad patch soils and could potentially used for bioindicators of habitat perturbations.

Transmission of Ranavirus between Ectothermic Vertebrate Hosts

Transmission is an essential process that contributes to the survival of pathogens. Ranaviruses are known to infect different classes of lower vertebrates including amphibians, fishes and reptiles. Differences in the likelihood of infection among ectothermic vertebrate hosts could explain the successful yearlong persistence of ranaviruses in aquatic environments. The goal of this study was to determine if transmission of a Frog Virus 3 (FV3)-like ranavirus was possible among three species from different ectothermic vertebrate classes: Cope’s gray treefrog (Hyla chrysoscelis) larvae, mosquito fish (Gambusia affinis), and red-eared slider (Trachemys scripta elegans). We housed individuals previously exposed to the FV3-like ranavirus with naïve (unexposed) individuals in containers divided by plastic mesh screen to permit water flow between subjects. Our results showed that infected gray treefrog larvae were capable of transmitting ranavirus to naïve larval conspecifics and turtles (60% and 30% infection, respectively), but not to fish. Also, infected turtles and fish transmitted ranavirus to 50% and 10% of the naïve gray treefrog larvae, respectively. Nearly all infected amphibians experienced mortality, whereas infected turtles and fish did not die. Our results demonstrate that ranavirus can be transmitted through water among ectothermic vertebrate classes, which has not been reported previously. Moreover, fish and reptiles might serve as reservoirs for ranavirus given their ability to live with subclinical infections. Subclinical infections of ranavirus in fish and aquatic turtles could contribute to the pathogen’s persistence, especially when highly susceptible hosts like amphibians are absent as a result of seasonal fluctuations in relative abundance.

Use of track tubes to detect changes in abundance of beach mice

Track tubes may help measure impacts of disturbances on small mammals by providing information about the magnitude and timing of fluctuations in relative abundance. We evaluated live-capture and track-tube data from 8 trapping grids to test the utility of track tubes for monitoring abundance of beach mice (Peromyscus polionotus) through time and for comparing relative abundance of beach mice among sites. When averaged across grid sites, track indexes and abundance estimates exhibited strong covariation through time. Associations between the track index and estimated abundance on different grids varied from strong to very poor depending on sample period. Associations between track indexes and abundance estimates were strong when mouse densities on all grids were relatively low, but they were weak during other sample periods when densities were highly variable among grids. This study points to the need to understand factors that influence the relationship between indexes from track tubes and mouse abundance before track tubes can be used to compare abundances among sites.

Statistical analysis of polychaete population density: dynamics of dominant species and scaling properties in relative abundance fluctuations

Statistical analysis of polychaete population density: dynamics of dominant species and scaling properties in relative abundance fluctuations

Statistical analysis of polychaete population density: dynamics of dominant species and scaling properties in relative abundance fluctuations

Abstract. We consider here the dynamics of two polychaete populations based on a 20 yr temporal benthic survey of two muddy fine sand communities in the Bay of Morlaix, Western English Channel. These populations display high temporal variability, which is analyzed here using scaling approaches. We find that population densities have heavy tailed probability density functions. We analyze the dynamics of relative species abundance in two different communities of polychaetes by estimating in a novel way a "mean square drift" coefficient which characterizes their fluctuations in relative abundance over time. We show the usefulness of using new tools to approach and model such highly variable population dynamics in marine ecosystems.

Effects of multi‐year droughts on fish assemblages of seasonally drying Mediterranean streams

Summary1. This study analysed changes occurring in Mediterranean stream fish assemblages over a sequence of dry years followed by a generally wet period (1991–98). Variations in assemblage attributes were quantified at the basin and stream reach scales, and related to variables reflecting the occurrence of unusually dry or wet conditions.2. Assemblage variability increased along with the resolution of analysis, with little changes in species richness, composition and rank abundances, but significant variation in individual species abundances. Fluctuations in relative abundances were significantly affected by variables reflecting the severity of summer droughts and the occurrence of rainy springs. These patterns were evident at the basin scale, while variability at individual stream reaches tended to be higher and less related to rainfall patterns.3. At least three response guilds to rainfall variation could be identified: two of the four abundant and widespread species (chub and loach) declined following dry years, whereas the two other core species (nase and eel) declined after rainy spring; one scarce native species (stickleback) increased in dry years.4. Except at the two most upstream reaches, the assemblages tended to recover quickly to previous configuration after the changes occurring during the sequence of dry years.5. Temporal variability of local assemblages was concordant among reaches but did not follow any consistent spatial pattern. Instead, spatial patterns in assemblage attributes changed over time in response to environmental variability, with a tendency for a disruption of upstream–downstream gradients following dry years.6. Results supported the view that present‐day droughts cause relatively small and transient changes to Mediterranean stream fish assemblages. However, longer and more severe droughts expected under altered future climates, may result in declines or local extinctions of the most sensitive species and their potential replacement by more resistant species. Changing drought regimes thus need to be duly considered in the development of conservation strategies for Mediterranean stream fish.

Long-term study on a brachyuran crab community (Crustacea: Decapoda) inhabiting a boulder shore in the Eastern Mediterranean: relative abundance

A crab community inhabiting a cobble shore was studied from samples taken on 41 collecting trips in six individual years over a long period of 16 years (1984–1999). The aim of this long-term study was to find whether oscillations in abundance can be distinguished from decline in numbers. Quantitative changes taking place in the relative abundance of the four most common species during that period are described. Thus, 352 Pachygrapsus transversus, 233 P. marmora- tus, 48 Eriphia verrucosa and 155 Xantho poressa specimens were collected throughout the study. Some oscillations were observed in the relative abundance of both Pachygrapsus species and in E. verrucosa. In X. poressa a marked decline was noticeable after 1986. The sex ratio was male-biased in most cases except in P. marmoratus and in E. verrucosa in August. Since apparently no major change took place in the habitat itself, the main conclusion of this study is that the fluctuations in relative abundance of all species may be due to fluctuations in the sex ratio that occur naturally. Thus, the fact that during five months no E. verrucosa females were found is sufficient to effect such fluctuations in their population numbers. On the other hand, the decline in Xantho populations after 1986 could have its cause in the zoea larvae that were somehow affected, with the consequence that fewer megalops settled successfully in that specific habitat.

Lower-middle Eocene benthic foraminifera from the Fortuna Section (Betic Cordillera, southeastern Spain)

Research Article| January 01, 2006 Lower-middle Eocene benthic foraminifera from the Fortuna Section (Betic Cordillera, southeastern Spain) Silvia Ortiz; Silvia Ortiz 1Departamento de Ciencias de la Tierra, Universidad de Zaragoza, E-50009 Zaragoza, Spain Search for other works by this author on: GSW Google Scholar Ellen Thomas Ellen Thomas 2Center for the Study of Global Change, Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA, email: silortiz@unizar.es, ellen.thomas@yale.edu Search for other works by this author on: GSW Google Scholar Micropaleontology (2006) 52 (2): 97–150. https://doi.org/10.2113/gsmicropal.52.2.97 Article history received: 17 Jun 2005 accepted: 03 Jan 2006 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Silvia Ortiz, Ellen Thomas; Lower-middle Eocene benthic foraminifera from the Fortuna Section (Betic Cordillera, southeastern Spain). Micropaleontology 2006;; 52 (2): 97–150. doi: https://doi.org/10.2113/gsmicropal.52.2.97 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyMicropaleontology Search Advanced Search Abstract Benthic foraminifera were studied as part of an investigation of the Fortuna Section (Betic Cordillera, Southeastern Spain), undertaken to document a candidate section for the Global Stratotype Section and Point (GSSP) for the base of the Lutetian Stage (Ypresian/Lutetian boundary). Benthic foraminiferal assemblages in this section are mostly dominated by calcareous taxa (~50–96%) and by infaunal morphotypes (~55–88%). The most abundant calcareous taxa are Globobulimina species, Aragonia aragonensis, Cibicidoides species, Osangularia dominicana, and various Lenticulina species. The most common agglutinated species are Clavulinoides angularis, Gaudryina arenata, and Thalmannammina subturbinata. Globobulimina species, A. aragonensis, and C. angularis show strong fluctuations in relative abundances, with a pronounced peak in the relative abundances of Globobulimina species and Aragonia aragonensis, and low species richness across the Ypresian/Lutetian boundary. The occurrence of several peaks in abundance of Globobulimina species suggests that food supply to the benthos fluctuated, and the single peak in abundance of A. aragonensis across the Ypresian/Lutetian boundary might be indicative of a hyperthermal event. Paleodepths inferred from the assemblages range from lower upper bathyal for the lower part of the section to sublitoral for its upper part, and there is no evidence for a sea-level fall at the Ypresian/Lutetian boundary.We selected 82 out of 175 benthic foraminiferal taxa for detailed taxonomic description, based on their relative abundances and/or paleoecological or paleobathymetrical importance. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

PREY ECOLOGY OF MEXICAN SPOTTED OWLS IN PINE–OAK FORESTS OF NORTHERN ARIZONA

Abstract We studied Mexican spotted owl (Strix occidentalis lucida) diets and the relative abundance and habitat associations of major prey species in a ponderosa pine (Pinus ponderosa)–Gambel oak (Quercus gambelii) forest in north-central Arizona, USA, from 1990 to 1993. The owl's diet was comprised of 94% mammals by biomass and consisted of primarily the deer mouse (Peromyscus maniculatus), brush mouse (P. boylii), Mexican woodrat (Neotoma mexicana), and Mexican vole (Microtus mexicanus). Spotted owl prey in our study area were smaller on average than prey in other locations, and the total biomass of potential prey was less than that reported in other areas within the owl's geographic range. Although all prey populations exhibited seasonal fluctuations in relative abundance, only the deer mouse exhibited significant temporal variation in population abundance. The general pattern was for prey populations to rise during spring, peak during summer, decline in fall, and reach a winter low. Deer mice exhibit...

A high-resolution record of the last deglaciation in the Sicily Channel based on foraminifera and calcareous nannofossil quantitative distribution

Relative abundance fluctuations in planktic foraminiferal and calcareous nannofossil assemblages are reported on the basis of a high-resolution study of ODP Leg 160 Hole 963D, drilled in the Sicily Strait, near Capo Rossello (southern Sicily). With its 8 m of undisturbed sediments, the core covers the interval from 1.5 to 23 kyr, allowing a 50–100-yr sampling resolution. All the short warm and cold events and sub-events recorded in this time interval in the GRIP Greenland ice core and at several Mediterranean sites were recognized. On this basis, a total number of nine ecozones based on planktic foraminifera and seven ecozones based on calcareous nannofossils were identified. A short interval of climatic amelioration, identified above the base of the Holocene, precedes the segment contemporaneous with the deposition of sapropel S1, which has no lithological expression in the local sedimentary record. This segment has been subdivided into the two warm S1a and S1b phases, separated by a cold intermediate event. A gradual climatic deterioration is recorded above the top of the interval correlated with sapropel S1. It is interrupted by short cold spells, tentatively correlated with similar short duration cold events reported for this interval in the recent literature.

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.

Variations in calcareous dinoflagellate associations from the Maastrichtian to Middle Eocene of the western South Atlantic Ocean (São Paulo Plateau, DSDP Leg 39, Site 356)

Calcareous dinoflagellates often dominate the dinoflagellate cyst assemblage in Cretaceous to Recent oceanic sediments. However, their distribution in Paleogene sediments has scarcely been studied. The investigation of samples from DSDP Site 356 for their calcareous dinoflagellate content revealed 35 mainly long-ranging taxa. The associations and characteristic wall types (pithonelloid, oblique, radial, tangential) fluctuate quantitatively and qualitatively in distinct stratigraphic patterns. Significant shifts, primarily at the K/T boundary and the Paleocene/Eocene boundary, reflect changes in environmental conditions. Certain dinoflagellates forming calcareous cysts, such as Operculodinella operculata, were well adapted to the relatively rapid change of environmental conditions at the K/T boundary, thus blooming to dominate the carbonate flux to the ocean floor. In contrast to the stable Paleocene associations, Eocene calcareous dinoflagellates show fluctuations in relative abundances. These fluctuations can possibly be attributed to redeposition related to increased seaward transport of specimens, due to strengthened western boundary currents. The flora includes two new genera, one new species, and two new forms: Retesphaera diadema Hildebrand-Habel, Willems et Versteegh, gen. et. sp. nov., Cervisiella saxea (Stradner, 1961) Hildebrand-Habel, Willems et Versteegh, gen. et comb. nov., Sphaerodinella? tuberosa forma elongata Hildebrand-Habel, Willems et Versteegh, comb. et forma nov., Sphaerodinella? tuberosa forma variospinosa Hildebrand-Habel, Willems et Versteegh, comb. et forma nov. Three new combinations are proposed: Cervisiella saxea (Stradner, 1961) Hildebrand-Habel, Willems et Versteegh, gen. et comb. nov., Operculodinella operculata (Bramlette et Martini, 1964) Hildebrand-Habel, Willems et Versteegh, comb. nov., and Sphaerodinella? tuberosa (Kamptner, 1963) Hildebrand-Habel, Willems et Versteegh, comb. nov. The genus Operculodinella Kienel, 1994 is emended.

Pulleniatina obliquiloculata as a paleoceanographic indicator in the southern Okinawa trough during the last 20,000 years

Pulleniatina obliquiloculata, a tropical species of planktonic foraminifera, is indicative of the Kuroshio current and sensitive to winter sea surface temperature for the late Quaternary in the Okinawa Trough. Its relative abundance fluctuations are significant and correlatable between three gravity cores (cores 255, 170, 253) raised from the southern Okinawa Trough. Four major changes in its abundances with paleoceanographic significance have been recognized during the last 20,000 years: an abrupt increase at the Pleistocene/Holocene boundary, a short-term decrease indicative of a ‘Younger Dryas’-type climate reversal at about 11.4-9.6 ka B.P., the P. obliquiloculata maximum zone (around 7-6 ka B.P.) corresponding to the mid-Holocene climate optimum and the P. obliquiloculata minimum zone (around 4-2 ka B.P.) correlated probably to the Neoglacial cooling. The widespread occurrence of these events in the western Pacific and the correlated variations of its abundance between sea areas suggest that P. obliquiloculata is well promising as a paleoceanographic and climatic monitor for high-resolution reconstruction and sea-land correlation.

Trade wind forcing of upwelling, seasonally, and Heinrich events as a response to sub‐Milankovitch climate variability

Planktonic foraminifera recovered from two cores in the northern Benguela upwelling system reveal a history of rapid events with a variability at sub‐Milankovitch frequencies during the last 140 kyr. The “cold‐water” planktonic foraminifer, left coiling Neogloboquadrina pachyderma (Ehrenberg), shows rapid fluctuations in relative abundance, indicating changes in upwelling intensity. The periods of high abundance in left coiling N. pachyderma are referred to as “PS events” (pachyderma sinistral) and indicate increased intensity and zonality of the South Atlantic trade winds controlling the Benguela upwelling system. The good correlation between PS events, the North Atlantic Heinrich events, and the Dansgaard‐Oeschger cycles from the Greenland Ice Sheet Program (GISP2) ice core suggests large‐scale global oceanographic or climatic teleconnections between the South and North Atlantic via the trade wind system. The radiocarbon constrained timing of PS events younger than 45 kyr indicates that the South Atlantic leads the North Atlantic's response to trade wind changes, particularly during isotope stages 4–2 when the Earth was dominated by large ice sheets. At times of increased trade wind strength, tropical and subtropical waters are forced across the equator enhancing the pool of warm water to be transferred to the high latitudes of the North Atlantic via the Gulf Stream and North Atlantic Drift, increasing the pull of the thermohaline convective conveyor. The increased supply of warm water to the polar regions of the northern hemisphere increases the ice‐ocean moisture gradient and accelerates ice sheet growth, leading to eventual instability and collapse.