Hannelore Håkansson

A compilation and evaluation of species in the general Stephanodiscus, Cyclostephanos and Cyclotella with a new genus in the family Stephanodiscaceae

This paper includes species investigated through many years starting with original material from museum collections. A great number of species in the genera Stephanodiscus Ehrenberg, Cyclostephanos Round and Cyclotella (Kutzing) Brebisson, all in the family Stephanodiscaceae Makarova, are presented with holo-, lecto- and/or neotypes, type locality and information concerning nomenclature. A typus generis is given for each genus. Errors in the literature are pointed out and corrected. Species in the genus Stephanodiscus have the characteristic fasciculate morphology, they have fultoportulae, rimoportulae with their external tubular (long or short) extension and areolae internally with a domed cribrum both on the valve face and on the mantle. The typus generis is Stephanodiscus niagarae. Species in the genus Cyclostephanos have also the fasciculate structure, the fultoportulae, but their areolae have a domed cribium only on the valve face and are flat on the mantle, their rimoportulae have only a simple external opening, often connected with a spine and a marginal alveolate structure, even though in some species weakly developed, which differentiate these species from those in the genus Stephanodiscus. The typus generis: Cyclostephanos novaezeelandiae. The genus Cyclotella includes species which have a smooth central area, none to several valve face fultoportulae, a simple alveolate marginal striation and the rimoportulae with an external round or slit-like opening, located on the mantle interrupting the ring of mantle fultoportulae. In some species complexes, however, the external opening of the rimoportula can be found on the valve face, near the valve face/mantle junction. The typus generis selected is C. distinguenda. Some other described species are transferred to the new genus Puncticulata Hakansson gen. nov. encompassing the species with an areolate central area consisting of areolae and valve face fultoportulae, in some species only fultoportulae in others only areolae, a complex alveolate striated marginal area, rimoportulae on the valve face at the end of a shortened interstriae. The Opus generis: Puncticulata comta (Ehrenberg) Hakansson comb.Systems of classification are not hat racks, objectively presented to us by nature. They are dynamic theories developed to express particular views about the history of organisms. Evolution has produced a set of unique species ordered by differing degrees of genealogical relationship. Taxonomy, the search for this natural order, is the fundamental science of history. D.J. Gould (1987) This paper includes species investigated through many years starting with original material from museum collections. A great number of species in the genera Stephanodiscus Ehrenberg, Cyclostephanos Round and Cyclotella (Kützing) Brébisson, all in the family Stephanodiscaceae Makarova, are presented with holo-, lecto- and/or neotypes, type locality and information concerning nomenclature. A typus generis is given for each genus. Errors in the literature are pointed out and corrected. Species in the genus Stephanodiscus have the characteristic fasciculate morphology, they have fultoportulae, rimoportulae with their external tubular (long or short) extension and areolae internally with a domed cribrum both on the valve face and on the mantle. The typus generis is Stephanodiscus niagarae. Species in the genus Cyclostephanos have also the fasciculate structure, the fultoportulae, but their areolae have a domed cribrum only on the valve face and are flat on the mantle, their rimoportulae have only a simple external opening, often connected with a spine and a marginal alveolate structure, even though in some species weakly developed, which differentiate these species from those in the genus Stephanodiscus. The typus generis: Cyclostephanos novaezeelandiae. The genus Cyclotella includes species which have a smooth central area, none to several valve face fultoportulae, a simple alveolate marginal striation and the rimoportulae with an external round or slit-like opening, located on the mantle interrupting the ring of mantle fultoportulae. In some species complexes, however, the external opening of the rimoportula can be found on the valve face, near the valve face/mantle junction. The typus generis selected is C. distinguenda. Some other described species are transferred to the new genus Puncticulata Håkansson gen. nov. encompassing the species with an areolate central area consisting of areolae and valve face fultoportulae, in some species only fultoportulae in others only areolae, a complex alveolate striated marginal area, rimoportulae on the valve face at the end of a shortened interstriae. The typus generis: Puncticulata comta (Ehrenberg) Håkansson comb. nov.

A late Holocene lake sediment sequence from Livingston Island, South Shetland Islands, with palaeoclimatic implications

Analysis of a 1.5 m thick sediment sequence from Midge Lake, Byers Peninsula, Livingston Island, shows that the lake and its catchment have undergone significant changes during the last 4000 years. Radiocarbon dating (AMS), sediment lithology, and microfossil analyses indicate that the lake was deglaciated over 4000 14 C years ago. Distinct peaks in accumulation rates of sediment, Pediastrum algae, pollen and spores, as well as changes in the diatom assemblage, suggest significant environmental changes between ca 3200 and 2700 y BP. These changes are interpreted as reflecting a milder and more humid, maritime climate. The increased humidity can explain independent observations of glacier growth during this period. The combined data also indicate that between ca 1500 and 500 y BP the area might have experienced more continental conditions with slightly colder and drier climate than today. Since the 14 C dates from the Midge Lake sediments are regarded as reliable and the sediment sequence is rich in tephra layers this sediment sequence will be critical for a forthcoming tephra chronology of the region.

Palaeoclimatic studies in South Shetland Islands, Antarctica, based on numerous stratigraphic variables in lake sediments

The hitherto longest found lake sediment sequence on Byers Peninsula, Livingston Island, South Shetland Islands, was analysed with respect to lithology, chronology, diatoms, Pediastrum, pollen and spores, mosses, mineralogy, and sediment chemistry. During the ca. 5000 year long development the sediments were influenced by frequent tephra fall-outs. This volcanic impact played a major role in the lakes history during two periods, 4700–4600 and 2800–2500 BP, but was of importance during the lakes entire history with considerable influence on many of the palaeoenvironmentally significant indicators. The large and complex data set was analysed and zonated with different types of multivariate analysis. This resulted in a subdivision of the sequence into 8 time periods and 21 variables. Redundancy analysis (RDA) of this data set, both without and with the tephra periods, and with 4–6 of the variables as explanatory environmental variables, reveal that climatic/environmental signals are detectable. The palaeoclimatic picture that emerged out of the tephra ‘noise’ suggests that the first 100 years were characterized by mild, humid conditions. This was followed by a less mild and humid climate until ca. 4000 BP when a gradual warming seems to have started, coupled with increased humidity. These mild and humid conditions seem to have reached an optimum slightly after 3000 BP. At ca. 2500 BP a distinct climatic deterioration occurred with colder and drier conditions and long seasons with ice cover. This arid, cold phase probably reached its optimum conditions at ca. 1500 BP, when slightly warmer conditions might have prevailed for a while. Except for the modern sample with rather mild climate, the last 1400 years seem to have been fairly arid and cold, and the effects of the frequent volcanic activity during this period is only vaguely seen in the records.

A LIGHT AND ELECTRON MICROSCOPE STUDY OF CYCLOTELLA SPECIES (BACILLARIOPHYCEAE) FROM CENTRAL AND NORTHERN CANADIAN LAKES

Cyclotella species from several Canadian lakes have been investigated and their morphology illustrated by light and electron microscopy. Cyclotella antiqua W. Smith, C. bodanica var. bodanica Grun., C. bodanica var. affinis Grun., C. meneghiniana Kutz. and C. michiganiana Skvortzow have been identified with certainty although their cell diameters were smaller than those previously recorded in the literature. Differences and similarities in the C. bodanical/radiosa (Syn. comta)/comensis complex and in the Cyclotella stelligera group are discussed. One species appears to be identical to the newly described Stephanocostis chantaicus Genkal and Kuzmina. A new species (Cyclotella gordonensis nov. sp.) has been found so far only in deep, oligotrophic lakes of the Canadian Arctic.

DIATOM STRATIGRAPHY IN A LAKE ON HORSESHOE ISLAND, ANTARCTICA: A MARINE-BRACKISH-FRESH WATER TRANSITION WITH COMMENTS ON THE SYSTEMATICS AND ECOLOGY OF THE MOST COMMON DIATOMS

The diatom stratigraphy of Holocene sediment cores from an Antarctic lake (S 67° 48′40″, W 67° 18′50″) on Horseshoe Island has been investigated. The diatom analyses show evidence of a transition from a marine to a brackish to a fresh water environment. The diatom stratigraphy is divided into three different assemblage zones (marine, brackish and fresh water), which are mostly in accordance with the lithostratigraphical units as well as with chemical analyses. The diatom succession in the sediment of “Skua Lake” is not a response to climatic changes. The sediment cores represent a short time scale and the development from a marine to freshwater environment is so dominant that other changes in the diatom flora are hidden. Radiocarbon dates indicate that the isolation of the basin occurred some time after 1860 BP. The possible isostatic uplift resulting from deglaciation of the “Marguerite Bay” Glacier ca 10,000 year BP is discussed. Systematics and ecology for some of the species in the genera Achnanthes, ...

TAXONOMIC INTERPRETATION OF THE RIMOPORTULA OF FRESHWATER GENERA IN THE CENTRIC DIATOM FAMILY THALASSIOSIRACEAE

It is difficult to delineate Cyclotella, Cyclostephanos and Stephanodiscus—the three most taxonomically diverse, predominantly freshwater genera of the Thalassiosiraceae. Cyclostephanos was originally distinguished by a Stephanodiscus-like exterior of fasciculate striae and raised radiating costae, and a Cyclotella-like interior, with chambers near the margin. After the creation of Cyclostephanos, Cyclotella and Stephanodiscus were each diagnosed by absence of characters. However, this study suggests that most Cyclostephanos and Stephanodiscus species can be classified on the basis of their unusual rimoportula morphology. Over half of the Cyclostephanos species known, including the lectotype, have the rimoportula underneath a spine; we call this the ‘subspina’ species group. The rimoportula of all known Cyclostephanos species lacks a long external tube, regardless of its location, although some may have a small raised annulus around the external opening. In contrast, the lectotype of Stephanodiscus and al...

A late Holocene record of land-use history, soil erosion, lake trophy and lake-level fluctuations at Bjäresjösjön (south Sweden)

Land-use history, soil erosion, lake trophy and lake-level fluctuations during the last 3000 years were reconstructed through a multidisciplinary palaeolimnological study (pollen, plant macrofossils, diatoms, physical and chemical analysis, magnetic measurements and radiometric methods) of a small eutrophic lake in southern Sweden (Bjäresjösjön, Scania). There are striking responses in diatom, chemical, sediment yield and magnetic records to land-use changes documented by pollen analysis or historical sources, and to lake-level changes identified from sedimentary changes. Our multidisciplinary approach assists interpretation of the processes controlling long-term changes and separation of the effects of different factors (land-use changes, lake-level fluctuations) on individual biostratigraphical records. Climate has controlled processes in the lake indirectly, through lake-level fluctuations, from the Late Bronze Age to the Viking Age (700 BC-AD 800). Since the Viking Age, land-use controlled most of the changes observed in the lakes development and soil erosion processes. Major changes in lake development occurred during the last 200 years, due to a drastic increase in soil erosion and water eutrophication during a period of agricultural modernization.

THE CURRENT STATUS OF SOME VERY SMALL FRESHWATER DIATOMS OF THE GENERA STEPHANODISCUS AND CYCLOSTEPHANOS

The morphology of some very small Stephanodiscus and Cyclostephanos species from Canada, USA and Europe has been studied by light and electron microscopy. The possible influence of the environment on the variation of those characters often used in the diagnosis of species is discussed and an attempt is made to improve understanding of the features used to characterize such species as Cyclostephanos tholiformis Stoermer, Hakansson & Theriot, C. delicatus (Genkal) comb. nov. and S. cf. minutulus (Kutz.) Cieve & Molier. Two new species are described: Stephanodiscus binatus Hakansson & Kling sp. nov. and S. nipigonensis Kling & Hakansson sp. nov. Similarities and differences to original material are further shown and discussed.

CYCLOTELLOID SPECIES FROM A DIATOMITE IN THE HARZ MOUNTAINS, GERMANY, INCLUDING PLIOCAENICUS GEN. NOV.

Three centric taxa are described from a diatomite deposit in the Harz mountains, Niedersachsen, Germany. The delimination of Cyclotella, Cyclostephanos and Stephanodiscus is discussed and a tighter generic circumscription proposed. Cyclotella radiosa cf. var. pliocaenica is explored and a new genus Pliocaenicus is erected together with two new species, P. hercynicus and P. undulatus. Cyclostephanos pygmaea, C. costatus, C. omarensis and C. pantocseki are also transferred to Pliocaenicus.

A STUDY OF VARIATION IN VALVE MORPHOLOGY OF THE DIATOM CYCLOTELLA MENEGHINIANA IN MONOCLONAL CULTURES: EFFECT OF AUXOSPORE FORMATION AND DIFFERENT SALINITY CONDITIONS

Cyclotella meneghiniana was collected from an artificial pond Kosharskij near Feodosia, Crimea, isolated and cultured under different conditions (freshwater, 2‰, 4‰, 8‰, and 16‰ salinity). Detailed light- and electron microscope investigations were made of specimens from monoclonal cultures of C. meneghiniana and comparisons made with type material of both C. meneghiniana and C. kuetzingiana, which are conspecific. Cells cultured under different salinity conditions were very similar, showing only slight differences in the number of valve face fultoportulae, in the pattern of the satellite pores of the mantle fultoportulae, and in the lengths of the striae. New cultures started from single initial cells isolated from 3 of the 4 monoclonal cultures at 8‰or 16‰ salinity conditions, and grown in 16‰ salinity showed greater morphological variability, with differences in the pattern and number of the marginal striae, the number and placement of valve face fultoportulae, and the pattern of the satellite pores of...