Mark B. Edlund

Oxygen Isotope Fractionation Between Diatomaceous Silica and Water

Abstract The temperature dependence of the oxygen isotope fractionation between diatomaceous silica and water was determined by analyzing frustules of freshwater diatoms cultured in the laboratory at temperatures ranging from 3.6 to 20.0°C. Within the limits of experimental reproducibility, measured oxygen isotope fractionations were independent of species and of the isotopic composition of the water. The fractionation varied regularly with temperature according to the equation 1000lnα (silica-water) = 15.56 (10 3 T −1 ) − 20.92. This relation corresponds to a temperature coefficient of roughly 0.2‰ per °C, significantly lower than published coefficients estimated from analyses of fossil diatoms from sediments and from extrapolation of experimentally determined quartz-water fractionations to low temperatures. The magnitude of the fractionation at a given temperature was 3–8‰ lower than previously published fractionations that were determined from analyses of fossil diatoms and from experimental data for quartz. The discrepancies between the new results and those of previous studies are attributed mainly to intrinsic differences in the oxygen isotope characteristics of fresh and fossil diatoms. Fresh diatomaceous silica appears to have an isotopically anomalous surficial layer containing large amounts of readily exchangeable, relatively low- 18 O oxygen, including abundant oxygen in hydroxyl groups, with the result that partial dissolution or diagenesis may systematically shift the δ 18 O values of fossil diatom frustules to higher values by removing this relatively unstable surficial material. If the effects of partial dissolution and diagenesis are regular or predictable, then the temperature information recorded during diatom growth may prove useful for paleoclimate studies.

ECOLOGICAL, EVOLUTIONARY, AND SYSTEMATIC SIGNIFICANCE OF DIATOM LIFE HISTORIES1

In spite of high species diversity and many years of study, most diatom life history strategies remain unknown. Floristics, taxonomy, and applied studies continue to dominate scientific efforts on diatoms; however, as Stebbins implied, it is difficult to pursue these ventures without an understanding of diatom reproductive biology. Recent research has added significantly to classic literature and has further expanded understanding of both the diversity of sexual behavior and the importance of life history strategies to ecology, evolution, taxonomy, and systematics of diatoms. Since the turn of the century, when reduction division was first convincingly demonstrated during gametogenesis in Rhopalodia (Klebahn 1896) and Surirella (Karsten 1912), it has been accepted that most diatoms have a diplontic life history (van den Hoek et al. 1995). Before and since that time, a great deal of work has been directed toward description and classification of diatom life histories (reviewed by Klebahn 1896, Geitler 1932, 1973, 1984, Fritsch 1935, Drebes 1977, Round et al. 1990, Hori 1993, Mann 1993, Roschin 1994), whereas far less effort has been directed at the ecologic and systematic implications of diatom life histories. The purpose of this review is not to reiterate or classify all the reported types of sexual reproduction in diatoms, but to provide current perspectives relating diatom life history strategies to systematic, ecologic, and evolutionary issues.

Siliceous microfossil distribution in the surficial sediments of Lake Baikal

Examination of surficial sediments at 16 stations shows minor, but consistent differences in the numbers and kinds of siliceous microfossils deposited in different regions of Lake Baikal. There is a general north-south decreasing trend in total microfossil abundance on a weight basis. Endemic plankton diatom species are the most abundant component of assemblages at all stations. Chrysophyte cysts are present at all stations, but most forms are more abundant at northern stations. Non-endemic plankton diatom species are most abundant at southern stations. Small numbers of benthic diatoms and sponge spicules are found in all samples. Although low numbers are present in offshore sediments, the benthic diatom flora is very diverse. Principal components analysis confirms primary north-south abundance trends and suggests further differentiation by station location and depth.

AULACOSEIRA SKVORTZOWII SP. NOV. (BACILLARIOPHYTA), A POORLY UNDERSTOOD DIATOM FROM LAKE BAIKAL, RUSSIA1

Aulacoseira skvortzowii sp. nov. is a diatom taxon present in modern plankton assemblages and sedimentary deposits from Lake Baikal, Russia. It has been previously reported as A. islandica (O. Müll.) Simonsen, A. islandica ssp. helvetica (O. Müll.) Simonsen, a sporangial frustule of A. baicalensis (K. Meyer) Simonsen, and Aulacoseira “spore”. However, its microstructure, ecology, and ability to form true resting spores provide ample criteria to describe this diatom as Aulacoseira skvortzowii sp. nov.

Siliceous microfossil succession in the recent history of two basins in Lake Baikal, Siberia

As part of the international cooperative Baikal Drilling Project, siliceous microfossil assemblage succession was analyzed in two short (∼ 30-cm) sediment cores from Lake Baikal. One core was recovered from the north basin (Core 324, 55°15′N, 109°30′E), a second from between the central and southern basins (Core 316, 52°28′N, 106°5′E). The northern core had higher amounts of biogenic silica (40 g SiO2 per 100 g dry weight sediment) compared to the southern core, and increased deposition in the more recent sediments. Weight percent biogenic silica was lower in the southern core, ranging from approximately 20–30 g SiO2 per 100 g dry weight sediment throughout the entire core. Trends in absolute microfossil abundance mirror those of biogenic silica, with generally greater abundance in the northern core (86–275×106 microfossils g−1 dry sediment) compared to the southern core (94–163×106 microfossils g−1 dry sediment).Cluster analyses using relative abundance of the dominant diatom and chrysophyte taxa revealed four zones of microfossil succession in each core. Microfossil assemblage succession in the north basin may be reflecting shifts in nutrient supply and cycling driven by climatic changes. The most recent sediments in the northern basin (Zone 1,c. 1890s–1991 A.D.) were characterized by an increased abundance ofAulacoseira baicalensis andAulacoseira ‘spore’. Zone 3 (c. 1630s–1830s A.D.) was dominated by the endemicCyclotella spp. and reduced abundance of theAulacoseira spp. Zone 3 corresponds approximately to the Little Ice Age, a cooler climatic period. The microfossil assemblages between Zones 1 and 3 (Zone 2,c. 1830s–1890s A.D.) and below Zone 3 (Zone 4,c. 830s–1430s A.D.) are similar to one another suggesting they represent transitional intervals between warm and cold periods. Southern basin sediments record similar changes in the endemic taxa. However, the increased abundance of non-endemic planktonic taxa (e.g.Stephanodiscus binderanus, Synedra acus, Cyclostephanos dubius) during two periods in recent history (post World War II and late 1700s) suggests evidence for anthropogenic induced changes in southern Lake Baikal.

SEXUAL REPRODUCTION IN STEPHANODISCUS NIAGARAE (BACILLARIOPHYTA) 1

We observed sexual reproduction in a clonal culture of Stephanodiscus niagarae Ehrenb. and used light and scanning electron microscopy to absent flagellated male cells, auxospore growth, initial valve structure and production, and subsequent daughter cell division. Free auxospores were spherical and nonsiliceous throughout growth, producing hemispherical initial valves devoid of spines and with nonfasciculate striae. Pregametangial cells averaged 43% of the diameter of the daughter cell population and were 1/9 the biovolume of initial, cells. This paper is the first confirmed report of sexual reproduction in S. niagarae, although it appears that specimens of Actinocyclus niagarae H. L. Smith, described from Lake Erie in 1878, are actually initial valves of S. niagarae.

Resting spores of the freshwater diatomsAcanthoceras andUrosolenia

Diatom resting spores are a widespread, but sometimes misconstrued component of siliceous microfossil assemblages. We illustrate and discuss resting spore morphology found in populations ofAcanthoceras andUrosolenia, two widely distributed freshwater genera. Taxonomic status of these genera and the potential paleolimnologic interpretation of resting spores are discussed.

Assessing the performance of a diatom transfer function on four Minnesota lake sediment cores: effects of training set size and sample age

Paleolimnological information is often extracted from diatom records using weighted averaging calibration and regression techniques. Larger calibration sample sets yield better inferences because they better characterize the environmental characteristics and species assemblages of the sample region. To optimize inferred information from fossil assemblages, however, it is worth knowing if fewer calibration samples can be used. Furthermore, confidence in environmental reconstructions is greater if we consider the relative importance of (A) similarity between fossil and calibration assemblages and (B) how well fossil taxa respond to the environmental variable of interest. We examine these issues using ~200-year sediment profiles from four Minnesota lakes and a 145-lake surface sediment training set calibrated for total phosphorus (TP). Training set sample sizes ranging from 10 to 145 were created through random sample selection, and models based on these training sets were used to calculate diatom-inferred (DI) TP data from fossil samples. Relationships between DI-TP variability and sample size were used to determine the minimum sample size needed to optimize the model for paleo-reconstruction. Similarly, similarities between fossil and modern assemblages were calculated for each size training set. Finally, fossil and modern assemblages were compared to determine whether older fossil samples had poorer similarity with modern analogs. More than 50–80 samples, depending on lake, were needed to stabilize variability in DI-TP results, and >110 training set samples were needed to minimize modern-fossil assemblage dissimilarities. Dissimilarities appeared to increase with sample age, but only one of the four studied cores displayed a significant trend. We have two recommendations for future studies: (1) be cautious when dealing with smaller training sets, especially if they are used to interpret older fossil assemblages and (2) understand how well fossil taxa are attuned to the variable of interest, as it is critical to evaluating the quality of the diatom-inferred data.

TAXONOMY AND MORPHOLOGY OF CYMBELLA STUXBERGII FROM LAKES IN THE BAIKAL RIFT ZONE

The morphology and ultrastracture of Cymbella stuxbergii (Cleve in Cleve & Grunow) Cleve were investigated using light and scanning electron microscopy. Cymbella stuxbergii is one characteristic component of a species complex within the nearshore periphyton in large lakes in the Baikal Rift Zone. Its distribution also includes northern mid-Asian continental rivers. Ultrastructure of C. stuxbergii supports the maintenance of this taxon within Cymbella sensu stricto because it has a single bilobed chloroplast, a ventral nucleus, dorsi-ventral valve shape, many ventral stigmata, dorsally deflected distal raphe ends, well-differentiated apical pore fields, and lacks an intermissio. Within Cymbella, C. stuxbergii appears closely allied with C. proxima Reimer because these taxa share similar raphe, stigmatal opening, and areolae configurations.

RECOGNITION OF TAXONOMICALLY SIGNIFICANT CLUSTERS NEAR THE SPECIES LEVEL, USING COMPUTATIONALLY INTENSE METHODS, WITH EXAMPLES FROM THE STEPHANODISCUS NIAGARAE COMPLEX (BACILLARIOPHYCEAE)1

Since the early 1960s, numerical techniques have produced a wide variety of methods to suggest classifications of organisms based on quantitative measurements. A long‐recognized shortcoming of these methods is that they will suggest classifications for any group of organisms and any set of measurements, whether or not the clusters in the suggested classification have any natural meaning or significance. Some progress has been made in assessing the reality of clusters determined by various methods. Data simulated to reflect known cluster structure have been used to test the accuracy of different methods, Various methods have been applied to the same data sets to compare how well they realize various desirable properties. Here we define a data‐based model of randomness to represent what might be meant by “no natural basis for subdivision into clusters” and use it to compare an observed measure of cluster distinctness to the distribution of this measure predicted by this model of randomness. In this way, unwarranted subdivision can be statistically avoided, and significant subdivisions can be investigated with confidence. Our methods are illustrated with some examples from the Stephanodiscus niagarae Ehrenb. species complex. Significant differences in morphologic expression are identified in S. reimerii Theriot and Stoermer in Theriot, S. superiorensis Theriot and Stoermer and S. yellowstonensis Theriot and Stoermer. In addition, statistically significant clusters are identified in S. niagarae populations from different geographic locations and in members of the same population grown in different environments. These results suggest current criteria for resolving diatom taxa may not be sufficient to discern subtle differences that occur between real species.