Phytoplankton aggregate formation: observations of patterns and mechanisms of cell sticking and the significance of exopolymeric material

Flocculation of ‘sticky’ phytoplankton cells intorapidly sinking aggregates has been invoked as a mechanism explainingmass sedimentation of phytoplankton blooms in the ocean. Phytoplanktonstickiness, defined as the probability of adhesion upon collision,is one key factor determining the potential for aggregate formation.In the laboratory, we examined variation in stickiness in fivespecies of diatoms and two species of flagellates grown in batchcultures. We also investigated the production of paniculatemucus by phytoplankton cells and its role in aggregate formation,and we studied the effects of solute exudates on cell stickiness.Four of the five diatoms investigated were significantly sticky,while one diatom and both of the flagellates were not sticky.Stickiness varied considerably within species. In the diatomSkeletonema costatum, the typical but not entirely consistentpattern was that stickiness decreased with age of the batchcultures. We were otherwise unable to establish consistent relationshipsbetween cell stickiness and the growth stage of the algae, environmentalconcentrations of inorganic nutrients, and abundances of suspendedand epiphytic bacteria. We showed that the diatom S.costatumat times excretes a solute substance that depresses flocculation.This may reduce cell losses from the euphotic zone during thegrowth phase due to flocculation and sedimentation. We demonstratedtwo different mechanisms of phytoplankton aggregate formation.In the diatom S.costatum, the cells are sticky in themselves,and coagulation depends on cell-cell sticking and does not involvemucus. Aggregates are composed solely of cells. Cells of thediatom Chaetoceros affinis, on the other hand, are not in themselvessticky. Transparent exopolymeric particles (TEP), produced bythe diatom, cause the cells to aggregate and coagulation dependson TEP-cell rather than cell-cell sticking. Aggregates are formedof a mixture of mucus and cells. We found several species ofdiatoms and one flagellate species to produce copious amountsof TEP. TEP from some species (e.g. Coscinodiscus sp.) is stickyand may cause other, non-sticky particles to coagulate. Thisemphasizes the potential importance of diatom-derived paniculatemucus for particle flocculation in the ocean.     

Quantifying marine snow as a food choice for zooplankton using stable silicon isotope tracers

Aggregates of biogenic origin >0.5 mm, known as marine snow,represent a concentrated potential source of food for zooplankton.Little is known, however, about whether aggregates are commonlygrazed by zooplankton in the field. While previous laboratorystudies have shown that the euphausiid Euphausia pacifica, andthe copepod, Calanus pacificus, common crustacean zooplankters,consume marine snow if it is the only food source available,it is not known if euphausiids will select marine snow in thepresence of edible dispersed cells, as readily occurs in nature.To examine this question, we offered E. pacifica the diatom,Nitzschia angularis in aggregated and dispersed form as preysimultaneously. Aggregates and dispersed food contained identicalcell types so that differing prey quality, taste or motilitywould not be a factor. A new method was developed to track foodsources by labeling the frustules of aggregated cells with differentnaturally occurring, but rare, stable isotopes of silicon, ³⁰Siand ²⁹Si. Food selection was then estimated by measuring theisotopic composition of silica within fecal pellets producedby animals feeding on mixtures of the two labeled foods. Resultsindicate that E. pacifica consumed both aggregates and dispersedcells, even when more cells were made available in dispersedform than in aggregated form. This suggests that aggregatesmay indeed be a food source in the field, even when dispersedcells are relatively abundant. The method of labeling diatomcells with stable isotopes of silica may prove useful for futuregrazing experiments to distinguish identical cell types.     

Using laser scattering to identify diatoms and conduct aggregation experiments

Laser in situ scattering and transmissometry (LISST) instruments are used to measure particle size distributions (PSDs) and volume concentrations in water. For populations of regularly shaped non-spherical particles, such as phytoplankton, the PSD produces a ‘scattering signature’ that corresponds to the shape of the particles. The objectives of this research were to describe the scattering signatures of six diatom species and to determine whether LISST instruments can be used as a tool to measure the aggregation of diatoms into larger particles. The scattering signatures of Chaetoceros muelleri var. subsalsum, Coscinodiscus wailesii, Thalassiosira weissflogii, Phaeodactylum tricornutum, Skeletonema costatum and S. marinoi were measured. The scattering signatures of individual species were consistent over time in batch culture and there were clear differences between species in terms of peak location, peak width, and relative peak height in the PSD. LISST was used to non-destructively follow the formation of diatom aggregates in the laboratory. Both rolling and warming cultures of S. costatum caused the cell chains to form aggregates, resulting in a change in the PSD, with a shift in peak position towards larger size bins. These experiments showed that the scattering signatures of unaggregated diatom species are conservative and that LISST instruments are useful tools to investigate the factors affecting diatom aggregation and disaggregation, with potential applications both in the laboratory and field.     

Biofilm and capsule formation of the diatom Achnanthidium minutissimum are affected by a bacterium

Photoautotrophic biofilms play an important role in various aquatic habitats and are composed of prokaryotic and/or eukaryotic organisms embedded in extracellular polymeric substances (EPS). We have isolated diatoms as well as bacteria from freshwater biofilms to study organismal interactions between representative isolates. We found that bacteria have a strong impact on the biofilm formation of the pennate diatom Achnanthidium minutissimum. This alga produces extracellular capsules of insoluble EPS, mostly carbohydrates (CHO), only in the presence of bacteria (xenic culture). The EPS themselves also have a strong impact on the aggregation and attachment of the algae. In the absence of bacteria (axenic culture), A. minutissimum did not form capsules and the cells grew completely suspended. Fractionation and quantification of CHO revealed that the diatom in axenic culture produces large amounts of soluble CHO, whereas in the xenic culture mainly insoluble CHO were detected. For investigation of biofilm formation by A. minutissimum, a bioassay was established using a diatom satellite Bacteroidetes bacterium that had been shown to induce capsule formation of A. minutissimum. Interestingly, capsule and biofilm induction can be achieved by addition of bacterial spent medium, indicating that soluble hydrophobic molecules produced by the bacterium may mediate the diatom/bacteria interaction. With the designed bioassay, a reliable tool is now available to study the chemical interactions between diatoms and bacteria with consequences for biofilm formation.     

SUGAR-CONTAINING COMPOUNDS ON THE CELL SURFACES OF MARINE DIATOMS MEASURED USING CONCANAVALIN A AND FLOW CYTOMETRY

Some diatom exudates may remain attached to the exterior cell surface, potentially altering cell stickiness and affecting important aspects of the diatom's ecology such as aggregation rates and grazing rates. We measured the accumulation of cell-surface sugar-containing compounds by labeling cultured marine diatoms with fluorescent-tagged sugar-binding lectins and measuring the fluorescence associated with each cell using flow cytometry. The binding of FITC-labeled concanavalin A (FITC-ConA), a lectin that binds to glucose and mannose residues, varied 5-fold among different diatom species in exponential growth (on a per-cell basis) and 2–3-fold within a given species in different physiological states. Although transparent exopolymer particles followed a simple accumulation curve over time in batch culture, FITC-ConA. cell^-1 did not follow the same pattern, suggesting that surface sugar accumulation is not driven simply by the accumulation of such particles in the medium. For Thalassiosira pseudonana (Hustedt) Hasle and Heimdal (3H clone), the amount of sugar-containing compounds on the cell surface increased transiently as growth rate slowed in early stationary phase under both N and Si limitation. For Chaetoceros neogracile (Schuett) van Landingham, FITC-ConA. cell^-1had a strong inverse relationship with growth rate across several Si-limited batch culture experiments. Both results suggest some biological mediation of cell-surface sugar-containing compounds. Our study reveals the great potential for using lectin binding to investigate cell-surface sugars on diatoms. Lectins allow us to investigate noninvasively the role of cell-surface sugar-containing compounds in modifying cell stickiness and aggregation, as well as the partitioning of exuded phytoplankton carbon. We suggest that cell-surface sugar accumulation may be related to diatom stickiness, based on a correlation between our FITC-ConA measurements and stickiness estimates in the literature on several of the species we studied.     

Structural organization of developing acetylcholine receptor aggregates

We report the first quantitative ultrastructural analysis of newly formed acetylcholine receptor aggregates. Aggregates were induced in Xenopus muscle cell cultures with agrin, labeled with gold particles, and detected using high resolution scanning electron microscopy. Aggregates are readily discernible at the ultrastructural level within 2 h of stimulation by agrin. The size and density profiles of the developing aggregates show that receptors reach maximal density very quickly in small “nano-aggregates” and that the aggregation process is not limited by the diffusion rate of the receptor. Quantitative analysis of label locations indicates that the receptor distribution within aggregates is nonrandom. Instead, the newly aggregated receptors appear to be bound to a localized scaffold conforming to a hexagonal (close-packed) geometry with a spacing of approximately 9.9 nm. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 613–626, 1997     

Physical characterization and diel dynamics of different fractions of extracellular polysaccharides in an axenic culture of a benthic diatom

The excretion of extracellular polymeric substances (EPS) by an axenic culture of the benthic diatom Cylindrotheca closterium was investigated. Two sequential extraction steps proved to be suficient to remove the bulk of the EPS present. Soluble EPS was recovered by a simple centrifugation step and represented a fraction that was not or was only loosely associated with diatom cells. For the extraction of bound EPS, different procedures were compared. The best results were obtained using distilled water as extraction solvent (1?h, 30?°C). The sugars that were recovered using this procedure were typically associated with aggregates of diatoms. In addition to the distinct differences in localization of the different types of EPS, their temporal dynamics differed in relation to the light–dark cycle. Soluble EPS were continuously released into the medium at a rate of 1.6?pg?cell^?1?day^?1. In contrast, the production of bound EPS was highly light-dependent. In the dark, this bound EPS rapidly disappeared, probably as the result of its utilization by the diatoms.     

Novel function of eubacterial flagella: role in aggregation of a marine bacterium

Pseudomonas marina (ATCC 27 129) rapidly aggregates when suspended in buffered artificial seawater (ASW). Light microscopic observations of stained preparations, showed that flagella-flagella contact was responsible for this phenomenon. Aggregation did not occur if flagella were sheared off, or if motility was inhibited with NaN₃. Aggregates were not observed when Mg²⁺ was omitted from ASW, even though the bacteria remained motile. Other divalent cations, including Ca²⁺, Mn²⁺, and Ba²⁺ could replace Mg²⁺. However, there is no absolute requirement for divalent cations, since aggregation occurred in ASW containing Cs⁺ or Li⁺ instead of Mg²⁺. P. marina aggregates developed from pH 5.8–8.4, but not below pH 5.8 even though motility continued unimpaired to pH 4.5.Abbreviation ASW artificial seawater     

RELATIVE INCREASE OF DEOXY SUGARS DURING MICROBIAL DEGRADATION OF AN EXTRACELLULAR POLYSACCHARIDE RELEASED BY A TROPICAL FRESHWATER THALASSIOSIRA SP. (BACILLARIOPHYCEAE)1

The aim of this study was to characterize the extracellular polysaccharides (EPS) released by a freshwater Thalassiosira sp. (Bacillariophyceae) and evaluate their degradation by heterotrophic microbial populations from the same habitat of Thalassiosira sp., a tropical eutrophic reservoir. The EPS were purified by anion exchange column chromatography, the monosaccharide composition was determined by GC, and the linkages of the monosaccharides by GC‐MS. The EPS is a mannose‐rich heteropolysaccharide composed of two different acidic fractions. Both of these fractions are composed of mannose, rhamnose, fucose, xylose, galactose, glucose, glucuronic acid, and N‐acetyl glucosamine but with different proportions. N‐acetyl galactosamine occurs only in fraction 1 and galacturonic acid only in fraction 2. We monitored the concentrations of the monosaccharides in the EPS during its degradation using pulse amperometric detection in an HPLC. The decay patterns of the monosaccharides were varied and the deoxy sugars, fucose and rhamnose, were degraded at a slower rate than the other components, increasing their relative concentrations and the hydrophobic feature of the EPS. The possibility of a selective degradation, which enhances the stickiness of the EPS, promoting transparent exopolymeric particles and aggregate formation, is discussed based on the literature data.     

The role of transparent exopolymer particles (TEP) in the increase in apparent particle stickiness ({alpha}) during the decline of a diatom bloom

The termination of diatom spring blooms in temperate watershas been connected with the formation and subsequent rapid sedimentationof aggregates. According to coagulation theory, the rate ofaggregate formation depends on the probability of particle collisionand on the efficiency with which two particles adhere once theyhave collided (stickiness). During this study, the variationin particle stickiness was determined over the decline of adiatom bloom using the Couette Chamber assay with low shear(G = 0.86 s^–1). A mixed diatom population, dominated bySkeletonema costatum, was sampled during the spring bloom inthe Baltic Sea and incubated in the laboratory for 18 days.Measurements of diatom species composition, transparent exopolymerparticles (TEP) and bulk particle abundance, as well as chemicaland biological variables, were conducted in order to revealthe determinants of coagulation efficiency. The investigationshowed that an increase in TEP concentration relative to conventionalparticles at the decline of the bloom significantly enhancedapparent coagulation efficiencies. High proportions of TEP ledto apparent values of stickiness >1, which indicates thatcollision rates can be substantially underestimated when thestickiness parameter     

The diatom Thalassiosira rotula induces distinct growth responses and colonization patterns of Roseobacteraceae,Flavobacteria and Gammaproteobacteria

Diatoms as important phytoplankton components interact with and are colonized by heterotrophic bacteria. This colonization has been studied extensively in the past but a distinction between the bacterial colonization directly on diatom cells or on the aggregated organic material, exopolymeric substances (EPS), was little addressed. Here we show that the diatom Thalassiosira rotula and EPS were differently colonized by strains of Roseobacteraceae and Flavobacteriaceae in two and tree partner treatments and an enriched natural bacterial community as inoculum. In two partner treatments, the algae and EPS were generally less colonized than in the three partner treatments. Two strains benefitted greatly from the presence of another partner as the proportions of their subpopulations colonizing the diatom cell and the EPS were much enhanced relative to their two partner treatments. Highest proportions of bacteria colonizing the diatom and EPS occurred in the treatment inoculated with the enriched natural bacterial community. Dissolved organic carbon, amino acids and carbohydrates produced by T. rotula were differently used by the bacteria in the two and three partner treatments and most efficiently by the enriched natural bacterial community. Our approach is a valid model system to study physico-chemical bacteria-diatom interactions with increasing complexity.     

DAILY FLUCTUATIONS OF EXOPOLYMERS IN CULTURES OF THE BENTHIC DIATOMS CYLINDROTHECA CLOSTERIUM AND NITZSCHIA SP. (BACILLARIOPHYCEAE)1

Dynamics in the production of extracellular polymeric substances (EPS) were investigated for the benthic diatoms Cylindrotheca closterium (Ehrenberg) and Nitzschia sp. The effect of growth phase and light:dark conditions were examined using axenic cultures. Two EPS fractions were distinguished. Soluble EPS was recovered from the culture supernatant and represented polysaccharides that were only loosely associated with the cells. Bound EPS was extracted from the cells using warm (30° C) water and was more closely associated with the diatom aggregates. Concentrations of EPS exceeded internal concentrations of sugar throughout growth, indicating that EPS production is important in these organisms. Soluble and bound EPS revealed distinct differences in daily dynamics during the course of growth. Soluble EPS was produced continuously once cultures entered the stationary phase. During the stationary phase, chl a‐normalized EPS production rates equaled 6.4 and 3.4 d ^{? 1} for C. closterium and Nitzschia sp., respectively. In contrast, production of bound EPS occurred only in the light and was highest during the exponential phase. Up to 90% of the attached EPS that was produced in the light was degraded during the subsequent dark period. The monosaccharide distribution of EPS was constant during the course of the experiment. The soluble EPS consisted of high amounts of galactose and glucuronic acid, relative to rhamnose, glucose, xylose/mannose, and galacturonic acid. In contrast, glucose was the dominant monosaccharide present in the bound EPS. These differences suggest that the production of the two distinct EPS fractions is under different metabolic controls and probably serves different cellular functions.     

Diatom communities in small water bodies at H. Arctowski Polish Antarctic Station (King George Island, South Shetland Islands, Antarctica)

An investigation of ponds, puddles and slow-flowing waters situated in the area of the Polish Antarctic Station distinguished two groups of diatom communities. The first group characterized waters poor in nutrients and brackish. The number of taxa, abundance of species and diatom biomass index were all low. Nitzschia homburgiensis, Achnanthes laevis var. quadratarea and A. delicatula prevailed. The second group characterized water richer in nutrients and brackish. The number of species was also low, but the diatom biomass index was higher. Nitzschia gracilis, Navicula gregaria and Navicula wiesneri formed large populations. Received: 17 May 1996 / Accepted: 20 September 1997     

Listeria monocytogenes exopolysaccharide: origin,structure, biosynthetic machinery and c‐di‐GMP‐dependent regulation

Elevated levels of the second messenger c‐di‐GMP activate biosynthesis of an unknown exopolysaccharide (EPS) in the food‐borne pathogen Listeria monocytogenes. This EPS strongly protects cells against disinfectants and desiccation, indicating its potential significance for listerial persistence in the environment and for food safety. We analyzed the potential phylogenetic origin of this EPS, determined its complete structure, characterized genes involved in its biosynthesis and hydrolysis and identified diguanylate cyclases activating its synthesis. Phylogenetic analysis of EPS biosynthesis proteins suggests that they have evolved within monoderms. Scanning electron microscopy revealed that L. monocytogenes EPS is cell surface‐bound. Secreted carbohydrates represent exclusively cell‐wall debris. Based on carbohydrate composition, linkage and NMR analysis, the structure of the purified EPS is identified as a β‐1,4‐linked N‐acetylmannosamine chain decorated with terminal α‐1,6‐linked galactose. All genes of the pssA‐E operon are required for EPS production and so is a separately located pssZ gene. We show that PssZ has an EPS‐specific glycosylhydrolase activity. Exogenously added PssZ prevents EPS‐mediated cell aggregation and disperses preformed aggregates, whereas an E72Q mutant in the presumed catalytic residue is much less active. The diguanylate cyclases DgcA and DgcB, whose genes are located next to pssZ, are primarily responsible for c‐di‐GMP‐dependent EPS production.     

THE COMPLEX EXTRACELLULAR POLYSACCHARIDES OF MAINLY CHAIN‐FORMING FRESHWATER DIATOM SPECIES FROM EPILITHIC BIOFILMS1

Diatoms are dominant organisms in phototrophic biofilms in aquatic habitats. They produce copious amounts of extracellular polymeric substances (EPS), which mainly consist of carbohydrates and traces of proteins and glycoproteins. This study focuses on the characterization of EPS from a total of 14 diatoms belonging to the six genera Achnanthes, Cymbella, Fragilaria, Punctastriata, Staurosira, and Pseudostaurosira, all of which were isolated from epilithic biofilms of the littoral zone of Lake Constance. EPS from all isolates were extracted by a sequential extraction procedure resulting in five different fractions. The monosaccharide composition of each fraction was analyzed by HPLC equipped with a pulse amperiometric detector, yielding results similar to those obtained by probing the EPS structures with monomer‐specific fluorophore‐linked lectins. Significant differences in carbohydrate composition occurred in the different fractions of single isolates. Most of the diatom isolates in our study form chain‐like colonies in which the cells are attached to each other by intercellular pads. Here we demonstrate that these pads can be dissolved in hot bicarbonate and that they show a heterogeneous composition of monosaccharides in contrast to other fractions, which mostly were dominated by one or two monosaccharides. Principal component analysis indicates a correlation between carbohydrate composition of EPS fractions and the phylogenetic relationship of the respective species, indicating that EPS analyses under defined culture conditions may support taxonomic analyses.     

Diatom Communities and Water Quality Assessment in Mountain Rivers of the Upper Segre Basin (La Cerdanya,Oriental Pyrenees)

Epilithic diatoms of mountain rivers from the upper Segre catchment (Oriental Pyrenees) were studied in 1998, during three different seasons: March, July and September. Four rivers, the river Segre and its three most important tributaries, Duran, Molina and Querol, were sampled in upstream and downstream stretches. The diatom communities were comparable in all upstream stretches of these mountain rivers draining siliceous substrates. Dominant taxa were Achnanthidium subatomus, Diatoma mesodon, Encyonema cf. minutum, E. silesiacum, Fragilaria arcus, F. capucina, Gomphonema calcifugum, G. pumilum, Meridion circulare and Nitzschia pura. Changes in water quality in the downstream stretches lead to the appearance of pollution tolerant taxa, such as Eolimna minima, Gomphoneis minuta, Navicula gregaria, and Nitzschia inconspicua. As a result, the values obtained with the diatom water quality indices (IPS Specific Polluosensitivity Index, CEE and IBD Biological Diatom Index) decreased. The diatom community composition and the derived water quality values did not change in the upstream stretches over the year. In contrast, significant changes were observed in the downstream stretches with best water␣quality in July, during high flows due to melting snow, and worst values in September, during low␣discharge. The diatom indices, especially the IPS, showed a good performance in these mountain rivers.     

Aggregation of algae released from melting sea ice: implications for seeding and sedimentation

Summary Factors influencing the fate of ice algae released from melting sea ice were studied during a R V Polarstern cruise (EPOS Leg 2) to the northwestern Weddell Sea. The large-scale phytoplankton distribution patterns across the receding ice edge and small-scale profiling of the water column adjacent to melting ice floes indicated marked patchiness on both scales. The contribution of typical ice algae to the phytoplankton was not significant. In experiments simulating the conditions during sea ice melting, ice algae revealed a strong propensity to form aggregates. Differences in the aggregation potential were found for algal assemblages collected from the ice interior and the infiltration layer. Although all algal species collected from the ice were also found in aggregates, the species composition of dispersed and aggregated algae differed significantly. Aggregates were of a characteristic structure consisting of monospecific microaggregates which are likely to have formed in the minute brine pockets and channels within the ice. Sinking rates of aggregates were three orders of magnitude higher than those of dispersed ice algae. These observations, combined with the negligible seeding effect of ice algae found during this study, suggest that ice algae released from the melting sea ice are subject to rapid sedimentation. High grazing pressure at the ice edge of the investigation area is another factor eliminating ice algae released during melting.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

EFFECTS OF BROMIDE AND IODIDE ON STALK SECRETION IN THE BIOFOULING DIATOM ACHNANTHES LONGIPES (BACILLARIOPHYCEAE)1

Extracellular polymeric substance (EPS) secretion was examined in the stalked marine diatom Achnanthes longipes Ag. in defined medium. This common biofouling diatom exhibited an absolute requirement for bromide for stalk production and substratum attachment, whereas elevated iodide concentrations in the growth medium inhibited stalk formation and adhesion. Varying EPS morphologtes resulted from altering bromide and iodide levels: pads, stalk-pads, stalks, and no EPS. Cells showed no differences in growth with bromide or iodide concentrations, indicating that they were not physiologically stressed under conditions that impaired EPS secretion. Cells grown in elevated iodide secreted significantly more soluble extracellular carbohydrate into the medium, suggesting that the EPS was soluble and unable to be polymerized into a morphologically distinct gel. By replacing sulfate with methionine, the diatom lost its ability to form stalks even in the presence of bromide, indicating that free sulphate may be required for proper cross-linking of stalk polymers. Lotus-FITC, a fluorescent-tagged lectin, preferentially labeled the EPS and, thus, was used to visualize and quantify EPS secretion along a bromide gradient in conjunction with an image analysis system. This technique demonstrated a direct correlation between the amount of bromide present in the medium and the specific EPS morphology formed.     

CHANGES IN SOIL AGGREGATION ASSOCIATED WITH TALLGRASS PRAIRIE RESTORATION

To investigate the dynamics of soil aggregation associated with the restoration of cultivated soil to tallgrass prairie, changes in soil aggregation and aboveground production were compared in a corn field, restored prairie plantings of various ages (second, fifth, eighth, and eleventh growing season), and an uncultivated prairie remnant. The restored prairie was also compared with a long-term (fourteenth growing season) ungrazed pasture dominated by Eurasian grasses. All plots were located on similar soils. The regression model, Y = 95.8 - 56.2/X (R² = 0.93), best described the relatively rapid recovery of water-stable soil aggregates >0.2 mm diameter with time (in years) since cultivation. Similar models were also found to describe changes in the percentages of aggregates > 1 and > 2 mm diameter. Aggregates > 0.2 and > 2 mm diameter were more closely associated with prairie graminoids than with other vegetation categories. However, time without disturbance may be a more important factor in soil aggregate formation than vegetation type, but it was difficult to separate the effects of these two factors in this study. The percentage of aggregates > 0.2 mm diameter was found to be significantly higher (P = 0.0553) in the oldest restored prairie than in ungrazed pasture although the former had been cultivated more recently. This suggests that C₄ prairie graminoids may confer some advantage over introduced C₃ Eurasian grasses for the development of water-stable aggregates in soils of the Prairie Peninsula.     

The construction of a diatom-based chlorophyll a transfer function and its application at three lakes on Signy Island (maritime Antarctic) subject to differing degrees of nutrient enrichment

1. Canonical correspondence analysis of a diatom and water chemistry dataset from fifty-nine maritime Antarctic lakes situated on Signy and Livingston Islands showed that nutrients and functions of nutrients (NH₄⁺, chlorophyll a) accounted for a significant fraction of the variance in the diatom data. 2. Weighted averaging regression was used to construct a diatom-based transfer function for inferring chlorophyll a concentrations from sediment core diatom assemblages. 3. The transfer function was applied to ²¹⁰Pb-dated sediment cores from three lakes (Moss, Sombre and Heywood) receiving different levels of nutrient input from fur seal populations, i.e. low, medium and high, respectively. 4. Moss Lake showed relatively stable reconstructed chlorophyll a values, and no evidence of recent eutrophication, agreeing with measured chlorophyll a concentrations at the site. 5. Changes in diatom assemblages and results of chlorophyll a reconstructions at Sombre Lake suggested that nutrient enrichment had occurred, which could be clearly linked to fluctuations in the measured water chemistry over the last 10–14 years. 6. Despite recorded increases in recent nutrient inputs there was no apparent diatom response at Heywood Lake.