Diatom assemblages and water depth in Lake 239 (Experimental Lakes Area,Ontario): implications for paleoclimatic studies

Diatom assemblages in surface sediments were sampled along three transects in Lake 239, from the Experimental Lakes Area (NW Ontario), and analyzed in order to explore the relationship between modern species distributions and water depth. Approximately 170 diatom species were identified in surficial sediments at lake depths from 2 to 30 m. The species composition varied with sample depth but remained highly similar across all three transects. The main patterns of variation in the diatom assemblages across transects, derived from a detrended correspondence analysis (DCA), showed that assemblages were highly correlated (r = 0.97 to 0.98). At depths > 8 m the pattern of predominantly benthic composition changed to a planktonic assemblage dominated by Cyclotella stelligera. This depth currently corresponds to the depth of 1% light penetration as assessed from extinction coefficient measurements. Diatom species diversity increases with the switch to the near-shore benthic taxa in all three transects. Additionally, there is a large decrease in the ratio of chrysophyte scales to diatoms at depths < 8 m. Light transmission data from wet and dry periods over the last 35 years suggests that during dry periods the extent of the littoral zone should change by over 2 m. We suggest that cores along a transect from 8 to 14 m should provide a highly sensitive location for detailed paleoclimatic study.     

Diatom-based models for reconstructing past water quality and productivity in New Zealand lakes

The trophic status of lakes in New Zealand is, on average, low compared to more densely populated areas of the globe. Despite this, trends of eutrophication are currently widespread due to recent intensification in agriculture. In order to better identify baseline productivity and establish long-term trends in lake trophic status, diatom-based transfer functions for productivity-related parameters were developed. Water quality data and surface sediment diatom assemblages from 53 lakes across the North and South Islands of New Zealand were analysed to determine species responses to the principal environmental gradients in the data set. Repeat sampling of water chemistry over a 12-month period enabled examination of species responses to annual means as well as means calculated for stratified and mixed periods. Variables found to be most strongly correlated with diatom species distributions were chlorophyll a (Chl a), total phosphorus (TP), dissolved reactive phosphorus (DRP), ionic concentration (measured as electrical conductivity (EC)) and pH. These variables were used to develop diatom-based transfer functions using weighted averaging regression and calibration (simple, tolerance down-weighted and with partial least squares algorithm applied). Overall, models derived for stratified means were weaker than those using annual or isothermal means. For specific variables, the models derived for the isothermal mean of EC (WA-tol r²_jack = 0.79; RMSEP = 0.15 log₁₀ S cm^–1),the annual mean of pH (WA r²_jack = 0.72; RMSEP = 0.25 pH units) and the isothermal mean of Chl a (WA r²_jack = 0.71; RMSEP = 0.18 log₁₀ mg m^–3 Chl a) performed best. The models derived for TP were weak in comparison (for the annual mean of TP: WA r²_jack = 0.50; RMSEP = 0.24 log₁₀ mg m^–3 TP) and residuals on estimates for this model were correlated with several other water quality variables, suggesting confounding of species responses to TP concentrations. The model derived for the isothermal mean of DRP was relatively strong (WA-tol r²_jack = 0.78; RMSEP = 0.17 log₁₀ mg m^–3 DRP); however, residual values for this model were also found to be strongly correlated with several other water quality variables. It is concluded that the poor performance of the TP and DRP transfer functions relative to that of the Chl a model reflects the coexistence of nitrogen and phosphorus limitation within the lakes in the data set. In spite of this, the suite of transfer functions developed from the training set is regarded as a valuable addition to palaeolimnological studies in NewZealand.     

The role of shoreland development and commercial cranberry farming in a lake in Wisconsin,USA

Musky Bay in Lac Courte Oreilles, Wisconsin, USA, is currently eutrophic. This large, shallow bay of an oligotrophic lake possesses the densest aquatic plant growth and a floating algal mat. Paleoecological reconstructions encompassing the last 130 years, were based on multiproxy analyses of sediment cores from three coring sites, two within the bay and one in the lake itself. These data were compared to historical records of the construction and expansion of two commercial cranberry bogs and shoreline residential homes to identify temporal and causal relations of eutrophication. The proxies investigated included: minor and trace elements; biogenic silica; and the diatom community. Post-depositional diagenesis of organic carbon, nitrogen, and phosphorus in the upper 30 cm of the core obscured records of historical ambient nutrient concentrations in the bay obviating their usefulness for this purpose. In contrast, calcium, magnesium, and potassium concentration profiles appeared to reflect runoff of soil amendments applied to the cranberry bogs and aerial fertilizer spraying over the eastern bog adjacent to Musky Bay. The increase in aluminum content since about 1930 coincided with the historical trend in shoreland development and construction of the original commercial cranberry farm. The biogenic silica profile recorded a steady increase of nutrients to Musky Bay over the last several decades. Stratigraphic changes in the diatom community indicated that nutrient input began to increase in the 1940s and accelerated in the mid-1990s with the onset of a noxious floating algal mat. The diatom community indicates the bay has possessed a significant macrophyte community for at least the last 200 years, but increased nutrient input was manifested by a change in the composition, and an increase in the density of the epiphytic diatom community. Cranberry farming appeared to be the major source of nutrients because the diatom community changes occurred prior to the significant increase in residential housing.     

Diatom paleoecology of Laguna Zoncho,Costa Rica

We analyzed diatoms in a sediment profile from Laguna Zoncho in southern Pacific Costa Rica (lake elevation 1190 m asl, depth 2.6 m, area 0.75 ha) spanning some 3240 cal yr. Diatoms are common in the profile, which we subdivide into three zones. Zone C (3240–1020 cal yr B.P.) is dominated by Staurosira construens var. venter and Aulacoseira spp.; during this time, the lake was dilute and circumneutral. Benthic and acidophilous taxa increase gradually in the upper section of this zone. Zone B (1020–460 cal yr B.P.) almost totally lacks Aulacoseira, and instead is dominated by combinations of Eunotia minor, Encyonema lunatum, Gomphonema gracile, and Pinnularia braunii. Previous pollen and charcoal analysis indicates that this zone falls within the peak of prehistoric agricultural activity at the lake, but diatoms may also reflect climate change. During this period, the lake was likely shallower and more acidic, but not eutrophic. Finally, Zone A (460 cal yr B.P. to AD 1997) begins near a 1.5-cm tephra layer from nearby Volcán Barú; diatom assemblages are dominated by Aulacoseira spp., and suggest deepening of the lake and return to conditions similar to Zone C. This was a time of indigenous population decline and forest recovery in the Zoncho region, probably reflecting the impact of European diseases on the native population, although climate change and impacts of the tephra deposition cannot be wholly discounted.     

Temporal and spatial variability in export production in the SE Pacific Ocean: evidence from siliceous plankton fluxes and surface sediment assemblages

Flux of siliceous plankton and taxonomic composition of diatom and silicoflagellate assemblages were determined from sediment trap samples collected in coastal upwelling-influenced waters off northern Chile (30°S, CH site) under “normal” or non-El Niño (1993–94) and El Niño conditions (1997–98). In addition, concentration of biogenic opal and siliceous plankton, and diatom and silicoflagellate assemblages preserved in surface sediments are provided for a wide area between 27° and 43°S off Chile. Regardless of the year, winter upwelling determines the maximum production pattern of siliceous microorganisms, with diatoms numerically dominating the biogenic opal flux. During the El Niño year the export is markedly lower: on an annual basis, total mass flux diminished by 60%, and diatom and silicoflagellate export by 75%. Major components of the diatom flora maintain much of their regular seasonal cycle of flux maxima and minima during both sampling periods. Neritic resting spores (RS) of Chaetoceros dominate the diatom flux, mirroring the influence of coastal-upwelled waters at the CH trap site. Occurrence of pelagic diatoms species Fragilariopsis doliolus, members of the Rhizosoleniaceae, Azpeitia spp. and Nitzschia interruptestriata, secondary components of the assemblage, reflects the intermingling of warmer waters of the Subtropical Gyre. Dictyocha messanensis dominates the silicoflagellate association almost year-around, but Distephanus pulchra delivers ca. 60% of its annual production in less than three weeks during the winter peak. The siliceous thanatocoenosis is largely dominated by diatoms, whose assemblage shows significant qualitative and quantitative variations from north to south. Between 27° and 35°S, the dominance of RS Chaetoceros, Thalassionema nitzschioides var. nitzschioides and Skeletonema costatum reflects strong export production associated with occurrence of coastal upwelling. Both highest biogenic opal content and diatom concentration at 35° and 41°–43°S coincide with highest pigment concentrations along the Chilean coast. Predominance of the diatom species Thalassiosira pacifica and T. poro-irregulata, and higher relative contribution of the silicoflagellate Distephanus speculum at 41°–43°S suggest the influence of more nutrient-rich waters and low sea surface temperatures, probably associated with the Antarctic Circumpolar Water.     

Spatial heterogeneity of diatom stratigraphy in varved and non-varved sediments of a small,boreal-forest lake

Spatial and temporal heterogeneity of recent diatom stratigraphy were assessed for a small (23 ha) dimictic lake in Northern Sweden (Kassjön). Varves are confined to water depths > 10 m. Six freeze cores were taken along a transect covering a range of water depths (3–12 m) and both varved and non-varved sediments. Core profiles were compared for dry mass accumulation, loss-on-ignition (LOI), and diatom relative frequency stratigraphy and accumulation rate. Excluding the 3.2 m water depth, non-varved core, all parameters showed good repeatability between cores, apart from diatom accumulation rates which were more variable. The 3.2 m core was atypical and had lower LOI values, low planktonic diatom percentages and high values of benthic taxa that were not abundant in the deep-water sediments. Detrended Correspondence Analysis (DCA) and Canonical Correspondence Analysis (CCA) ordinations were compared; both methods clearly differentiated the shallow water core, and showed the general similar ecological trends of the deeper-water cores. CCA axes constrained by environmental (i. e. core location) data resulted in slightly lower eigenvalues than that obtained by Correspondence Analysis (CA), but the axes were significantly non-random. A Partial-CCA of the four varved cores alone (with effects of sediment depth, i.e. age, partialled out), indicated that there was no significant difference between their diatom assemblages.     

Global Radiation and Onset of Stratification as Forcing Factors of Seasonal Carbonate and Organic Matter Flux Dynamics in a Hypertrophic Hardwater Lake (Sacrower See,Northeastern Germany)

A 2-year (October 2003–October 2005) high-resolution sediment trap study was conducted in Sacrower See, a dimictic hardwater lake in northeastern Germany. Geochemical and diatom data from sediment trap samples were compared with a broad range of limnological and meteorological parameters to quantify the impact of single parameters on biochemical calcite precipitation and organic matter production. Our goals were to disentangle how carbonaceous varves and their sublaminae form during the annual cycle to better understand the palaeorecords and to detect influences of dissolution, resuspension as well as of global radiation and stratification on lake internal particle formation. Total particle fluxes in both investigated years were highest during spring and summer. Sedimentation was dominated by autochthonous organic matter and biochemically precipitated calcite. Main calcite precipitation occurred between April and July and was preceded and followed by smaller flux peaks caused by resuspension during winter and blooms of the calcified green algae Phacotus lenticularis during summer. In some of the trap intervals during summer up to 100% of the precipitated calcite was dissolved in the hypolimnion. High primary production due to stable insolation conditions in epilimnic waters began with stratification of the water column. Start and development of stratification is closely related to air and water surface temperatures. It is assumed that global radiation influences the onset and stability of water column stratification and thereby determining the intensity of primary production and consequently of timing and amount of calcite precipitation which is triggered by phytoplanktonic CO₂ consumption. Sediment fluxes of organic matter and calcite are also related to the winter NAO-Index. Therefore these fluxes will be used as a proxy for ongoing reconstruction of Holocene climate conditions.     

Consensus among multiple trophic levels during high- and low-water stands over the last two millennia in a northwest Ontario lake

We investigated the modern distribution of fossil midges within a dimictic lake and explored downcore patterns of inferred lake depths over the last 2000 years from previously published proxies. Modern midge distribution within Gall Lake showed a consistent and predictable pattern related to the lake-depth gradient with recognizable assemblages characteristic of shallow-water, mid-depth and profundal environments. Interpretations of downcore changes in midge assemblages, in conjunction with quantitative lake-depth inferences across a priori defined (based on diatom data) ~ 500-yr wet and dry periods, demonstrated that both invertebrate and algal assemblages exhibited similar timing and nature of ecological responses. Midges were quantified by their relative abundance, concentrations and an index of Chaoborus to chironomids, and all showed the greatest differences between the wet and dry periods. During the low lake-level period of the Medieval Climate Anomaly (MCA: AD 900 to 1400), profundal chironomids declined, shallow-water and mid-depth chironomids increased, chironomid-inferred lake level declined and the Chaoborus-to-chironomid index decreased. The coherence between multiple trophic levels provides strong evidence of lower lake levels in Gall Lake during the MCA.     

Multi-proxy record of postglacial environmental change, south-central Melville Island, Northwest Territories, Canada

A sediment core from Lake BC01 (75°10.945′N, 111°55.181′W, 225 m asl) on south-central Melville Island, NWT, Canada, provides the first continuous postglacial environmental record for the region. Fossil pollen results indicate that the postglacial landscape was dominated by Poaceae and Salix, typical of a High Arctic plant community, whereas the Arctic herb Oxyria underwent a gradual increase during the late Holocene. Pollen-based climate reconstructions suggests the presence of a cold and dry period ~12,000 cal yr BP, possibly representing the Younger Dryas, followed by warmer and wetter conditions from 11,000 to 5000 cal yr BP, likely reflective of the Holocene Thermal Maximum. The climate then underwent a gradual cooling and drying from 5000 cal yr BP to the present, suggesting a late Holocene neoglacial cooling. Diatom preservation was poor prior to 5000 cal yr BP, when conditions were warmest, suggesting that diatom dissolution may in part be climatically controlled. Diatom concentrations were highest ~4500 cal yr BP but then decreased substantially by 3500 cal yr BP and remained low before recovering slightly in the 20th century. An abrupt warming occurred during the past 70 yr at the site, although the magnitude of this warming did not exceed that of the early Holocene.     

Diatom responses to 20th century climate-related environmental changes in high-elevation mountain lakes of the northern Canadian Cordillera

Diatom responses to 20th century climate-related environmental change were assessed from three high-elevation lakes in the northern Canadian Cordillera. Dominance of small benthic Fragilaria diatoms reflect the generally cold conditions with long periods of ice cover that have characterized these mountain lakes over at least the last ~300 years until the period of recent warming. At the turn of the 20th century, salient shifts in the diatom assemblages reveal individualistic limnological responses with the onset of climate warming trends in northwest Canada. At YK3 Lake, an oligotrophic, chemically dilute, alpine lake, increased representation of the planktonic Cyclotella pseudostelligera may reflect longer ice-free conditions and/or more stable thermal stratification. By contrast, in the more productive, alkaline lakes (BC2 and Deadspruce lakes), changes to more diverse assemblages of periphytic diatoms suggest greater benthic habitat availability, most likely associated with the enhanced growth of aquatic plants with lengthening of the growing seasons. In addition, diatom assemblages from these lakes suggest less alkaline conditions following the onset of 20th century climate warming. Continued alkalinity reduction throughout the 20th century is qualitatively inferred at the lower elevation, treeline lake (Deadspruce Lake), while greater representation of alkaliphilous Fragilaria diatoms after ~1950 suggested increased alkalinity at the alpine BC2 Lake. Our results confirm the sensitivity of diatoms from high-elevation mountain lakes to regional climate change in northwest Canada. Individualistic limnological responses to 20th century warming are potentially attributed to differences in their physical setting (e.g., bedrock geology, elevation, catchment vegetation) in this complex mountain environment.