Primary production in Lake La Cruz (Spain) over the last four centuries: reconstruction based on sedimentary signal of photosynthetic pigments

We analysed photosynthetic pigments in annually laminated sediment of meromictic Lake La Cruz, Spain, to cope with the timing and characterisation of primary productivity changes over the last four centuries. The photosynthetic pigments identified included chlorophyll a and b (and its derivatives) and specific carotenoids of different algal groups, such as zeaxanthin, lutein, alloxanthin, diadinoxanthin and diatoxanthin among others. Marker pigments of phototrophic sulfur bacteria were also observed, including bacteriochlorophyll a derivatives, homologue series of bacteriophaeophytins d and bacterial carotenoids okenone and chlorobactene. We investigated the diagenetic processes of pigment alteration in anoxic sediments and the possible implications for paleoproductivity reconstruction and interpretation. The lack of systematic down-core changes in diagenetic indicators suggests that variability of sedimentary pigment concentrations is the result of changes in lake productivity. The lower concentration of algal photosynthetic pigments in the bottom of sediment sequence corresponds to the onset of meromictic conditions. Before that, sediment was not continuously anoxic and the preservation of pigments was reduced. Regarding photosynthetic bacteria, green sulfur bacteria derivatives indicate that population growth was limited until the settled organic matter was sufficient to provide enough sulphide. Moreover, the presence of bacterioviridine, an oxidised derivative of bacteriochlorophyll a, suggest the competitive relation among photosynthetic bacteria. The high relative abundance of zeaxanthin indicates the dominance of picocyanobacteria in the primary productivity over the last centuries. Fluctuations of the pigment signal observed in the sediment sequence during the meromictic period were well correlated with fluctuations of solar activity.     

Signature pigments of green sulfur bacteria in lower Pleistocene deposits from the Banyoles lacustrine area (Spain)

Signature pigments of photosynthetic green sulfur bacteria (GSB) were found in ancient sediments collected from an abandoned clay quarry located in the Banyoles lacustrine area (Spain). The sediments belong to the Interglacial Waalian of the lower Pleistocene (0.7–1.5 millions years old) and were deposited after a marshy event occurring during that geologic period. Reverse-phase high performance liquid chromatography (RP-HPLC) analyses of acetone:methanol sediment extracts revealed that the main pigments found were carotenoids of both eukaryotic and prokaryotic photosynthetic organisms. In particular, isorenieratene (Isr) and β-isorenieratene (β-Isr) constituted the larger bacterial fraction (35–40% of the total carotenoid content), whereas okenone, a signature pigment of purple sulfur bacteria, accounted for less than 2%. Xanthophylls from oxygenic photosynthetic organisms accounted for the remaining carotenoids. Likewise, traces of degradation products of both bacteriochlorophyll (BChl) and chlorophyll (Chl) were also detected. The low concentration of Chl degradation products made proper identification of these compounds impossible. In contrast, degradation products of BChl-e such as bacteriochlorophyllide-e and bacteriopheophorbide-e were identified in the HPLC analyses, suggesting that chemical degradation of naturally occurring BChl aggregates of GSB was slower in the clay quarry sediments. The presence of signature pigments of brown-colored species of GSB (Isr, β-Isr and degradation products of BChl-e) in the sediments suggests an ancient aquatic environment where GSB were present and where episodes of sulfide-rich anoxia reaching the photic zone could be envisaged. Similarly, the large percentage of algal xanthophylls supports that algae could be an important part of the microbial photosynthetic community in this ancient ecosystem.     

Phototrophic activity and redox condition in Lake Hamana, Japan, indicated by sedimentary photosynthetic pigments and molybdenum over the last ∼250 years

We analyzed photosynthetic pigments, total organic carbon (TOC), biogenic silica, and Mo, a redox-sensitive element, in ²¹⁰Pb-dated sediment cores to reconstruct the historical changes in primary productivity and anoxia in the central basin of Lake Hamana, a brackish lake in Shizuoka Prefecture, Japan, over the last 250 years. The algal photosynthetic pigments we analyzed included chlorophyll a (and its derivatives), chlorophyll b (and its derivatives), and carotenoids such as -carotene, lutein, zeaxanthin, diatoxanthin, fucoxanthin, alloxanthin, and -carotene. Marker pigments for phototrophic sulfur bacteria were also recorded, including okenone and bacteriopheophytin a, originating from Chromatium (a genus of purple sulfur bacteria), and isorenieratene and bacteriochlorophylls e ₁, e ₂ and e ₃ (and corresponding bacteriopheophytins) from brown Chlorobium (a brown-colored group of green sulfur bacteria). The occurrence of these pigments throughout the length of all cores indicates that the anoxia in Lake Hamana has existed over at least the last 250 years. The indicators related to primary productivity – TOC and pigments of aerobic and anaerobic phototrophs – and an indicator of anoxia, Mo, increased after 1860, indicating that productivity had increased in both the oxic and anoxic (sulfidic) zones. The depth profiles of the indicators in the sediment cores showed that among phototrophic sulfur bacteria, Chromatium preferentially increased relative to brown Chlorobium when the lake productivity was high, and hence high anoxia existed in the lake. This can be explained by a shallowing of the oxic/anoxic boundary zone due to changes in temporal and/or spatial extents of seasonal anoxia, which made the light intensity in the upper anoxic zone high enough for Chromatium to grow. The upper Chromatium layer may absorb the wavelengths of light that favor the growth of brown Chlorobium in the water column, resulting in a relative decrease in brown Chlorobium. During the 1950s, the trends among the indicators changed significantly. This change is attributed to the construction of training walls, built to direct tidal currents into the lake, on the Imagire-guchi Channel, the sole inlet of seawater to the lake, during 1954–1956, and the resultant increases of seawater intrusion and lake salinity. A decrease in okenone and bacteriopheophytin a, or in okenone/isorenieratene ratio, after 1960 accompanying a decrease in Mo, is attributed to a deepening of the anoxic zone, where the light intensity became too low for growth of Chromatium, more light-demanding than the brown Chlorobium. A decrease of zeaxanthin (cyanobacteria) after 1960 relative to lutein (green algae) and diatoxanthin (diatoms and dinoflagellates) indicated a change in algal assemblage, presumably due to the increased salinity. Principal component analysis with a data set of total algal carotenoids, okenone, isorenieratene, and Mo also suggested that a major change occurred around the 1950s.     

Multi-proxy Reconstruction of Trophic State,Hypolimnetic Anoxia and Phototrophic Sulphur Bacteria Abundance in a Dimictic Lake in Northern Germany over the past 80 Years

During monthly investigations from 1996 to 2000, a hypolimnetic layer of phototrophic sulphur bacteria (Chromatium spp.) were observed in Lake Dudinghausen, a small dimictic lake in northern Germany. This paleolimnological study was initiated to detect if the occurrence of sulphur bacteria was related to cultural eutrophication or reflected natural conditions. Therefore, diatoms, algal pigments, okenone, geochemical proxies, and ²¹⁰Pb and ¹³⁷Cs were used in four sediment cores to investigate historical changes in trophic development, hypolimnetic redox conditions, anoxia and phototrophic sulphur bacteria abundances. Fossil diatoms, pigments, the ratio of chlorophyll derivatives to total carotenoids and the ratio of chlorophyll a to its derivatives suggest two phases of eutrophication coupled with hypolimnetic anoxia over the last ~80 years: a first phase from about 1923–1932 and a second from 1952 to 1982. In the first phase the ratios of Fe–Mn as well as Fe–Ca increased, suggesting seasonal anoxia. However, hypolimnetic anoxia was only weak because low levels of okenone suggest no mass development of sulphur bacteria. In contrast, sulphur bacteria increased during the early stages of the second eutrophication phase, suggesting increased temporal and spatial hypolimnetic anoxia. Surprisingly, the ratios of Fe–Mn as well as Fe–Ca decreased during this time. Possibly Fe, Mn and Ca were equally reduced through the intense anoxia. In the final stage, sulphur bacteria decreased again. As these bacteria need both anoxic conditions and a certain amount of light, the increased nutrient load probably led to low Secchi depth and therefore insufficient light conditions. In more recent years, diatoms and pigments suggest a decrease in nutrient levels. A second mass development of sulphur bacteria occurred, probably due to improved light conditions and continued anoxia in the upper hypolimnion. We conclude that the recent development of phototrophic sulphur bacteria do not represent natural conditions in Lake Dudinghausen. Furthermore, the upper sediments contain a completely new diatom flora that never occurred in older sediments of Lake Dudinghausen. Therefore, nutrient levels may eventually reach natural conditions, however they may not represent biological background reference conditions.     

Climate signal in varve thickness: Lake La Cruz (Spain), a case study

Lake La Cruz is a meromictic, karstic lake with annually laminated sediment formed by summer pulses of calcite deposition. The aim of this study was to explore the potential use of the laminated sediment from Lake La Cruz as a quantitative climate proxy, by calibrating lamina thickness against instrumental climate data. Statistical analysis of the relation between lamina thickness and the meteorological dataset indicated a high correlation between calcium carbonate lamina thickness and rainfall from December to March (r  = 0.725, P < 0.01, n = 35). Winter rainfall anomalies in the area are, in turn, highly negatively correlated with the North Atlantic Oscillation index (NAO, r = −0.832; P < 0.01; n = 53). We propose a regression model to infer past winter rainfall from calcium carbonate laminae thickness. These results highlight new possibilities for paleoenvironmental research using calcite laminated sediment records as climate proxies, especially to study past rainfall variability.     

Bacterial dominance of phototrophic communities in a High Arctic lake and its implications for paleoclimate analysis

The phototrophic communities in meromictic, perennially ice-covered Lake A, on Ellesmere Island in the Canadian High Arctic, were characterized by pigment analysis using high performance liquid chromatography. Samples were taken to determine the vertical changes down the water column as well as a variation between years. These analyses showed that Lake A had distinct phototrophic communities in its oxic and anoxic layers. The pigment analyses indicated that phototrophic biomass in the upper, oxic waters was dominated by picocyanobacteria, while in the lower, anoxic layer photosynthetic green sulphur bacteria were dominant. Interannual variation in pigment concentrations was related to the penetration of photosynthetically active radiation in the water column, suggesting that light availability may be limiting the net accumulation of photosynthetic bacterial biomass in Lake A. Pigment analysis of the surface sediments indicated that deposition was dominated by the photosynthetic sulphur bacterial contribution. The sedimentary record of bacterial pigments in polar meromictic lakes offers a promising tool for the reconstruction of past changes in ice cover and therefore in climate.     

Analysis of fossil pigments from algae and bacteria in meromictic Lake Fidler, Tasmania, and its application to lake management

Lake Fidler is an ectogenic meromictic lake with a monimolimnion maintained by periodic incursions of brackish water from the lower Gordon River estuary. A dam across the middle reaches of the Gordon River has restricted these incursions of brackish water and meromictic stability has rapidly declined. A palaeolimnological study was carried in order to assess the historical development of meromixis and the impact of the dam on the microbiological communities in the lake. Fossil pigments in a 17 m sediment core were analysed using reverse phase high performance liquid chromatography (rp-HPLC) and mass spectrometry (MS). In addition, taphonomic studies of pigment production, deposition and degradation in the water column and surface sediments were used to identify planktonic and benthic pigment degradation processes and constrain the stratigraphic interpretation. Results comparing the pigment composition of pelagic sediment traps and littoral surface sediments indicated that the core from the centre of the lake would permit a historical reconstruction of planktonic bacterial and algal communities. Marked increases in prokaryotic pigments ca 3500 yr B.P. suggested the possible colonisation of a chemocline by phototrophic bacteria. Further changes in chlorophyll: carotenoid ratios and changes in relative abundances of both chlorophyll a and bacteriochlorophyll c derivatives also indicated that a change in the depositional environment had occurred; possibly due to altered stratification or anoxia. From this we infer the onset of either intermittent or permanent meromixis. Further increases in prokaryotic pigment abundance suggested that the present state of permanent meromixis was firmly established by 2070 ±50 ¹⁴C yr B.P., and diatom analysis confirmed the development of a stable mixolimnion. High resolution studies of the top 10 cm of sediments measured pigments in mean concentrations of 15.1 ng g^-1 with a mean S.D. of only 2.78 indicating little change in pigment abundance since the construction of the dam. Thus, Lake Fidler still retains most of the features of meromixis. However, evidence from nearby Lake Morrison and Sulphide Pool has shown that any further declines in meromictic stability will cause a rapid reversion to holomixis. Palaeolimnological evidence from the early stages of meromictic development of Lake Fidler suggests that such reversion to holomixis may not permanently eliminate all the microbiological communities, and that, given time, they may return and prosper with re-establishment of a suitable chemocline. These studies will guide recommendations for a management strategy to prevent the further decay of meromixis in the Gordon River lakes.     

Sedimentary Steryl Chlorin Esters (SCEs) and Other Photosynthetic Pigments as Indicators of Paleolimnological Change Over the Last 28,000 Years from the Buguldeika Saddle of Lake Baikal

Detailed depth profiles of photosynthetic pigments in a sediment core (G-12) collected at the BDP93 site, the Buguldeika saddle, of south Lake Baikal, along with depth profiles of total organic carbon (TOC) and biogenic silica, were studied to elucidate the temporal changes of phytoplankton assemblages in the lake during the past 28 kyr. In addition to the quantification of carotenoids by high-performance liquid chromatography with photodiode-array detection (HPLC-PDA), steryl chlorin esters (SCEs) were analyzed by HPLC-PDA, HPLC-mass spectrometry (LC-MS) and sterols in SCEs by gas chromatography–mass spectrometry (GC–MS) to enrich the taxonomical information on the phytoplankton composition. Allochthonous input of organic matter from the Selenga River resulted in the higher TOC contents in core G-12 than in a previously reported core (G-6) collected at another site from the southern basin. The poorer correlation in core G-12 than in G-6 between TOC and chlorophyll-a-originating pigments, which are indicative of autochthonous production, also indicated a significant allochthonous input at the site. The abundance of lutein among the carotenoids detected, and the good correlation of total chlorophyll a and b shows that green algae represented a significant portion of the phytoplankton, accompanying the diatoms at the G-12 site, after the last glacial period. The presence of cryptomonads and cyanobacteria were confirmed from marker carotenoids in the sediment core. GC–MS analysis of sterols in SCEs detected marker sterols of diatoms, green algae, chrysophytes and dinoflagellates. The depth profiles of the measured indicators gave consistent features for temporal changes in phytoplankton assemblage at the G-12 site of Lake Baikal after the last glacial maximum. Notably, the profile of a chrysophyte-specific sterol in SCEs was consistent with the reported distribution of chrysophyte cysts during the Holocene. The presence of phytoplankton, such as green algae, diatoms and chrysophytes, in Lake Baikal during the late last glacial period was indicated by the analysis of sterols in SCEs. Sedimentary carotenoids and sterols in SCEs were found to give complementary information about phytoplankton composition. These molecular indicators allow us to reconstruct past lake phytoplankton assemblages responding to environmental changes with a time resolution as high as age–depth relationship in sediments attainable at present.     

The misrepresentation of the planktonic diatom assemblage in traps and sediments: southern Lake Malaŵi,Africa

Sediment trap collections near Cape Maclear, Lake Malai, were compared to phytoplankton and surface sediment diatoms to assess taphonomic variations. The sedimenting diatom community became progressively different from the diatom plankton with increasing depth: long Nitzschia species were strongly under-represented in the traps (annually, 53% among planktonic diatoms vs. 14% in the offshore 29 m trap; p0.005 by Kruskal-Wallis test), while Melosira was greatly over-represented in traps (32% vs. 57%; p<0.005). The abundances of the minor taxa (Rhopalodia, Fragilaria, Cymbella, and Surirella) were greatly enhanced in traps relative to the plankton, but they were still relatively uncommon (<3% of all diatoms each). Differences in grazing, dissolution, and sinking rates alone are insufficient to account for these distortions; a combination of these, plus perhaps unknown factors, strongly influence the deposited assemblage.These misrepresentations were also present at the sediment surface. The greatest discrepancy was noted for Melosira (32% of plankton vs. 53% of sediment surface diatoms; p<0.005) and for elongate Nitzschia species (53% of plankton vs. 0.8% in sediments; p<0.005). In Lake Malai, at least, paleolimnologists must not assume a straightforward correlation of modern and fossil assemblages.     

Sediment Fluxes and Varve Formation in Sihailongwan,a Maar Lake from Northeastern China

Data derived from monthly sediment traps in Sihailongwan, a maar lake in northeastern China, yielded a detailed record of seasonal sediment fluxes. Sediment fluxes correspond to seasonal climatic variations. The diatom flux shows two distinct peaks in September and November, whereas the flux of chrysophyte stomatocysts shows a maximum in May. The blooms of diatoms may be related to the subsequent deepening of the thermocline in September and lake overturn in spring and November, and influx of nutrient-rich groundwater sometime after the onset of the summer monsoon. The fluxes of organic matter and siliciclastics show a distinct seasonal pattern. They are varying between 0.03 and 0.56 g m⁻² d⁻¹ and reach a maximum in May. Quartz in the trap samples indicates that the siliciclastic matter may originate from distant aeolian sources. Sediment trap data and thin section investigations confirm the seasonality of Lake Sihailongwan sediments. Dark-colored layer, which mainly consists of valves of Cyclotella comta, might be deposited during autumn, and then is followed by a light-colored mixed layer starting with siliciclastics deposited after ice-out. The varved sediments in the U-shaped Lake Sihailongwan represent a sensitive siliciclastic and geochemical archive of paleoenvironmental variability in this data-sparse area. Detailed investigations of varved sediments should provide decadal to annual records of seasonal sediment flux and its relation to climatic parameters. Especially the diatomaceous layer is regarded to indicate summer climatic fluctuations, while the thick siliciclastic layer could be an indictor of dust events.     

Phytoplankton response to climate changes in Lake Baikal during the Holocene and Kazantsevo Interglacials assessed from sedimentary pigments

Lake Baikal, an ancient pristine lake in Siberia, has accumulated sediment deposits that span 25 million years. These deposits have the potential to provide a long-term record of climate changes and their interaction with the ecology of the lake. In order to investigate whether sedimentary phytoplankton pigments could be used to reconstruct past changes in total phytoplankton abundance and productivity, we compared the spatial variability in sedimentary pigment distributions in Holocene cores from three separate regions of the lake; Vidrino in the south, Posolski on Selenga Delta and Continent Ridge in the north. Furthermore, we present the first continuous sedimentary pigment and organic carbon sequence of the Kazantsevo interglacial (roughly a time equivalent to the European Eemian, and Marine Isotopic Stage MIS5e) at a resolution of approximately 150 years. Results of the spatial study showed marked differences in the sediment pigment deposition. Lowest chlorophyll a plus its degradation products versus organic carbon ratios (Chlas/TOC) indicating lowest production, but highest variability with time (indicating strongest climatic oscillations) were found at Continent Ridge. The study of sedimentary pigments deposited during the last two interglacial periods at Continent Ridge showed Chlas/TOC ratios 50–1000 times higher during the Kazantsevo Interglacial compared to the glacial periods, whereas the TOC content was only five times higher, thus indicating the significance of the Chlas/TOC ratio for the reconstruction of the phytoplankton abundance and productivity. Strong oscillations occurred during the Kazantsevo Interglacial within centennial time scales. Chlorophyllb plus its degradation products provided additional information on the past development of Chlorophyceae. Highest Chlas/TOC ratios were found during the early Holocene at approximately 9 kyr BP. Indications of short phytoplankton production maxima were also found during the late Atlantic (6 kyr BP) and at the Subboreal/Subatlantic transition (3 kyr BP). From this we conclude that sedimentary chlorophyll a is a reliable indicator of phytoplanktonic response to climate changes and may serve for␣validation of future climate scenarios in continental regions.     

南极普里兹湾北部深海沉降颗粒物的垂直通量研究I.沉积物捕获器的误差来源及相关建议

捕获器是目前研究沉降颗粒物的生物地球化学循环最直接的手段。捕获器不仅能够定量不同时间、空间以及不同深度的颗粒物通量 ,而且还能够收集到进行物理、化学分析的样品。但是尽管捕获器被广泛地认为是海洋中的“颗粒物测量器” ,然而关于捕获器的绝对精确度仍存在异议。本文综合了当前的众多研究 ,对有关捕获器偏差的误差来源及相关建议做了一定的总结。重点讨论了一些大家关注的诸如“swimmers”的去除以及样品保护的问题     

Late‐Summer Peak in Sediment Accumulation in Two Lakes with Contrasting Watersheds,Alaska

The timing of clastic sedimentation in two glacial‐fed lakes with contrasting watersheds was monitored using sequencing sediment traps for two consecutive years at Allison Lake (Chugach Range, Alaska) and four months at Shainin Lake (Brooks Range, Alaska). Shainin Lake is a weakly stratified lake fed by distant glaciers, whereas Allison Lake is more strongly stratified and fed predominantly by proximal glaciers. At Shainin Lake, sediment accumulation started in late June and reached its maximum in mid‐August, just before lake mixing and during a period of low river discharge. The grain size of the sediment reaching the sediment trap in Shainin Lake was homogenous throughout the summer. At Allison Lake, pulsed sedimentation of coarse particles during late summer and early fall storms were superimposed on the fine‐grained sedimentation pattern similar to that observed at Shainin Lake. These storms triggered underflows that were observed in the thermal structure of the lake and deposited abundant sediment. The sequencing sediment traps reveal a lag between fluvial discharge and sediment deposition at both lakes, implying limitations to interpreting intra‐annual sedimentary features in terms of inflow discharge.     

The History of Recent Limnological Changes and Human Impact on Upper Klamath Lake, Oregon

Hypereutrophic Upper Klamath Lake has been studied for almost 50 years to evaluate the nature, cause, and effects of its very productive waters. Mitigation of undesirable effects of massive cyanobacterial blooms requires understanding their modern causes as well as their history. Knowledge of the pre-settlement natural limnology of this system can provide guidelines for lake restoration and management of land and water use strategies to maximize the benefits of this aquatic resource. This investigation uses a paleolimnological approach to document the nature and chronology of limnological and biological changes in Upper Klamath Lake for the past 200 years, covering the time when the lake was first described until today. A 45-cm gravity core, dated by ²¹⁰Pb and diatom correlations, was analyzed for diatoms, pollen, akinetes (resting spores) of the cyanobacterium Aphanizomenon flos-aquae, reworked tephra shards, and sediment magnetic characteristics. Pollen profiles show little vegetation change during this time. In contrast, diatoms indicative of increased nutrient fluxes (P and Si) increase moderately, coinciding with the settlement of the region by Euro-Americans. Numerous settlement activities, including draining of lake-margin marshes, upstream agriculture and timber harvest, road construction, and boat traffic, may have affected the lake. Magnetic properties and reworked tephra suggest riparian changes throughout the basin and increased lithogenic sediment delivery to the lake, especially after 1920 when the marshes near the mouth of the Williamson River were drained and converted to agricultural and pasture land. Drainage and channelization also decreased the ability of the marshes to function as traps and filters for upstream water and sediments. Akinetes of Aphanizomenon flos-aquae record progressive eutrophication of Upper Klamath Lake beginning in the 20th century and particularly after 1920 when lake-margin marsh reclamation more than doubled. The coincidence of limnological changes and human activities following European settlement suggests a major impact on the Upper Klamath Lake ecosystem, although ascribing specific limnological changes to specific human activities is difficult.     

The use of sedimentary algal pigments to infer historic algal communities in Lake Apopka,Florida

The primary producer community of Lake Apopka, a large (125 km²), shallow (mean depth, 1.7 m), polymictic Florida lake, shifted from macrophyte dominance to phytoplankton dominance in the 1940s. Today, frequent wind resuspension of highly organic, unconsolidated sediments supports a meroplanktonic community that is predominantly diatoms, but during calm periods the algal community is dominated by planktonic cyanobacteria. Sedimentary algal pigments (chlorophyll derivatives and carotenoids) and chemical proxies for nutrient enrichment (polyphosphate, total phosphorus and biogenic silica) in three sediment cores were used to investigate historic changes in primary producers. Sediments were separated into three stratigraphic zones using multivariate statistical techniques. Stratigraphic zonation was established in each core although sediment deposition at one site was insufficient to adequately resolve temporal changes. These results show the importance of selecting suitable sites for paleolimnological studies. The oldest zone represents macrophyte-derived sediments, and the two overlying zones represent phytoplankton-derived sediments deposited since the 1940s. Algal pigments in the most recent sediment zone show little degradation, which might be due to the presence of viable meroplankton in the sediment. After the initial primary producer shift from macrophytes to phytoplankton, the lake experienced a short period of cyanobacterial dominance followed by a period of benthic diatom abundance before being replaced by the present algal community consisting of cyanobacteria and meroplanktonic diatoms. Chlorophyll derivatives and carotenoids were highly correlated with total phosphorus. Historic trends inferred from the data include algal and cyanobacterial productivity that increased with increased phosphorus loading. The study demonstrates that valid paleolimnological proxies for historic eutrophication are available in loosely consolidated sediments of shallow, subtropical lakes.     

Hyperspectral imaging of sedimentary bacterial pigments: a 1700-year history of meromixis from varved Lake Jaczno,northeast Poland

Hypoxia in freshwater systems is currently spreading globally and putting water quality, biodiversity and other ecosystem services at risk. Such adverse effects are of particular concern in permanently stratified meromictic lakes. Yet little is known about when and how meromixis and hypoxia became established (or vanished) prior to anthropogenic impacts, or how human activities such as deforestation, erosion and nutrient cycling affected the mixing regimes of lakes. We used calibrated hyperspectral imaging (HSI) data in the visible and near infrared range from a fresh, varved sediment core taken in Lake Jaczno, NE Poland, to map sedimentary pigments at very high resolution (sub-varve scale) over the past 1700 years. HSI-inferred bacteriopheophytin a (bphe a, produced by anoxygenic phototrophic bacteria) serves as a proxy for meromixis, whereas HSI-inferred green pigments (chlorophyll a and diagenetic products) can be used as estimators of aquatic productivity. Meromixis was established and vanished long before significant human disturbance in the catchment was observed in the late eleventh century AD. Under pre-anthropogenic conditions, however, meromixis was interrupted frequently, and the lake mixing regime flickered between dimixis and meromixis. During two periods with intense deforestation and soil erosion in the catchment, characterised by sedimentary facies rich in clay and charcoal (AD 1070–1255 and AD 1670–1710), the lake was mostly dimictic and better oxygenated than in periods with relative stability and a presumably closed forest around the lake, i.e. without human disturbances. After ca. AD 1960, meromixis became established quasi-permanently as a result of eutrophication. The persistent meromixis of the last ~60 years is unusual with respect to the record of the last 1700 years.     

Experimental calibration of lake-sediment spectral reflectance to chlorophyll a concentrations: methodology and paleolimnological validation

Chlorophyll a preserved in lake sediments reflects, in part, past primary production. This study assesses the spectral properties of sedimentary chlorophyll a using visible-near infrared reflectance (VNIR) spectroscopy, with the objective of establishing a new, non-destructive paleolimnological proxy. Reflectance spectra were determined from a dilution series (n = 10) involving incremental additions of pulverized modern algae to a lake sediment matrix of low organic content. This enabled an assessment of the development of sediment reflectance spectra in relation to different sediment chlorophyll a concentrations, and subsequent regression of spectral features against measured concentrations of chlorophyll a and derivatives obtained by high performance liquid chromatography (HPLC). The experiment demonstrates that ubiquitous troughs in sediment reflectance near 675 nm are attributable to chlorophyll a and derivative compounds. A significant correlation (r ² = 0.98, P < 0.01) was obtained between the area of the reflectance trough in the 650–700 nm interval and summed concentrations of chlorophyll a, all derivative isomers, and degradational pheopigments. A simple linear inference model derived from this experiment was applied to a down-core sequence of VNIR spectra from a productive prairie lake (Alberta, Canada), where it produced inferred sediment chlorophyll a concentrations in concordance with HPLC measurements. Although a larger training set is desirable to further refine the inference model, the analyses reported here demonstrate that reflectance spectroscopy provides a rapid, semi-quantitative method for assessing the chlorophyll a content of lake sediments.     

Shallow lake trophic status linked to late Holocene climate and human impacts

Lake Mattamuskeet, North Carolina, USA is a large (162 km²) and shallow (mean depth = 1 m) coastal lake, which was significantly modified to support agricultural activities following European settlement in 1850. Paleolimnological proxies measured on a 400-cm sediment core collected from Lake Mattamuskeet reveal shifts in organic matter input and primary producer community structure in response to climatic and human impacts on the lake during the late Holocene. Stratigraphic changes in organic matter content, nutrients, metals, lignin phenols and photosynthetic pigments were used to divide the sediment core into three intervals. Interval I includes sediment deposited between A.D. 360–1584 and indicates a clear-water, sand-bottom state with low algal abundance. In addition, the lake catchment area experienced two significant fires during this interval that were recorded as charcoal layers in the core around A.D. 360 and A.D. 1435 (calibrated ¹⁴C AMS dates). Trophic structure changed with the onset of Interval II (A.D. 1584–1860) when total algal abundance increased, and the primary producer community was comprised primarily of diatoms, chrysophytes, cryptophytes and cyanobacteria. During this interval there was also an increase in terrestrial organic material input into the lake as well as a shift in plant type from woody gymnosperms to non-woody angiosperms as determined from lignin data. Sediment deposited in Lake Mattamuskeet following European settlement (Interval III, A.D. 1860-present) suggests a dramatic increase in organic-matter deposition, metals, primary-producer abundance and the onset of cyanobacterial dominance. Sedimentary evidence indicates that shallow-water primary producers can respond rapidly to climate change and human development.     

Phytoplankton dynamics in Lake Biwa during the 20th century: complex responses to climate variation and changes in nutrient status

We examined algal remains and fossil pigments in ²¹⁰Pb-dated sediment cores from Lake Biwa to explore historical changes in the phytoplankton community of the lake over the past 100 years and to identify environmental factors that caused those changes. Fluxes of fossil pigments and algal remains were very low before the 1960s, but increased through the 1960s and 1970s, indicating that the lake had eutrophied in the 20 years since 1960. After 1980, however, fluxes of all fossil pigments and algal remains decreased or stabilized. Redundancy analysis with meteorological and limnological variables explained more than 70% of the variation of these fluxes and showed that the decrease in fluxes of most algal taxa that occurred in the 1980s was related to changes in meteorological variables such as wind velocity, rather than changes in the lake’s trophic state. Sedimentary records of algal remains also revealed that Aulacoseira nipponica, an endemic diatom species that grows in winter, decreased dramatically after 1980, while Fragilaria crotonensis, a cosmopolitan spring diatom species, became dominant. Replacement of one dominant diatom species by another could not be explained simply by changes in the lake trophic state, but was reasonably strongly related with an increase in winter water temperature. These results suggest that the phytoplankton community in Lake Biwa was influenced by changes in local environmental conditions (nutrient loading) through the 1960s and 1970s, but more so by regional (meteorological) and global (climate warming) factors since 1980.     

Processes affecting deposition of sediment in a small, morphologically complex lake

Devil Lake is morphologically complex as a result of Pleistocene glacial erosion of the Frontenac Axis of the Canadian Shield. In order to assess the processes causing highly variable sedimentation in the lake, we monitored currents, suspended sediment and temperature in the lake before and during autumn overturn in 2002. Strong summer thermal stratification (stability number to 0.11 s^–1 declining with the approach of overturn) was insufficient to prevent a dynamic response in the hypolimnion to wind forcing. Superimposed on a gradual increase in suspended sediment concentration in the last weeks of stratification from less than 2 g/l to about 30 g/l were shorter-term rises lasting up to several days. Associated with these events was an increase in particle size of the sediment from a mode of 40–50 to 150–200 m ascribed to flocculation from primary particles. These events culminated in rapid (<1 h) clearing of the water associated with strong, sustained winds over the lake, especially from the southwest. After overturn, the events were more frequent, and flocculation was unable to develop as well in the more vigorous circulation. However, currents in the hypolimnion occurred throughout the period before, during and after overturn with speed related to wind speed, but direction largely independent of wind direction. The results represent an approach to understanding the nature of sedimentary processes and thus to strengthening the use of sedimentary records as proxy in environmental and paleoenvironmental assessment.