Sedimentary Pigments Research Articles

Long‐term effects of climate change on carbon flows through benthic secondary production in small lakes

Abstract A palaeolimnological study, covering the last c. 12,000 years, was conducted in a small subalpine lake located in the Alps to study climate change impacts on carbon flows through food webs in small lakes. We used analysis of sedimentary pigments and carbon stable isotopic composition of chironomid remains (δ13CHC) to reconstruct past dynamics of phytoplankton community and carbon sources sustaining benthic consumers. Chironomid biomass was sustained by a combination of allochthonous, autochthonous and CH4‐derived organic matters, and their relative contributions were correlated to changes in temperature. Relatively high terrestrial contributions to chironomid biomass were observed during period of the Holocene when in‐lake production was low. Relatively high incorporation of CH4‐derived carbon to chironomid biomass was found during anoxic events co‐occurring with the Holocene thermal maximum. Results were then compared with those collected in a small boreal lake in Estonia. We tested the hypothesis that responses in carbon flows through benthic food web to past climate change would be similar between these lakes. We found a negative correlation between δ13CHC values of both lakes and inferred air temperature, suggesting that temperature was the major driver to different food sources being incorporated into chironomid biomass. Our study demonstrated that air temperature was the principal driver of the energy flows through benthic food web in the studied small lakes. We conjectured that carbon cycling in food webs of small lakes might be strongly sensitive to climate change.

Distribution of Sedimentary Pigments and Macrozoobenthos in the Deep Water Part of the Rybinsk Reservoir

Distribution of Sedimentary Pigments and Macrozoobenthos in the Deep Water Part of the Rybinsk Reservoir

Paleolimnology of a freshwater estuary to inform Area of Concern nutrient delisting efforts

The St. Louis River Estuary (SLRE), a freshwater estuary bordering Duluth, Minnesota, Superior, Wisconsin, and the most western point of Lake Superior (46.74°, − 92.13°), has a long history of human development since Euro-American settlement ~ 200 years ago. Due to degradation from logging, hydrologic modification, industrial practices, and untreated sewage, the SLRE was designated an Area of Concern in 1987. Action has been taken to restore water quality including the installation of the Western Lake Superior Sanitary District in 1978 to help remove beneficial use impairments. A better understanding of historical impacts and remediation is necessary to help document progress and knowledge gaps related to water quality, so a paleolimnological study of the SLRE was initiated. Various paleolimnological indicators (pigments, diatom communities, and diatom-inferred phosphorus) were analyzed from six cores taken throughout the SLRE and another from western Lake Superior. Reductions in eutrophic diatom taxa such as Cyclotella meneghiniana and Stephanodiscus after 1970 in certain cores suggest an improvement in water quality over the last 40 years. However, in cores taken from estuarine bay environments, persistence of eutrophic taxa such as Cyclostephanos dubius and Stephanodiscus binderanus indicate ongoing nutrient problems. Sedimentary pigments also indicate cyanobacteria increases in bays over the last two decades. Diatom model-inferred phosphorus and contemporary monitoring data suggest some of the problems associated with excess nutrient loads have been remediated, but modern conditions (internal phosphorus loading, changing climate) may be contributing to ongoing water quality impairments in some locations. The integrated record of biological, chemical, and physical indicators preserved in the sediments will aid state and federal agencies in determining where to target their resources.

Natural and anthropogenic forcing of Holocene lake ecosystem development at Lake Uddelermeer (The Netherlands)

Lake Uddelermeer (The Netherlands) is characterized by turbid conditions and annual blooms of toxic cyanobacteria, which are supposed to be the result of increased agricultural activity in the twentieth century AD. We applied a combination of classic palaeoecological proxies and novel geochemical proxies to the Holocene sediment record of Lake Uddelermeer (The Netherlands) in order to reconstruct the natural variability of the lake ecosystem and to identify the drivers of the change to the turbid conditions that currently characterize this lake. We show that the lake ecosystem was characterized by a mix of aquatic macrophytes and abundant phytoplankton between 11,500 and 6000 cal year BP. A transition to a lake ecosystem with clear-water conditions and relatively high abundances of ‘isoetids’ coincides with the first signs of human impact on the landscape around Lake Uddelermeer during the Early Neolithic (ca. 6000 cal year BP). An abrupt and dramatic ecosystem shift can be seen at ca. 1030 cal year BP when increases in the abundance of algal microfossils and concentrations of sedimentary pigments indicate a transition to a turbid phytoplankton-dominated state. Finally, a strong increase in concentrations of plant and faecal biomarkers is observed around 1950 AD. Canonical Correspondence Analysis suggests that reconstructed lake ecosystem changes are best explained by environmental drivers that show long-term gradual changes (sediment age, water depth). These combined results document the long-term anthropogenic impact on the ecosystem of Lake Uddelermeer and provide evidence for pre-Industrial Era signs of eutrophication.

Reconstructing recent environmental changes using non-biting midges (Diptera: Chironomidae) in two high mountain lakes from northern Patagonia, Argentina

Fil: Montes de Oca, Fernanda. Administracion de Parques Nacionales; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina

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.

Influence of seabird colonies and other environmental variables on benthic community structure, Lancaster Sound Region, Canadian Arctic

The Canadian Arctic shelters millions of seabirds each year during the breeding season. By the excretion of important quantities of guano, seabirds locally concentrate nutrient-rich organic matter in the marine areas surrounding colonies. Seabirds, acting as biological vectors of nutrients, can markedly affect terrestrial ecosystems, but their influence on the structure of marine benthic communities is still under-studied. Sessile and long-lived megabenthic species can integrate environmental variation into marine food webs over long time frames. The objectives of this study were (1) to characterize the epifaunal and infaunal communities of the Lancaster Sound Region (LSR) and (2) to test the influence of the presence of seabird colonies and other environmental parameters on the structure of those benthic communities. Our prediction was that benthic diversity, number of taxa, total biomass of infauna and total density of epifauna and infauna, would be higher in areas with colonies present. Photos of the seafloor (data on epifauna) and grab samples (data on infauna) were taken at three control areas and at five areas near seabird colonies, within a depth range of 122 to 442m. A database of 26 environmental parameters was built to study the environment-benthos relationships. Infauna, which was relatively uniform across the LSR, was numerically dominated by Annelida. Epifauna was much patchier, with each study area having unique epibenthic assemblages. Brittle stars were highly abundant in epifaunal communities, reaching 600 individuals per square meter. The presence of seabird colonies was not a major driver of benthic community structure in the LSR at the depths studied. Negative effects of colonies were detected on the density and number of taxa of infauna, perhaps due to top-down effects transmitted by the seabirds which feed in the water column and can directly reduce the quantity of food reaching the seabed. Sediment concentration of pigment, percent cover of gravel and boulders, depth, temperature and duration of open water explained a substantial part of the observed variation across the LSR. Food availability, as expressed by sediment pigment concentration, is a factor driving benthic communities, even if potential pathways through seabirds did not broadly affect the benthos at the point source.

Tracking trends in eutrophication based on pigments in recent coastal sediments

Summary Eutrophication in two different coastal areas – the Gulf of Gdansk (southern Baltic) and the Oslofjord/Drammensfjord (Norway) – both subject to human pressure and with restricted water exchange with adjacent seas, was investigated and compared. Sediment cores (up to 20 cm long) were collected at 12 stations using a core sampler, 6 in each of the two areas, and divided into sub-samples. The physicochemical parameters characterizing the adjacent water column and near-bottom water, i.e. salinity, oxygen concentration and temperature, were measured during sample collection. Chlorophylls-a, -b and -c, their derivatives and selected carotenoids were determined for all the samples, as were additional parameters characterizing the sediments, i.e. Corg, Ntot, δ13C and δ15N, grain size. 210Pb activity was also determined and on that basis sediment mixing and accumulation rates were estimated. The distribution of pigments in sediments was related to environmental conditions, the sampling site location and sediment characteristics. The results are in agreement with other observations that eutrophication in the Gulf of Gdansk has increased, especially since the 1970s, whereas in the Oslofjord it decreased during the same period. The pigments are better preserved in inner Oslofjord sediments than in those from the Gulf of Gdansk. The results demonstrate that the sum of chloropigments-a in sediments calculated per dry weight of sediments is a valuable measure of eutrophication, providing that the monitoring site is selected properly, i.e. sediments are hypoxic/anoxic and non-mixed. Besides, the results confirm previous observations that the percentages of particular chlorophyll-a derivatives in the sum of chloropigments-a are universal markers of environmental conditions in a basin. The ratios of chloropigments-b and chlorophylls-c to the sum of chloropigments-a (ΣChlns-b/ΣChlns-a; Chls-c/ΣChlns-a) may by applied as complementary markers of freshwater and marine organic matter input, respectively.

Phytoplankton Pigments in Maryland Coastal Bay Sediments as Biomarkers of Sources of Organic Matter to Benthic Community

ABSTRACT Chen, J.; Oseji, O.; Mitra, M.; Waguespack, Y., and Chen, N., 2016. Phytoplankton pigments in Maryland coastal bay sediments as biomarkers of sources of organic matter to benthic community. In this study, the sources of pelagic organic matter and their transport to the surface sediment in the Maryland Coastal Bays (MCB), part of the U.S. mid-Atlantic coastal lagoon system, were examined. Photosynthetic pigments (chlorophylls a and b), accessory pigments (peridinin, fucoxanthin, zeaxanthin, alloxanthin, etc.), and chlorophyll a decomposition products (chlorophyllide a, pheophorbide a, pheophytin a, and pyropheophytin a) from surface sediments collected at 13 sites in the MCB in March 2013 were analyzed using high performance liquid chromatography. The spatial distributions of diagnostic pigments in surface sediments indicated that organic matter was mainly derived from nano- and picophytoplankton (i.e. cyanobacteria, cryptophytes, and chlorophytes) at sites characterized by high nutrient input, al...

Sedimentary pigments and nature of organic matter within the oxygen minimum zone (OMZ) of the Eastern Arabian Sea (Indian margin)

Sedimentary pigments, carbon and nitrogen content and their stable isotopes were studied in three short cores collected from the oxygen minimum zone (OMZ) of the Eastern Arabian Sea (EAS). Nine pigments including chlorophyll a and their degradation products were quantified using High Performance Liquid Chromatography (HPLC). Astaxanthin followed by canthaxanthin and zeaxanthin were the major carotenoids detected in these cores. The total pigment concentration was high in the core collected from 500 m water depth (6.5 μgg−1) followed by 800 m (1.7 μgg−1) and 1100 m (1.1 μgg−1) depths respectively. The organic carbon did not have considerable control on sedimentary pigments preservation. Pigment degradation was comparatively high in the core collected from the 800 m site which depended not only the bottom dissolved oxygen levels, but also on the faunal activity. As reported earlier, the bottom water dissolved oxygen and presence of fauna have good control on the organic carbon accumulation and preservation at Indian margin OMZ sediments. The C/N ratios and δ13C values for all the cores conclude the marine origin of organic matter and δ15N profiles revealed signature of upwelling associated denitrification within the water column.

Increase in benthic trophic reliance on methane in 14 French lakes during the Anthropocene

Summary Lakes play a key role in the regulation of the global carbon cycle. However, their functioning can be strongly impacted by anthropogenic pressures and climate variability. Understanding the response of the carbon cycle to environmental changes remains a crucial, elusive goal for both ecosystem managers and aquatic ecologists. In particular, the relations among lake physical and chemical properties, landscape structure and lake carbon cycling must be studied to predict future trends in lake functioning. Sediment cores were collected from the deepest part of 14 small French lakes that differed in lake properties (elevation, conductivity, area, area of the watershed) and land‐use class (forest, wetland, agricultural land and urban area). The sampling strategy employed the top‐bottom approach (a comparison between present‐day conditions and ‘reference’ conditions at Medieval period, c. AD 1000). For each sample, the following variables were analysed: isotopic carbon composition of sedimentary organic carbon (δ13COM), δ13C of chironomid remains (δ13CHC), and sedimentary pigments (total carotenoids, TC). Stepwise multiple regression analysis showed that the size of the catchment area may affect δ13COM values for the Medieval samples (R2 = 0.36, P < 0.05), such that the flux of terrestrial organic matter increases with the size of the watershed. However, this relation is not observed in the present‐day samples, and the influence apparently becomes largely anthropogenic. For these present‐day samples, the proportion of agricultural land in the watershed appears to be the primary driver of lake biogeochemical cycles through a direct effect on nutrient availability (R2 = 0.24, P < 0.05) and through an indirect effect on the benthic carbon cycle. The results also confirm the widespread existence of a pathway for methane‐derived carbon contribution to chironomid biomass (up to 61% of chironomid biomass) and suggest that high‐conductivity lakes are highly sensitive to the presence of this pathway (more than 75% of the lakes in our dataset have benthic food webs apparently dependent on biogenic methane; CH4). The results may indicate that the high‐conductivity lakes (from the Jura Mountains) are more vulnerable to anthropogenic activities than low‐conductivity lakes because high‐conductivity water provides an excellent nutritive medium for the development of photoautotrophic production. Studying within‐lake CH4 dynamics and the response of the CH4 cycle in high‐conductivity lakes appear to be crucial for understanding both regional carbon budgets and lake trophic functioning.

Application of orthogonal design to the extraction and HPLC analysis of sedimentary pigments from lakes of the Tibetan Plateau

Sedimentary pigments are useful proxy indicators of phytoplankton biomass, community structure, primary productivity and human influence in lacustrine and oceanic ecosystems. Pigments are relatively labile due to their complex chemical structures, which makes the extraction and analysis of sedimentary pigments challenging. In addition, it is important to select appropriate methods to study sedimentary pigments in regions which lack previous investigations. In this study, we adopted the L9(34) orthogonal design to develop methods of extraction and HPLC (high performance liquid chromatography) analysis of sedimentary pigments at two lakes on the Tibetan Plateau: meromictic lake-Dagze Co and dimictic lake-Jiang Co. The orthogonal design comprises 9 combinations of various parameters for extraction and HPLC analysis. The results show that the type and volume of solvent are the most important factors for pigment extraction, and the mobile phase and column selection are the most important for HPLC analysis. For the study sites, we found that the best methods to extract sedimentary pigments are as follows: the use of a mixture of acetone:methanol:water (80:15:5, v:v:v) as the extraction solvent; solvent/ sample ratio of 10 mL/g; sonication for 30 s and standing extraction for 6 h. The best methods for HPLC analysis are as follows: Zorbax Eclipse plus C8 column with mobile phase A, methanol:acetonitrile:0.25 M aqueous pyridine (50:25:25, v:v:v) and mobile phase B, methanol: acetonitrile: acetone (20:60:20, v:v:v); pH of mobile phase A adjusted to 6 with acetic acid; and HPLC column temperature maintained at 40°C. The study provides an experimental basis for future investigations of past changes in primary productivity and the response of lake ecosystems to climate change and human activities on the Tibetan Plateau.

Impacts of land use and climate variability on algal communities since ~1850 CE in an oligotrophic estuary in northeastern New Brunswick, Canada

We used sedimentary pigments to evaluate algal abundance since 1868 in a relatively unperturbed, oligotrophic northeastern New Brunswick (Canada) estuary and to understand how variations relate to past land transformation or climate factors. Environmental variables included sedimentary pigments (β-carotene, chlorophyll a, diatoxanthin, fucoxanthin, alloxanthin, canthaxanthin, echinenone, lutein–zeaxanthin, and chlorophyll b), climate variability (precipitation, temperature during the months of April, May, June, July, August) and land-based activities (relative land surface area occupied by hay, oats, wheat, barley, peat extraction). A sediment core was taken downstream of the Pokemouche Estuary in 2010 and sectioned into 2-cm slices whose ages spanned the interval 1868–2010, determined by 210Pb dating. Most pigments showed a marked peak in the 1920s, around the time of most widespread agricultural activities, and declined thereafter at the same time that peat extraction increased. Relative land surface area occupied by oat crops was a statistically significant environmental variable that accounted for 40 % of algal community variability (p = 0.009 by redundancy analysis). Correlation analysis suggests that expansion of non-industrial agriculture was associated with increased abundance of algal photoautotrophs in the 1920s, possibly via soil nutrient leaching. In contrast, expansion of mechanized peat extraction since the 1940s was associated with declining abundance of photoautotrophs, possibly a consequence of turbidity or color-induced light limitation, and increasing abundance of groups known to include mixotrophic algae. Overall, long-term changes in algal abundance were better correlated with land transformation variables than with climate variables. Even though resource extraction activities (agriculture, peat mining) historically involved relatively small areas of this coastal catchment (<10 %), algal abundance seems to have fluctuated in response to changes in flux of land-derived matter.

Algal biomass and pigments along a latitudinal gradient in Victoria Land lakes, East Antarctica

It is generally accepted that Antarctic terrestrial diversity decreases as latitude increases, but latitudinal patterns of several organisms are not always as clear as expected. The Victoria Land region is rich in lakes and ponds and spans 8 degrees of latitude that encompasses gradients in factors such as solar radiation, temperature, ice cover and day length. An understanding of the links between latitudinally driven environmental and biodiversity changes is essential to the understanding of the ecology and evolution of Antarctic biota and the formulation of hypotheses about likely future changes in biodiversity. As several studies have demonstrated that photosynthetic pigments are an excellent, although underused, tool for the study of lacustrine algal communities, the aim of the present study was to investigate variations in algal biomass and biodiversity across the latitudinal gradient of Victoria Land using sedimentary pigments. We test the hypothesis that the biodiversity of freshwater environments decreases as latitude increases. On the basis of our results, we propose using the number of sedimentary pigments as a proxy for algal diversity and the sum of chlorophyll a and bacteriochlorophyll a with their degradation derivatives as an index of biomass. Overall, our data show that biomass and diversity decrease as latitude increases but local environmental conditions, in particular, natural levels of eutrophy, can affect both productivity and diversity. Keywords: Biodiversity; photosynthetic pigments; proxy; continental Antarctica; sediments; biogeography. (Published: 13 May 2016) To access the supplementary material for this article, please see the supplementary files in the column to the right (under Article Tools). Citation: Polar Research 2016, 35 , 20703, http://dx.doi.org/10.3402/polar.v35.20703

Asynchronous onset of eutrophication among shallow prairie lakes of the Northern Great Plains, Alberta, Canada

Coherent timing of agricultural expansion, fertilizer application, atmospheric nutrient deposition, and accelerated global warming is expected to promote synchronous fertilization of regional surface waters and coherent development of algal blooms and lake eutrophication. While broad-scale cyanobacterial expansion is evident in global meta-analyses, little is known of whether lakes in discrete catchments within a common lake district also exhibit coherent water quality degradation through anthropogenic forcing. Consequently, the primary goal of this study was to determine whether agricultural development since ca. 1900, accelerated use of fertilizer since 1960, atmospheric deposition of reactive N, or regional climate warming has resulted in coherent patterns of eutrophication of surface waters in southern Alberta, Canada. Unexpectedly, analysis of sedimentary pigments as an index of changes in total algal abundance since ca. 1850 revealed that while total algal abundance (as β-carotene, pheophytin a) increased in nine of 10 lakes over 150 years, the onset of eutrophication varied by a century and was asynchronous across basins. Similarly, analysis of temporal sequences with least-squares regression revealed that the relative abundance of cyanobacteria (echinenone) either decreased or did not change significantly in eight of the lakes since ca. 1850, whereas purple sulfur bacteria (as okenone) increased significantly in seven study sites. These patterns are consistent with the catchment filter hypothesis, which posits that lakes exhibit unique responses to common forcing associated with the influx of mass as water, nutrients, or particles.

Impact of Deepwater Horizon spill on food supply to deep-sea benthos communities

Deep-sea ecosystems encompass unique and often fragile communities that are sensitive to a variety of anthropogenic and natural impacts. After the 2010 Deepwater Horizon (DWH) oil spill, sampling efforts documented the acute impact of the spill on some deep-sea coral colonies. To investigate the impact of the DWH spill on quality and quantity of biomass delivered to the deep-sea, a suite of geochemical tracers (e.g., stable and radio-isotopes, lipid biomarkers, and compound-specific isotopes) was measured from monthly sediment trap samples deployed near a high-density deep-coral site in the Viosca Knoll area of the north–central Gulf of Mexico prior to (Oct-2008 to Sept-2009) and after the spill (Oct-10 to Sept-11). Marine (e.g., autochthonous) sources of organic matter (OM) dominated the sediment traps in both years, however after the spill, there was a pronounced reduction in marine-sourced OM, including a reduction in marine-sourced sterols and n-alkanes and a concomitant decrease in sediment trap organic carbon and pigment flux. Results from this study indicate a reduction in primary production and carbon export to the deep-sea in 2010–2011, at least 6–18 months after the spill started. Whereas satellite observations indicate an initial increase in phytoplankton biomass, results from this sediment trap study define a reduction in primary production and carbon export to the deep-sea community. In addition, a dilution from a low-14C carbon source (e.g., petro-carbon) was detected in the sediment trap samples after the spill, in conjunction with a change in the petrogenic composition. The data presented here fills a critical gap in our knowledge of biogeochemical processes and sub-acute impacts to the deep-sea that ensued after the 2010 DWH spill.

Climate and human land-use as a driver of Lake Narlay (Eastern France, Jura Mountains) evolution over the last 1200 years: implication for methane cycle

This study aims at reconstructing the limnological conditions of Lake Narlay, a lake of which a previous study has shown major changes in the trophic reliance on methane of the benthic food web. The evolution of environmental and limnological conditions in and around Lake Narlay is reconstructed using combined analyses of subfossil chironomid remains, sedimentary pigments and organic carbon accumulation rates; whereas potential driving factors are reconstructed by pollen and coprophilous analyses and the use of climate paleo-records. The results suggest a complex combination of climate and anthropogenic pressure to explain the 1200 years of ecological trajectory of Lake Narlay. This ecological trajectory seems to be firstly control by climatic variability, because the colder event of the Little Ice Age induces a significant impact on chironomid assemblage, organic matter influx, and lake productivity. In a more recent time, the anthropogenic control seems to become predominant. The intensification of the agro-pastoral practices in the catchment area of Lake Narlay seems to induce the degradation of the oxygen conditions at the water/sediment interface. In addition, the present study gives also lines of evidence that the human intensification in the watershed of Lake Narlay seems to be the main cause of the major change in the contribution of biogenic methane to the benthic food web. However, the transition stage between the two trophic pathways (from absence to transfer of biogenic methane) involves a progressive change with intermediate limnological conditions, making the assessment of a tipping point in the temporal evolution difficult. Better knowledge of the relationships between the temporal evolution of limnological conditions and the activation of the trophic reliance on methane will require the study of other sites with different conditions.

Benthic Oxygen Uptake in the Arctic Ocean Margins - A Case Study at the Deep-Sea Observatory HAUSGARTEN (Fram Strait).

The past decades have seen remarkable changes in the Arctic, a hotspot for climate change. Nevertheless, impacts of such changes on the biogeochemical cycles and Arctic marine ecosystems are still largely unknown. During cruises to the deep-sea observatory HAUSGARTEN in July 2007 and 2008, we investigated the biogeochemical recycling of organic matter in Arctic margin sediments by performing shipboard measurements of oxygen profiles, bacterial activities and biogenic sediment compounds (pigment, protein, organic carbon, and phospholipid contents). Additional in situ oxygen profiles were performed at two sites. This study aims at characterizing benthic mineralization activity along local bathymetric and latitudinal transects. The spatial coverage of this study is unique since it focuses on the transition from shelf to Deep Ocean, and from close to the ice edge to more open waters. Biogeochemical recycling across the continental margin showed a classical bathymetric pattern with overall low fluxes except for the deepest station located in the Molloy Hole (5500 m), a seafloor depression acting as an organic matter depot center. A gradient in benthic mineralization rates arises along the latitudinal transect with clearly higher values at the southern stations (average diffusive oxygen uptake of 0.49 ± 0.18 mmol O2 m-2 d-1) compared to the northern sites (0.22 ± 0.09 mmol O2 m-2 d-1). The benthic mineralization activity at the HAUSGARTEN observatory thus increases southward and appears to reflect the amount of organic matter reaching the seafloor rather than its lability. Although organic matter content and potential bacterial activity clearly follow this gradient, sediment pigments and phospholipids exhibit no increase with latitude whereas satellite images of surface ocean chlorophyll a indicate local seasonal patterns of primary production. Our results suggest that predicted increases in primary production in the Arctic Ocean could induce a larger export of more refractory organic matter due to the longer production season and the extension of the ice-free zone.

Tracking past changes in lake-water phosphorus with a 251-lake calibration dataset in British Columbia: tool development and application in a multiproxy assessment of eutrophication and recovery in Osoyoos Lake, a transboundary lake in Western North America

Recently there has been an active discussion about the potential and challenges of tracking past lake-water trophic state using paleolimnological methods. Herein, we present analyses of the relationship between modern-day diatom assemblages from the surface sediments of 251 fresh-water lakes from British Columbia and contemporary limnological variables. Total phosphorus (TP) was significantly related to the modern distribution of diatom assemblages. The large size of this new calibration dataset resulted in higher abundances and occurrences of many diatom taxa thereby allowing a more accurate quantification of the optima of diatom taxa to TP in comparison to previous smaller calibration datasets. Robust diatom-based TP inference models with a moderate predictive power were developed using weighted-averaging regression and calibration. Information from the calibration dataset was used to interpret changes in the diatom assemblages from the north and south basins of Osoyoos Lake, in conjunction with fossil pigment analyses. Osoyoos Lake is a large salmon-bearing lake that straddles the British Columbia-Washington border and has undergone cultural eutrophication followed by recovery due to substantial mitigation efforts in managing sources of nutrients. Both diatom assemblages and sedimentary pigments indicate that eutrophication began c. 1950 in the north basin and c. 1960 in the southern basin, reaching peak levels of production between 1960 and 1990, after which decreases in sedimentary pigments occurred, as well as decreases in the relative abundance and concentrations of diatom taxa inferred to have high TP optima. Post-1990 changes in the diatom assemblage suggests conditions have become less productive with a shift to taxa more indicative of lower TP optima in concert with measurements of declining TP, two of these diatom taxa, Cyclotella comensis and Cyclotella gordonensis, that were previously rare are now abundant.

An 800‐year ultraviolet radiation record inferred from sedimentary pigments in the Ross Sea area, East Antarctica

High performance liquid chromatography (HPLC) combined with mass spectrometry (MS) was used to analyse deposited pigments (including chlorophyll a, phaeophytin a, canthaxanthin, echinenone, zeaxanthin, scytonemin and mycosporine‐like amino acids) from two sediment profiles of ponds in the Ross Sea area, East Antarctica. We explored the sources and characteristics of each pigment, reconstructed an 800‐year record of ultraviolet radiation (UVR) and total incoming light intensity, and identified the possible factors that may have influenced historical UVR changes in this region. The results indicated at least four UVR peaks during the past 800 years, corresponding to c. AD 1950–2000, 1720–1790, 1560–1630 and 1350–1480, with the intensity from the most recent sediments being the highest. A comparison between the changes in UVR and total incoming light intensity showed similar patterns between AD 1720 and 1830, suggesting that factors controlling the UVR intensity in the Ross Sea area may be related to insolation fluctuation at that time. The two proxies are, however, weakly correlated during other periods. Historically, there is a relationship between the reconstructed UVR and solar activity, but this natural process may be strongly affected by multiple factors, including climate parameter change and anthropogenic activities during the modern times.