Sediment distribution and accumulation in lagoons of the Southern Mediterranean Region (the MELMARINA Project) with special reference to environmental change and aquatic ecosystems

Surface sediments and sediment cores were collected from coastal lagoons and lakes located in the Southern Mediterranean Region (SMR) as part of the MELMARINA Project which involved integrated eco-hydrological monitoring and modelling. This study uses surface sediments and sediment cores to infer spatial characteristics and temporal changes at the MELMARINA primary sites, Merja Zerga in Morocco, Ghar El Melh in Tunisia and Lake Manzala in Egypt. In addition, surface sediment sampling was undertaken at Egyptian Lake Bardawil and sediment cores were collected from the Lagune de Nador (Morocco). Sediment distribution patterns are investigated using GIS with georeferenced sample locations to facilitate display and resurvey. Major variations in sedimentary organic matter and, particularly, carbonate content, occur within and between sites. Local landscapes combined with hydrological and biogeochemical processes influence the distributions of sediment bulk components (carbonates, organic material and clastic matter) and molluscan shells and shell debris are an important source of sedimentary carbonate at all three primary sites. Sediment cores were dated using natural (²¹⁰Pb) and artificial (¹³⁷Cs) radionuclides, and sediment accumulation rate changes indicate that sources of sediment supply varied markedly through the twentieth century but have generally diminished after the mid-1960s. Sedimentary siliceous microfossils (diatoms) were generally poorly preserved, but mollusc shell remains were well represented. Sediment chronologies and sediment bulk composition allow discussion of some recent changes in bulk, minerogenic and biogenic sediment accumulation patterns in the SMR lagoons. Sediment accumulation rates also varied between sites and multiple cores from Lake Manzala indicated that rates showed considerable spatial variability. Low-level sediment contamination by fossil fuel combustion particulates and trace metals was demonstrated for Ghar El Melh and Lagune de Nador where Pb and Zn accumulation rates were highest in twentieth century sediment. It is emphasized that sediment quality and quantity have strong influences on lagoon ecosystem function and sedimentation is relevant to hydromorphology and to concepts of ecological quality. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Guest editors: J. R. Thompson & R. J. Flower Hydro-ecological Monitoring and Modelling of North African Coastal Lagoons     

DIATOM PALEOLIMNOLOGY OF TWO FLUVIAL LAKES IN THE ST. LAWRENCE RIVER: A RECONSTRUCTION OF ENVIRONMENTAL CHANGES DURING THE LAST CENTURY

Water quality degradation is a serious concern for the St. Lawrence River. While some environmental data are available for the St. Lawrence ecosystem, long-term monitoring data are generally lacking. To infer past environmental changes, we undertook a paleolimnological assessment of diatom assemblages preserved in four ²¹⁰Pb- and ¹³⁷Cs-dated sediment cores from two fluvial lakes in the river, and used diatom transfer functions to infer past shoreline habitat characteristics. At sites in Lake Saint-François, a fluvial lake downstream from Cornwall, water quality decreased this century in response to human impacts (e.g. macrophyte density and nutrient levels increased). These trends were apparent from an increase in epiphytic diatom taxa, followed by an increase in eutrophic planktonic taxa. Water quality, however, appears to have improved somewhat in response to rehabilitation measures during the last two decades. From a sediment core near Montréal (Lake Saint-Louis), we also noted a large proportion of eutrophic and epiphytic taxa, but less evidence was recorded of a recent improvement in water quality. The diatom-based inference model for habitat characteristics appeared to reconstruct environmental conditions in the St. Lawrence River during the last century. The most notable shift has been an increase in diatom taxa commonly associated with macrophyte substrates. Trends in some of the planktonic diatoms were similar to those recorded in paleolimnological investigations from Lake Ontario, but cores from the river also may be reflecting local environments. This study shows that diatom-based paleolimnological studies are possible in large river systems, if coring sites (e.g. fluvial lakes) are carefully selected.     

Release of sediment-phosphorus and the influence of algal growth on this process

Summary Phosphorus release from undisturbed sediment cores was studied in a continuous flow system. Various lakes and polder ditches were compared with each other. Peaty lakes exchanged their particulate phosphorus rapidly with the overlying water as compared with other sediment types. This makes them suitable for phosphorus load reduction measures. There was found to be no simple direct relation between chemically extracted phosphorus fractions and the phosphorus released during the experiment. Algal growth in the overlying water stimulated the release of phosphorus.     

Radiometrically determined dates and sedimentation rates for recent sediments in nine North African wetland lakes (the CASSARINA Project)

Sediment cores were collected from nine wetland lakes in Morocco, Tunisia and Egypt for the CASSARINA project investigating environmental change in Northern African wetlands. The cores were dated radiometrically by using natural (²¹⁰Pb) and artificial (¹³⁷Cs and ²⁴¹Am) radionuclides. At sites in Morocco and Tunisia with mean annual rainfall totals ranging from 500–1000 mm yr^–1, fallout records were generally satisfactory and it was possible to develop independent sediment chronologies based on the radiometric data alone. At the Egyptian sites, rainfall was less than 200 mm yr^–1 and fallout records were much less distinct. At these sites the radiometric data could only be used to give an indication of mean sedimentation rates during the past 30–40 years. By using a combination of fallout radionuclide, pollen, and macrofossil stratigraphic records it was however possible to determine a credible sediment chronology spanning the major part of the 20th century. Applying this chronology to records of spheroidal carbonaceous particles (SCP) from the same sediment cores, the onset of significant levels of atmospheric pollution in the Nile Delta is dated in all three cores to the mid 1950s. Results from a number of lakes (Sidi Bou Rhaba, Ichkeul and Korba) revealed high and accelerating siltation rates, threatening their continued existence beyond the next few decades. In contrast, sedimentation rates at all three Nile Delta sites appear to have declined in recent decades, most probably due to the impact of the Nile barrages.     

Zooplankton (Cladocera, Ostracoda), Chironomidae and other benthic faunal remains in sediment cores from nine North African wetland lakes: the CASSARINA Project

Palaeolimnological studies were carried out on sediment cores from nine North African wetland lakes. The lakes represented a variety of habitats ranging from freshwater to mixohaline conditions and with and without direct connections to the sea. Sediment cores were examined for records of recent environmental change during the 20th century period. Faunal remains analysed in the sediments included those of Cladocera, Ostracoda, Chironomidae, Mollusca and, at the sites with marine connections, Foraminifera. Cyprideis torosa littoralis was the most common ostracod, occurring in sediments from the seven sites linked with the sea and also in brackish water Sidi Bou Rhaba. In acid Megene Chitane ostracods were scarce, being represented by a single species (Cypria ophtalmica). Candona neglecta completely disappeared at two sites (Sidi Bou Rhaba and Bokka) during the 20th century. Of the Cladocera, Chydorus sphaericus was common in the moderately mixohaline sites and but Heterocypris salina occurred only where marine salinities were occasionally achieved (in Zerga, Ichkeul and Korba). Microinvertebrate assemblages in the Nile Delta lake cores and to a less extent in Zerga showed a clear response indicative of more fresher conditions occurring during the latter part of the 20th century. However the freshening trend began prior to construction of the Aswan High Dam (mid 1960s).With the exception of the most saline site (Korba), microinvertebrate communiuties at all the CASSARINA sites have experienced major disturbances during the 20th century. Changes in freshwater availability associated with increased human usage of water resources is perceived as a major factor regulating the abundance and occurrence of aquatic microinvertebrate species at these sites     

Recent environmental change in North African wetland lakes: diatom and other stratigraphic evidence from nine sites in the CASSARINA Project

All nine wetland lakes in the CASSARINA North African suite of sample sites have been disturbed strongly by human activity during the 20th century. Dated lake sediment core were used to provide evidence of the extent of recent environmental change at each site. Sedimentary diatoms at seven sites were useful for inferring salinity change trends during the last century. At two sites preservation problems severely degraded the sedimentary diatom record. Sediment core integrity was otherwise established.Lithostratigraphic measurements indicated some site specific changes in soil erosion and sediment composition but, for the Egyptian Delta lakes, no physical signal synchronous with Aswan High Dam construction was found. Sedimentary diatom assemblages were generally site characteristic and halophilous taxa were common. At two sites planktonic diatoms indicated some recent eutrophication but generally the assemblages were more indicative of salinity changes. Diatom-inferred salinity trends for the seven sites typically indicated that reductions in water salinity occurred sometime during the early or mid 20th century.Rather than climate, hydrological modification of water resources is implicated as the primary driver of salinity changes during most of the 20th century. In the western North African region these modifications were mainly local land drainage and water diversion programmes to alleviate winter flooding and/or promote summer water availability. In the Delta region, the Nile has been intensively exploited since antiquity and intensively so from the late 19th century to release more fresh water for agriculture. Here, diatom records indicate that freshening began well before the Aswan High Dam but salinity fluctuations have tended to diminish during the latter part of the 20th century. A small reversal in the water freshening trend in the 1980/90s was possibly a response to land subsidence/sea-level change or to reduced freshwater supply.Freshwater supply to the sites is generally diminishing as former freshwater surpluses switch to deficit. One site (Merja Bokka, Morocco) became completely dry in 1998 as agriculture encroached and Megene Chitane, the only acid lake in Tunisia, is currently affected by excessive inflow abstraction. At the beginning of the 21st century, eight of the nine CASSARINA sites persist as viable but modified aquatic ecosystems. They nevertheless continue to support valuable aquatic biodiversity, especially in the Delta sites and in Chitane. The modern diatom communities are clearly tolerant of considerable environmental change but the remaining sites are increasingly threatened by major hydrological disturbance. Base-line floristic data for the late 20th century are given but continuous biomonitoring combined with effective management is needed urgently to help conserve North Africa's diminishing natural wetland lake resources.     

PERIPHYTIC DIATOM ASSEMBLAGES FROM BATHURST ISLAND, NUNAVUT, CANADIAN HIGH ARCTIC: AN EXAMINATION OF COMMUNITY RELATIONSHIPS AND HABITAT PREFERENCES

Diatoms are potentially the most important biomonitors of environmental change in high arctic lakes and ponds, but to date few autecological data are available. Because of the shallow nature of many of these water bodies, a large proportion of taxa are periphytic and planktonic diatoms are absent for the most part. By determining the microhabitat and substrate preferences of these benthic diatom taxa, the potential exists to infer past changes in available habitats from fossil diatom assemblages collected from sediment cores and ultimately to reconstruct past environmental and climatic changes responsible for these shifts in habitat availability. To refine our understanding of high arctic diatom habitat preference, the common diatom taxa found on submerged moss (bryophyte), sediment, and rock substrates from lakes and ponds on Bathurst Island, Nunavut, Canadian High Arctic were examined. The relationships among key limnological variables and the common taxa from each habitat were examined. Many diatom taxa exhibited varying degrees of microhabitat preference, with moss representing the more unique habitat. In addition, the following limnological variables significantly ( P ≤ 0.05) explained the species variance for each of the three substrates: Na ⁺ and total nitrogen for moss; total phosphorus (filtered) and pH for rock; and Fe^{3 +} , total phosphorus (unfiltered), total nitrogen, temperature, and pH for sediment. These data can be used to help interpret monitoring and paleolimnological studies in this environmentally sensitive region.     

Concentration and annual accumulation values of heavy metals in lake sediments: Their significance in studies of the history of heavy metal pollution

The first part of the paper discusses the significance of using either concentration or accumulation values for expressing the results of investigations of lake sediment cores aimed at studying the history of heavy metal pollution. Neither the values for heavy metal concentration in the lake sediment, whether expressed per gram dry sediment or per gram soluble (organic) sediment, nor the values for their total annual accumulation per unit area of the lake bottom, can, on their own, provide an accurate picture of past pollution conditions, but when considered in combination they render a fairly reliable and detailed interpretation. The second part of the paper deals with Pb, Cd and Hg analyses of cores of varved sediments from several lakes in N. Sweden. Pb and Cd pollution increased during the second half of the 19th century. In most lakes, Hg pollution seems first to have started during the 20th century. Marked increases in both concentration and accumulation of heavy metals took place during the 20th century. For recent decades, the estimated accumulation rates of heavy metals from anthropogenic sources are: Pb 0.5–1.5 µg cm^–2 yr^–1, Cd 15–30 ng cm^–2 yr^–1 and Hg 1–2 ng cm^–2 yr^–1, Higher values were recorded in lakes affected by local emissions.     

Expanded spatial extent of the Medieval Climate Anomaly revealed in lake‐sediment records across the boreal region in northwest Ontario

Multi‐decadal to centennial‐scale shifts in effective moisture over the past two millennia are inferred from sedimentary records from six lakes spanning a ~250 km region in northwest Ontario. This is the first regional application of a technique developed to reconstruct drought from drainage lakes (open lakes with surface outlets). This regional network of proxy drought records is based on individual within‐lake calibration models developed using diatom assemblages collected from surface sediments across a water‐depth gradient. Analysis of diatom assemblages from sediment cores collected close to the near‐shore ecological boundary between benthic and planktonic diatom taxa indicated this boundary shifted over time in all lakes. These shifts are largely dependent on climate‐driven influences, and can provide a sensitive record of past drought. Our lake‐sediment records indicate two periods of synchronous signals, suggesting a common large‐scale climate forcing. The first is a period of prolonged aridity during the Medieval Climate Anomaly (MCA, c. 900‐1400 CE). Documentation of aridity across this region expands the known spatial extent of the MCA megadrought into a region that historically has not experienced extreme droughts such as those in central and western north America. The second synchronous period is the recent signal of the past ~100 years, which indicates a change to higher effective moisture that may be related to anthropogenic forcing on climate. This approach has the potential to fill regional gaps, where many previous paleo‐lake depth methods (based on deeper centrally located cores) were relatively insensitive. By filling regional gaps, a better understanding of past spatial patterns in drought can be used to assess the sensitivity and realism of climate model projections of future climate change. This type of data is especially important for validating high spatial resolution, regional climate models.     

Heavy metals (Cu and Zn) in recent sediments of Llangorse Lake,Wales: non-ferrous smelting,Napoleon and the price of wheat — a palaeoecological study

Elevated concentrations of Cu and Zn have been found in the upper part of three sediment cores collected from Llangorse Lake, in south Wales. Palaeomagnetic evidence from one of the cores and ²¹⁰Pb analysis of another, suggests that the increase in sediment Cu and Zn concentrations began during the eighteenth century. A sharp increase in the concentrations of these metals in the sediment profile appears to have occurred during the latter part of the eighteenth century and these concentrations remained high until the mid to late nineteenth century.The absence of known ore deposits and industry around the lake suggests that the lake and catchment soils were increasingly contaminated by long-range aerial transport of emissions from the expanding activity of Cu and Zn smelters located some 80 km upwind in the Swansea area during the Industrial Revolution. Evidence from agricultural crop returns indicates a significant increase in the amount of land devoted to tillage in the catchment, particularly to cereal production, during the late eighteenth and the first half of the nineteenth century which included the Napoleonic Wars. This agricultural shift appears to coincide with increased concentrations of Cu and Zn in the lake sediments. It is suggested that newly ploughed soils, contaminated with metals for many years by long-range aerial transport from the Swansea area, eroded, and were carried into the lake by catchment run-off and added to the sediment burden of Cu and Zn. A subsequent decline of Cu and Zn emissions due to the collapse of the non-ferrous smelting industry and reduced soil erosion because of a 50% reduction of tillage due to an agricultural depression in the second half of the 19th century may explain the fall in Cu and Zn concentrations in the upper part of the sediment profile. The most recent sediments (20th century) show the increase in heavy metals characteristic of many lakes around the world.     

Phosphorus release from sediments in hardwater eutrophic lakes: the effects of redox-sensitive and -insensitive chemical treatments

1. Phosphorus (P) release from bottom sediments is an important source of nutrient enrichment in many lakes in sedimentary basins, such as those in western Canada. On the Boreal Plain, sediment P release is particularly strong during periods of seasonal anoxia.
2. In this study, the effects of reduction–oxidation (redox)-sensitive and -insensitive chemicals on P release were examined in sediment cores collected from three eutrophic lakes.
3. Contrary to expectations, redox-sensitive treatments were no more effective at lowering total phosphorus (TP) in sediment cores than some redox-insensitive treatments. Redox-sensitive treatments with FeCl₃ and FeCl₃ + O₂ reduced TP to 8 and 6%, respectively, of reference concentrations, whereas redox-insensitive treatments with alum or lime + alum reduced TP to 14% of reference concentrations. Lime and O₂ treatments reduced TP concentrations to 35 and 52% of reference concentrations, respectively.
4. The fraction of P that adsorbed and co-precipitated with iron and aluminium in the sediment cores was low (non-apatite phosphorus fractions < 5%), suggesting that P release was controlled by apatite solubility and bacterial metabolism.     

ABSENCE OF EVIDENCE IS NOT EVIDENCE OF ABSENCE: IS STEPHANODISCUS BINDERANUS (BACILLARIOPHYCEAE) AN EXOTIC SPECIES IN THE GREAT LAKES REGION?1

The eutrophic, freshwater diatom species Stephanodiscus binderanus (Kütz.) Willi Krieg. has long been considered a nuisance exotic alga introduced from Eurasia to the Great Lakes in North America in the early to mid‐20th century. However, our paleolimnological data from Lake Simcoe, Ontario, provide unequivocal evidence that this taxon has been present in the Great Lakes region since at least the late 17th century. Subfossil diatom valves were identified and enumerated at high resolution in ²¹⁰Pb‐dated sediment cores from four sites across the lake. The taxonomic identification of S. binderanus was confirmed using SEM. The historical presence of this species in Lake Simcoe indicates somewhat naturally productive conditions and also refutes the idea that S. binderanus is a nonindigenous species to North America. This study underscores the caution that should be applied to questions of diatom (and protistan) distributions in time and space. Clearly, the absence of evidence is not evidence of absence.     

Iron:phosphorus ratio in surface sediment as an indicator of phosphate release from aerobic sediments in shallow lakes

Analysis of Danish lakes showed that both mean winter and mean summer concentrations of lake water total phosphorus in the trophogenic zone correlated negatively with the total iron to total phosphorus ratio (Fe:P) in surface sediments. No correlation was found between the water total phosphorus concentration and either the sediment phosphorus concentration alone or with sediment calcium concentration. The increase in total phosphorus from winter to summer, which is partly a function of net internal P-loading, was lowest in lakes with high Fe:P ratios in the surface sediment.A study of aerobic sediments from fifteen lakes, selected as representative of Danish lakes with respect to the sediment Fe and phosphorus content, showed that the release of soluble reactive phosphorus was negatively correlated with the surface sediment Fe:P ratio. Analysis of phosphate adsorption properties of surface sediment from 12 lakes revealed that the capability of aerobic sediments to buffer phosphate concentration correlated with the Fe:P ratio while the maximum adsorption capacity correlated with total iron. Thus, the Fe:P ratio may provide a measure of free sorption sites for orthophosphate ions on iron hydroxyoxide surfaces.The results indicate that provided the Fe:P ratio is above 15 (by weight) it may be possible to control internal P-loading by keeping the surface sediment oxidized. Since the Fe:P ratio is easy to measure, it may be a useful tool in the management of shallow lakes.     

Recovery of UK lakes from acidification: An assessment using combined palaeoecological and contemporary diatom assemblage data

We assess the recovery of UK lakes from acidification using the combined data from sediment cores and sediment traps to track changes in diatom assemblages in 11 UK upland lakes from pre-acidification times (prior to ca. 1850 AD) to the present (2008 AD). We projected the data into a Principal Component Analysis (PCA) of diatom assemblage data from 121 low-alkalinity lakes in the UK to show how the floristic composition of the core and trap diatom assemblages for each site has changed through time. The results show that the degree of recovery from acidification varies amongst sites but in all cases its extent is limited when compared with the pre-acidification reference. In most cases the recovery, although usually slight, is characterised by a decline in acid tolerant taxa and a return towards taxa that occurred previously at each respective site. In a few cases, however, the floristic composition of recent samples is different from those that occurred during and before the acidification phase. The reasons for this are not yet clear but it is possible that nutrient enrichment from atmospheric N deposition and/or climate change is beginning to play a role in driving water quality as acidity decreases. More generally the results show that annually recovered samples from sediment traps can be successfully combined with sediment core data to provide a continuous record of environmental change in lake systems, and that diatoms collected in sediment traps can be used to provide a very powerful lake monitoring tool.     

Organic carbon burial in lake sediments in the middle and lower reaches of the Yangtze River Basin, China

Lakes are important in the global and regional carbon cycle, and lake sediments potentially store substantial quantities of organic carbon. The middle and lower reaches of the Yangtze River basin (MLYB) are some of the largest agricultural areas in China with an extremely high density of lakes and rivers. The lakes in the region have undergone dramatic changes over the past several decades. In this study, six cores from five lakes (the macrophyte-dominated: Shijiuhu Lake and Honghu Lake; the algae-dominated: Chaohu Lake, Taihu Lake, and Nanyihu Lake) in the MLYB were collected from 2002 A.D. to 2008 A.D. Mass accumulation rates (MARs) of sediment derived from ²¹⁰Pb and ¹³⁷Cs along with total organic carbon content (TOC) were used to determine organic carbon accumulation rates (OC ARs) over the last 100 years. The TOC in the five lakes exhibited a significant increase since the mid or late 20th century, which was consistent with the increase in the lake water trophic status due to nutrient input. The average organic carbon accumulation rates for the Taihu Lake, Nanyihu Lake, Chaohu Lake, Shijiuhu Lake, and Honghu Lake were calculated to be 16.6, 28.9, 9.8, 25.4, and 113.2 g C m^?2 year^?1, respectively, over the past 100 years. Based on the average OC AR of 32.1 g C m^?2 year^?1 from the five lakes, carbon burial in lake sediments may be as much as 6.8 × 10¹³ g C in the MLYB over the past 100 years.     

Paleolirnnological records of carotenoids and carbonaceous particles in sediments of some lakes in Southern Alps

Stratigraphic analyses of organic carbon, organic nitrogen and algal and bacterial carotenoids in short cores of profundal sediments of four alpine lakes (Tovel, Leit, Paione superiore and Tom) were used to reconstruct their trophic history. In addition, depth distribution of carbonaceous particle concentrations provided information on lake contamination from atmospheric deposition. In three lakes (Tovel, Leit and Tom), sedimentary carotenoids unique to sulfur photosynthetic bacteria (okenone and isorenieratene) provide evidence of changes in the oxygen, light and sulfide conditions in the water column. All the lakes are oligotrophic or moderately productive, and the algal community is dominated by Chlorophyta, Pyrrhophyta and Cryptophyta. Cyanobacteria are rather poorly represented. The steep increase of carbonaceous particles in the uppermost sediment layers of all the lakes suggests that lake contamination by atmospheric transport of pollutants began in the 1940s to 1950s. These data, coupled with those from a parallel study on Chrysophycean scale-inferred pH, indicate recent acidification in those which are poorly buffered (Paione superiore and Leit).     

Limnology of high mountain tropical lake, in Ecuador: characteristics of sediments and rate of sedimentation

Equatorial high mountain lakes are a special type of lake occurring mainly in the South American Andes as well as in Central Africa and Asia. They occur at altitudes of a few thousand meters above sea level and are cold-water lakes (< 20 degrees C). Relatively little is known about them. A long-term limnological study was therefore undertaken at Lake San Pablo, Ecuador, to analyze the basic limnological processes of the lake, which has a tendency for eutrophication. Sediment quality of San Pablo Lake is given under consideration of horizontal and vertical distribution using sediment cores. Significance of sediments for eutrophication process of lakes is demonstrated using phosphorus concentration of sediments as well as the phosphorus retention capacity of the sediments by ratio Fe/P. Dating of the sediments is done using 137Cs and 210Pb, but the activity of 137Cs in the sediment was very low nearly at the detection level. Sedimentation rate is determined to be 3.5 mm/year and the sediment cores represent about 110 years. P concentration of the sediments is high (approximately 5 g/kg dry substance), and P retention capacity by Fe is insufficient (Fe/P = 4). The sediment quality did not change significantly during the past decades, and the trophic state of San Pablo Lake was already less or more eutrophic 110 years ago. The contamination of the lake sediments by heavy metals is insignificant.     

Sedimentary losses of phosphorus in some natural and artificial Iowa lakes

Phosphorus sedimentation in four natural and four artificial Iowa lakes was measured by using sediment traps to determine if sedimentary phosphorus losses were greater in artificial lakes than in natural lakes and the limnological factors influencing phosphorus loss rates. Mean phosphorus sedimentation rates ranged from 13.3 to 218 mg · m^–2 day^–1. Although phosphorus sedimentation rates for the natural lakes as a group did not differ significantly from the rates for artificial lakes, there were significant differences among individual lakes. Phosphorus sedimentation rates also varied significantly during different seasons at different locations within a lake and at different depths within a location. Despite the variance, phosphorus sedimentation rates were strongly correlated with inorganic sediment concentrations and inorganic matter sedimentation rates, thus suggesting that inorganic sediments influence phosphorus sedimentation rates. When Iowa data were combined with data from published studies, mean sedimentation rates were directly correlated with mean chlorophyll a concentrations of the lakes. These data strongly suggest that sedimentation rates as measured by sediment traps are strongly influenced by the trophic status of a lake. Though sedimentation rates were higher in the more productive lakes, it is suggested that these rates represent only gross sedimentation rates rather than net sedimentation rates because of resuspension and resedimentation of bottom sediments.     

Fluxes of mercury to lake sediments in central and northern Canada inferred from dated sediment cores

Sediment cores were collected from lakes in central and northern Canada and from Hudson Bay to compare current and historic net depositions of mercury. Cores from most locations were enriched in mercury in the upper layers deposited recently relative to deeper, historic layers. The lakes with the greatest enrichments in mercury were located in central/southern Canada. This enrichment was interpreted as being of anthropogenic origin. Mercury inputs at the Experimental Lakes Area (ELA) in northwestern Ontario inferred from a core profile agreed well with inputs calculated independently from precipitation and runoff data. Anthropogenic inputs of mercury to northwestern Ontario were calculated to be about 9 µg m^-2 y^-1. Considering all the locations over the geographic range, the core profiles infer that fluxes of mercury have increased on average by about 2 fold over the past half century. This is consistent with results from other sites in North America and Europe.     

The vegetation history of monastic and secular sites in the midlands of Ireland over the last two millennia

Historic Icelandic tephrochronology forms the timeframe for four palynological investigations of peat profiles from the Irish midlands to trace vegetation history and land use at contemporary monastic and secular sites during the last two millennia. Scrubby woodlands changed little in nature and extent until the late 17th or early 18th century. Pastoral agriculture has a long record at all of the sites studied, with arable agriculture being sustained throughout the 2nd millennium. All sites show evidence for scrub clearance with associated agricultural expansion in the late 18th and early 19th centuries. At this time, trees not native to Ireland were planted.