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.     

Whole-basin,mass-balance approach for identifying critical phosphorus-loading thresholds in shallow lakes

Lake Lochloosa, Florida (USA) recently underwent a shift from macrophyte to phytoplankton dominance, offering us the opportunity to use a whole-basin, mass-balance approach to investigate the influence of phosphorus loading on ecosystem change in a shallow, sub-tropical lake. We analyzed total phosphorus (TP) sedimentation in the basin to improve our understanding of the forcing factor responsible for the recent shift to phytoplankton dominance. We measured ²¹⁰Pb activity, organic matter (OM), organic carbon (OC) and TP in short sediment cores from 20 locations to develop a comprehensive, whole-basin estimate of recent mass sedimentation rates (MSR) for bulk sediment, OM, OC and TP. The whole-basin sedimentation models provided insights into historic lake processes that were not evident from the limited, historic water quality data. We used Akaike’s Information Criteria to differentiate statistically between constant MSR and exponentially increasing MSR. An eightfold, exponential increase in TP accumulation over the past century provided evidence for the critical role of increased P loading as a forcing factor in the recent shift to phytoplankton dominance. Model results show increased TP retention and decreased TP residence time were in-lake responses to increased TP loading and the shift from macrophyte to phytoplankton dominance in Lake Lochloosa. Comparison of TP loading with TP retention and historic, diatom-inferred limnetic TP concentrations identified the TP loading threshold that was exceeded to trigger the shift to phytoplankton dominance.     

Response of the cladoceran community to trophic state change in Lake Apopka, Florida

A paleolimnological evaluation of cladoceran microfossils was initiated to study limnological changes in Lake Apopka, a large (125 km²), shallow (mean depth = 1.6 m), warm, polymictic lake in central Florida. The lake switched from macrophyte to algal dominance in the late 1940s, creating a Sediment Discontinuity Layer (SDL) that can be visually used to separate sediments derived from macrophytes and phytoplankton. Cladoceran microfossils were enumerated as a means of corroborating extant eutrophication data from the sediment record. Inferences about the timing and trajectory of eutrophication were made using the cladoceran-based paleo-reconstruction. The cladoceran community of Lake Apopka began to change abruptly in both total abundance and relative percent abundance just before the lake shifted from macrophyte to algal dominance. Alona affinis, a mud-vegetation associated cladoceran, disappeared before the SDL was formed. Planktonic and benthic species also began to increase below the SDL, indicating an increase in production of both planktonic and benthic species. Chydorus cf. sphaericus, an indicator of nutrient loading, increased relative to all other cladocerans beginning in the layer below the SDL and continuing upcore. Changes in the transitional sediment layer formed before the lake switched to phytoplankton dominance, including an increase in total phosphorus concentration, suggest a more gradual eutrophication process than previously reported. Data from this study supported conclusions from other paleolimnological studies that suggested anthropogenic phosphorus loading was the key factor in the hypereutrophication of Lake Apopka.     

Spatial variability of chironomid death assemblages in the surface sediments of a fluctuating tropical lake (Lake Naivasha,Kenya)

Studies addressing within-lake variability of fossil chironomid assemblages are very few, and all deal with hydrologically stable temperate lakes where the question of spatial integration mostly relates to the mixing of faunal assemblages associated with shallow, warm-water habitat and those associated with deeper, cold-water habitat. Here we study within-lake variability of surface-sediment chironomid assemblages in the fairly large (∼100–170 km² since 1983) and shallow (Z _max = 5–8 m) fluctuating tropical lake basin of Lake Naivasha, Kenya, and compare the patterns observed with those in two smaller adjacent basins, one similarly shallow (Lake Oloidien, 5.1–5.7 km2, 5–8 m), the other deep and stratified (Crescent Island Crater, 1.9 km², 14–17 m). Chironomid assemblages were analysed in core-top samples and surface sediments along inshore to offshore transects, and how well individual samples represented the total (basin-wide mean) subfossil assemblage was considered both in terms of taxon richness and taxon percent composition. Within-lake variability of subfossil chironomid concentrations (with generally higher absolute values in nearshore samples) could be explained by effects of sediment winnowing and focusing, whereas between-lake variability reflected their relative susceptibility to wind-driven sediment disturbance or bottom anoxia. In all study lakes, but most significantly in lakes Naivasha and Oloidien, species distribution in the subfossil chironomid assemblages showed a strong nearshore to offshore gradient, which in these shallow lakes, reflects the dominant control of substrate and food quality on species distribution in the living community. Particularly in the larger basins, nearshore samples better represented the total lake assemblage than offshore samples, because the former always contained a component of mud-dwelling species whereas the latter often lacked a component of macrophyte-dwelling species. Our results show that although sedimentation dynamics in the shallow, wind-stressed Lake Naivasha is dominated by frequent resuspension and random sediment redistribution, the near- to offshore gradient in chironomid habitat remains imprinted on subfossil assemblages. We conclude that also in shallow fluctuating lakes, given sufficient size, incomplete pre-burial spatial integration of habitat-specific chironomid assemblages can be exploited for within-lake calibration of environmental gradients.     

Identifying sources of organic matter in sediments of shallow lakes using multiple geochemical variables

We analyzed ²¹⁰Pb-dated sediment cores from four relatively shallow lakes (z_max < 10 m) in the Upper Ocklawaha River Basin, Florida, USA to compare primary producer community structure before and after anthropogenic impacts. We measured physical and chemical sediment variables including density, organic matter (OM), water-soluble phosphorus, polyphosphate (Poly-P), total P (TP), total carbon to total nitrogen mass ratios of OM (TC:TN), biogenic silica (diatoms, sponge spicules), total amorphous silica, and stable carbon and nitrogen isotope ratios of bulk OM. Principal component analysis showed that diatom biogenic silica, TC:TN, Poly-P and TP displayed discernible stratigraphic changes associated with the shift in the primary producer community. We applied k-means cluster analysis to these variables to identify macrophyte-derived, transitional, and phytoplankton-derived sediments. Our approach provides an objective method for identifying sediment sources that may be applied to shallow lakes in other regions. The four study lakes shifted from a macrophyte-dominated state to a transitional state before major anthropogenic disturbances, and became phytoplankton-dominated after ~1950.     

Mechanisms for Organic Matter and Phosphorus Burial in Sedimentsof a Shallow, Subtropical, Macrophyte-Dominated Lake

We studied the role that submersed aquatic vegetation (SAV) plays in the sedimentation of organic matter (OM) and phosphorus (P) in Lake Panasoffkee, Florida (USA), a shallow, hard-water, macrophyte-dominated water body. Carbon/Nitrogen ratios (C/N) and stable isotope signatures (δ¹³C and δ¹⁵N) in algae, higher plants, and surface sediments were measured to identify sources of OM to the lake mud. Pollen, plant macrofossils, and geochemistry in sediment cores indicated that primary productivity and SAV abundance in Lake Panasoffkee increased in the late 1800s, probably as a response to increased P loading from human settlement and forest clearance. SAV and associated periphyton served as temporary sinks for soluble P, maintaining relatively clear-water, low-nutrient conditions in the lake. P accumulation in Lake Panasoffkee sediments increased together with indicators for greater SAV presence. This suggests that SAV and associated epiphytes promote P burial and retention in sediments. Although it might be assumed that rooted submersed macrophytes are directly responsible for P uptake from water and transfer to sediments, C/N and stable carbon isotope results argue for the importance of other macrophyte growth forms, and perhaps epiphytic algae, in permanent OM and P sequestration. For instance, high rates of photosynthesis by epiphytes in hard-water systems consume CO₂ and promote CaCO₃ precipitation. Sloughing of accumulated carbonates from macrophyte leaves transfers epiphytes and associated P to the sediment. Our paleolimnological findings are relevant to restoration efforts in the Florida Everglades and support the claim that constructed SAV wetlands remove P from waters effectively.     

Highly variable sediment distribution in shallow, wind-stressed lakes: a case for sediment-mapping surveys in paleolimnological studies

We examine sediment distribution patterns in seven Florida lakes and discuss implications for paleolimnological studies of shallow, subtropical lakes. The study lakes are highly productive and should exhibit thick organic sediment deposits, but organic sediments are often grossly lacking because basins are shallow, and frequent mixing, lack of stratification, and warm temperatures lead to breakdown of organic material.Organic sediment distribution patterns are highly variable. We observe three types of distribution patterns. When organic sediments are abundant, there may be (1) uniform sediment distribution. In lakes lacking organic sediments, there are (2) distribution to deeper areas if present, or (3) distribution to peripheral areas and embayments when deep waters are absent. We advocate the use of systematic mapping surveys to locate optimal coring sites for paleolimnological studies of shallow, wind-stressed lakes. Because numerous factors affect diagenesis and sediment redistribution, sediment abundance and location are not predictable. Sediment chronologies may be discontinuous and disturbed even in accumulation zones. The extent to which sedimentary records are discontinuous or disturbed is not quantifiable in any practical manner. ²¹⁰Pb and ¹³⁷Cs radioisotopic profiles provide qualitative evidence of the degree of stratigraphic disturbance. Total excess ²¹⁰Pb inventories show that sediments are focused into depositional zones when sediment distribution is uneven. Excess ²¹⁰Pb inventories are not informative about the completeness of sedimentary profiles unless small inventories suggest discontinuous sedimentation or erosional events. We present examples of disturbed and undisturbed profiles, and discuss how we use radioisotopic and geochemical evidence, and multiple cores to assess stratigraphic continuity.Journal Series No. R-04815 of the Florida Agricultural Experiment Station.     

Reconstructing changes in macrophyte cover in lakes across the northeastern United States based on sedimentary diatom assemblages

Macrophytes are a critical component of lake ecosystems affecting nutrient and contaminant cycling, food web structure, and lake biodiversity. The long-term (decades to centuries) dynamics of macrophyte cover are, however, poorly understood and no quantitative estimates exist for pre-industrial (pre-1850) macrophyte cover in northeastern North America. Using a 215 lake dataset, we tested if surface sediment diatom assemblages significantly differed among lakes that have sparse (<10% cover; group 1), moderate (10–40% cover; group 2) or extensive (>40% cover; group 3) macrophyte cover. Analysis of similarity indicated that the diatom assemblages of these a priori groups of macrophyte cover were significantly different from one another (i.e., difference between: groups 1 and 3, R statistic = 0.31, P < 0.001; groups 1 and 2, R statistic = 0.049, P < 0.01; groups 3 and 2, R statistic = 0.112, P < 0.001). We then developed an inference model for macrophyte cover from lakes classified as sparse or extensive cover (145 lakes) based on the surface sediment diatom assemblages, and applied this model using the top-bottom paleolimnological approach (i.e., comparison of recent sediments to pre-disturbance sediments). We used the second axis of our correspondence analysis, which significantly divided sparse and extensive macrophyte cover sites, as the independent variable in a logistic regression to predict macrophyte cover as either sparse or extensive. Cross validation, using 48 randomly chosen sites that were excluded from model development, indicated that our model accurately predicts macrophyte cover 79% of the time (r ² = 0.32, P < 0.001). When applied to the top and bottom sediment samples, our model predicted that 12.5% of natural lakes and 22.4% of reservoirs in the dataset have undergone a ≥30% change in macrophyte cover. For the sites with an inferred change in macrophyte cover, the majority of natural lakes (64.3%) increased in cover, while the majority of reservoirs (87.5%) decreased in macrophyte cover. This study demonstrates that surface sediment diatom assemblages from profundal zones differ in lakes based on their macrophyte cover and that diatoms are useful indicators for quantitatively reconstructing changes in macrophyte cover.     

Determination of carbon, carbonate, nitrogen, and phosphorus in freshwater sediments by near-infrared reflectance spectroscopy: Rapid analysis and a check on conventional analytical methods

Sediments are typically analyzed for C, N, and P for characterization, sediment quality assessment, and in nutrient and contaminant studies. Cost and time required for analysis of these constituents by conventional chemical techniques can be limiting factors in these studies. Determination of these constituents by near-infrared reflectance spectroscopy (NIRS) may be a rapid, cost-effective method provided the technology can be applied generally across aquatic ecosystems. In this study, we explored the feasibility of using NIRS to predict total C, CO₃ ^–2 organic C, N, and P in deep-water sediment cores from four Canadian lakes varying over 19 degrees of latitude. Concentration ranges of constituents in the samples (dry weight basis) were total C, 12-55; CO₃ ^–2, 6-26; organic C, 7-31; N, 0.6-3.1; and P, 0.22-2.1 mg g^–1. Coefficients of determination, r2, between results from conventional chemical analysis and NIR-predicted concentrations, based on calibrations across all the four lakes, were 0.97-0.99 for total C, organic C, and N. Prediction for CO₃ ^–2 was good for the hard water lake from a calibration across all four lakes, but this constituent in the three soft water lakes was better predicted by a calibration across the soft water lakes. The NIR calibration for P fell below acceptable levels for the technique, but proved useful in the identification of outliers from the chemical method that were later removed with the re-analysis of several samples. This study demonstrated that NIRS is useful for rapid, simultaneous, cost-effective analysis of total C, CO₃ ^–2, organic C, N, and P in dried sediments from lakes at widely varying latitudes. Also, this study showed that NIRS is an independent analytical tool useful for the identification of outliers that may be due to error during the analysis or to distinctive composition of the samples.     

Sedimentary carbon forms in relation to climate and phytoplankton biomass in a large,shallow, hard-water boreal lake

Carbon storage in lakes can have huge implications for the global carbon cycle, as lakes annually accumulate up to one half the amount of organic carbon buried in marine sediments. Yet little is known of the effect of recent climate change on carbon storage in lakes. We analyzed century-scale time series of climate variables (precipitation, temperature, NAO winter index) and profiles of sediment characteristics in a dated sediment core from shallow, eutrophic Lake Võrtsjärv, south Estonia. We used path analysis to evaluate the effect of climate conditions on phytoplankton biomass in the lake and accumulation of organic and inorganic carbon in the sediment. Changes in winter and spring climate influenced the lake’s phytoplankton growth significantly. Carbon pathways in hard-water Lake Võrtsjärv were influenced by both hydrological (most significant in colder periods) and biogeochemical processes. Increased nutrient and water input to Lake Võrtsjärv, anticipated with projected climate warming, favours greater in-lake productivity, larger accumulation of inorganic carbon in sediments, and an increase in organic carbon mineralisation, which fuels atmospheric greenhouse gas emissions from the lake.     

Landscape-scale patterns of sediment sulfur accumulation in Swedish lakes

Sulfur has played a central role in the acidification of many lakes in Scandinavia and other regions. As part of the research into sulfur cycling, numerous studies have analyzed the sediment record in order to develop insights into past in-lake cycling of sulfur, particularly in the context of reconstructing past deposition rates. Although many of these studies have shown that it is not easy to interpret the sediment record in terms of past sulfur deposition rates, analyses of sulfur in sediment still provide valuable information on the response of lakes to anthropogenic sulfur deposition. Here, we have analyzed sulfur in top and bottom samples from short surface cores (25–35 cm, representing ≥250 years) as well as bulk cores from ∼110 lakes located throughout Sweden, which were collected during 1986, as well as in more-detailed profiles from six lakes. The lakes with the highest surface sediment concentrations (9–24 mg S g⁻¹ dry mass) and the highest calculated inventories of ‘excess’ sulfur (20–180 g S m⁻²) are found in southern Sweden and around one industrial area along the northeastern coast where sulfur deposition rates and lake-water concentrations have been highest. For many lakes in the central and northern inland region it is common that the sediment cores exhibit either no enrichment or even a decline in sulfur concentrations in near-surface sediments, which we suggest was the pre-pollution norm for lakes. Although interpreting sulfur sediment profiles is problematic for reconstructing deposition, a more-comprehensive spatial sampling approach shows that there is a good geographic agreement between sulfur deposition, lake-water chemistry and sediment sulfur accumulation.     

Controls on loss-on-ignition variation in cores from two shallow lakes in the northeastern United States

Loss-on-ignition analysis of Quaternary lake sediments provides an inexpensive and easy way to investigate past environmental changes. The mass loss on ignition at 550 °C (LOI) from lake sediment cores may vary because of temporal changes in: (1) sediment composition controlled by factors such as productivity, inorganic inputs, and decomposition; and (2) the patterns of sediment accumulation controlled by factors such as basin morphology and water level. Climatic changes can alter both. Here, modern surface samples and transects of sediment cores, collected across small (<10 ha), shallow (<4 m) lakes in the northeastern United States, show that LOI varies little (2–5%) across the deep portions of these small lakes at a given time. Large changes in LOI occur only at the transition into the littoral (shallow) zone. LOI variations in sediment cores that exceed 2–5%, therefore, appear to represent meaningful environmental changes. However, because of the many possible controls, changes in the LOI of a single core are often hard to interpret. Multiple cores increase the interpretability. At lakes studied here, similar LOI trends among several cores confirm that some LOI changes resulted from basin-wide shifts in sediment composition. Differences among cores, however, developed during the early- and mid-Holocene and indicate that the edge of the littoral zone moved towards the centers of the lakes during two periods of low lake levels, at ca. 11 000–8000 and ca. 5400–3000 cal yr B.P. The basin-wide balance of sediment sources controlled the LOI from deep-water sediments, but sedimentation patterns, which changed as lake levels changed, were also important. LOI differences among cores may therefore help identify past lake-level changes in other lakes.     

Sedimentary evidence for recent eutrophication in the northern basin of Lake Taihu,China: human impacts on a large shallow lake

Environmental change in Lake Taihu and its catchment since the early to middle part of the twentieth century has left a clear geochemical record in the lake sediments. The human activities in the lake and its catchment responsible for the change include agriculture, fishery, urbanisation, sewage and industrial waster disposal. Sediment cores were collected from Meilian Bay of northern Lake Taihu to investigate the record of anthropogenic impacts on the lake’s ecosystem and to assess its natural, pre-eutrophication baseline state. Two marked stratigraphic sediment units were identified on the basis of total phosphorus concentration (TP), pigments, total organic carbon (TOC)/total nitrogen (TN), δ¹³C and δ¹⁵N corresponding to stages in the lake history dominated by phytoplankton, and by aquatic macrophytes. Results show that as TP loading increased from the early 1950s the lake produced sediments with increasing amounts of organic matter derived from phytoplankton. In the early 1950s, the first evidence for eutrophication at the Meilian Bay site is recorded by an increase in C/N values and in sediment accumulation rate, but there is little change in phosphorus concentrations, pigments, δ¹³C and δ¹⁵N at this time. After 1990 a more rapid increase in trophic status took place indicated by increased levels of phosphorus, pigments, δ¹⁵N and by decreased δ¹³C and TOC/TN values in the lake sediments. The first increase in trophic status of the early 1950s results mainly from agricultural development in the catchment. In contrast, the acceleration from ca. 1990 originates from the recent development of fisheries and the urbanisation and industrialisation of the catchment.     

Palaeolimnological evidence for submerged plant loss in a floodplain lake associated with accelerated catchment soil erosion (Murray River,Australia)

Shallow lakes have been described as existing in two alternative equilibrium states, dominated by either submerged plants or phytoplankton. Causes of, often catastrophic, shifts between these states have been widely debated but may often result from displacement of the dominant community by stochastic influence. In Australian cut-off river meanders (also known as ‘billabongs’), anecdotal and palaeolimnological evidence suggests widespread loss of aquatic macrophytes since European occupation of the region c. post-1800. Our detailed and high-resolution stratigraphic study of a sediment core from Hogan’s Billabong (Murray River, Australia) seeks to identify the causes of the loss of aquatic macrophytes. Little direct evidence of the past extent and composition of submerged macrophyte communities was recovered. Nevertheless, results derived from other sediment proxies, including declines in the abundance of epiphytic diatoms and in plant-associated invertebrates, provide further indirect evidence of macrophyte disappearance. Despite limitations with radiometric dating, the sequence of events in the derived record suggests that a period of high abiotic turbidity, leading to a critical reduction in water transparency and caused by widespread erosion during the late 19th century, is the most likely factor contributing to loss of submerged vegetation from this billabong.     

A record of phosphorus dynamics in oligotrophic lake sediment

Historical phosphorus (P) dynamics were studied using sediment cores from three oligotrophic, acidic lakes in Maine, USA. Long-term oligotrophy of these lakes is consistent with high sediment aluminum (as Al(OH)₃) concentrations, as Al inhibits internal P loading, even under reducing conditions. The role of microbially-mediated reactions in controlling redox conditions was evaluated by estimating microbial biomass and relative abundance of specific functional groups. Sediments were fractionated using a sequential chemical extraction technique and all lakes met criteria for P retention based on threshold sediment concentrations of Al, Fe, and P fractions as determined by (Kopáček et al. (2005) Limnol Oceanogr 52: 1147–1155). Sediment NaOH-extractable molybdate-reactive P (rP) and non-reactive P (nrP) represent P associated with non-reducible phases, and organic matter-related P, respectively. Total P (TP) does not decrease with sediment depth, as is typical of eutrophic lake sediments; however, nrP/TP decreases and rP/TP increases for all three lakes, indicating nrP mineralization without any significant upward diffusion and release into the hypolimnion; i.e. diagenesis of P is conservative within the sediment. Two diagenetic models were developed based on nrP and rP concentrations as a function of sediment age. The first model assumes a first-order decay of nrP, the rate coefficient being a function of time, and represents irreversible nrP mineralization, where the produced PO₄ is permanently sequestered by the sediment. The second model assumes a first-order reversible transformation between nrP and rP, representing biotic mineralization of organic P followed by incorporation of inorganic P into microbial biomass. Both models reflect preservation of TP with no loss to overlying water. The rate coefficients give us insight into qualities of the sediment that have affected mineralization and sequestration of phosphorus throughout the ²¹⁰Pb-dateable history of each lake. Similar models could be constructed for other lakes to help reconstruct their trophic histories. Paleolimnological reconstruction of the sediment P record in oligotrophic lakes shows mineralization of nrP to rP, but unlike the case in eutrophic lake sediments, sediment TP is preserved in these sediments.     

Geochemical and biological consequences of groundwater augmentation in lakes of west-central Florida (USA)

We studied sediment cores from four Florida (USA) lakes that have received groundwater hydrologic supplements (augmentation) for >30 years to maintain lake stage. Top samples (0–4 cm) from sediment cores taken in Lakes Charles, Saddleback, Little Hobbs, and Crystal had ²²⁶Ra activities of 44.9, 17.5, 7.6, and 8.5 dpm g⁻¹, respectively, about an order of magnitude greater than values in deeper, older deposits. The surface sample from Lake Charles yielded the highest ²²⁶Ra activity yet reported from a Florida lake core. Several lines of evidence suggest that groundwater augmentation is responsible for the high ²²⁶Ra activities in recent sediments: (1) ²²⁶Ra activity in cores increased recently, (2) the Charles, Crystal, and Saddleback cores display ²²⁶Ra/²¹⁰Pb disequilibrium at several shallow depths, suggesting ²²⁶Ra entered the lakes in dissolved form, (3) cores show recent increases in Ca, which, like ²²⁶Ra, is abundant in augmentation groundwater, and (4) greater Sr concentrations are associated with higher ²²⁶Ra activities in recent Charles and Saddleback sediments. Sr concentrations in Eocene limestones of the deep Floridan Aquifer are high relative to Sr concentrations in surficial quartz sands around the lakes. Historical water quality inferences for the lakes were based on diatom assemblages in sediments. Recent alkalization in Lakes Charles, Saddleback, Little Hobbs, and Crystal was inferred from weighted-averaging calibration (WACALIB). The lakes also show recent trophic state increases based on WACALIB-derived estimates for limnetic total P. Although residential and agricultural sources might contribute to increased P loading, P in augmentation waters probably has had significant influence on eutrophication. Dystrophic diatoms were abundant in the early history of Lakes Saddleback, Little Hobbs, and Crystal, which suggests that these lakes contained more tannic waters during the past than at present, perhaps as a consequence of greater inflows from surrounding wetlands. Ionic content of lake waters increased, as indicated by diatom autecological analysis. Recent geochemical and biological changes detected in cores from these lakes probably are a result of deliberate groundwater augmentation, although inputs of groundwater pumped for agricultural and residential development in the watersheds also might have contributed to limnological changes.     

Paleoshoreline evidence for postglacial tilting in Southern Manitoba

Detailed air photo interpretation and four seasons of field mapping and surveying in southern Manitoba have revealed that the once-level paleoshorelines of Lake Winnipegosis and Dauphin Lake and the Burnside shoreline of former Lake Agassiz have been tilted up to the northeast by postglacial differential rebound. Our investigation has also revealed that Lake Winnipegosis has the best preserved paleoshoreline record of any of the large lakes in southern Manitoba, including lakes Winnipeg and Manitoba. This is because northeasterly uptilting shifts the region's lakes to the southwest. Lakes with southern outlets, like Lake Winnipegosis, undergo general regression as the outlet is lowered relative to the rest of the basin. Lakes with northern outlets, like lakes Winnipeg and Manitoba, undergo general transgression as northeasterly uptilting raises the outlet relative to the rest of the basin. Along the northeastern shore of Lake Winnipegosis a staircase of at least 32 abandoned Winnipegosis shorelines exists that is consistent with northeasterly tilting. The Dawson level represents the major mid-Holocene highstand on Lake Winnipegosis. It persisted for about 500 years, peaking at 5290 ¹⁴C yr B.P. (early Dawson) and then falling about 3 m by 4740 ¹⁴C yr B.P. (late Dawson). The early Dawson shoreline is tilted at 13.5 cm km^-1 in a direction N24.3°E. Three other shorelines informally named shoreline 4, shoreline 3, and shoreline 2 are also tilted up to the northeast. Their radiocarbon ages (and slopes in cm km^-1) are 3330 yr B.P. (2.2), 1510 yr B.P. (1.3), and 1080 yr B.P. (0.7), respectively. On Dauphin Lake shoreline IV is the oldest level mapped for this study. It has a ¹⁴C age of 7910 yr B.P. and is tilted at 21.7 cm km^-1 in a direction N44.4°E. The Id shoreline marks the major mid-Holocene highstand for Dauphin Lake. It peaked at 4640 ¹⁴C yr B.P. followed by a rapid decline of about 1 m to the Ib shoreline, which is dated at 4320 ¹⁴C yr B.P. Id is tilted up at 8.8 cm km^-1 in a direction N53.4°E. The next major shoreline is Ia3 which has a ¹⁴C age of 3020 yr B.P. and is tilted up at 5.3 cm km^-1 in a direction N62.3°E. Tilt directions are significantly more easterly for the Dauphin Lake shorelines than those from Lake Winnipegosis or any of the much older Lake Agassiz shorelines. Taken together, the Winnipegosis and Dauphin isobases indicate that the direction of tilt in southern Manitoba is more complex than a simple uni-directional pattern. The observed pattern of tilting for paleoshorelines in southern Manitoba agrees better with predictions derived from the recently revised loading history model ICE-4G than with those from its predecessor ICE-3G. In general, the calculated tilt based on the ICE-3G load tends to exceed the observed tilt, while ICE-4G tends to underestimate it. Both ice load models appear to disagree most with our observed tilts in this region during the interval before about 9000 cal yr B.P., when deglaciation was proceeding rapidly and the large water load associated with Lake Agassiz covered the region. Because both of these ice load models have been estimated mainly from a global data set of relative sea level curves from marine coast sites, it is not unexpected that model tilts derived from them do not agree well with observations in the North American continental interior. The pattern of postglacial crustal deformation for southern Manitoba described in this paper could be used to further refine ice load models for the North American continental interior.     

Sedimentary macrofossil records reveal ecological change in English lakes: implications for conservation

Aquatic macrophytes play a key role in providing habitat, refuge and food for a range of biota in shallow lakes. However, many shallow lakes have experienced declines in macrophyte vegetation in recent decades, principally due to eutrophication. As changes in macrophyte composition and abundance can affect overall ecological structure and function of a lake, an assessment of the timing and nature of such changes is crucial to our understanding of the wider lake ecosystem. In the typical absence of historical plant records, the macro-remains of macrophytes preserved in lake sediments can be used to assess long-term changes in aquatic vegetation. We generated recent (150–200 years) plant macrofossil records for six English lakes subject to conservation protection to define past macrophyte communities, assess trajectories of ecological change and consider the implications of our findings for conservation targets and strategies. The data for all six lakes reveal a diverse submerged macrophyte community, with charophytes as a key component, in the early part of the sedimentary records. The stratigraphies indicate considerable change to the aquatic vegetation over the last two centuries with a general shift towards species more typically associated with eutrophic conditions. A common feature is the decline in abundance of low-growing charophytes and an increase in tall canopy-forming angiosperms such as fine-leaved Potamogeton species, Zannichellia palustris and Callitriche species. We hypothesise, based on findings from long-term datasets and palaeoecological records from enriched shallow lakes where plants are now absent, that the observed shifts provide a warning to managers that the lakes are on a pathway to complete macrophyte loss such that nutrient load reduction is urgently needed. It is the sound understanding of present-day plant ecology that affords such reliable interpretation of the fossil data which, in turn, provide valuable context for current conservation decisions.     

The biota of Romanian saline lakes on rock salt bodies: A review

An inventory of the aquatic and circum-lacustrine flora and aquatic fauna of 23 karst- and anthroposaline lakes formed on Miocene rock salt bodies in Romania was undertaken. Simultaneous field determinations of water and sediment pH, total dissolved solids, Secchi transparency and lake maximum depth were also made. Environmental conditions for phytoplankton, microphytobenthos, macrophyte and cormophyte species were defined. Relations between the numbers of plankton, microbenthos, macrophyte and cormophyte species differentiated pelogenous (usually, therapeutic) and non-pelogenous lakes. High densities and occurrence frequencies characterized the most representative species. For three of these species, Amphora veneta, Artemia sp. and Potamogeton pectinatus, ecological conditions were noted. In the circum-lacustrine vegetation, glycophytes were more important than halophytes.     

Unexpected post-settlement freshening and increase in charophytes in Bombah Broadwater (Myall Lakes,New South Wales,Australia)

Bombah Broadwater is a shallow coastal lake within the Ramsar-listed Myall Lakes system on the mid-north coast of New South Wales, Australia. Increased nutrient and sediment loads resulting from catchment modification are thought to have instigated the loss of aquatic plants in the lake, causing it to “switch” from a clear, macrophyte dominated system (similar to the conditions in present day Myall Lake) to a turbid, phytoplankton dominated system. To assess this hypothesis, charophytes, foraminifera and aquatic fauna remains from an 800 year sediment record were examined. The sediment chronology was established using ¹⁴C, ²¹⁰Pb and ¹³⁷Cs radiometric dating and sediment composition. Interestingly, a clear increase in charophytes since European arrival conflicted with the hypothesised aquatic plant loss. Hence, it appears Bombah Broadwater has not undergone a change in stable state since European arrival. An additional and unexpected finding in the patterns of the foraminifera and testate amoeba suggest that Bombah Broadwater has freshened substantially since European arrival. This freshening may have resulted from increased catchment run off as a result of the clearance of catchment vegetation. Since catchment vegetation clearance is widespread in Australia, this finding raises the possibility that post-settlement freshening of coastal lakes may be a common occurrence.