Inferring long-term nutrient changes in southeastern Ontario lakes: comparing paleolimnological and mass-balance models

Lake eutrophication continues to be a major concern in many lake regions, but long-term monitoring data are often lacking. Therefore, indirect proxy methods must be used to infer these missing data sets. Two methods were applied to infer pre-industrial and present-day lakewater total phosphorus concentrations (TP) in a suite of 50 hardwater lakes in southern Ontario (Canada). One method inferred TP from the diatom species composition in the tops (present-day inferences) and bottoms (pre-1850 inferences) of sediment cores. The other method applied the Lakeshore Capacity Model (LCM), which is a mass-balance model based on phosphorus export coefficients that relate lakes and their watershed characteristics to epilimnetic nutrient concentrations. Diatom-based estimates of preindustrial to present-day change show that 78% of the lakes increased in TP (29% significantly) and 8% decreased. According to model error, 63% of the lakes have not significantly changed. LCM estimates show that 56% of the lakes have increased in TP, and the remainder (44%) have not changed. The average inferred increase in TP was similar for both models, but a lake-by-lake comparison indicated marked differences in model output. In particular, a paired comparison of diatom-based and LCM-based inferences of preindustrial TP shows no correlation. It is suggested that lake managers be thorough when collecting data for either model, and model selection should be carefully considered. The LCM and diatom-based models perform better in regions that are geologically similar to where the respective models were calibrated. Advantages and disadvantages of each model are further discussed.     

The ratio of diatom frustules to chrysophycean statospores: A useful paleolimnological index

Stratigraphic analyses of the postglacial sediments of Little Round Lake and the recent sediments of two additional southern Ontario lakes were used to compare the relative abundance of fossil diatom frustules to chrysophycean statospores. Relatively few statospores were recorded during eutrophic phases in the lakes' histories such as during periods of anthropogenic activity in the lakes' catch me nt, when diatom frustules were abundant. The increased relative abundance of statospores during the early postglacial in Little Round Lake was attributed to both oligotrophy and a short growing season, conditions that currently exist in many high arctic lakes. From these data it is seen that the ratio of fossil diatom frustules to chrysophycean statospores may provide a useful paleolimnological index of trophic status in temperate lakes. The advantages of this index include its simplicity, the fact that frustules and statospores can be enumerated from the same microscope slides, and that the ratio is independent of sedimentation rates.     

Sedimentary diatom assemblages and their utility in computing diatom-inferred pH in Sudbury Ontario lakes

Surface sediment diatoms from 30 Sudbury lakes were analyzed to establish a relationship between diatoms and limnological characteristics of these lakes. Factor analysis of various chemical and physical variables suggested that most of the variance was associated with the pH, pH-metal, and conductivity factors. Factor analysis of common diatom taxa indicated that the distribution of most of these diatoms is closely related to lake water pH and/or pH related factors. Regression analysis of various pH indicator assemblages with measured lake water pH also demonstrated significant relationships. Among the transfer functions employed to compute diatom-inferred pH, multiple regressions using pH indicator assemblages proved to be the best. Using this method an accuracy of about 0.3 pH unit can be achieved, even for lakes which have received large trace metal inputs concurrent with increase in acidity. The presence of planktonic diatoms in acidic lakes suggests that the widely accepted hypothesis that planktonic diatoms are greatly reduced in waters at pH below 5.6 is not always true.     

Associations of diatoms in the surficial sediments of lakes and peat pools in Atlantic Canada

The distribution of diatom frustules in the surficial sediments of lakes and ombrotrophic peat pools in Atlantic Canada are examined especially in relation to pH and bog lake succession. Sedimentary diatom associations for peat pools, strongly acidic lakes, and weakly acidic lakes are characterized. These associations and the derived diatom-pH calibration functions will facilitate improved paleoecological interpretation of natural ontogenetic pathways and assessments of recent anthropogenic impacts.     

Methods of pH calibration of sedimentary diatom remains for reconstructing history of pH in lakes

The pH history of lakes can be inferred from diatom remains in dated sediment cores. To derive transfer functions for pH inference in acidic lakes, we counted diatoms in surface-sediment from 31 soft-water lakes in n. New England (NE) and 36 in Norway (N), covering pH 4.4–7.1. Cluster analysis of each data set indicates that pH 6 is an upper limit for a group of similar diatom assemblages. For each set, we developed multiple linear regressions to relate three versions of the diatom data to pH of surface-waters: (1) relative frequencies of selected diatom taxa, (2) the first principal component (1 PC) of these frequencies, and (3) the frequencies of Hustedt pH groups. Also, simple linear regressions were developed for two versions: (1) Index B and (2) Index Alpha, both based on pH groups. Regressions were run separately for lakes with pH 6; these are most relevant for pH inference in acidic lakes. The best regressions (N: taxa & 1 PC taxa) have r² 0.69–0.91 and S_e 0.24–0.31 pH units, the worst (NE: log alpha) have r² 0.27–0.57 and S_e 0.51. In all cases, errors for NE are greater than N, partly due to greater diversity of NE lakes. Regressions based on pH groups (directly & by indices) have smaller r² and larger S_e than those based on taxa and 1 taxa. The Index Alpha is least useful because its requirement for alkaline diatom units is unsatisfied at many acidic lakes. Regressions based on taxa may give erratic pH inferences due to sensitivity to unusual frequencies of individual taxa; this effect is reduced by using 1 PC taxa. Four regressions based on pH 6 lakes were used for inferring pH in a ²¹⁰Pb dated core from Nedre Målmesvatn, N (now pH 4.6). There is good agreement among three of the four (not for the regression based directly on taxa) that there has been a decrease of ca. 0.6 pH units starting in the late 1800's.     

The influence of catchment size on lake trophic status during the hemlock decline and recovery (4800 to 3500 BP) in southern Ontario lakes

We investigated the ecological effects of terrestrial ecosystem change during the hemlock decline and recovery (4,800–3,500 BP) on lake communities (diatoms and chrysophytes). This study specifically assessed the role of catchment area and slope in determining the magnitude of lake eutrophication during the hemlock decline by analyzing sediment cores from five alkaline, holomictic lakes in southeastern Ontario, Canada. The study lakes were similar in most limnological aspects, but differed widely in the relative sizes of their catchments. Diatoms were used to quantitatively infer past lake-water total phosphorus (TP) concentrations.All five lakes showed shifts in their algal communities during the hemlock decline, but most lakes exhibited only minor changes in trophic status. The magnitude of the limnological response appears to be related to catchment size and slope. Long Lake, Burridge Lake, and Gunter Lake possess the smallest catchments and exhibited the weakest responses to the hemlock decline. The catchment area of Flower Round Lake is considerably larger and steeper than these lakes, and was the only lake to show a marked eutrophication. Aulacoseira ambigua bloomed and diatom-inferred TP concentration increased by 14 µg 1^–1.Catchment slope appears to have influenced the type of material exported into the lakes. Lake basins draining catchments with gentle relief received proportionally greater amounts of organic matter, whereas steeper catchments supplied relatively greater proportions of mineral matter. Faster water flow associated with steeper catchment slope may have enhanced mineral erosionFollowing the hemlock decline, nutrient supplies to most of the study lakes were reduced. The period of forest recovery was associated with an 11 µg 1^–1 reduction in diatom-inferred lake-water TP concentration in Flower Round Lake, and algal populations decreased. Our results generally support the ecological theory of forest ecosystem development and secondary succession developed from long-term data collected at the Hubbard Brook Experimental Ecosystem.     

Correlation of surface sediment diatoms with the present lake water pH in 28 Algoma lakes,Ontario, Canada

The surface sediment diatom analysis of 28 Algoma lakes (pH 4.40–8.13) indicates that even though each lake has a widely different aquatic environment and characteristic diatom assemblage, a definite relationship exists between the lake water pH and their diatom assemblages. In the acidic lakes acidobiontic and acidophilous diatom species predominate whereas in circumneutral and alkaline lakes circumneutral and alkaliphilous diatoms were most common. Cluster analysis of the pH indicator diatom assemblages grouped the study lakes into three distinct cluster groups. These groups also closely corresponded to lake water pH. On the basis of published ecological information as well as their presence in our study lakes, the pH indicator status of a number of diatom taxa have been discussed. A detailed listing of the diatom taxa identified and their pH indicator status is provided in order to facilitate their use in future diatom-inferred pH studies.     

Stratigraphy of specific algal and bacterial carotenoids in the sediments of Lake Varese (N. Italy)

A study on the succession of some phytoplankton populations in the very eutrophic Lake Varese over the past 80–100 years, was based on thin-layer chromatography of four group-specific carotenoids (echinenone, lutein, myxoxanthophyll and oscillaxanthin). The development of diatoms was traced by microscope counts of their frustules. In addition, the carotenoid okenone, peculiar to some Chromatium species, was identified and related to lake trophic conditions (the Chromatium species are obligate anaerobic bacteria, and use H₂S).     

Sedimentary records of accelerated nutrient loading in Florida lakes

Transfer functions relating trophic state (Carlson's TSI_{chlorophyll-a}) to present day accumulation rate of (1) nutrients, (2) cations, and (3) organic sediment, are computed using Binford's ²¹⁰Pb-dilution method. As computed from surficial sediments of 27 lakes, former trophic states are reconstructed for recent (²¹⁰Pb-dated) sedimentary histories of 14 lakes. Of the three kinds of models potentially available, model (3) (TSI vs. organic accumulation) is the weakest statistically, and may be unduly influenced by exceptional deposition and/or preservation of allochthonous organic matter. At present, however, it is the only model applicable to all 14 of the histories tested. Results are encouraging in that model accurately predicts observed TSI's in several mesotrophic and eutrophic lakes. Clearly significant increases (accelerations) are inferred only for a of the most eutrophic lakes of the set, while the model consistently overpredicts TSI's of the 7 most oligotrophic lakes. As Whitmore's diatom-assemblage index is a better predictor of TSI than is model (3) in the one eutrophic lake in which it has been tested, we expect more persuasive results when models (1) and (2) can be tested within a more complete set of analytical data. We were surprised to find 3 severely disturbed lakes among the 12 that show little or no acceleration in rate of eutrophication in recent decades, but we defer attempts at explanation until former nutrient loading can be tested by model (1).     

Paleolimnological reconstruction of the effects of atmospheric deposition of acids and heavy metals on the chemistry and biology of lakes in New England and Norway

Sediment cores from nine lakes in southern Norway (N) and six in northern New England (NE) were dated by ¹³⁷Cs, ²¹⁰Pb and in NE also by pollen, and were analyzed geochemically and for diatoms. Cores from two N and three NE lakes were analyzed for cladocerans. ¹³⁷Cs dating is unreliable in these lakes, probably due to mobility of Cs in the sediment. In Holmvatn sediment, an up-core increase in Fe, starting ca. 1900, correlates with geochemical indications of decreasing mechanical erosion of soils. Diatoms indicate a lake acidification starting in the 1920's. We propose that soil Fe was mobilized and runoff acidified by acidic precipitation and/or by soil acidification resulting from vegetational succession following reduced grazing. Even minor land use changes or disturbances in lake watersheds introduce ambiguity to the sedimentary evidence relating to atmospheric influences. Diatom counts from surface sediments in 36 N and 31 NE lakes were regressed against contemporary water pH to obtain coefficients for computing past pH from subsurface counts. Computed decreases of 0.3–0.8 pH units start between I890 and I930 in N lakes already acidic (pH 5.0–5.5) before the decrease. These and lesser decreases in other lakes start decades to over a century after the first sedimentary indications of atmospheric heavy metal pollution. It is proposed that the acidification of precipitation accompanied the metal pollution. The delays in lake acidification may be due to buffering by the lakes and watersheds. The magnitude of acidification and heavy metal loading of the lakes parallels air pollution gradients. Shift in cladoceran remains are contemporary with acidification, preceding elimination of fishes.     

Effects of increased salinity on the diatom assemblage in Fonda Lake,Michigan

A salt storage facility has been located adjacent to Fonda Lake since 1953. In February 1981 a core was taken from the profundal sediments of the lake and analyzed to determine the effects of salt perturbation on the diatom community over a 32-year period. Diatom assemblages from different levels were compared using multivariate techniques including cluster analysis and principal component analysis. Shifts in diatom composition related to salinification were revealed most clearly by subdominant taxa. Five distinct groups of diatom taxa were found to correspond with 5 depth intervals. The diatom component of the lake up to 1960 included two groups of taxa which were alkaliphilous and chloride indifferent. A reduction in species diversity beginning in 1960 may indicate a salt effect. By 1968, when diversity reached a minimum, a variety of halophilic taxa (including Diatoma tenue, Navicula gregaria and Synedra fasciculata) attained their highest relative abundances. At the top of the core, diversity increased slightly and some halophilic taxa decreased in relative abundance, which suggests a possible decrease in salt loading to the lake.     

Material transfer from water to sediment in Florida lakes

We describe statistical relationships between chemical aspects of surficial sediments from 34 Florida lakes, and trophic state of the overlying waters. Trophic state is expressed by Carlson's TSI-Chl a. The objectives of the study are two-fold: 1) to understand processes that govern the transfer of material across the mudwater interface, and 2) to develop transfer functions for inferring historical TSI measures in chemically analyzed, ²¹⁰Pb-dated cores. Simple regression of organic matter content or nutrient (C, N, P) concentration in surface sediments vs. TSI yields nonsignificant or weak positive correlations. However, using a novel application of ²¹⁰Pb assay, net accumulation rates of the materials are estimated and show a better correlation with trophic state. Cation (Ca, Mg, Fe, K) and sulfur concentrations in surface muds are poorly related to corresponding TSI's. Net accumulation rates of these elements (Ca, Mg, Fe, K, S) are positively correlated (P < 0.01) with TSI-Chl a. Chemical data from the Florida surficial sediment survey suggest that inferred levels of past trophic state should be based on net accumulation rates of chemical constituents rather than their simple sediment concentrations.     

Biogenic silica and phosphorus accumulation in sediments as indices of eutrophication in the Laurentian Great Lakes

Biogenic silica (BSi), total phosphorus (TP), and biologically available phosphorus (AVP) were measured in short cores from Lake Michigan, Lake Erie, and Lake Ontario. Peaks in BSi concentration and peaks in BSi:TP or BSi:AVP ratios provided stratigraphic signals of water column silica (Si) depletion as a response of increased diatom production to P enrichment and decreased diatom production resulting from silica depletion. By contrast the stratigraphic record of P accumulation provided very weak signals of the historical nutrient enrichment in the water column. These results indicate that system P recycling has a higher rate constant than Si recycling and, as a consequence, that relatively small levels of P enrichment can increase diatom production and sedimentation eventually causing Si depletion and Si-limited diatom production in the water mass.     

Spatial and temporal patterns of sediment and nutrient accumulation in shallow lakes of the Upper St. Johns River Basin, Florida

We used paleolimnological methods to investigate spatial and temporal patterns of bulk sediment and nutrient (C, N, P) accumulation in Lakes Hell n Blazes (A = 154 ha, zmax = 240 cm), Sawgrass (A = 195 ha, zmax = 157 cm) and Washington (A = 1766 ha, zmax = 322 cm), in the Upper St. Johns River Basin, Florida. The study was designed to evaluate long-term changes in sedimentation and nutrient storage in the basin, and was one component of a larger project addressing flood control, wetland restoration, and water quality improvement. These three study lakes are wide, shallow waterbodies in the upper reaches of the St. Johns River channel. Sediment mapping indicates soft, organic deposits are distributed uniformly throughout Lakes Hell n Blazes and Sawgrass. In contrast, much of Lake Washington is characterized by sandy bottom, and organic sediment is largely restricted to the north end of the lake. Lakes Hell n Blazes and Sawgrass are effective sediment traps because dense submersed macrophytes and their associated epiphytes reduce flow velocity, intercept suspended particles, and utilize dissolved nutrients. Abundant Hydrilla, combined with short fetch, prevents resuspension and downstream transport of sediments. Larger Lake Washington is probably wind-mixed and resuspended organic sediments are redeposited to downstream sites. 210Pb-dated sediment cores show that organic sediment accumulation began in all three lakes before 1900, but that bulk sediment and nutrient accumulation rates have generally increased since then. The increases are probably attributable, in part, to anthropogenic activities including 1) hydrologic modifications that reduced flow rates in the channel, 2) discharge of nutrient-rich waters from urban, agricultural and ranching areas, and, 3) introduction and periodic herbicide treatment of the exotic macrophytes Eichhornia and Hydrilla.     

Hemispheric‐scale patterns of climate‐related shifts in planktonic diatoms from North American and European lakes

A synthesis of over 200 diatom‐based paleolimnological records from nonacidified/nonenriched lakes reveals remarkably similar taxon‐specific shifts across the Northern Hemisphere since the 19th century. Our data indicate that these diatom shifts occurred in conjunction with changes in freshwater habitat structure and quality, which, in turn, we link to hemispheric warming trends. Significant increases in the relative abundances of planktonic Cyclotella taxa (P<0.01) were concurrent with sharp declines in both heavily silicified Aulacoseira taxa (P<0.01) and benthic Fragilaria taxa (P<0.01). We demonstrate that this trend is not limited to Arctic and alpine environments, but that lakes at temperate latitudes are now showing similar ecological changes. As expected, the onset of biological responses to warming occurred significantly earlier (P<0.05) in climatically sensitive Arctic regions (median age=ad 1870) compared with temperate regions (median age=ad 1970). In a detailed paleolimnological case study, we report strong relationships (P<0.005) between sedimentary diatom data from Whitefish Bay, Lake of the Woods (Ontario, Canada), and long‐term changes in air temperature and ice‐out records. Other potential environmental factors, such as atmospheric nitrogen deposition, could not explain our observations. These data provide clear evidence that unparalleled warming over the last few decades resulted in substantial increases in the length of the ice‐free period that, similar to 19th century changes in high‐latitude lakes, likely triggered a reorganization of diatom community composition. We show that many nonacidified, nutrient‐poor, freshwater ecosystems throughout the Northern Hemisphere have crossed important climatically induced ecological thresholds. These findings are worrisome, as the ecological changes that we report at both mid‐ and high‐latitude sites have occurred with increases in mean annual air temperature that are less than half of what is projected for these regions over the next half century.     

Non-siliceous algae in a five meter core from Lake Kinneret (Israel)

The composition and succession of non-siliceous algae, studied in a five meter core from Lake Kinneret (Israel), are described. Only Chlorophyta species were recorded, probably due to the standard palynological sample processing which was used. In the lower part of the core, from the bottom to 300 cm (interval 5500–2500 years B.P.), Botryococcus braunii was the only common alga. Relevant changes in algal diversity and abundance occur at 300 cm. Many species of green algae were recorded for the first time (Pediastrum, Scenedesmus, Coelastrum, etc.). These changes may be related to an increase in nutrient concentration as a consequence of cultural disturbance. In the interval 300-0 cm, a succession of Pediastrum species is followed. The recovered green algae are extant in the present plankton of Lake Kinneret. They also constitute an important part of the algae found in the profundal sediments today.     

The chironomid fauna of four shallow,humic lakes and their representation by subfossil assemblages in the surficial sediments

A comparison of the present chironomid fauna of four shallow, humic lakes with the subfossils of surficial sediments is made. The chemistry and benthos of the lakes investigated varied greatly. Results indicate a reasonable correlation between biocenoses and thanatocenoses, but a marked under-representation ofProcladius is apparent. It is suggested that, at least in shallow, humic lakes,Procladius be excluded in attempts to reconstruct chironomid communities from subfossils.     

Diatom assemblages and their relationship to environmental variables in lakes from the boreal forest-tundra ecotone near Yellowknife,Northwest Territories,Canada

The relationship between diatom (Bacillariophyceae) taxa preserved in surface lake sediments and measured limnological and environmental variables in 22 lakes near Yellowknife (N.W.T.) was explored using multivariate statistical methods. The study sites are distributed along a latitudinal gradient that includes a strong vegetational gradient of boreal forests in the south to arctic tundra conditions in the north. Canonical correspondence analysis (CCA) revealed that lakewater concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) each accounted for independent and statistically significant proportions of variation in the distribution of diatom taxa. Weighted-averaging (WA) models were developed to infer DIC and DOC from the relative abundances of the 76 most common diatom taxa. These models can now be used to infer past DIC and DOC concentrations from diatom assemblages preserved in sediment cores of lakes in the Yellowknife area, which may provide quantitative estimates of changes in lakewater chemistry related to past vegetational shifts at treeline.     

The effect of land use on the diatom communities in lakes

Data on recenr diatom community structure and relevant environmental characteristics from the lakes and their catchments have been collected from 151 oligotrophic lakes in eastern Finland. The pattern of frequency distribution of diatoms as a function of environmental variables, including land use in the catchment, differs between diatom taxa and indicates the optimum conditions and amplitude of occurrence for particular species. This kind of study should lead to increased understanding of the environmental requirements of diatom species and will be useful in the interpretation of historical changes in lakes as well as in forecasting possible future changes.     

Paleolimnology of mid-elevation lakes in the Kenya Rift Valley

Three small lakes of the Kenya Highlands have been subject to intermittent paleoenvironmental investigation for more than 50 years. In this report, the stratigraphic, core-based history of Lake Naivasha (elev. 1 890 m) is extended backward to 20 000 B.P. and is related to less complete 30 000-year core stratigraphies from nearby lakes Nakuru and Elmenteita. The paleolimnological reconstructions, primarily diatom-based, include geochemical parameters and the first ostracode stratigraphies from tropical Africa. The analyses from all three lakes converge on a history of low and fluctuating water levels over approximately the past 4 500 years, preceded by a major Holocene high stand (ca. 10 000-6 000 B.P.) and a brief earlier rise centered on 12 500 B.P. The lakes were mostly very low during the late Pleistocene (13 000–30 000 B.P.), though modestly elevated levels may have occurred early in this time period. The ostracode record, comprising at least 15 species, was relatively continuous in the Nakuru core but was restricted to sharply demarcated regions in cores from the other lakes.