Effects of hypolimnetic oxygenation on water quality: results from five Danish lakes

Stratified eutrophic lakes often suffer from hypolimnetic oxygen depletion during summer. This may lead to low redox conditions and accumulation of phosphate and ammonia in the hypolimnion. Hypolimnetic oxygenation has been used as a lake management strategy to improve the water quality in five eutrophic dimictic Danish lakes where oxygenation was conducted for 4–20 years. In one lake, the hypolimnetic oxygen concentration clearly improved by oxygenation, whereas the other four lakes still exhibited low mean summer levels (<2.2 mg O₂ l⁻¹). Oxygenation generally increased the hypolimnetic water temperature by 0.5–2°C, but in one lake it increased by 4–6°C. In all lakes, oxygenation significantly reduced the hypolimnetic concentrations of phosphorus and ammonia during stratification. The accumulation of phosphorus and ammonia typically decreased by 40–88%. In two lakes oxygenation was stopped for 1–2 years and here hypolimnion concentrations of both phosphorus and ammonia increased again. Surface water quality only improved in one lake, but was likely also influenced by simultaneously occurring changes in external nutrient loading. Overall, it is concluded that hypolimnetic oxygenation reduces the hypolimnetic accumulation of phosphorus and ammonia and may prevent anoxia in the deeper parts of the lake. However, long-term oxygenation is required and it is uncertain whether the overall lake water quality can be improved by oxygenation. Reduction of the external nutrient loading is still essential to improve lake water quality. Handling editor: Luigi Naselli-Flores     

Relationship between hypolimnetic phosphorus and iron release from eleven lakes in Maine, USA

We studied five eutrophic (high phosphorus) and six mesotrophic/oligotrophic (low phosphorus) lakes in Maine, USA, all of which are dimictic and develop anoxic hypolimnia during stratification. The lakes were sampled during the stratified period from May to September 1999. Late summer hypolimnetic total phosphorus (P) concentrations in the high-P lakes ranged from 185 to 460ppb; epilimnetic total P increased up to 30ppb from the spring to the fall overturn. During the same period, the low-P lakes had hypolimnetic total P concentrations in the range of 6–19ppb.Individual high-P lakes demonstrated strong temporal correlations between aqueous hypolimnetic dissolved Fe and total P concentrations (R ²0.88) with an average molar Fe:P ratio of 11.9±4.2. For the combined data, the high-P lakes exhibited strong correlation between hypolimnetic Fe and P concentrations (R ²=0.82). The low-P lakes, however, did not show a good correlation between the hypolimnetic Fe and P concentrations. Among the low-P lakes two lakes had hypolimnetic Fe fluxes comparable to the Fe fluxes of the high-P lakes. These two lakes had considerably higher hypolimnetic Fe:P ratios than all other lakes studied here. There were no significant differences in surface sediment Fe(III) or P fractions that correlated with the differences in the relationship between aqueous concentrations of Fe and P in these two outlier low-P lakes. A model for the generation of hypolimnetic acid neutralization capacity (ANC) was developed based on microbially-catalyzed reduction of Fe(III) hydroxide, Mn(IV) oxide and sulfate. Reduction of Fe(III) hydroxide was the most important contributor to the increase in the hypolimnetic ANC in all high-P and the two outlier lakes. Assuming that all hypolimnetic P was due to the reduction of Fe(III) hydroxide by bacteria and sulfide, average summer hypolimnetic P flux for each lake was predicted using the sediment reducible Fe(III):P ratio. The observed and predicted average P fluxes in the high-P lakes corresponded reasonably, suggesting that in these lakes internal P release is closely related to the reduction of Fe(III) hydroxide. Other release or sequestration mechanisms may operate for the release and availability of P in the low-P lakes.     

High variability in iron-bound organic carbon among five boreal lake sediments

Being both stable carbon sinks and greenhouse gas sources, boreal lake sediments represent significant players in carbon (C) cycling. The release of dissolved organic carbon (DOC) into anoxic water is a widespread phenomenon in boreal lakes with impact on sediment C budgets. The association of OC with iron (Fe) is assumed to play an important role for this anoxic OC release via the dissimilatory reduction of Fe, but also to influence the stabilization of OC in sediments. To investigate the role of Fe–OC association for OC dynamics in different boreal lake sediments, we compared the content of Fe-bound OC [Fe–OC, defined as citrate bicarbonate dithionite (CBD) extractable OC] and the extent of reductive dissolution of solid-phase Fe and OC at anoxia. We found high among-lake variability in Fe–OC content, and while the amount of Fe–OC was high in three of the lakes (980–1920 µmol g^?1), the overall contribution of Fe–OC to the sediment OC pool in all study lakes was not higher than 11%. No linkages between the amount of the Fe–OC pool and lake or sediment characteristics (e.g., pH, DOC concentration, sediment OC content, C:N ratio) could be identified. The observed release of OC from anoxic sediment may be derived from dissolution of Fe–OC in the lake sediments with high Fe–OC, but in other lake sediments, OC release during anoxia exceeded the sediment Fe–OC pool, indicating low contribution of reductive Fe dissolution to OC release from these lake sediments. The range of the investigated boreal lakes reflects the high variability in the size of the sediment Fe–OC pool (0–1920 µmol g^?1) and CBD-extractable Fe (123–4050 µmol g^?1), which was not mirrored in the extent of reductive dissolution of Fe (18.9–84.6 µmol g^?1) and OC (1080–1700 µmol g^?1) during anoxia, suggesting that Fe-bound OC may play a minor role for sediment OC release in boreal lakes. However, studies of redox-related OC cycling in boreal lake sediments should consider that the amount of Fe–OC can be high in some lakes.     

Persistent internal phosphorus loading during summer in shallow eutrophic lakes

Nutrient availability, in particular of phosphorus (P), is a key factor for the structure and functioning of shallow lakes, and not least the sediment plays an important role by acting as both a nutrient source and sink. We used 21 years of monthly mass balance and lake water data from six shallow (mean depth = 1.2–2.7 m) and fast flushed (mean hydraulic retention time = 0.6–2.6 months) eutrophic Danish lakes (mean summer P concentrations ranging from 0.09 to 0.61 mg/l) to investigate long-term trends in yearly and seasonal patterns of P retention. To one of the lakes, the external P input was reduced by 70% in the early 1990s, whereas none of the other lakes have experienced major changes in external P loading for more than 20 years. All lakes showed a distinct seasonal pattern with high P concentrations and typically negative P retention during summer (up to ?300% of the external loading from May to August). During winter, P retention was overall positive (up to 50% of the external loading from December to April). Internal P loading from the sediment delayed lake recovery by approximately 10 years in the lake with the most recently reduced external loading, but in all the lakes net release of P from the sediment occurred during summer. P release in the six lakes has not abated during the past decade, indicating that the sediment of eutrophic and turbid shallow lakes remains a net source of P during summer. The seasonal variations in P retention became more pronounced with increasing P levels, and retention decreased with increasing temperature, but increased if clear water conditions were established.     

Laboratory-determined Phosphorus Flux from Lake Sediments as a Measure of Internal Phosphorus Loading

Eutrophication is a water quality issue in lakes worldwide, and there is a critical need to identify and control nutrient sources. Internal phosphorus (P) loading from lake sediments can account for a substantial portion of the total P load in eutrophic, and some mesotrophic, lakes. Laboratory determination of P release rates from sediment cores is one approach for determining the role of internal P loading and guiding management decisions. Two principal alternatives to experimental determination of sediment P release exist for estimating internal load: in situ measurements of changes in hypolimnetic P over time and P mass balance. The experimental approach using laboratory-based sediment incubations to quantify internal P load is a direct method, making it a valuable tool for lake management and restoration.Laboratory incubations of sediment cores can help determine the relative importance of internal vs. external P loads, as well as be used to answer a variety of lake management and research questions. We illustrate the use of sediment core incubations to assess the effectiveness of an aluminum sulfate (alum) treatment for reducing sediment P release. Other research questions that can be investigated using this approach include the effects of sediment resuspension and bioturbation on P release.The approach also has limitations. Assumptions must be made with respect to: extrapolating results from sediment cores to the entire lake; deciding over what time periods to measure nutrient release; and addressing possible core tube artifacts. A comprehensive dissolved oxygen monitoring strategy to assess temporal and spatial redox status in the lake provides greater confidence in annual P loads estimated from sediment core incubations.     

Sediment diatom assemblages and composition of pore-water dissolved organic matter reflect recent eutrophication history of Lake Peipsi (Estonia/Russia)

A paleolimnological approach was used for the assessment of the recent eutrophication history and identification of possible reference conditions in the large, shallow, eutrophic Lake Peipsi. Lake Peipsi is the fourth largest lake by area, and the largest transboundary lake in Europe, being shared between Estonia and Russia. Lake Peipsi has been anthropogenically impacted over a longer time-scale than that covered by instrumental limnological monitoring. The ²¹⁰Pb record and down-core distribution of fly-ash particles in the 40-cm core from the middle part of the lake suggest 130 years of sediment accumulation. Diatom assemblages indicate alkaline mesotrophic conditions and a well-illuminated water column, sediment pore-water fluorescence index values suggest low autochthonous productivity and a stable aquatic ecosystem similar to natural reference conditions during the second half of 19th and early 20th century. Near-synchronous stratigraphic changes including the expansion of the eutrophic planktonic diatom Stephanodiscus parvus, the appearance of new species associated with eutrophic lakes and the decrease in the relative abundance of littoral diatoms, together with changes in the fluorescence properties of sediment pore-water dissolved organic matter, imply increased nutrient availability, enlarged phytoplankton crops, reduced water-column transparency and the onset of human-induced disturbances in the lake since the mid-20th century. The most conspicuous expansion of eutrophic planktonic diatoms and maximum concentration of siliceous microfossils occur simultaneously with changes in the fluorescence indexes of pore-water dissolved organic matter, indicating a pronounced increase in the contribution of autochthonous organic matter to the lake sediment. This implies that nutrient loading and anthropogenic impact was at a maximum during the 1970s and 1980s. Sedimentary diatom flora may reflect a reduction of phosphorus loading since the 1990s. However, the absolute abundance of planktonic diatoms and sediment pore-water fluorescence index values vary greatly implying that the lake ecosystem is still rather unstable.     

Phosphorus inactivation in a eutrophic lake by the direct addition of ferric aluminium sulphate: impact on iron and phosphorus

SUMMARY.

• 1 Following addition of ferric aluminium sulphate (FAS) in February 1980 to a eutrophic lake, White Lough, Northern Ireland, the cycles of phosphorus (P) and iron (Fe) over 34 months were compared to those in the 18 months preceding the application.
• 2 During the summer of 1980, P release to the hypolimnion was reduced by 92% compared to 1979 but by 1982 had returned to pre-treatment levels. Phosphorus budget calculations for the post- treatment period indicated that P was accumulating in the sediment at a rate sufficient to permit the recovery of P release.
• 3 In the three summers following FAS addition, Fe released into the hypolimnion increased by 56–71% compared to 1979 but this increase represented less than 10% of Fe added in the FAS.
• 4 Prior to FAS treatment, hypolimnetic Fe and P were mixed throughout the lake at overturn, resulting in high winter concentrations, but after treatment, overturn was accompanied by large reductions in the lake Fe and P content due to precipitation. As a result, overwintering P concentrations in 1980/81 and 1981/82 were in the range 31–53μg P1^-1 compared to 111–194μg P1^-1in 1978/79 and 1979/80.
• 5 The reduction of hypolimnetic P release in 1980 and 1981 and lower overwintering P had little impact on summer epilimnetic P concentrations which for 1979–82 were 51, 36, 47 and 48μg P1^-1respectively. Phosphorus concentrations in the epilimnion were raised between April and June in each year by a release of P into the epilimnion. August and September was the only period when reductions in hypolimnetic P were reflected in lower P concentrations in the epilimnion.

     

Sediment phosphorus cycling in a large shallow lake: spatio-temporal variation in phosphorus pools and release

Sediment and water column phosphorus fractions were recorded monthly for one year (April 2004–April 2005) in a shallow lake recovering from nutrient pollution (Loch Leven, Scotland). Equilibrium phosphate concentration (EPC0) and gross sediment phosphorus (P) release rates were estimated from laboratory experiments. Pore water and organic P pools were lowest during warm water periods whereas bottom water P was lowest during cold water periods. Reductant-soluble, organic, metal oxide-adsorbed, residual and sediment total phosphorus pools all varied significantly with overlying water depth. Short-term, high magnitude, redox initiated P release events occurred in late summer and winter as a result of anoxic sediment conditions. Lower magnitude long-term release conditions were maintained for most of the year, most likely as a result of organic P cycling and maintenance of high concentration gradients between the pore and bottom water P pools. Estimates of summer P uptake/release rates, across an intact sediment-water interface, suggested that maximum gross internal release was ~12 mg SRP m⁻² lake surface area d⁻¹ with EPC0 values ranging between 180 and 270 μg P L⁻¹. This study highlights the biological mediation of internal loading in shallow eutrophic lakes, and in particular, the role of sediment algae in decreasing, and sediment bacteria in enhancing, sediment P release.     

Effectiveness of Al,Ca, and Fe salts for control of internal phosphorus loading in shallow and deep lakes

Internal P loading can maintain high P concentrations and delay eutrophic lake recovery following abatement of external loading. Sediment P inactivation with Al salts has been shown to provide long-term (5–14 years) control of sediment P release; long-term effectiveness of Fe and Ca salts has not been reported. Al toxicity problems are possible unless pH is maintained in the 6–8 range. Vertical transport of hypolimnetic P is unlikely in small, deep, dimictic lakes (\-ZA₀ > 8), and effectiveness of P inactivation in lowering their mid-summer epilimnetic P has not been demonstrated. To date, P inactivation has been found to be most effective in improving trophic state in shallow, softwater, polymictic lakes where control of sediment P release affects the entire water column. Abatement of external loading, where necessary, is essential for a successful P inactivation treatment.     

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.     

Temporal and spatial variations of aquatic environmental characteristics and sediment bacterial community in five regions of Lake Taihu

Sediment bacterial community and their relation with environmental factors were investigated in the five different trophic status lake regions sediment, Meiliang Bay, Wuli Lake, Gonghu Bay, Western Lake Taihu and Xukou Bay in a large, shallow, eutrophic freshwater lake (Lake Taihu, China). Water and surface sediment samples were collected at 35 sampling sites in January 2014 (winter) and July 2015 (summer). The physicochemical characterization showed that there were obvious changes in the trophic status and eutrophic index of five lake regions, which was mainly due to the difference of organic matter source. Based on the analysis of aquatic environmental characteristics, the organic nitrogen or nitrate nitrogen was the main storing form in the overlying water of five lake regions. In addition, nitrate nitrogen in pore water was lower than in overlying water, while ammonia nitrogen in pore water was higher than in overlying water. According to the DGGE profiles, temporal and spatial variations of bacterial community were apparent. Bacterial diversity was higher in summer than in winter and increased with the decrease in the lake region trophic status. The dendrogram of the bacterial community similarities revealed that samples were almost all grouped into two defined clusters (summer and winter), which indicated that season rather than region was the dominant factor. Canonical correspondence analysis demonstrated that ammonia nitrogen and nitrate–nitrite nitrogen in the sediment and pore water, organic matter and temperature significantly influenced the sediment bacterial community in the five lake regions.     

Deepwater sediments and trophic conditions in Florida lakes

Sediment cores were taken from near maximum depth in 15 Florida lakes representing a wide range of trophic conditions. Chemical analyses of surface sediments showed Al, Fe, and Ca to be the most abundant elements in all samples, and the ratio of Al to Ca to be smaller for eutrophic lakes. Sediment organic matter increased with trophic state, as did the degree to which it was enriched in nitrogen. Corresponding sediment C/N ratios decreased with increasing lake trophic state and showed significant negative correlation with chlorophylla, total N, and total P in the water column. Concentrations of sedimentary chlorophyll derivatives showed some relation to trophic state but differences in basin morphometry hinder its use as an inter-lake index of chlorophyll production.     

Nitrogen mass balances and denitrification rates in central Ontario Lakes

Nitrogen mass balances for seven unproductive lakes and 20 forested catchments in central Ontario were measured between 1977 and 1989. Average annual lake denitrification rates calculated with the N/P ratio method were strongly correlated with summer anoxic factor (extent of surficial sediment anoxia) whereas denitrification rates calculated with a²¹⁰Pb sediment N accumulation method were poorly correlated with the anoxic factor suggesting that the N/P method is superior. Substantial denitrification occurred in all lakes — an average of 36% of TN inputs or 75% of the net gain. On a regional area-weighted basis, 67% of bulk atmospheric TN deposition was stored or denitrified terrestrially, 12% was denitrified in lakes, 4% was stored in lake sediments, and 17% was exported from lakes. N/P ratios were generally less in streams than in precipitation suggesting preferential N retention in catchments, whereas the N/P ratios in lake outputs were slightly higher than lake input ratios, suggesting preferential P retention in lakes. This is consistent with the notion that P-limited lakes can exist adjacent to N-limited forests.     

Interannual variations in atmospheric forcing determine trajectories of hypolimnetic soluble reactive phosphorus supply in a eutrophic lake

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Hypolimnetic phosphorus and nitrogen dynamics in a small,eutrophic lake with a seasonally anoxic hypolimnion

In situ estimates of sediment nutrient flux are necessary to understand seasonal variations in internal loading in lakes. We investigated the sources and sinks of nutrients in the hypolimnion of a small (0.33 km²), relatively shallow (18 m max. depth), eutrophic lake (Lake Okaro, New Zealand) in order to determine changes in sediment nutrient fluxes resulting from a whole lake sediment capping trial using a modified zeolite phosphorus inactivation agent (Z2G1). Sediment nutrient fluxes in the hypolimnion were estimated as the residual term in a nutrient budget model that accounted for mineralisation of organic nutrients, nutrient uptake by phytoplankton and mixing, nitrification, adsorption/desorption and diffusion of dissolved nutrients at the thermocline. Of the total hypolimnetic phosphate and ammonium fluxes during one period of seasonal stratification (2007–08), up to 60 and 50%, respectively, were derived from the bottom sediments, 18 and 24% were due to mineralisation of organic species, 36 and 28% were due to phytoplankton uptake and 9 and 6% were from diffusion across the thermocline. Adsorption/desorption of phosphate to suspended solids and nitrification were of minor (<8%) importance to the total fluxes. Any reduction in sediment nutrient release by Z2G1 was small compared with both the total sediment nutrient flux and the sum of other hypolimnetic fluxes. Uneven sediment coverage of Z2G1 may have been responsible for the limited effect of the sediment capping layer formed by Z2G1.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

Seasonal changes in the biochemistry of lake seston

1. The quantity of seston was measured and the elemental carbon, nitrogen and phosphorus (C, N, P) and biochemical composition (carbohydrate, protein, lipid) of the < 53 μm size fraction in three temperate lakes during one year was analysed. The lakes differed in nutrient concentration and were characterized as oligotrophic, mesotrophic and eutrophic. Linear regression analyses defined associations between seston composition and either lake trophic status, depth or season. 2. The concentration of particulate organic seston was greatest during spring and autumn and lowest during the clear water period in early summer. Seasonal patterns in seston elemental and biochemical percentage composition (quality) were observed to be independent of differences in seston quantity. 3. Concentrations of seston C, N and P were high in most cases in the spring and autumn and low in summer. Concentrations of P were particularly high during late summer and early autumn in the metalimnion, perhaps because of recovery of P from anaerobic sediments and hypolimnetic waters. Because seston C and N did not increase as markedly as P, C : P and N : P ratios both declined in the autumn. Primary production was thought to be co-limited by N and P in all three of these lakes; however, the data suggested that N might be more important as a major limiting nutrient in the eutrophic lake as the metalimnion increased in depth in late summer and autumn. 4. Concentrations of protein, carbohydrate, polar lipid and triglyceride generally increased with lake type as expected (greatest in the eutrophic lake), but showed no relationship with water depth. As the year progressed, no significant changes were measured in protein and carbohydrate concentrations; however, the concentration of polar lipid decreased and triglyceride increased significantly with time of year. 5. The biochemical composition of seston varied during the year and among lakes; for example, in Lake Waynewood the proportion of protein composing the seston (percentage protein by weight) varied from < 10% to > 40%. No statistically significant patterns in the percentage protein or carbohydrate were found. However, the proportion of seston comprised of triglyceride decreased with lake type and increased during the year; whereas the proportion of seston as polar lipid increased with lake type and decreased during the year. Triglyceride comprised most of the lipid. Both protein : lipid and protein : carbohydrate ratios tended to be greatest in summer and lowest in the spring and autumn. 6. Relationships between samples and biochemical composition analysed by Canonical Correspondence Analysis (Canoco) indicated similar patterns in seasonal changes in seston biochemistry for the three lakes, with samples separated primarily by vectors for lake type (oligotrophic to eutrophic) and the percentage polar lipid (proportion of total lipid) and secondarily by vectors for date and water depth (epilimnion or metalimnion). 7. These seasonal biochemical changes in the seston food base were compared with biochemical changes known to occur in algae grown under N-or P-limited conditions in the laboratory, and the resultant quality of this algal food for suspension-feeding consumers (zooplankton). It was concluded that zooplankton were likely to be physiologically challenged by these distinct seasonal shifts in the quality of lake seston.     

Distribution of Sediment Phosphorus Fractions in Hypertrophic Strongly Stratified Lake Verevi

Lake Verevi is a hypertrophic and strongly stratified (partly meromictic) small temperate lake. Vertical distribution of sediment phosphorus fractions as well as iron, manganese, organic matter and calcium carbonate of the deep bottom sediment was determined. The study focused on the ecologically important layer of the sediment [<20(45) cm]. In the uppermost layers of the sediment, NaOH-NRP (organic P) dominated while HCl-RP (apatite-P) became dominant in some deeper layers below 7 cm. Extremely high concentrations of labile phosphorus fraction (NH₄Cl-RP) indicated the low binding capacity of phosphorus by lake sediment. Due to sediment and hypolimnion anoxia, the internal load of phosphorus in this lake is most likely. Potentially mobile phosphorus fractions (NH₄Cl-RP, BD-RP, NaOH-NRP) formed 301 kg in upper 10 cm thick sediment layer of hypolimnetic bottom sediment (40% of lake bottom area).     

A palaeolimnological record of human disturbance from Harvey's Lake, Vermont: geochemistry, pigments and diatoms

• 1 Stratigraphic analyses of inorganic geochemistry, pigments and fossil diatoms in a 0.7 m core of profundal sediments are used to reconstruct the limnological history of Harvey's Lake, Vermont, over the last 1000 years. The lake is moderately productive, deep (44 m) and clear, and the phytoplankton today is dominated by the blue-green alga, Oscillatoria rubescens. Sedimentary pigments unique to blue-green algae, oscillaxanthin and myxoxanthophyll, provide a detailed history of changes in the O. rubescens population. Accurate sediment chronology is derived from ²¹⁰Pb, ¹³⁷Cs and ¹⁴C dating and from the stratigraphy of pollen and sawmill wastes.
• 2 Primary production increased in Harvey's Lake in 1780 following European settlement and again after 1945, as shown by greater accumulation of sedimentary pigments and diatom frustules, and changes in fossil algal assemblages. Blue-green algae first appeared in abundance about 1945, indicating nutrient enrichment from dairy wastes and shoreline development. Increased deposition of elements associated with classic minerals also suggests greater soil erosion during both of these intervals.
• 3 Two episodes of increased sedimentary anoxia (1820–1920 and 1945–present) are marked in the sedimentary record by enhanced pigment preservation, changes in authigenic Fe and Mn stratigraphy,’and the development of laminated sediments. The earlier episode of oxygens depletion is correlated with the discharge of sawmill wastes into the lake, and the later episode is associated with increased primary production.
• 4 Based on these data a new model for Fe and Mn sediment stratigraphy is proposed for lakes that do not undergo complete hypolimnetic anoxia.
• 5 Fine-scale resolution of recent diatom and oscillaxanthin stratigraphy provides historical evidence for a long-term negative interaction between diatom and blue-green algal populations in Harvey's Lake.

     

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).