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Experimental removal of early instar black fly larvae from 3 oligotrophic lake-outlet streams in southern Quebec showed that larvae reduced phytoplankton biomass, but had little effect on bacterioplankton biomass. The rates of decline in bacterioplankton and phytoplankton biomass with distance downstream from 6 lake outlets were determined before the insecticide Bacillus thuringiensis var. israelensis was used to remove black fly larvae from 3 of the streams. The rates of decline in bacterioplankton and phytoplankton biomass were measured again after the larvae were removed, and the differences in the rates of decline between before and after were used as indicators of the rates of biomass ingestion by the black fly larvae. The proportion of bacterioplankton ingested/m stream length was

Characterizing dissolved organic matter (DOM) composition remains a major unresolved problem in aquatic ecology and is one of the key impediments to developing a good understanding of DOM production and consump- tion by heterotrophic bacteria. Fluorescence spectroscopy has been proposed as a promising method for character- izing DOM, but few links have been demonstrated between DOM fluorescence and DOM composition or the processes affected by DOM composition. In 28 southern Quebec lakes, tryptophan-like DOM fluorescence (T- FDOM) was found to be a much better descriptor of rates of heterotrophic bacterial metabolism than dissolved organic carbon, describing 52%, 44%, 51%, and 55% of the variability in bacterial production, bacterioplankton respiration, total bacterial carbon consumption, and total plankton community respiration, respectively. In addition, evidence from a series of bacterial regrowth cultures suggests that T-FDOM represents a product of bacterial activity as well as, to a lesser extent, a bioavailable substrate. Our results instead raise the intriguing possibility that T- FDOM concentration reflects a balance between its production and consumption by bacteria. We demonstrate here that fluorescence spectroscopy can be used to identify a highly dynamic fraction of DOM related to bacterial metabolism in lakes.

Submerged macrophytes are a major component of freshwater ecosystems, yet their net effect on water column phosphorus (P), algae, and bacterioplankton is not well understood. A 4-month mass-balance study during the summer quantified the net effect of a large (∼5.5 ha) undisturbed macrophyte bed on these water-column properties. The bed is located in a slow-flowing (0.05–0.1 cm s−1) channel between two lakes, allowing for the quantification of inputs and outputs. The P budget for the study period showed that, despite considerable short-term variation, the macrophyte bed was a negligible net sink for P (0.06 mg m−2 day−1, range from −0.76 to +0.79 mg m−2 day−1), demonstrating that loading and uptake processes in the weedbed roughly balance over the summer. Chlorophyll a was disproportionately retained relative to particulate organic carbon (POC), indicating that the algal component of the POC was preferentially trapped. However, the principal contribution of the weedbed to the open water was a consistent positive influence on bacterioplankton production over the summer. Conservative extrapolations based on measured August specific exports (m−2 day−1) of P and bacterial production exiting the weedbed applied to five regional lakes varying in lake morphometry and macrophyte cover suggest that even in the most macrophyte dominated of lakes (66% cover), P loading from submerged weedbeds never exceeds 1% day−1 of standing epilimnetic P levels, whereas subsidization of bacterioplankton production can reach upward of 20% day−1. The presence of submerged macrophytes therefore differentially modifies algae and bacteria in the water column, while modestly altering P dynamics over the summer.

The relationships between long-term (;115-year) phosphorus and sediment accumulation and submerged macrophyte biomass were determined both within a submerged macrophyte bed and among beds in Lake Memphremagog, Quebec, Canada. Strong relationships between present submerged macrophyte biomass and littoral-zone sediment and phosphorus accumulation rates both within and among macrophyte beds yielded simple models for estimating long-term littoral-zone accumulation rates. Mean sediment particle size at the depth of maximum submerged macrophyte biomass, estimated from water content, was comparable with the mean particle size in profundal zone of Lake Memphremagog, suggesting that energy environments differ little between vegetated littoral zones and profundal zones. The mean total sediment-accumulation rate of 1.2 mm yr 21 in the macrophyte beds was indistinguishable from average profundal rates of sediment accumulation in North American lakes. Although dense beds accumulated twice as much bulk sediment per unit area (g m 22 yr 21 ) compared with their profundal counterparts, phosphorus accumulation per unit bulk sediment in macrophyte beds with a biomass of 1,000 g m 22 (wet weight) was one sixth that measured in profundal sediments, pointing to large postdepositional losses of sedimented phosphorus (.70%) from macrophyte beds. Finally, extrapolation of the present results to five nearby lakes suggests, based on still-limited data, that, in lakes in which half of the sediment surface is colonized by submerged macrophytes, the littoral zone accounts for roughly one third of whole-lake phosphorus and two thirds of whole-lake bulk sediment accumulated annually.

The effect of submerged macrophytes on interactions among epilimnetic phosphorus, phytoplankton, and heterotrophic bacterioplankton has been acknowledged, but remains poorly understood. Here, we test the hypotheses that the mean summer phytoplankton biomass (chlorophyll a): phosphorus ratios decrease with increased macrophyte cover in a series of nine lakes. Further, we test that both planktonic respiration and bacterioplankton production increase with respect to phytoplankton biomass along the same gradient of increasing macrophyte cover. Increased macrophyte cover was associated with a lower fraction of particulate phosphorus in epilimnia, with total particulate phosphorus declining from over 80% of total phosphorus in a macrophyte free lake to less than 50% in a macrophyte rich lake. Phytoplankton biomass (chlorophyll a) too was lower in macrophyte dominated lakes, despite relatively high levels of total dissolved phosphorus. Planktonic respiration and bacterioplankton production were higher in macrophyte rich lakes than would be expected from phytoplankton biomass alone, pointing to a subsidy of bacterioplankton metabolism by macrophyte beds at the whole lake scale. The results suggest that the classical view of pelagic interactions, which proposes phosphorus determines phytoplankton abundance, which in turn determines bacterial abundance through the production of organic carbon, becomes less relevant as macrophyte cover increases.

Although there are a number of single system and regional studies on the importance of picophytoplankton (PP; 0.2–2 µm), there has been little effort to quantify their importance on a larger scale and to integrate marine and freshwater studies. We developed empirical models from data in the literature to assess the absolute and relative importance of PP in marine and freshwater. The PP biomass (chlorophyll a [Chl a]) increases with trophic status, here represented by the total Chl a, in both marine and freshwater systems. In addition, PP abundance in freshwater, and primary production in marine waters, increases with trophic status. Even so, the relative importance of the PP biomass and primary production declines systematically with increasing trophic status in both marine and freshwater systems. Both the absolute biomass of PP and the relative contribution of PP biomass to the total phytoplankton community biomass are larger in freshwater at any particular value of total Chl a. In eutrophic lakes, which are typically shallow and particularly susceptible to the greatest phytoplankton resuspension and sediment recruitment, the on average small relative contribution of PP biomass appears affected by a relatively high but variable contribution of resuspended microplankton.

As human activities continue to alter the global nitrogen cycle, the ability to predict the impact of increased nitrogen loading to freshwater systems is becoming more and more important. Nitrogen retention is of particular interest because it is through its combined processes (denitrification, nitrogen sedimentation and uptake by aquatic plants) that local and downstream nitrogen concentrations are reduced. Here, we compare the magnitude of nitrogen retention and its components in wetlands, lakes and rivers. We show that wetlands retain the highest proportion of total nitrogen loading, followed by lakes and then rivers. The differences in the proportion of N retained among systems is explained almost entirely by differences in water discharge. Denitrification is the primary mechanism of nitrogen retention, followed by nitrogen sedimentation and uptake by aquatic plants.

We monitored the biomass and distribution of submerged macrophyte communities in five lakes in the Eastern Townships of Quebec, Canada, in two climatically different growing seasons. Early season warm temperatures allowed for much deeper macrophyte colonization (25–170%), an average of 300 g m−2 greater wet weight biomass (g m−2) and a 45–1160% increase in whole lake biomass in warm 1998 compared to a cooler 1997. In contrast, the inter-year variation in underwater light climate had no effect on macrophyte colonization as water column turbidity was generally higher in the warmer year, although irradiance did affect the distribution and biomass within seasons. The positive relationship between phytoplankton and whole system macrophyte biomass observed in this study cautions that the negative relationship in highly eutrophic shallow systems does not appear applicable to deeper systems characterized by lower macrophyte biomass to lake volume ratios. Earlier growing seasons, as predicted by climate change models, would result in greater biomass and distribution of submerged macrophyte communities, thereby modifying the structure and functioning of north temperate lakes. The effect will be a function of lake morphometry and most pronounced in shallow systems.

1. To determine the influence of macrophyte beds on plankton abundance within fluvial lakes of the St Lawrence River, planktonic components (macrozooplankton, heterotrophic bacteria, and phytoplankton as chlorophyll-a [Chl-a]) were sampled in Lake St Francis and Lake St Pierre during summer 1998. We tested the hypothesis that the abundance of planktonic components was higher within macrophyte beds in comparison to the more rapidly flushed open water areas of the fluvial lakes. 2. The large cross channel variation in zooplankton biomass was indeed correlated with the presence of dense beds of submerged macrophytes. Total macrozooplankton biomass was nine-fold greater within the beds (mean = 180 ?g L-1 dry mass) than in either the open water or areas with only sparse vegetation (mean = 20 ?g L-1 dry mass). 3. Chl-a and heterotrophic bacterial abundance were also higher in the beds, but only slightly so. There was no difference in total phosphorus or dissolved organic carbon concentrations between areas of dense vegetation, sparse vegetation or open water. 4. Macrophyte beds on the margins of the fluvial lakes allow the development of high planktonic abundance relative to the fast flowing central channel. Macrozooplankton biomass was much higher at the outflows of the lakes (? 50 ?g L-1 dry mass) in comparison to the inflows (< 20 ?g L-1 dry mass). The increase is due to the transfer of organisms from submerged macrophyte beds into the central channel in the downstream quarter of the two lakes where the marginal littoral waters enter central channel waters. 5. Along rivers, shallow fluvial lakes appear to act as sources of plankton which is exported downstream during years of extensive littoral macrophyte development.

Midsummer planktonic communities were sampled in 1997 along a 425-km reach of the St. Lawrence River between Lake Ontario and Trois-Rivières (Quebec) to describe longitudinal patterns of plankton development. Chlorophyll a (Chl a) quickly declined as water flowed from Lake Ontario into the river. Thereafter, in the reach upstream from Montreal, Chl a was ~1 µg·L-1and total phosphorus (TP) was ~10 µg·L-1. Downstream from Montreal, TP increased to 20-40 µg·L-1and Chl a rose to 2-5 µg·L-1. Although Chl a increased concomitant with TP, phytoplankton may have been light limited at the more turbid downstream stations. Zooplankton biomass decreased from 40 µg dry mass·L-1in Lake Ontario to 16 µg dry mass·L-1at Brockville, 90 km downstream. Zooplankton biomass continued to decline and downstream from Montreal remained -1, despite the increased TP and Chl a. Rotifers dominated the zooplankton community, but as in other temperate rivers, total zooplankton biomass was low (-1). There was no longitudinal pattern in heterotrophic bacterial or flagellate abundance. A comparison of the inflow and outflow biomass of two fluvial lakes showed the lakes to contribute little to the downstream river plankton during summer.

Pharmacokinetic models provide novel approaches in the environmental arena for evaluating toxicological problems. For example, pharmacokinetic parameters play an important and even determinant role in risk assessment and policy making. The present contribution reviews the most basic pharmacokinetic models, their development, and their applications in toxicology. The theory, principles, and data requirements are critically discussed. The fundamental differences among various pharmacokinetic models are compared and contrasted. The techniques and philosophy for model validation are discussed and illustrated. Resume : Les modeles pharmacodynamiques offrent de nouvelles approches au forum environnemental portant sur l'evaluations des problemes toxicologiques. Par exemple, les parametres pharmacodynamiques jouent un role important et meme determinant dans l'evaluation des risques et la definition des politiques. Les auteurs revoient les modeles pharmacodynamiques de base, leur developpement et leurs applications en toxicologie. La theorie, les principes et les besoins en donnees font l'objet d'une discussion critique. Ils comparent et font ressortir les distinctions fondamentales entre les differents modeles pharmacodynamiques. Ils discutent et illustrent enfin les techniques et la philosophie necessaire pour valider ces modeles. Mots-cles : Pharmacodynamiques, toxicologie, simulation, philosophie, statistique, validation. (Traduit par la Redaction) Wen et al. 18

This study quantifies the role of lake morphometry and submerged macrophyte beds on the accumulation of sediments in the littoral zone. Stable Pb, a historical marker of lacustrine sediments in southern Quebec, was used to date the sediments (;110 yr) and to calculate three long-term sediment accumulation rates (SARs) in Lake Memphremagog (located in Quebec and Vermont). The anthropogenic Pb burden in the littoral zone of Lake Memphremagog was found to be two to eight times greater (per m 2 ) than the Pb burdens in the profundal zone of surrounding eastern township lakes. Pb concentrations were nearly fourfold higher than background Pb concentrations (115 and 31 m gg 21 , respectively), providing a reliable marker for littoral sediment core analysis. Lake morphometry is related to the three accumulation rates measured by providing distinct threshold limits (i.e., littoral slope, .10%; exposure, .10 km 2 ) where sediments are unable to accumulate. Macrophyte beds are shown to disproportionately accumulate sediments at rates 2 to 20 times greater (per m 2 ) than in the profundal zone. Linear regression models show that both the total SAR (mean, 1.7 mm yr 21

Organic matter mineralization rates were measured by the accumulation of DIC + CH, in the water overlying intact cores taken from littoral and protimdal sediments of nine Quebec lakes. The variability in areal carbon mineralization is much greater within lakes than among lakes varying in trophic richness. Organic matter mineralization in littoral sediments is more variable and, on average, threefold higher than in the profundal sediments. Sixty percent of the variation in mean summer mineralization rates is explained by site depth, a surrogate variable that incorporates the effect of temperature and may also be reflecting substrate quality and(or) supply. The lakespecific characteristics most strongly correlated to the residuals of the regression with depth are catchment area-tolake area ratio (CA : LA) and water residence time. In lakes with a larger CA : LA and a shorter residence time, the amount and(or) the quality of organic matter settling to the sediments at a given depth may be reduced, resulting in the lower observed mineralization rates. Total mineralization in the sediments is, not surprisingly, greater in larger lakes but the rate per unit area is smaller, reflecting the decreased importance of the littoral zone. More than half (54-100%) of the DIC + CH, produced in the sediments is from the littoral zone. Yet, because of the large biomass in epilinmetic waters, the littoral sediments account for

Bacterioplankton and total planktonic community respiration were measured in 14 Canadian lakes during the summer. Total planktonic community respiration (O2 consumption and CO2 production) was best predicted by total phosphorus (TP), with respiratory rates increasing at higher TP levels. The bacterioplankton respiration alone was less well linked to nutrient concentrations. Yet, the rates of both planktonic and bacterial CO2 production changed similarly with changes in the C:N ratio, with the bacterial fraction, on average, contributing about 42% of the total planktonic respiratory rates. Bacterial carbon respired was uncoupled from oxygen consumption, with resultant average planktonic respiratory quotient (RQ) close to 2. Bacterioplankton respiratory rates are affected by a negative relationship between specific aerobic respiration and bacterial biomass. Higher bacterial specific oxygen consumption rates (µg O2 cell-1 d-1) were observed with increasing C:N and C:P ratios, suggesting an increase in maintenance cost at the low cell densities observed in oligotrophic waters. Phosphorus appeared as the limiting nutrient in these lakes and determined total planktonic respiratory rates, however, epilimnetic bacterial respiration was mainly related to the DOC: nutrient ratios.

Des specimens de macrozooplancton filtreur ont ete preleves dans 24 lacs du secteur centre-sud de l'Ontario en vue de la determination des liens existant entre des facteurs environnementaux et la bioaccumulation du methylmercure. La concentration de cette substance dans le zooplancton variait entre 19 et 448 ng g en masse seche -l ; c'est dans les specimens provenant des lacs aux eaux brunes et acides qu'elle etait la plus elevee. La couleur de l'eau et le pH pris conjointement ont constitue les meilleurs predicteurs de la concentration du methylmercure dans le zooplancton; ils permettaient d'expliquer 73% de la variation. Il existe une correlation positive entre ces concentrations et la couleur de l'eau, et une correlation negative entre elles et le pH de l'eau. Il existe egalement une correlation positive entre la concentration du methylmercure dans le zooplancton et le coefficient d'ecoulement ainsi que le pourcentage de la superficie du bassin occupee par des milieux humides; cela parait indiquer que l'apport de mercure provenant du bassin hydrographique est important dans les concentrations de methylmercure mesurees dans le biote lacustre. Nos resultats font la demonstration pour la premiere fois que, dans les lacs recepteurs, les effets de la couleur de l'eau sur la concentration du methylmercure dans la partie du biote correspondant aux premiers echelons du reseau trophique depassent ceux du pH. Il existe une bonne correlation entre la concentration de cette substance dans le zooplancton et la concentration de mercure chez l'achigan a petite bouche (Micropterus dolomieui) et a grande bouche (Micropterus salmoides) dans 11 des lacs d'etude; cela nous montre que le zooplancton est un bon indicateur de l'assimilabilite relative du mercure a la base de la chaine alimentaire.

A sampling algorithm requiring only bathymetric and catchment parameters has been derived from several recent empirical developments and should drastically reduce the number of sites and replicates necessary to characterize the distribution, trace element concentrations, and sediment accumulation rates of lake sediments. A test of this algorithm with sediments from 96 sites in three basins of morphometrically complex Jack Lake, Ontario, demonstrates the utility of this approach. A priori application of the algorithm would have reduced the number of sample sites on our sampling grid from 96 to 22 (a reduction of 77%), without losing precision. This reduction in sample sites (and in time and money expended) would allow for sampling additional lakes or sites of local significance (e.g., river or creek mouths, pollution point sources), or measuring additional parameters. The relationships between lake morphometry, sediment distribution, and sediment texture, along with the relationships between sediment texture and inter- and intra-site variability, can also be used to extrapolate sediment characteristics from a few samples to the whole lake or to predict the zone to which sediment is focused.

Sediment focusing is a process whereby water turbulence moves sedimented material from shallower to deeper zones of a lake. Sediment focusing occurs in lakes in both the erosional (coarse-grained sediments) and the transportational zone with the latter characterized by discontinuous sedimentation and resuspension. The zone of accumulation is diefined as the zone where sedimentation is final (i.e. no resuspension) and where there is no further sediment focusing. A geochemical tracer (Pb) was used to trace sediment focusing patterns in 12 lakes of different morphomctries. The area occupied by the zone of accumulation is predicted from the mean basin slope (a’,) with 86% of the variance explained. Only in large, exposed lakes in which turbulence from waves is more severe are the resulting sediment distributions more erratic. This study is the first to provide a general model of sc:diment focusing patterns among lakes. The study of lake-bottom dynamics has received attention in the contexts of sediment, contaminant, and eutrophication studies. The fact that sediments are major depositories for contaminants and nutrients has made them a focus for research in limnology. Also, because sediments provide a record of past events in lakes, an understanding of the manner in whllch sediments accumulate is essential for hindcasting. Interpretations of the historical record in sediments will be confounded if the record has been obscured by sediment resuspension. The term “sediment focusing” was coined by Likens and Davis ( 1975) to describe the resuspension of sediments in shallower zones by waves and water currents with subsequent transport to and settling in the deeper zones of lakes. Much of the work

We used sediment chronology data from fourteen published studies of lake cores across much of North America and Scandinavia in order to make a large scale comparison of the different dating techniques. The uncertainty of210Pb derived dates was determined using common sediment event markers: the stable Pb rise, the137Cs rise, and theAmbrosia pollen rise. For all data combined, the 95% confidence intervals for the stable Pb rise and theAmbrosia rise, were approximately 30 years. These 95% confidence intervals are slightly higher than those derived by First-Order Error analysis performed by others on210Pb derived dates. When comparing the concordance of two210Pb models (CRS and CIC) against markers of known history, we found that the CRS model dates (constant rate of supply) had consistently better agreement than the CIC model dates (constant initial concentration). Major discrepancies between137Cs and210Pb were common, but were consistently more severe in sediments of soft water lakes pointing to an inability of sediments with low mineral content to ‘immobilize’ Cs.

To test the hypothesis that elements can be transferred from freshwater sediments to macrophytes and to epiphytes, we grew Myriophyllum spicatum in natural sediments containing 60Co and 54Mn (essential) and 137Cs, 152Eu, 154Eu, and 155Eu (nonessential). We found no differences between shoot or epiphyte to sediment bioaccumulation factors (BAFs) and only found differences between root and shoot to sediment BAFs for 60Co and 54Mn, probably because these elements adsorb onto root surfaces. We found no differences in BAFs for any elements between shoots and epiphytes, suggesting no differences in the transfer of essential and nonessential elements. There were, however, interelement differences for the roots, again probably due to surface adsorption related to redox sensitivity. Using isotope specific activity ratios, we calculated that over 75% of the Co and 100% of the Mn in M. spicatum was obtained from the sediments and that all Co and Mn in the epiphytes was sediment derived through the host. Although the net flux of trace elements from the sediments to the water via macrophytes has been shown to be small in most situations, buried pulses of nonessential elements may be reintroduced to the littoral food web by epiphytes being grazed by invertebrates and fish.

A mass-balance approach was used to examine the role of macrophyte beds as a sink or source for 7 metals over time scales varying from two months (the growing season of the plants) to one year. During the growing season the macrophyte beds were found to be net sinks for particulate metals but were net sources of dissolved metals. During senescence, ca. 15–20% of the Al, Fe, and Mn and ca. 25–30% of the Cr, Cu, Ni and Zn within the macrophyte tissues at maximum seasonal biomass was lost to the surrounding waters in a dissolved form. The export of metals from the weeds during senescence was a very small fraction (

The trophic structure of pelagic communities in glaciated regions is highly variable due to restricted dispersal of glacial relict taxa and recent species introduction. Much of the between-lake variation in Hg in lake trout (Salvelinus namaycush) flesh from the St. Lawrence system (non-point-source contaminated lakes), which spans more than two orders of magnitude (0.03–3.96 μg/g), results from differences in the length of pelagic food chains. Hg levels from the longest food chains where pelagic forage fish and the crustacean Mysis relicta were present were about 3.6-fold higher than those from the shortest food chains where these last two trophic components were missing. A lack of correlation between Hg levels in small-mouth bass (Micropterus dolomieu), which do not exploit the pelagic food chain, and the length of the pelagic food chain confirmed that the positive relationship observed in lake trout could not be attributed to the confounding effect of some physicochemical factors covarying with our pelagic food chain classification. Our results confirmed that the food chain biomagnification model developed by Rasmussen et al. (1990. Can. J. Fish. Aquat. Sci. 47: 2030–2038) to predict PCB levels in lake trout can also be applied to Hg biomagnification.

Concentrations of Al, Fe, Mn, Zn and Cu were measured monthly at 24 sites in 21 rivers in Ontario and Quebec. Relationships between metal and suspended particulate matter (SPM), turbidity, colour (g440), temperature and system hydrology were quantified, and used to derive empirical models for predicting metal concentrations. In a test of the models using an independent data set, they explained a significant proportion of the variation in Al (90%), Fe (85%), and Mn (57%), but only 37% of the variation in riverine Zn concentrations. Metals concentrations are most strongly associated with SPM concentrations. The proportion of the total metal load associated with particulates (>0.45 μm) is highly variable below 10 ppm SPM, indicating that this concentration approximates the division between systems dominated by weathered (solution) versus eroded (particulate) inputs. Annual metal exports were calculated, and empirical models for predicting catchment exports were developed using system hydrology and average SPM concentrations. These simple models can be used to estimate metals concentrations and exports from routine water quality monitoring data, without requiring chemical analyses. They also serve to distinguish background levels from those indicating metal contamination, and will, therefore, be useful for water quality evaluation.

SUMMARY 1 Sediments and eleven species of rooted submerged aquatic macrophytes were sampled from twenty-five lakes in southern Quebec to examine species patterns in metal content. 2 Linear regression models relating the content of metals in the macrophytes to their sediments demonstrated that the isoetoid (Eleocharis acicitlaris, Eriacaulon septangulare, Isoetes sp. and Sagittaria graminae) and the non-isoetoid species had different plant-sediment relationships. Canopy-forming (Myriophyllum spicatum, Potamogeton amplifalius, P. crispus and P. perfoliatus) and understory species (Elodea canadensis, P. robbinsii and Vallisneria americana) had similar linear regression model slopes, but the understory species had significantly higher intercepts, indicating higher plant metal contents for a given sediment metal concentration. 3 We grouped species by their biomass density (biomass (kg)/height (m)), a measure of the packing of the plant canopy, and averaged values for each metal, thereby producing one model per group. Comparison of the models by analysis of covariance revealed no significant differences in metal content. However, the canopy-forming species that occurred in monospecific and mixed species stands (M. spicatum and P. amplifolius) had significantly higher metal content for a given sediment metal content when an understory was present. 4 These results show that sediment metal content explains the largest proportion of variation in plant metal content across all sites and species. Deviations from what would be predicted by the most general model are not related to large-scale differences in sediment geochemistry, but to the physical structure of the weedbed within geochemically similar sediments.

We have analyzed our own data set as well as a number of previously published ones to examine factors related to the numbers and biomass of sediment bacteria in a variety of ecosystems. We have chosen to express sediment variables either on an areal or per unit sediment volume basis to avoid bias due to the more common standard- ization to sediment dry mass (also known as gram dry weight). Cross-system comparisons show that the amount of organic matter present and the sediment water content are im- portant to sediment bacterial abundance. The C:N ratio of the sediment organic matter, possibly reflecting organic matter quality, was found to explain variation in bacterial num- bers in addition to that explained by organic content. Over a broad range, sediment water content is related to sediment bacterial numbers in a unimodal fashion indicating dilution of sediments by water at high (> 80%) water content sites and dilution by inorganic particles at low (< 50%) water content sites. In lakes, the most important factors explaining sediment bacterial biomass were sediment water content and hydraulic flushing rate, a finding sup- ported by analysis of an independent data set of English lakes. Planktonic chlorophyll a and planktonic bacterial numbers (both reflecting autochthonous organic matter levels) were not significantly related to sediment bacterial biomass. We found that sediments overlain by epilimnetic water had significantly higher bacterial biomass than those overlain by hypolimnetic water. A strong positive relationship between macrobenthos and sediment bacterial biomass was found for lakes in Ontario and Quebec. Calculations of annual bacterial production and P:B ratios necessary to sustain the macrobenthic population, based on the observed bacterial and macrobenthic biomass and temperature at our sites, indicate that the bacterial biomass present is more than sufficient to maintain the macrobenthos. In addition, based on the observed ratios of bacterial to detrital carbon at our sites and on a range of published assimilation efficiencies for macrobenthos grazing on these carbon sources, we calculated that the mean percentage of carbon of bacterial origin assimilatable by macrobenthos at our sites is 53% with a maximum of 79% and a minimum of 12%. Our results suggest a tighter trophic linkage between sediment bacteria and macrobenthos in our lakes than in most marine habitats, as indicated by published studies. From these analyses, the general picture emerges of a decoupling between pelagic and benthic bacterial processes in lakes, the pelagic ones being driven mainly by autochthonous production and the benthic ones by allochthonous inputs.

We measured bacterial growth rates by labelled thymidine incorporation, grazing loss rates with fluorescent latex particles, and bacterial cell size and abundance within narrow size-fractions of freshwater bacterioplankton. Contrary to the predictions of standard allometric relationships, the smallest bacteria showed the lowest incorporation rate per cell and per unit DNA content. Cells trapped by 1- and 5-μm filters grew the fastest and were responsible for 86% of detected thymidine biosynthesis. Grazing studies in six lakes showed that grazing pressure from flagellated protozoans and mixotrophic algae on large bacterial cells was probably 2–40 times more intense than that on the smallest cells. We suggest that reduced grazing pressure on the smallest bacteria allows them to dominate numerically, despite their slower growth.

We have evaluated the role of sediment pH (4.7–7.1) and redox potential (88–305 mV) in determining the bioavailability of five metals to four species of isoetoid macrophytes. The four species (Eleocharis acicularis, Eriocaulon septangulare With., Isoetes sp., and Sagittaria graminae) did not differ significantly in the relationship between the contents of metals in plants and sediment. When data for all species were pooled, 28–80% of the variation in the log of plant metal content was explained by the log of sediment metal (Al, Fe, and Mn) or by the log of sediment organic content (Cu and Zn). Sediment pH explained 45% (Al), 10% (Cu), 20% (Fe), 5% (Mn), and 12% (Zn) variation over and above that explained by the sediment metal content. These results demonstrate that lower sediment pH, in the presence of mildly oxic redox conditions, increases the bioavailability of these five trace metals to rooted aquatic plants. Rooted macrophytes subject to acidification contain higher metal concentrations which may be transferred by grazing to higher trophic levels.

Surficial sediments (0 to 2 cm) from 189 sites in 52 Quebec and Ontario lakes were analyzed for Al, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn by atomic adsorption spectrometry after extraction in dilute aqua regia. Empirical models using sediment texture (water content), site morphometry (depth) and geology (categorical variables) as predictors explain 82 to 87% of the between-site and between lake variation in sediment concentrations of Cu, Cr, Ni, Pb, and Zn, and 52 to 68% of the variation of Al, Co, Fe, and Mn. Over a broad geologic range, geologic variables are only significant in models predicting Co, Cr, and Ni. These three elements are low in concentration in lakes with catchments on the Grenville shield, while Cr and Ni are enriched in catchments containing ophiolite geology. These models explain both between-site and between-lake variation in sediment metal concentrations, and due to the lack of geologic influence on most metals, may be valid for other regions. The models appear promising as a means to identify point source contamination without assumptions about the relevant sediment fractions or inter-element relationships.

The effect of aboveground plant structure on near-bottom water flow and surficial sediment composition was evaluated in Lake Memphremagog (Quebec–Vermont) at 34 nearshore sites within three mixed-species macrophyte beds. Plant surface area accounted for 70% of the variance in flow reduction when the effect of changing water depth was removed. Relationships between plant surface area and surficial sediment clay content were evident within each of the three multispecies beds. The equations predicting within-bed clay concentrations were not significantly different from a general among-bed model (r2 = 0.74) developed at 25 sites of high biomass from other nearshore locations in Lake Memphremagog. The study provides quantitative evidence for the importance of macrophytes as sites of sedimentation of fine particles and their associated nutrients and contaminants.

The mud deposition boundary depth (mud DBD) is the depth in lakes at which the boundary occurs between high-energy erosive environments (coarse-grained noncohesive sediments) and low-energy depositional zones where fine-grained cohesive sediments accumulate. We have derived a model from the theory of waves and sediment thresholds that predicts the upper limit to the distribution of fine-grained sediments in lakes of any size. Our results suggest that the several biggest storms each year, rather than extremely rare events, are responsible for the upper limit to the distribution of mud. However, significant areas of coarse-grained sediments and many mud DBDs occur deeper than this upper limit, usually on slopes greater than 3%. For sediment at the mud DBD (23 μm), we have developed an empirical relationship between slope and maximum horizontal velocity that demonstrates the significant effect of slope on reducing either sediment threshold velocity or sediment stability.

An analysis of profundal sediment data from 83 north-temperate lakes shows that increasing inorganic sedimentation and exposure (or lake surface area) results in lower organic content and water content, and greater bulk density. Because sedimentation rates are unavailable for most lakes, we estimate sedimentation rates from readily available catchment sediment loads using a mass-balance model. The mass-balance estimate of sediment retention (per square metre of depositional zone) is an excellent predictor of measured inorganic sedimentation rates for a data set covering 19 lakes (R2 = 0.92). Multiple regressions are used to predict organic content, water content, and bulk density of profundal sediment from inorganic sedimentation rates and either exposure or lake surface area, which are surrogates for the energy of the depositional environment. These analyses explain 76, 74, and 66% of the between-lake variation in the three sediment parameters, respectively. Sediment organic content is not related to lake trophic status (chlorophyll a) and is negatively correlated with net organic matter sedimentation rates. The common occurrence of organic-rich sediments in oligotrophic shield lakes is, therefore, not a reflection of high organic matter inputs, but rather the extremely low inputs of mineral sediments to these lakes.

A method for estimating flow rate in freshwater was refined by calibrating the rate of dissolution of inexpensive and easily made gypsum cylinders at flows of 0–10 cm∙s−1 and at temperatures between 3 and 25 °C. Gypsum flux was linearly related to flow (r2 > 0.94); thus, weight loss combined with the surface area of the gypsum source and exposure period allows an estimate of flux. Temperature-corrected estimates of flow, obtained from calibration equations and observed dissolution of gypsum cylinders, were virtually identical to flow measured with an electromagnetic current meter in five of eight comparisons conducted in a lake and in three rivers; the three other comparisons were within 0.95 cm∙s−1. The calibrations are suitable for inexpensive and quantitative estimates of flow in freshwater within the range of temperature and flow conditions typically encountered. We suggest that the method can be used for comparing flow regimes in virtually any marine or freshwater environment as long as salinity and temperature among water bodies are comparable.

Lacustrine sediment focusing presumes the removal of fine-grained littoral particles (silt and clay) to storage sites in the deeper, less productive zone of a lake. In the littoral zone, shorter term sediment storage, on the time scale of biological transfers, demands attention as these clays and silts represent potential vectors in aquatic contaminant transfers. Two empirical models that predict surficial fine-grained sediment composition of the littoral were developed in Lake Memphremagog, Quebec. A 5-variable model explained 67% of the variance in the proportion of clay at 62 nearshore sites. A 3-variable model including measures of fetch, depth, and slope explained 58% of the variance while the additional variables of plant presence or absence and sediment organic matter contributed the remaining 9% of explained variation. The two models performed exceedingly well when tested at 41 sites not used in the original derivation of the model. Of these 41 test sites, 13 represent samples from littoral zones of five eastern Canadian lakes (maritime provinces) representing different geologies and morphologies.

(1991). The utility of latitude and other environmental factors as predictors of nutrients, biomass and production in lakes worldwide: Problems and alternatives. SIL Proceedings, 1922-2010: Vol. 24, No. 2, pp. 1235-1239.

Attached algae play a minor role in conceptual and empirical models of lake ecosystem function but paradoxically form the energetic base of food webs that support a wide variety of fishes. To explore the apparent mismatch between perceived limits on contributions of periphyton to whole-lake primary production and its importance to consumers, we modeled the contribution of periphyton to whole-ecosystem primary production across lake size, shape, and nutrient gradients. The distribution of available benthic habitat for periphyton is influenced by the ratio of mean depth to maximum depth (DR = z/ z(max)). We modeled total phytoplankton production from water-column nutrient availability, z, and light. Periphyton production was a function of light-saturated photosynthesis (BPmax) and light availability at depth. The model demonstrated that depth ratio (DR) and light attenuation strongly determined the maximum possible contribution of benthic algae to lake production, and the benthic proportion of whole-lake primary production (BPf) declined with increasing nutrients. Shallow lakes (z < or =5 m) were insensitive to DR and were dominated by either benthic or pelagic primary productivity depending on trophic status. Moderately deep oligotrophic lakes had substantial contributions by benthic primary productivity at low depth ratios and when maximum benthic photosynthesis was moderate or high. Extremely large, deep lakes always had low fractional contributions of benthic primary production. An analysis of the world's largest lakes showed that the shapes of natural lakes shift increasingly toward lower depth ratios with increasing depth, maximizing the potential importance of littoral primary production in large-lake food webs. The repeatedly demonstrated importance of periphyton to lake food webs may reflect the combination of low depth ratios and high light penetration characteristic of large, oligotrophic lakes that in turn lead to substantial contributions of periphyton to autochthonous production.

The utility of biomass density (biomass per unit volume) as a quantitative descriptor of the growth form of submerged macrophytes is evaluated and confirmed. Biomass density (BD) is a species specific characteristic which does not appear to be influenced by plant development. Secondly, BD reflects the growth form of the dominant species in the stand, because stands of species with a similar growth form also have similar BD's. Lastly, the BD of submerged macrophyte stands is closely related to the flowering pattern of the dominant species, and has important implications for light capture and light competition with neighbouring plants. Thus, BD should be a useful tool in the quantitative analysis of submerged macrophyte community structure.

Quantifying periphyton (attached algal) contributions to autotrophic production in lakes is confounded by properties of substratum that affect community biomass (as chlorophyll content) and productivity. We compared chlorophyll content and productivity of natural algal communities (phytoplankton, epipelon, epilithon, epixylon, and epiphyton) experiencing high (.10%) incident radiation in lakes in the US, Greenland, and Quebec, Canada. Chlorophyll content and productivity differed significantly among regions, but they also differed consistently among communities independent of region. Chlorophyll content of periphyton on hard substrata (rocks and wood) was positively related to water- column total P (TP), whereas chlorophyll content of algae on sediment (epipelon) and TP were not significantly related. Chlorophyll content was up to 1003 higher on sediments than on hard substrata. Within regions, chlorophyll-specific primary productivity was highest for phytoplankton and lowest for epipelon. Periphyton on hard substrata and on macrophytes (epiphyton) had similar rates of chlorophyll- specific productivity that were intermediate to those of epipelon and phytoplankton. Area-specific productivity of epipelon was 5 to 103 higher than area-specific productivity of periphyton on hard substrata. This broad geographic comparison indicates that, in low to moderately productive lakes under high-light conditions, algal communities have predictable differences in area-specific and chlorophyll- specific productivity based on substratum. As such, chlorophyll alone is an inadequate predictor of the relative contributions of different algal communities to total primary production. Our results highlight the importance of the relative abundance and spatial distributions of substrata in determining the role of the littoral zones in nutrient and energy cycles in lakes.

Epilimnetic and hypolimnetic bacterial production (BP) were measured once in summer, by the incorporation of [3H] - Leucine in each of 14 Quebec (Canada) lakes varying in nutrient content and morphometry. The epilimnetic and hypolimnetic BP were evaluated at two scales: the common per unit volume and areal (m-2) scale. The per unit volume scale epilimnetic BP was best predicted by total phosphorus (TP, r2 = 0.63), and by water residence time (WRT r2 = 0.57), with WRT serving as a surrogate for the nutrient and organic matter supply from the catchments. Total phosphorus and lake mean depth (Zm) together explained 79% of the variation in epilimnetic BP (1-1). In contrast, hypolimnetic BP (1-1 ) was neither linked to nutrients (TP or TN) or dissolved organic carbon (DOC) but only to measures of lake morphometry and best of all to hypolimnetic thickness (Zh; r2 = 0.74). With increased Zh, there is an increased dilution of settling organic particles and their nutrients, resulting in a decrease in BP per litre. Conversely, when BP is expressed in areal units (m-2), hypolimnetic production increases with increasing hypolimnetic thickness. Water column thickness is a master variable, which together with Chl a (abundance of particles) determines hypolimnetic BP at the whole system scale even though the trophic status is the best single indicator of epilimnetic BP on a volumetric scale. Conclusions drawn invariably change with the scale of investigation. Moreover, it is clear that lake morphometry has a major impact on BP. A comparison of whole water column integrated BP with literature derived estimates of the equivalent sediment production (m-2) below suggests that if the estimated sediment rates are not complete technique artefacts, they are likely to be an order of magnitude higher than the water column rates (m-2) at the maximum depth sampling sites. The relative importance of the sediments could be expected to rise with a decline in the maximum depth of lakes, characterized by progressively thinner hypolimnia. The present findings point to both a primarily allocthonous fuelling of sediment production and an uncoupling of water and sediment BP.

Examination of particulate light absorption and microplankton metabolism in 36 northeastern Spanish aquatic ecosystems, ranging from alpine rivers to inland saline lakes and the open Mediterranean Sea, revealed the existence of general relationships between particulate light absorption and the biomass of phytoplankton and microplankton metabolism. The particulate absorption spectra reflected a dominance of nonphotosynthetic, likely detrital, particles in rivers and a dominance of phytoplankton in coastal lagoons. There was a strong relationship between the light absorbed by phytoplankton and the chlorophyll a (Chl a) concentration of the systems, which indicated an average (±SE) Chl a specific absorption coefficient of 0.0233 ± 0.0020 m2 · mg Chl a-1 for these widely diverse systems. Chl a concentration was a weaker predictor of the total particulate light absorption coefficient, pointing to an important role of nonphytoplanktonic particles in light absorption. Gross production was very closely related to the light absorption coefficient of phytoplankton, whereas community respiration was strongly correlated with the total particulate light absorption coefficient, indicating the optical signatures of sestonic particles to be reliable predictors of planktonic biomass and metabolism in aquatic ecosystems.

25 pages, 8 figures, 6 tables, The importance of resources versus that of predators in determining the abundance of organisms is modulated by several factors which vary over time and space. Here, we evaluate the abundance of heterotrophic nanoflagellates (HNF), the main predators of planktonic bacteria, in the plankton of 16 Quebec lakes as a function of the abundance of resources and zooplankton (predators). We analyze the data at two different scales: among lakes varying in resource supply and within lakes. Resource factors (total phosphorus, chlorophyll, bacterial concentration) best explained the abundances of HNF among lakes differing in resource availability. When the effect of resource availability was removed, the biomass of cladocerans became an important determinant of HNF abundance, with abundance declining and the ratio of bacteria per flagellate increasing, as the cladocerans increased. Within lakes, HNF abundance was not a direct function of resource concentration. Multiple regression models based on data for many lakes showed that both bottom-up (food) and top-down (predation) variables predicted HNF abundance quite well. An examination of the seasonal pattern among individual lakes showed each group of variables differing in importance: bottom-up variables were more important in spring and top-down variables in midsummer. An examination of the effects of different assemblages of zooplankton on HNF showed cladocerans to exert the highest negative impact on HNF and to be responsible for keeping low HNF numbers during most of the summer with that probably affecting the pathways of energy flux in those lakes dominated by cladocerans versus those dominated by other zooplankton. The absolute, as well as the relative, importance of the different biotic and abiotic factors examined in predicting the abundance of HNF is a function of the scale of the investigation. Among lakes varying greatly in resource supply, resources are found to better correlate with HNF abundance. When the analysis is performed at another scale, within lakes, the relative importance of resources and predation in determining HNF abundances changes depending on the variability superimposed by other factors (seasonality and lake food web structure)

Although catchments have been implicated as an important source of metals to lakes, the catchment contribution of different metals is poorly known, and the anthropogenic contribution is not known at all. We determine the anthropogenic lake sediment burdens of Zn, Cu, Ni, Cr, and Pb for several Quebec and Ontario lakes, not subject to point source loading, to obtain estimates of atmospheric loading and inputs from terrestrial sources. To do this, we first collected multiple cores across 11 lake basins to estimate the whole-lake Pb burdens. As the whole-lake Pb burdens did not differ among lakes that spanned over two orders of magnitude in drainage ratios (drainage basin area/lake area), we conclude that catchment retention of anthropogenic Pb is complete. The anthropogenic Pb burdens were then used as a correction for focusing for the other metals. Among the metals, Cr and Ni were the most readily exported from drainage basins, followed by Cu. Zn showed no increase with drainage ratio, indicating Zn to be effectively retained by catchments. The export coefficients of the Pb corrected metals correlate well with ocean residence time, revealing a similar metal sorption/precipitation sequence in both soils and oceans. Sediment metal burdens provide a relatively easy way to obtain not only metal export coefficients from drainage basins, but also the atmospheric deposition of anthropogenic metals (e.g. Pb: S.E. Quebec, 950 mg*m−2: Laurentians, north of Montreal, 420 mg*m−2). The export coefficients are not only simpler to obtain than by mass balance measurements, but, in addition, identify the anthropogenic component.

We collected benthic bacterial production data measured by (3)H thymidine incorporation (TTI) (25 studies), frequency of dividing cells (FDC) (3 studies), dark-C02 assimilation (1 study) and (3)H-adenine uptake (2 studies) from the literature, which included 18 marine, 6 river, and 2 lake studies. In all of the studies that used the TTI method, (3)H-DNA was isolated and incubations were carried out at in situ temperatures. Most of the researchers also determined (3)H-DNA extraction efficiencies and isotope dilution, thus interpretable estimates of bacterial production were used in the analysis. In marine sediments, bacterial production rates were linked to bacterial biomass, bacterial abundance, sediment organic matter, temperature, and sediment chlorophyll a, with these variables explaining between 40% and 68% of the variation in production rates. Simple relationships between production and bacterial biomass or bacterial abundance, or between production and sediment organic matter, were improved by also including temperature in the analysis of marine sediments. Sediment organic matter explained an appreciable fraction (58%) of the observed production in freshwater sediments. Temperature was the most powerful predictor of the observed variability in specific growth rates (r (2) = 0.48 and r (2) = 0.58) in marine and freshwater sediments, respectively. Thus, bacterial production and specific growth rates are most closely linked to substrate supply and temperature in marine and freshwater sediments.

A simple view of the role of rooted macrophytes in element cycling sees them as pumps retrieving buried elements from the sediment profile. To investigate the relationship between the elemental composition of plants and sediments, we analysed published data for 39 elements. The best general model explained 84% of the variance of the log of plant element concentration: LPE = - 0.81 + 0.90 Log Sediment Element (ug/g dry wt.) − 0.12 Sediment Organic Content (ug/g drt wt.) + 0.67 Atomic radius (nm) (r2 = 0.84; n = 39)

20 pages, 4 figures, 4 tables, We analyzed heterotrophic, pelagic bacterial production and specific growth rate data from 57 studies conducted in fresh, marine and estuarine/coastal waters. Strong positive relationships were identified between 1) bacterial production and bacterial abundance and 2) bacterial production and algal biomass. The relationship between bacterial production and bacterial abundance was improved by also considering water temperature. The analysis of covariance model revealed consistent differences between fresh, marine and estuarine/coastal waters, with production consistently high in estuarine/coastal environments. The log-linear regression coefficient of abundance was not significantly different from 1.00, and this linear relationship permitted the use of specific growth rate (SGR in day−1) as a dependent variable. A strong relationship was identified between specific growth rate and temperature. This relationship differed slightly across the three habitats. A substantial portion of the residual variation from this relationship was accounted for by algal biomass, including the difference between marine and estuarine/coastal habitats. A small but significant difference between the fresh- and saltwater habitats remained. No significant difference between the chlorophyll effect in different habitats was identified. The model of SGR against temperature and chlorophyll was much weaker for freshwater than for marine environments. For a small subset of the data set, mean cell volume accounted for some of the residual variation in SGR. Pronounced seasonality, fluctuations in nutrient quality, and variation of the grazing environment may contribute to the unexplained variation in specific growth.

(1998). Planktonic primary production and respiration in Spanish waters: from the mountains to the sea. SIL Proceedings, 1922-2010: Vol. 26, No. 4, pp. 1727-1727.

Two of three Kenyan lakes studied between November 1979 and October 1980 have very short 33PO4 turnover times, indicating a high phosphorus (P) demand throughout the year. The P turnover time in Lakes Oloidien and Sonachi is as rapid as in the most P deficient temperate zone lakes. The third lake, Lake Naivasha, has a lower overall P demand and a wide seasonal range, with lowest demand between November 1979 and February 1980 when a P deficiency was unlikely. On an annual basis the Lake Naivasha status is, however, not statistically different from that recorded during the summer in Lake Memphremagog, a generally P-limited temperate zone lake. Lake Naivasha and Lake Oloidien fit well to the line of best fit for the Dillon-Rigler relationship relating total phosphorus (TP) and chlorophyll a derived in temperate zone lakes. Thus, temperate zone models predicting aspects of lake behaviour on the basis of TP may also be applicable to these two tropical lakes. Saline lake Sonachi had not only a short P turnover time but also responded dramatically to the fertilization of enclosures with P. However, it does not fit the TP-chla or the total nitrogen-chla plots from the temperate zone. This suggests that, in this saline lake at least, much of the TP is unavailable to the algae, with some of it in a particulate form that is readily extracted with boiling water. The epilimnetic N:P ratios also characterize lakes Oloidien and Sonachi lakes as highly P deficient and lake Naivasha as more moderately P limited. A single set of measurements in Winam Gulf (Lake Victoria) also showed a rapid P turnover time and thus P limitation, but as in lake Sonachi much of the TP was in a non-algal particulate form. Occasional measurements in three other hypertrophic and saline lakes suggest them to be primarily light limited on the basis of their very high photosynthetic cover. These findings support the hypothesis of a primary P limitation for those lakes not light limited, and contradicts literature suggestions that nitrogen is the primary limiting element in tropical lakes.

In order to distinguish the activity of phytoplankton and bacterioplankton in 13 lakes of widely varying trophy, we size-fractionated orthophosphate uptake and alkaline phosphatase activity. In most lakes, orthophosphate uptake was consistently and overwhelmingly associated with the smallest particles. Based upon indicators of algal and bacterial presence in each size class, we inferred that the bacterioplankton are responsible for ≥95% of the orthophosphate uptake in situ, except in lakes that are not phosphorus-deficient. In contrast, a large portion of the alkaline phosphatase activity was free in solution (median 46%), and much of the remainder (median 44%) was apparently associated with algae.