Lake Primary Production Research Articles

An automata networks based preprocessing technique for artificial neural network modelling of primary production levels in reservoirs

Primary production in lakes and reservoirs develops as a result of complex reactions and interactions. Artificial neural networks (ANN) emerges as an approach in quantification of primary productivity in reservoirs. Almost all of the past ANN applications employed input data matrices whose vectors represent either water quality parameters or environmental characteristics. Most of the time, the components of input matrices are determined using expert opinion that implies possible factors that affect output vector. Major disadvantage of this approach is the possibility of ending-up with an input matrix that may have high correlations between some of its components. In this paper, an automata networks (AN) based preprocessing technique was developed to select suitable and appropriate constituents of input matrix to eliminate redundancy and to enhance calculation efficiency. The proposed technique specifically provides an apriori rough behavioral modeling through identification of minimal AN interaction topology. Predictive ANN models of primary production levels were developed for a reservoir following AN based pre-modeling step. The achieved levels of model precisions and performances were acceptable: the calculated root mean square error values (RMSE) were low; a correlation coefficient ( R) as high as 0.83 was achieved with an ANN model of a specific structure.

Transition from P‐ to N‐limited phytoplankton growth in an artificial lake on flooded cutaway peatland in Ireland

AbstractQuestion: Which nutrient limits primary production in a lake created by flooding industrial cutaway peatland?Location: Clongawny Lake (53°10’N, 07°53’W), County Offaly, IrelandMethods: Nutrient concentrations in lake water and the dynamics of phytoplankton populations were monitored over a 38‐month period. The ratio of dissolved inorganic nitrogen to total phosphorus (DIN:TP) and nutrient enrichment bio‐assays were used to investigate temporal changes in nutrient limitation.Results: Primary production in the new lake was phytoplankton‐driven due to the scarcity of recolonizing macrophytes. Phytoplankton growth was initially phosphorus‐limited. The runoff of phosphate fertilizer from an adjacent coniferous forestry plantation raised the TP concentration of lake water 5.5‐fold. Consequently, the biovolume of phytoplankton increased 30‐fold, and chlorophyll‐a concentrations increased eightfold, reaching hyper‐eutrophic levels. A concurrent depletion of nitrogen in lake water reduced the DIN:TP ratio from 17.8 to 0.6, and phytoplankton growth rapidly became nitrogen‐limited. Phytoplankton composition shifted from dinoflagellates to minute, unicellular chlorophytes, with a coincident decline in species diversity. Cyanobacteria did not proliferate, most likely due to the acidic nature of the lake.Conclusions: Results illustrated the vulnerability of newly created cutaway peatland lakes to developing severe phytoplankton blooms and coincident secondary nitrogen limitation in the presence of moderate external phosphorus inputs.

Coupling between Primary Terrestrial Succession and the Trophic Development of Lakes at Glacier Bay, Alaska

The natural eutrophication of lakes is still an accepted concept in limnology, arising as it does from the earliest efforts to classify lakes and place them in an evolutionary sequence. Recent studies of newly formed lakes at Glacier Bay, Alaska, only partially support this idea, and suggest more variable trends in lake trophic development which are under local (catchment-level) control. Here we use sediment cores from several lakes in Glacier Bay National Park to examine the relationship between successional changes in catchment vegetation and trends in water-column nitrogen (a limiting nutrient) and lake primary production. Terrestrial succession at Glacier Bay follows several different pathways, with older sites in the lower bay being colonized directly by spruce (Picea) and by-passing a prolonged alder (Alnus) stage that characterizes younger upper-bay sites. Sediment cores from three sites spanning this successional gradient demonstrate that the variability in nitrogen trends among lakes is a consequence of the establishment and duration of N-fixing alder in the lake catchment. In the lower-bay lakes, diatom-inferred nitrogen concentrations rise and then fall in concert with the transient appearance of alder in the catchment, while in the upper bay, high nitrogen concentrations are sustained by the continuous dominance of alder. Diatom accumulation, a proxy for whole-lake biological productivity, increases steadily at all three sites during the first century following lake formation, but declines in more recent times at the lower-bay sites in apparent response to the disappearance of alder and decreasing lake-water nitrogen. These results demonstrate a tight biogeochemical coupling between terrestrial succession and lake trophic change during the early developmental history of Glacier Bay lakes.

Urban geochemistry of Khoshk River, Shiraz, Iran

phosphorous is related to human activities marked by utilization of phosphoric fertilizers since 1952 A.D. The increase of 6 13Corg towards the sediment surface, together with increasing of OC and N accumulation, indicated the elevation of lake primary productivity due to excess phosphorous loading caused by utilization of phosphoric fertilizer. The decrease of fi ~SN during the primary productivity elevation process, especially after 1952, can be attributed to the discharged of nitrogen with lower 5 15N into the lake. Intensive human activities and enhanced anthropogenic input of nutrients played a great role in nutrient accumulation in sediments and primary productivity elevation.

Comparative influence of resuspended glacial sediment on physicochemical characteristics and primary production in two arctic lakes

We compared physicochemical properties and rates of phytoplankton and epipelic primary production in two shallow lakes in the Alaskan arctic on eight occasions over three years. The two morphometrically similar lakes lacked defined inlets and had a mean depth of 2.2 m. The lakes differed with respect to glacial influence. Lake GTH 112 was continuously turbid due to resuspension of glacial silt from the lake bed, while GTH 114 showed higher clarity as it was situated on coarser glacial drift. The two lakes contrasted sharply in euphotic zone nutrient concentrations. Soluble reactive phosphorus, NO3−-N and NH4+-N concentrations averaged 0.17, 2.5 and 12.6 μM, respectively, in GTH 112, but were generally at or below the detection limit of 0.05 μM in GTH 114. Reduced light limited the ability of phytoplankton in GTH 112 to use the increased nutrients, and volume-based rates of phytoplankton primary production were similar between lakes. High turbidity in GTH 112 decreased the average percentage of total lake volume and sediment surface within the euphotic zone to 63% and 47%, respectively, compared with values of 88% and 85%, respectively, for GTH 114. Consequently, the average whole lake (phytoplankton plus epipelic) primary production rate in GTH 112 (8.5 mmol m−2 d−1) was significantly lower than the mean rate (12.3 mmol m−2 d−1) for GTH 114. The increase in turbidity affected benthic and pelagic habitats proportionately, as epipelon accounted for about 25% of total whole lake primary production in both lakes.

Mid- to late-Holocene land-use change and lake development at Dallund Sø, Denmark: trends in lake primary production as reflected by algal and macrophyte remains

Diatom, macrofossil, pollen, Pediastrum and biogenic silica analyses were carried out on an 11-m sediment sequence from the Danish lake Dallund Sø, demonstrating major changes in the aquatic ecosystem over the last 7000 years. A diatom-phosphorus calibration model was applied to the fossil diatom record to reconstruct in-lake total phosphorus (TP) concentrations over this period. Prior to the introduction of agriculture to the region, c. 6000 years ago, the lake was relatively deep and had low diatom-inferred TP concentrations ( c. 20 υg TP/L), with limited macrophyte growth. Moderate nutrient enrichment of the lake was inferred during the Bronze Age (1700-500 Bc) and Iron Age (500 BC-AD 1050) periods and evidence for water-level lowering was observed. Marked eutrophication of the lake (reconstructed TP levels consistently > 100 υg/L) was associated with major changes in agriculture during the Mediaeval period (AD 1050-1536) and continued to the present day. These data document the long-term anthropogenic impact on Dallund Sø, a lake in an area with a long history of human activity.

The Biologists' Forum: A trophic cascade synthesis: review of top-down mechanisms regulating lake ecosystems

This paper evaluates the Trophic Cascade hypothesis and its predictive effects of food web structure regulation on primary productivity in lake ecosytems. The hypothesis prognosticates that the top of the aquatic food web, either with or without piscivorous fishes, should have cascading interactions through the trophic levels ultimately impacting algal biomass. Extensive experiments have shown validity to the hypothesis, while critics of the idea, through their own research, claim that it is too simplistic of a mechanism lost in the complexities of food web dynamics and nutrient loading models. I discuss the development of the Trophic Cascade hypothesis, its strengths and weaknesses, and conclude with the recognized and current synthesis. Many researchers now recognize the interactions of both top-down foraging mechanisms and bottom-up nutrient loading dynamics on lake primary production when assessing lacustrine systems.

Advance and application of lake optics research

The mainstreams of lake optics research in recent decades include optical properties of lakewater, observation, transmission and calculation of underwater radiation, determination of absorption coefficient S of yellow substance, influence of UV-B radiation of lake primary productivity by bio-optical model. Major lake optics applications, such as calculation of lake primary productivity and chl-a, analysis of factors restricting eutrophication, and protection against lake eutrophication are summarized.

Holocene glaciation and climate evolution of Baffin Island, Arctic Canada

Lake sediment cores and cosmogenic exposure (CE) dates constrain the pattern of deglaciation and evolution of climate across Baffin Island since the last glacial maximum (LGM). CE dating of erratics demonstrates that the northeastern coastal lowlands became ice-free ca.14 ka as the Laurentide Ice Sheet (LIS) receded from its LGM margin on the continental shelf. Coastal lakes in southeastern Baffin Island started to accumulate sediment at this time, whereas initial lacustrine sedimentation was delayed by two millennia in the north. Reduced organic matter in lake sediment deposited during the Younger Dryas chron, and the lack of a glacial readvance at that time suggest cold summers and reduced snowfall. Ice retreated rapidly after 11 ka but was interrupted by a widespread readvance of both the LIS and local mountain glaciers ∼9.6 ka (Cockburn Substage). A second readvance occurred just before 8 ka during a period of unusually cold summers, corresponding to the 8.2 ka cold event in the Greenland Ice Sheet. Most local glaciers were behind their present margins before 7 ka, and in some instances much earlier, although the Foxe Dome of the LIS continued to slowly retract toward the present day Barnes Ice Cap throughout the Holocene. Pollen in lake sediments is rare and dominated by exotic sources prior to 12 ka. Subsequently, grass tundra became established, followed by modern tundra vegetation ca. 8 ka, with subtle changes in pollen assemblages in the late Holocene. Lake primary productivity peaked in the early Holocene, before terrestrial vegetation or marine surface waters reached their apparent thermal maxima. Lacustrine, marine, and glacial proxies all reflect significant late Holocene cooling. The onset of Neoglaciation is well dated in lacustrine records at ca. 6 ka, with intensification after 2.5 ka. The expansion of local glaciers during the Little Ice Age represents the most extensive advance since 7 ka. We suggest that the replacement of Atlantic surface waters by cold, low-salinity Arctic Ocean water, coupled with the steady reduction of summer insolation, resulted in a significant positive sea-ice feedback that produced a larger late Holocene summer temperature depression over the Baffin region than in the Pacific sector of the Arctic.

Late Quaternary environmental and climatic changes in central Europe as inferred from the composition of organic matter in annually laminated maar lake sediments

Geochemical (elemental analysis, bulk δ13CTOC, thermal degradation techniques, molecular analysis of n‐alkanes) and microscopic investigations (maceral analysis, vitrinite reflectance) were performed (1) to characterize the organic matter (OM) in sediments from two maar lakes in the Westeifel volcanic field, Germany, and (2) to discuss paleoenvironmental processes which have controlled the accumulation and preservation of OM. The annually laminated sediments show a wide range of organic carbon contents between 0.3 and 21.4% TOC and Hydrogen Index values between 80 and 501 mg HC/g TOC. Well‐developed laminations mirror suboxic to anoxic bottom waters. Diagnostic compounds in the pyrolysates and microscopic analysis permit discrimination between the principal OM sources, and their variations over time provide important evidence for the reconstruction of the environmental histories of the lakes and their watershed. The significance of terrigenous OM is reflected by the occurrence of methoxyphenols in the insoluble as well as mid‐ and long‐chain n‐alkanes in the soluble OM which are indicative of land plant or macrophyte input to the lakes. The relative abundance of certain homologues together with the amount of terrigenous macerals can be used to show differences in higher plant input and vegetation changes through time. During the Pleniglacial and Younger Dryas period, high‐reflecting reworked OM was introduced by soil erosion in an environment with relatively open vegetation. The alkylphenols in the pyrolysates originate partly from the microbial reworking of proteinaceous tyrosine moieties and/or reflect a direct contribution from cyanobacteria. We assume that variations in the carbon isotopic composition of OM are mainly due to changes in the lake's primary productivity with the exception of the Pleniglacial. Various forms of land use are reflected by increasing amounts of terrigenous particles as well as chemical plant markers from 500 varve years B.P. until present. The organic geochemical and microscopic data correlate well with other proxy information about climate‐dependent fluctuations in the ecosystem of the studied sites.

Climatic forcing and primary productivity in a subalpine lake: Interannual variability as a natural experiment

We analyzed a 42‐yr record of primary productivity in small, subalpine Castle Lake to determine how climatic variability might influence lake primary productivity. A Pacific Decadal Oscillation (PDO) polarity reversal in 1977 significantly affected winter air and summer water temperatures in Castle Lake. The timing of lake ice‐out was explained by spring air temperature and winter total precipitation (r2 = 0.72) and significantly affected water temperature (r2 = 0.74). Primary productivity was negatively correlated with ice‐out date and positively correlated with primary productivity during the previous year (r2 = 0.47). Alternatively, primary productivity was positively correlated with water temperature and primary productivity during the previous year (r2 = 0.49). Ammonium availability immediately after ice‐out was significantly related to primary productivity from the previous and the current year, suggesting that nutrient availability is an important mechanism for the serial correlation. Daphnia and cyanobacteria biomass also increased during warmer years. Our results suggest that variability in air temperature and precipitation from global warming, PDO, and the El Niño Southern Oscillation (ENSO) influence primary productivity and plankton communities in North American dimictic lakes.

Compositional and interlake variability of zooplankton affect baseline stable isotope signatures

Zooplankton are commonly used to establish a baseline isotopic signature for pelagic production in lakes. Our objective was to evaluate this approach by quantifying among‐lake and within‐lake variability of δ13C and d15N for different taxa of pelagic zooplankton. We measured the δ13C and δ15N of Daphnia, Holopedium, and calanoid copepods from four lakes sampled from June to November 2001 and from eight additional lakes sampled once in midsummer. In the four lakes with temporal sampling, within‐lake differences due to taxonomic grouping accounted for 36.7% of the variance in δ15N and 41.7% of the variance in δ13C. Among all lakes, the δ15N of calanoid copepods was on average 2.55‰ and 2.44‰ higher than Daphnia or Holopedium, respectively, whereas the δ13C of calanoid copepods was 2.19‰ and 2.23‰ lower than Daphnia or Holopedium, respectively. If 15N fractionation is similar among species, the differences in δ15N suggest that calanoid copepods either feed at a higher trophic position in the food web or they have a consistently higher baseline δ15N signature than Daphnia or Holopedium among lakes. Differences in δ13C suggest that zooplankton taxa in the pelagia of lakes have different food sources. We conclude that species composition and feeding behaviors of the zooplankton community should be considered before making among‐lake comparisons of food web structure. We show that Daphnia is a useful isotopic baseline for organisms that rely on primary production in lakes.

Fluxes of methane, carbon dioxide and nitrous oxide in boreal lakes and potential anthropogenic effects on the aquatic greenhouse gas emissions

We have examined how some major catchment disturbances may affect the aquatic greenhouse gas fluxes in the boreal zone, using gas flux data from studies made in 1994–1999 in the pelagic regions of seven lakes and two reservoirs in Finland. The highest pelagic seasonal average methane (CH 4) emissions were up to 12 mmol m −2 d −1 from eutrophied lakes with agricultural catchments. Nutrient loading increases autochthonous primary production in lakes, promoting oxygen consumption and anaerobic decomposition in the sediments and this can lead to increased CH 4 release from lakes to the atmosphere. The carbon dioxide (CO 2) fluxes were higher from reservoirs and lakes whose catchment areas were rich in peatlands or managed forests, and from eutrophied lakes in comparison to oligotrophic and mesotrophic sites. However, all these sites were net sources of CO 2 to the atmosphere. The pelagic CH 4 emissions were generally lower than those from the littoral zone. The fluxes of nitrous oxide (N 2O) were negligible in the pelagic regions, apparently due to low nitrate inputs and/or low nitrification activity. However, the littoral zone, acting as a buffer for leached nitrogen, did release N 2O. Anthropogenic disturbances of boreal lakes, such as increasing eutrophication, can change the aquatic greenhouse gas balance, but also the gas exchange in the littoral zone should be included in any assessment of the overall effect. It seems that autochthonous and allochthonous carbon sources, which contribute to the CH 4 and CO 2 production in lakes, also have importance in the greenhouse gas emissions from reservoirs.

On the limnology of Lake Baringo (Kenya): II. Pelagic primary production and algal composition of Lake Baringo, Kenya

Primary production and algal composition were studied in the shallow, highly turbid tropical freshwater Lake Baringo (Kenya). The lake's primary productivity was found to be very low when compared to other neighbouring Rift Valley lakes. The mean value of daily gross primary production was 0.56 g O2 m−2 d−1 with a range of 0.22 – 0.70 g O2 m−2 d−1. The study showed that this lake's primary production is strongly influenced by the inorganic turbidity but not the self-shading of algae as observed in the neighbouring soda and alkaline lakes. The lake has a high aphotic:photic zone ratio of 19, thus leading to a low light adapted phytoplankton community capable of vertical movement. This was shown by a uniform chlorophyll-a concentration at night and a gradient with higher values near the water surface observed during day time, too. Mean chlorophyll-a concentration was 55 μg l−1. Lake Baringo is characterised by only low diversity of algae and cyanobacteria, the latter being the major group of phytoplankton in this lake. Few green algae and diatoms were recorded during this study, growing for the most part in gelatinous sheaths of the blue-greens (strategy to get some light), on the stones near the shores of the lake and attached to the macrophytes.

Seasonal and interannual mobility of arsenic in a lake impacted by metal mining.

Detailed examination of the water column, sediments, and interstitial waters was conducted in Balmer Lake, Ontario, Canada, in 1993-1994 and 1999 in order to assess the seasonal and interannual controls governing the behavior of As. High-resolution profiles of dissolved (<0.45 microm) Fe, Mn, SO4(2-), and sigmaH2S across the sediment-water interface indicate the presence of reducing conditions in close proximity to the benthic boundary during ice-free periods, which are characterized by fully oxygenated bottom waters. Dissolved As is remobilized as As(III) in suboxic sediment horizons via the redox-controlled dissolution of Fe (and perhaps Mn) oxide phases. During 1993-1994, As fluxes to the water column were relatively low (2-15 microg cm(-2) year(-1)) and contributed between 2 and 18% of the water column inventory. Dissolved As in the lake waters was derived primarily from external mining-related loadings during this period. Between 1993 and 1999, external loadings of As to Balmer Lake decreased while [As]aq within the lake increased, suggesting an increase in the proportion of sediment-derived As. Indeed, benthic dissolved As fluxes in 1999 ranged from 179 to 380 microg cm(-2) year(-1), representing approximately 33-60% of the water column burden. The relatively recent importance of sedimentary arsenic sources is suggested to reflect changes to sediment redox conditions associated with a postulated increase in lake primary productivity. Ironically, the increased contribution of dissolved arsenic to the water column appears to have resulted from an otherwise improvement in water quality. Reduced loadings of Cu, Zn, and Ni to the lake since 1994 appear to have allowed increased phytoplankton production that has stimulated arsenic release.

WHOLE-LAKE FERTILIZATION EFFECTS ON DISTRIBUTION OF PRIMARY PRODUCTION BETWEEN BENTHIC AND PELAGIC HABITATS

The perception that primary production in lakes is positively related to phosphorus loading is based almost entirely on studies of phytoplankton. This is partly because benthic and pelagic habitats in lakes are often treated as separate ecosystems, the processes of which can be evaluated independently. However, light and nutrients often limit primary producers in both benthic and pelagic habitats. We tested the hypothesis that reductions in light associated with increases in phytoplankton could cause compensatory decreases in benthic algal (periphyton) primary production. We monitored production of periphyton on sediments (epipelon), periphyton on wood (epixylon), and phytoplankton in four lakes in upper Michigan, USA, from 1991 to 1995. During the summers of 1993–1995, we stimulated phytoplankton production in three of the lakes by fertilizing with nitrogen and phosphorus (N:P ≥ 25 by atoms) at rates between 0.3 and 2.0 mg P·m−3·d−1. The response of periphyton to fertilization was substratum specific: epixylon increased with fertilization, but epipelon decreased. However, when area-specific production was extrapolated to the whole-lake scale, epixylon never constituted >4% of benthic primary production. Thus, the decline in epipelic production dominated the benthic response to fertilization. We also estimated whole-lake (epipelon + phytoplankton) primary production. Epipelic algae constituted 50–80% of whole-lake primary production at ambient nutrient levels. However, only 10–40% of primary production was benthic at the highest fertilization rates. The increase in whole-lake primary production caused by water column fertilization was greatly overestimated when we did not include the compensatory decline in epipelic algae as they were shaded by increases in phytoplankton concentrations.

Optimal conditions for primary production in a polymictic tropical lake (Lake Xolotlán, Nicaragua)

From 1987 to 1993 we assessed the variation of phytoplankton biomass, underwater irradiance and primary production in Lake Xolotlan (L. Managua, Nicaragua). Chlorophyll- a averaged 65 mg m-3 and maximum and minimum concentrations were 120 and 30 mg m-3, respectively. The variability over depths and weeks was low (CV < 20%). There were strong correlations between particulate carbon and chlorophyll- a (the ratio ≈ 100: 1) and between particulate carbon and particulate nitrogen and phosphorus (the ratio ≈ 100: 11: 1). Gross primary production averaged 6.8 g C m-2 d- 1 and was stable over the years (CV ≈ 10%). Algal cell growth was approximately 4–5 g C m-2 d- 1. Productivity was limited only by the availability of underwater light and the depth of the photic zone was mainly regulated by the chlorophyll- a concentration. Therefore, areal photic zone chlorophyll- a was the only factor directly correlated to the integral photosynthetic activity but, contrary to theoretical models, the production did not increase in proportion to chlorophyll- a. Data from African lakes show a similar pattern.

Secondary production and energetics of the shrimp Caridina nilotica in Lake Victoria, East Africa: model development and application

Measurements of body mass, carbon content, respiration, growth, and egestion are combined in a model of secondary production by the tropical freshwater shrimp Caridina. The model is developed to permit its direct application to empirical data for abundances and size frequency distributions of field populations. Model calculations combined with population data for offshore Lake Victoria over a period of two years indicate that Caridina consume the equivalent of 2.2% of annual lake primary production. Present net annual secondary production by the shrimp is an order of magnitude greater than the present fishery yield of the lake. Detritus-fed experimental organisms evidently had assimilation efficiencies as low as 10% by model calculation.

Relationships of Littoral Fish Abundance to Water Chemistry and Macrophyte Variables in Central Ontario Lakes

We collected data on the most frequently encountered medium- to large-bodied littoral fishes in two independent sets of central Ontario lakes; the first set (10 lakes) was sampled between 1979 and 1980 and the second (15 lakes) from 1982 to 1988. Empirical models were constructed by correlating the strongest with in-year, among-lake trends in species abundance with the strongest among-lake trends in abiotic variables determined by principal coordinates analyses; indices of nearshore vegetation cover were also used as predictor variables. From 1982 to 1988, variation in abundance of Lepomis (pooled pumpkinseed (L. gibbosus) and bluegill (L. macrochirus)) was the major among-lake trend, and this was strongly correlated with differences in lake primary productivity (P &lt; 0.02 for each year) and nearshore macrophyte cover (P &lt; 0.03 for each year). In 1979–80, variation in abundance of pumpkinseed was the major among-lake trend, and this was strongly correlated with among-lake variability in nearshore macrophyte cover (P = 0.006) and lake flushing rate (P = 0,008); bluegill were not present in these lakes. Among-lake variation in food availability and/or abundance of refuges from predators may be factors responsible for the among-lake patterns in Lepomis abundance.

Phytoplankton Utilization of Phosphorus Bound to Suspended Sediments from Selected Tributaries to Lake Geneva

Bioassays with natural phytoplankton exposed to suspended sediments were carried out to assess the effect of phosphorus associated with particles on algal productivity. Sediments were recovered in 1988 and 1989 by continuous flow centrifugation of water samples taken at the mouth of the major rivers in the Lake Geneva basin (Rhone, Dranse, Venoge and A ubonne) and at the effluent of the sewage treatment plant (STP) at Vidy-Lausanne. Sediment samples were analyzed for the forms of phosphorus (total P, organic P, apatite P, and non-apatite inorganic P) and used in an algal fractionation bioassay, involving a 3.5 hour incubation of lake phytoplankton in direct contact with the sediment. Change in growth rate in the > 20 μm and < 20 μm algal size fractions was measured by 14C uptake following filtration and dimethyl sulfoxide extraction, and compared to a lake water control. Results showed that the enhancement of algal uptake induced by the sediment P varies with river and the limnological conditions in the lake. Phytoplankton utilization of particulate nutrients appeared to proceed when soluble P in lake water dropped below a threshold of about 14 μg/L. This finding has significant implications for the direct involvement of particle associated P in primary production in lakes of differing trophic status. No growth was observed upon the addition of sediments from the Rhone River or the STP due to low concentrations of NAIP in the Rhone and to probable toxicity in the STP effluent. Growth was promoted with sediments from the Dranse and the Venoge in 1988, but inhibition occured with Venoge samples in 1989. The river at this time was characterized by low flow resulting in increased concentrations of toxic substances derived from STPs in the watershed. This study thus demonstrates the net effects or interaction of sediment associated nutrients and contaminants on phytoplankton productivity.