Lake Plankton Research Articles

Management of lakes and reservoirs during global climate change

Preface. Changes in Atmospheric Circulation and Climate over the North Atlantic and Europe T.D. Davies, et al. Linkages Between Atmospheric Weather and the Dynamics of Limnetic Phytoplankton C.S. Reynolds. The Influence of Weather Conditions on the Seasonal Plankton Development in a Large and Deep Lake (L. Constance): I. The Impact of Irradiance, Air Temperature and Wind on the Algal Spring Development - An Analysis Based on Long-Term Measurements U. Gaedke, et al. II. Water Column Stability Derived from One-Dimensional Hydrodynamical Models D. Ollinger, E. Bauerle. III. The Impact of Water Column Stability on Spring Algal Development U. Gaedke, et al. IV. The Response of Crustacean Zooplankton to Variations in Water Temperature and Algal Growth in Spring and Early Summer D. Straile, W. Geller. Fluctuations of Phytoplankton Production and Chlorophyll Concentrations in a Small Humic Lake During Six Years (1990-1995) J. Keskitalo, K. Salonen. Sudden and Gradual Responses of Phytoplankton to Global Climate Change: Case Studies from Two Large, Shallow Lakes (Balaton, Hungary, and the Neusiedlersee, Austria/Hungary) J. Padisak. Impact of Climate-Affected Flowrates on Phytoplankton Dynamics in Dam Reservoirs B. Desortova, P. Puncochar. Managing Reservoir Water Quality in the Vltava River Board Area J. Duras. The Mass Invasion of Several Blue-Green Algae in Two Drinking-Water Supply Reservoirs in Southern Poland H. Bucka. Water Quality in the Lakes and Reservoirs of Romania T.L. Constantinescu. Long-Term Variation of Chlorophyll Content in Rybinsk Reservoir (Russia) in Relation to its Hydrological Regime N.M. Mineeva, A.S. Litvinov. Long-Term Hydrobiological Investigations on Lakes in Northwest Russia V.G.Drabkova, et al. The Timing of Warming -- An Important Regulator of Change in Lake Plankton Communities R. Adrian. The Influence of Year-to-Year Changes in Position of the Atlantic Gulf Stream on the Biomass of Zooplankton in Windermere North Basin, UK D.G. George, D.P. Hewitt. Environmental Factors Influencing the Recruitment and Growth of Underyearling Perch (Perca fluviatilis) in Windermere North Basin, UK, from 1966 to 1990 I.J. Winfield, et al. Fluctuations in Lake Fisheries and Global Warming D. Gerdeaux. Interannual Synchronous Dynamics in North Temperate Lakes in Wisconsin, USA T.K. Kratz, et al. Working Group Topic 1: Modelling the Responses of Lakes to Day-to-Day Changes in Weather C.S. Reynolds, et al. Working Group Topic 2: Detecting and Predicting the Responses of Lakes to Global Climate Change U. Gaedke, et al. Working Group Topic 3: Managing Water Quality in a Changing World D.G. George, et al. Working Group Topic 4: International Networks of Lake Sites T.K. Kratz, et al. Index.

Seasonal dynamics of plankton in a mountain lake in the southern Alps (Laghetto Inferiore, Switzerland)

The pelagic populations in Laghetto Inferiore (southern Swiss Alps) were studied between 1996 and 1998 during the ice-free period. Investigations revealed a considerable number of phytoplankton species but few zooplankton species. The vertical distribution of the main species of algae shows a marked vertical stratification, with higher biomass concentrations in the deep zone. Along this profile, species in the same taxonomic class tend to be distributed at different depths, according to their specific light requirements. During the study period no clear seasonal succession of populations was observed, also because of the influence of precipitation on the biological cycles. On the whole there was a good correspondence between the biomass values of phytoplankton and those of chlorophyll-a, both in integral samples of the whole water column and in the vertical profile.

Planktonic nutrient regeneration and cycling efficiency in temperate lakes

Planktonic nutrient regeneration is a fundamental process that maintains most of the primary productivity in marine and freshwater environments. However, there is no robust predictive model to describe the pattern and efficiency of nutrient cycling across aquatic systems. Based on rather weak evidence, the efficiency of nutrient regeneration is believed to decline along a gradient of productivity, so that nutrient-poor environments are assumed to be more efficient at cycling their nutrients than are nutrient-rich environments1,2,3,4,5. Here we measure phosphorus regeneration directly and show that cycling efficiency does not vary with phosphorus concentration. In addition, we confirm that the phosphorus supply for lake plankton comes primarily from within the plankton community, rather than from external loading or from larger organisms such as fish.

Carbon stable isotopes reveal complex trophic interactions in lake plankton.

The lower trophic linkages in lake plankton food webs are generally described as relatively simple, even accounting for the additional complexity of potential 'microbial looping'. Crustacean zooplankton are frequently amalgamated into one trophic functional group as grazers of autotrophic production. The carbon stable isotope ratios for separated zooplankton species, particulate organic matter (POM) and phytoplankton from a number of lakes in Finland and the UK were analysed. These revealed greater complexity in trophic interactions than would otherwise be observed if the zooplankton had been represented by a mixed sample. Grazing zooplankton were usually depleted in (13)C relative to the bulk POM on which they might feed, with (13)C deviating by up to 17 per thousand There were no consistent differences between (13)C values for copepods and cladocerans. Predatory cladocerans were generally enriched by greater than 1 per thousand compared to their putative prey. We suggest that care in separating the zooplankton species for stable isotope analysis may expose otherwise undetected sources of carbon and facilitate unravelling trophic links further up the food web. Copyright 1999 John Wiley & Sons, Ltd.

Biomass and production of plankton in shallow and deep lakes : are there general patterns ?

Empirical models were established, which link the biomasses or productions of the successive planktonic trophic levels in lakes of different depth and trophic status. Published data on 56 lakes were analysed, using both least-square and reduced-major-axis regressions. Reduced-major-axes regression technique was used in order to take into account variability due to both bottom-up and top-down forces. In shallow lakes, the energy transfer efficiency between phytoplankton and zooplankton decreases with increasing trophy. An inverse tendency appears in deep lakes. However, when considering only the oligo- and mesotrophic temperate lakes, the mean primary and secondary productivities per unit of area are about the same in shallow and in deep sites. On the other hand, shallow water bodies are characterized by a smaller mean size of the crustacean species, a lower mean biomass of zooplankton and a higher P zoo/B zoo ratio than deep lakes, probably as a response to the predation by fish. While the consumers seem to have a strong influence on the size structure and biomass of plankton, the energy flows between the trophic levels could be largely controlled by bottom-up forces.

An unusual type of Daphnia head shields from plankton and sediments of Himalayan lakes.

The paper reports the finding of a particular type of Daphnia (Ctenodaphnia, Dybowski & Grochowski 1895) head shields, found in the plankton and in the sediments of two lakes in the Khumbu Region (Nepalese Himalayas). The capsules, all of the smallest size, were characterised by the presence of a hole in the medio-dorsal region, behind the posterior median groove. The location of the hole suggests a link with the nuchal organ, which we found in the neonates of the species. The finding is important for taxonomy and provides information for discussing the current hypothesis on the biogeography of the Ctenodaphnia subgenus. Being well preserved in sediment cores, the capsules are useful tools for reconstructing past phases of active reproduction which occurred also in the past.

Lake Shira, a Siberian salt lake: ecosystem structure and function. 1. Major physico-chemical and biological features

The specific composition, vertical distribution and daily migration of plankton in a meromictic salt lake, Lake Shira, have been investigated. The main structural components of the ecosystem are bacteria, algae and crustaceans. Since the late 1970s, blue-green algae are reported as dominant. Nowadays the phytoplankton is dominated by Lyngbya contorta, Microcystis ichtyoblabe (Cyanophyta), Cyclotella tuberculata (Bacillariophyta) and Dictyosphaerium tetrachotomum (Chlorophyta). The zooplankton of Lake Shira comprises Arctodiaptomus salinus (Copepoda), Brachionus plicatilis and Hexarthra oxiuris (Rotatoria). Investigation revealed a stratified vertical distribution of all dominant species except Cyclotella with a maximum at a depth of 12--15 m during the summer growth period. Zooplanktons differ in the pattern of distribution and daily migration, but most zooplankton tend to concentrate near the surface. The possible reasons for the vertical distribution of the plankton are discussed.

Comparative analysis of lake plankton structure vs. function

Four modeling approaches were used to describe variations in the plankton of a large lake. The objective was to determine whether information about ecological transfer efficiency could be predicted from metrics related to particle size, taxonomic composition, and food web structure. No relationships were found between the structural metrics and ratios of zooplankton to algal carbon uptake (one index of transfer efficiency). Ratios of zooplankton to algal biomass (another often-used index) increased significantly with food web complexity, zooplankton size, and percent dominance by Daphnia. Biomass ratios decreased with increasing algal cell size. The results provide insight into how structural properties of plankton may relate to community function.

UV damage in shallow lakes: the implications of water mixing

The plankton of shallow lakes is expected to experience increasing exposure to ultraviolet radiation (UVR) within the next decades. In addition to stratospheric ozone depletion, decreases in dissolved organic carbon (DOC), caused by climate warming or acidification, will result in greater UVR penetration in many lakes. Circulation patterns induced by moderate winds force the plankton of shallow lakes to be cycled through the whole water column. This translates into higher doses and fluctuating dosage rates. Here, we report the results of a series of ‘ in situ ’ UV exposure experiments in which two zooplankton crustaceans (one capable of photorecovery and the other not) were incubated either at fixed depths or under simulated mixing. Our main goal was to assess the extent to which the results from static incubations could be extrapolated to vertically moving zooplankton. Static incubations accurately predicted the survival of moving individuals for the species without photorecovery, but failed to do so for the species capable of photorecovery. The fluctuations in dosage rate for high doses tended to increase the survival of the species with photorecovery. We contend that neglecting the effects of vertical mixing may result in a biased perception of UVR effects in shallow lakes.

The zooplankton communities of an artificially divided subtropical coastal estuarine–lake system in South Africa

Nhlabane coastal lake and estuary system in South Africa is divided by a permanent barrage constructed at the outlet of the lake. This study compared the zooplankton assemblages of the lake and estuary systems after 15 years of separation. Before sampling commenced the mouth of the estuary closed and remained so throughout the duration of the study. The lake was completely fresh and the estuary had a low salinity not exceeding 4‰. Both the estuary and lake are shallow systems (less than 2 m) and were well mixed. Turbidity levels were generally below 10 NTUs. Temperature fluctuated seasonally between 18.5 and 30°C. Oxygen levels ranged between 4.3 and 9.9 mg/l but were mostly above 5 mg/l for both systems. The estuarine and lake plankton was dominated by copepod instars, especially of the calanoid Pseudodiaptomus hessei. During the entire study period the lake also supported high densities of rotifers. Rotifers only appeared in the estuarine plankton at the end of the sampling period. During the first sampling sessions zooplankton diversity in the estuary was higher but declined thereafter to levels within the range calculated for the lake. Estuarine relict species recorded reflect the estuarine history of the lake. Multivariate analyses indicated a change in the composition of the zooplankton community in the estuary during the duration of the study, while that of the lake remained comparatively stable. Species composition analyses showed that this change in the estuary was due to a shift from an estuarine to a more freshwater zooplankton assemblage. During this change some estuarine species declined in abundance or were not recorded anymore. These changes in the zooplankton community of the estuary may well illustrate the changes that occurred in the lower (south) section of the lake after completion of the barrage which isolated it from the estuary.

Copepod mortality induced by fluctuating levels of natural ultraviolet radiation simulating vertical water mixing

The effect of fluctuating ultraviolet radiation levels on the copepod Boeckella gracilipes was investigated in Lake Escondido (Patagonia, Argentina). The animals were incubated either at fixed depths or rotating in in situ plankton wheels of different diameters. The observed mortality was significantly higher in rotating treatments. Static incubations can be used to predict the mortality of vertically moving B. gracilipes, providing that the doses of UVA and UVB are known. The results suggest that under moderate wind conditions, the plankton of shallow lakes are exposed to potentially damaging levels of solar radiation, even in relatively turbid waters.

Seasonal patterns of mercury species in water and plankton from softwater lakes in Northeastern Minnesota

Twelve softwater lakes in NE Minnesota were sampled in spring, summer, and fall of 1992 and 1993 for labile (unextracted) methyl-Hg, total (extracted) methyl-Hg, and total Hg in lake water and net plankton (≥300 µm). The lakes are small (5.6–56 ha), low productivity, headwater drainage or seepage lakes. They are acid-sensitive (ANC ≤200 µeq/L) but not low pH lakes (average pH 6.6). The lakes ranged in color from 8.5 to 70 PCU. Statistical analysis of the water chemistry variables and mercury species support the conclusion that these were a homogeneous set of lakes; therefore, seasonality of mercury forms was analyzed on combined (mean) data from the 12 lakes. Methyl-Hg in water declined throughout the growing season. HgT also declined sharply from spring to summer but increased again in the fall. In contrast to the methyl-Hg and Hg in water, concentrations in plankton were at the lowest levels in spring and rose to higher levels in summer. The mass of mercury in plankton increased from spring to fall, as did the methyl-Hg fraction, which increased from 20% of HgT in spring to 52% in autumn. Bioaccumulation factors (BAF) for methyl-Hg in net plankton increased over the growing season. Overall, log BAF for HgT in net plankton (wet wt.) was 4.45. Log BAF for methyl-Hg in plankton was 4.90 to 5.43 depending on the analytical form of methyl-Hg in water (labile or total). Seasonal patterns of methyl-Hg and HgT did not covary in water, but did covary in plankton. These results support the conclusion that measurement of Hg in water is not adequate in itself to determine the amount of bioavailable Hg (i.e., methyl-Hg) in a lake. Labile (unextracted) methyl-Hg could be a useful measurement of bioavailable Hg. Labile methyl-Hg exhibits the same seasonal patterns as total methyl-Hg, but does not require the extraction steps necessary for measuring total methyl-Hg.

Midsummer zooplankton assemblages in four types of wetlands in the Upper Midwest, USA

The objective of this study was to characterize the zooplankton and phytoplankton assemblages of four different types of wetlands and to evaluate their use as environmental indicators. Total abundances, community composition, and species diversity were evaluated for zooplankton and phytoplankton assemblages from 24 wetlands and related to water quality variables. During August 1995, six representative sites were sampled from four types of wetlands designated as constructed, impacted, non-impacted, or temporary. The plankton assemblages of all wetlands were dominated by cosmopolitan crustacean, rotifer, and phytoplankton taxa typical of lake plankton communities. Species diversity, richness, and evenness of zooplankton and phytoplankton assemblages did not differ significantly among the wetland types. Total zooplankton abundance was significantly (p < 0.01) related to chlorophyll a and total phosphorus concentrations over the range of trophic conditions. Mean zooplankton densities and phytoplankton biovolumes were similar among the wetlands, however, the relative abundances of major zooplankton groups differed among the wetland types. Cyanophytes, primarily Oscillatoria spp., were a major component of the phytoplankton across all four wetland types, and were significantly more abundant within the constructed and temporary sites. On average, rotifers accounted for 79% of total zooplankton abundance within the constructed wetlands and were much less dominant in the non-impacted and temporary wetlands. Cladoceran, copepodite, and adult copepod concentrations were low in the constructed and impacted wetlands and increased in the non-impacted and temporary wetlands in conjunction with increased chlorophytes and cryptophytes. Our preliminary survey suggests that abiotic factors which are known to directly affect phytoplankton may indirectly affect zooplankton composition in such a way as to use zooplankton assemblages as indicators of water quality. However, further study incorporating seasonal dynamics and the influence of predators on zooplankton assemblages is needed to fully assess the use of zooplankton community composition as an environmental indicator for wetland systems.

Seasonal Dynamics of Daphnia and Algae Explained as a Periodically Forced Predator-Prey System

The classical seasonal sequence of events in lake plankton is an early spring bloom of algae followed by a peak of zooplankton, typically of the genus Daphnia, grazing down algal biomass resulting in the spring clear-water phase around early June. Subsequently, Daphnia numbers usually remain low and algal biomass increases until autumn when a second small Daphnia peak can cause another drop in algal biomass. In this paper we show that this entire scenario with the right timing of events comes out naturally from a minimal model of the Daphnia-algae interaction if we simply vary the parameters of the model sinusoidally over the year to mimic the effect of seasonal variation in temperature, light and predation pressure from planktivorous fish. The model also reproduces the observation that in lakes where fish biomass is low, Daphnia peaks can continue to occur regularly throughout the summer, while in lakes with very high fish biomass Daphnia can be virtually absent throughout the season.

The role of highly unsaturated fatty acids in aquatic foodweb processes

1. Polyunsaturated fatty acids (PUFA) are almost exclusively synthesized by plants. Animals can convert from one form of PUFA to another through elongation and desaturation, but very few can synthesize PUFA de novo. PUFA play an important role in regulating cell membrane properties, serve as precursors for important animal hormones and are essential for animals.2. In aquaculture studies, highly unsaturated fatty acids (HUFA), a subset of PUFA, have been found to be critical for maintaining high growth, survival and reproductive rates and high food conversion efficiencies for a wide variety of marine and freshwater organisms.3. The plankton literature suggests high food‐quality algae species are rich in HUFA and low food‐quality algae are poor in HUFA. Adding semi‐pure emulsions of HUFA to algae monocultures can markedly increase the growth rates of zooplankton feeding on these mixtures.4. A study measuring zooplankton biomass accrual when feeding on natural phytoplankton found a strong correlation between phytoplankton HUFA (specifically eicosapentaenoic acid) content and herbivorous zooplankton production.5. The aquatic ecology literature suggests that planktonic foodwebs with high HUFA content phytoplankton have high zooplankton to phytoplankton biomass ratios, while systems with low HUFA phytoplankton have low zooplankton biomass. Also, the seasonal succession of plankton in many temperate lakes follows patterns tied to phytoplankton HUFA content, with intense zooplankton grazing and ‘clear‐water‐phases’ characteristic of periods when the phytoplankton is dominated by HUFA‐rich species.6. Herbivorous zooplankton production is constrained by the zooplankton’s ability to ingest and digest phytoplankton. It is becoming increasingly clear, however, that much of the phytoplankton which is assimilated may be nutritionally inadequate. HUFA may be key nutritional constituents of zooplankton diets, and may determine energetic efficiency across the plant–animal interface, secondary production and the strength of trophic coupling in aquatic pelagic foodwebs.

A 3600 years paleoclimatic change inferred from organic δ13C and TOC/TN of the Gucheng Lake sediments, southeast China

A 6.2 m thick core of Gucheng Lake sediment provided a 3600 years record of climate change. The contents of the TOC in the core changed from 2.63% to 8.48%, and the δ13C values of organic matter were from −21.54% to −27.3%. The TOC/TN ratios indicated that the organic materials in sediments were from lake plankton and land-derived plants.

Introduction to papers from the GAP VI workshop: effects of physical forcing on primary production processes in inland and marine environments

The Group for Aquatic Primary Productivity (GAP) was established in 1980 at the Societas Internationalis Limnologiae’s (SIL) 21st Congress in Kyoto, Japan. GAP is a working group of both SIL and INTECOL (The International Association of Ecology) and is run by an international committee composed of seven scientists from Australia, Canada, Denmark, France, Israel, Japan, and the United States. The purpose of GAP is to assess the state of knowledge on a wide range of topics related to marine and freshwater primary production, to perform joint field and laboratory experiments using different techniques to test their comparability and reliability, to define major knowledge gaps and urgent research needs, and to publish these results in international journals. To carry out its mandate, GAP holds workshops approximately every 3 years, and each workshop addresses a specific theme. In addition, GAP also convenes smaller ad hoc workshops to address a specific issue or to investigate a regional aspect of primary production. An example of the latter was the investigation of the warm-core Cyrus Eddy in the Levantine Basin of the eastern Mediterranean Sea, organized by Tom Berman of the Israel Oceanographic and Limnological Research Ltd. GAP depends upon proposals from interested scientists, who volunteer themselves and an organizational team and their institutions to organize and host workshops. Past workshops have been held in several European countries and Israel, and results have been published in several journals. In 1993, GAP VI was held at Environment Canada’s National Hydrology Research Institute in Saskatoon, Saskatchewan, a location that provided easy access to a wide variety of saline lakes and to the South Saskatchewan River. The theme of GAP VI was “Effects of physical forcing on primary production processes in inland and marine environments.” The global concerns about the depletion of the ozone layer and the effects of increased ultraviolet (UV) radiation on aquatic systems resulted in many of the GAP VI participants examining this topic. Four papers containing data produced during GAP VI appear in this issue of the Canadian Journal of Fisheries and Aquatic Sciences. Each paper was reviewed by two or three referees before being accepted for publication. I thank these referees for their time and interest in helping the authors prepare their final manuscripts. Beardall et al. reported changes in respiratory activity and photosynthetic carbon assimilation of a cyanobacterium and a green alga when exposed to different levels of photosynthetically active radiation (PAR) and UV-A + UV-B radiation. The organisms differed in their responses, which led Beardall et al. to conclude that there is considerable interspecific variation in the response of autotrophic organisms to UV radiation and these responses are modulated by the level of concomitant exposure to PAR. Information on UV radiation effects on the plankton of prairie saline lakes in Canada is lacking. Two studies were conducted with water from Redberry Lake, an oligotrophic saline lake. Maske and Latasa presented data on changes in phytoplankton pigment content, and suggested that environmental factors other than UV-B, or the state of cellular adaptation, partially controlled reduction of pigment. Ferreyra et al. measured the effects of UV-B on phytoplankton carbon assimilation, chlorophyll a concentration, bacterial numbers, and bacterial electron transport system activity, and found different responses to UV-B in these two groups of organisms. In addressing the major GAP objective of conducting methodological comparisons, Bott et al. worked in the South Saskatchewan River to compare four methods to measure periphyton metabolism in in situ chambers. As a result of this research the authors were able to make recommendations for future research that is needed when measuring periphyton metabolism in chambers. GAP workshops offer an unique opportunity for aquatic scientists to come together in short-lived, tightly focused teams and effectively address key research questions, as demonstrated by these papers from GAP VI.

Possible impact of mild winters on zooplankton succession in eutrophic lakes of the Atlantic European area

1. We report on a long‐term study (1975–94) of water temperatures and plankton in a eutrophic lake (Heiligensee, Berlin, Germany). Using a phenomenological approach, we use historical data to infer how an increase in air temperature has influenced a natural zooplankton community.2. Air temperatures in Berlin showed a significantly rising trend between 1975 and 1994. Mean winter air temperatures in the last 8 years always exceeded the long‐term mean.3. A rising trend was also found for April water temperature, which increased significantly beginning in 1988–89. An increase of 2.58°C in the last 21 years was recorded using a linear model. A significantly decreasing trend was found in June but no trend was noted for the other summer months.4. Phytoplankton composition shifted from a dominance of diatoms and cryptophytes during winter and spring in the 1980s towards a dominance of cyanobacteria in 1990–94.5. The dominant zooplankton species in spring shifted in recent years from the large‐bodied Daphnia galeata to the smaller D. cucullata. Cyclops kolensis, previously the only invertebrate predator during winter, decreased in abundance while C. vicinus, usually present during spring and autumn, increased in abundance and was numerous during winter, a season passed in diapause in the earlier years.6. Because direct and indirect temperature effects are species specific, we put forward the hypothesis that zooplankton species, rather than functional groups, are the nexus between environmental stress, such as rising air temperatures, and ecosystem changes.

Effects of phosphorus and allochthonous humic matter enrichment on metabolic processes and community structure of plankton in a boreal lake (Lake Pääjärvi)

Pendant la saison libre de glaces, trois experiences durant chacune 16 ou 17 jours ont ete realisees dans quatre enclos en vue d'evaluer les effets sur le plancton de poussees moderees de matiere humique allochtone (5-29% de la concentration ambiante de carbone organique dissous) et de phosphore (PO 4 -P, passe de 1-2 a 15-20 μg.L -1 ). L'ajout de phosphore, seul ou accompagne de matiere humique, a nettement fait augmenter la production primaire en ete (54-56%) et, dans une moindre mesure, au printemps (17-23%) et a l'automne (17-58%). Une correlation significative etait manifeste entre la production primaire et la production bacterienne, ce qui indiquait que la production bacteriene etait controlee avant tout par les substrats organiques liberes par les algues plutot que par phosphore ou la matiere humique. Dans l'enclos temoin, la production bacterienne constituait 23-32% de la production primaire, tandis qu'avec l'ajout de phosphore et de matiere humique la proportion a monte a 59%, ce qui indique que la matiere humique etait utilisee par les bacteries comme source additionnelle d'energie et de carbone en plus des exudats des algues. L'enrichissement simultane avec phosphore et matiere humique a donne une production bacterienne nettement plus haute qu'avec la matiere humique seulement. A l'automne, l'ajout de matiere humique seule faisait monter la production bacterienne, ce qui indique que la matiere humique contenait alors des matieres organiques plus facilement utilisables qu'auparavant. Certains taxons algaux ont reagi positivement a l'enrichissement avec phosphore et (ou) matiere humique, mais les effets sur les communautes proto-et meta-zooplanctoniques etaient moins evidents.

Decreased carbon and nutrient input to boreal lakes from particulate organic matter following riparian clear-cutting

The plankton communities of oligotrophic Canadian Shield lakes are strongly regulated by the allochthonous supply of total phosphorus (TP) and dissolved organic carbon (DOC), a proportion of both of which originate from particulate organic matter. Although decreased inputs of allochthonous leaf litter have been documented for small streams whose riparian forests have been removed, no such data exist for boreal lakes. Through estimates of airborne litter input from forested and clear-cut shorelines and laboratory measurements of concentrations released from leaf leachate, we determined that riparian deforestation resulted in reductions of DOC from 17.8 to 0.4 g/m shoreline/yr and of TP from 2.9 to 0.3 g/m shoreline/yr. Previous predictive models indicate that such reductions may be substantial enough to decrease basic metabolic processes of lake plankton communities by as much as 9% in primary production and 17% in respiration.