Epilimnetic Phytoplankton Research Articles

Contrasting energy pathways suggest differing susceptibility of pelagic fishes to an invasive ecosystem engineer in a large lake system

Species invasions can lead to ecological regime shifts by altering food web structure and changing nutrient cycling. Stable isotopes are a powerful tool to understand the potential and realized impacts of invasive species on food webs, especially when used in tandem with other dietary tracers. An invasion by one of the most notorious freshwater invaders in North America, the quagga mussel (Dreissena bugensis), is imminent in Lake Champlain, United States. An invasion by this filter feeder has the potential to drastically alter energy pathways and destabilize pelagic fisheries via bottom-up impacts. However, the extent and magnitude of these impacts depend on the current food web structure of the mid-trophic pelagic food web, which was previously not well described. We used Bayesian stable isotope mixing models informed by stomach content analysis to identify which energy pathways are currently most important to mid-trophic level fishes. We determined that in the Main Lake basin, the spring phytoplankton bloom and deep chlorophyll layer – the resources most vulnerable to quagga mussels – provide a disproportionate amount of support to the pelagic food web via zooplankton and the migrating macroinvertebrate Mysis. The food web in the Northeast Arm of Lake Champlain is supported by epilimnetic phytoplankton, which is more protected from the filtration effects of quagga mussels than the deep chlorophyll layer. However, the Northeast Arm will likely not provide a high-quality foraging refuge to coldwater pelagic fish due to unfavorable oxythermal conditions. The mid-trophic food web of Lake Champlain—and consequently piscivores who rely on these prey—may be vulnerable to the impending quagga mussel invasion if migratory Mysis are not able to shift their diet to benthic resources.

Functional Groups of Phytoplankton and Their Relationship with Environmental Factors in the Restored Uzarzewskie Lake

Uzarzewskie Lake is a small, postglacial lake, located in western Poland. The lake is under restoration treatment since 2006. At first, iron treatment was done for 2 years. In the second stage, spring water was directed into the hypolimnion in order to improve water oxygenation near the bottom sediments. The purpose of our research was to determine changes in the contribution of functional groups to the total number of taxa and total biomass of phytoplankton due to changes in the physical and chemical characteristics of the restored lake. Phytoplankton composition was analyzed in three periods: (1) before restoration; (2) during the first method of restoration; and (3) when the second method was implemented in the lake. Epilimnetic phytoplankton was sampled every year monthly from March to November. The relationship between phytoplankton groups and environmental factors (water temperature, ammonium nitrogen, nitrate nitrogen, dissolved phosphorus, conductivity and pH) was examined, using the canonical analyses. The redundancy analysis indicated that the temperature, dissolved phosphates concentration, ammonium nitrogen and pH were the main determining factors of the phytoplankton community dynamics. During the study, 13 coda dominated the phytoplankton biomass. Cyanobacteria of the codon H1 with such species as Aphanizomenon gracile, Dolichospermum planctonicum, D. viguieri dominated the phytoplankton community before restoration. S1 group consisting of Planktolyngbya limnetica, Limnothrix redekei and Planktothrix agardhii mostly dominated during the period in which the first method was used. Improvement of water quality due to restoration efforts in the third period caused dominance of other groups, especially J (Actinastrum hantzschii and other Chlorococcales), C (Asterionella formosa and other diatoms), Y (Cryptomonas marssonii and other cryptophytes), Lo (Peridiniopsis cunningtonii and other dinophytes) and X2 (Rhodomonas lacustris).

Sedimentation of ballasted cells-free EPS in meromictic Fayetteville Green Lake.

Fayetteville Green Lake (FGL) is a recognized, extensively studied present-day model of the stratified Proterozoic ocean. Nonetheless, biomass sedimentation in FGL remains hard to explain: while virtually all sediment pigments belong to photosynthetic sulfur bacteria from a chemocline, the isotopic carbon signature of the bulk organic matter suggests its epilimnetic phytoplankton origin. To explain the epilimnetic origin of sedimented carbon, we studied the dominant Synechococci, isolated from FGL. Here, we present experimental evidence that FGL Synechococci produce copious extracellular polysaccharides (EPS) especially when availability of inorganic carbon (Ci ) is high relative to availability of other macronutrients, for example phosphorus. The accumulating EPS become impregnated with calcium, magnesium, and sodium cations and are released to the environment as ballasted cell coverings. Sedimentation of these cell-free EPS can constitute the bulk of pigment-free organic material in FGL sediment. Because increased availability of Ci specifically stimulates production of EPS and the accumulated EPS adsorb cations and become ballasted, we propose the universal role of cyanobacterial EPS in biomass sedimentation in the high-Ci Paleoproterozoic ocean as well as in modern aquatic systems like FGL.

Phytoplankton response to the summer 2015 heat wave – a case study from prealpine Lake Mondsee, Austria

Phytoplankton response to a heat wave in deep prealpine Lake Mondsee, Austria, was minor overall, although temperature significantly affected the epilimnetic phytoplankton assemblage at the deepest site of the lake. We detected no significant effect of nutrients. Using 3 complementary optical methods—light microscopy (Utermöhl technique), FlowCAM, and acoustic flow cytometry (AFC)—we found relatively low horizontal variation in the epilimnion but significant changes in phytoplankton community composition integrated over the 0–20 m water layer at the central station of the lake. These changes were mainly caused by a vertical shift in the Planktothrix rubescens peak from 11 to 16 m during the heat wave. Air temperatures reaching or exceeding 30 °C were measured in the area during 6 consecutive days at the end of June to the beginning of July, the first of 3 heat waves recorded during summer 2015; in-shore surface water temperature exceeded 27 °C. We sampled 9 stations across Lake Mondsee on 4 occasions during the heat wave and analysed temperature, nutrient levels, conductivity, pH, and the phytoplankton community. In addition to reporting the implications of increasing water temperatures for the algal assemblage in a deep stratified lake, we discuss the pros and cons of the different optical methods for phytoplankton identification and counting. For future field campaigns similar to the present study, we recommend using light microscopy to assess large or rare species, such as Ceratium hirundinella, and AFC, FlowCAM, or similar semi-automated devices for abundant small- to medium-sized species.

Macrozooplankton and the persistence of the deep chlorophyll maximum in a stratified lake

Summary Deep chlorophyll maxima (DCM) are common in deep, oligotrophic stratified lakes. The DCM refer to the maximal chlorophyll a concentration found at depth, and not at the lake surface. While control of the DCM is thought to be via physicochemical factors in many lakes, a role for zooplankton grazing in epilimnetic waters remains a possibility. The occurrence and dynamics of DCM are poorly documented in smaller lakes, where zooplankton grazing is likely to have a stronger structuring effect. In small, shallow stratified lakes, biological control by grazing may be magnified by the short vertical gradient and overall higher water temperature. The respective contributions of several physical, chemical and biological parameters to the vertical distribution of phytoplankton biomass in a small stratified lake were examined. Associations between phytoplankton depth distribution and vertical gradients in temperature, light and nutrients and the density of herbivorous zooplankton were established through regressions and generalised linear models. Colimitation of the DCM by light from above and nutrients from below was detected. A threshold was detected at 3% incident light (100 μmol photon m−2 s−1), below which the DCM disappeared. Epilimnetic biomass was related to nutrient availability, with a threshold concentration at 4 μg P L−1, below which the DCM dominated. Greater stability of the water mass and more zooplankton were associated with higher phytoplankton biomass in the DCM. Stability is likely to have controlled vertical nutrient fluxes, which were intercepted by the metalimnetic phytoplankton. Zooplankton grazing of epilimnetic biomass could have increased incident light reaching the top of the metalimnion, thereby favouring proliferation of photosynthetic biomass in the DCM. Wind mixing events, as detected by a reduction in Lake number (LN, a measure of the influence of wind forcing on vertical structure), induced vertical intrusions of metalimnetic water, rich in nutrients and phytoplankton, into the epilimnion. We can infer that dominance of phytoplankton in the epilimnion would have occurred earlier during the summer if grazing by zooplankton had not removed epilimnetic phytoplankton. Our results suggest that, while stable stratification is necessary for initial DCM formation, zooplankton grazing may promote the persistence of a DCM.

Photophysiology and its response to visible and ultraviolet radiation in freshwater phytoplankton from contrasting light regimes

Pulse amplitude-modulated (PAM) fluorometry was used to obtain rapid light curves (RLCs) for phytoplankton from small Canadian lakes of varying water clarity, including metalimnetic communities from two clear lakes. RLCs were measured before and after exposure to a number of experimental spectra containing ultraviolet radiation (UVR) and photosynthetically active radiation (PAR). Photochemical (qP) and non-photochemical (NPQ) quenching during RLCs were positively and significantly correlated with recent in situ light history, but the maximum quantum efficiency of Photosystem II (α) and maximum relative rate of electron transport (rETRmax) were not. rETRmax and α were diminished by experimental exposures to UVR and/or high PAR, but significantly less so in phytoplankton from brighter environments. UVR exposures diminished the inducible (photoprotective) NPQ of most epilimnetic phytoplankton. Both the inducible and total (photoprotective + photoinhibitory) NPQ of metalimnetic phytoplankton were stimulated by spectral exposures. Our results are the first obtained from natural communities of freshwater phytoplankton to show that aspects of PSII photophysiology (as inferred from RLCs) vary according to in situ light history.

Light and nutrient co-limitation of phytoplankton communities in a large reservoir: Lake Diefenbaker, Saskatchewan, Canada

The reliance on reservoirs to meet global water demands is increasing, as is the need to develop an understanding of factors influencing water quality in these understudied water bodies. This study assessed how the interaction between light and nutrients influences phytoplankton dynamics in a large, heterogeneous prairie reservoir; Lake Diefenbaker, Saskatchewan. A multiple indicator assessment of factors (light, phosphorus [P], and nitrogen [N]) influencing phytoplankton biomass, physiology, and gross primary productivity (GPP) was conducted. Light deficiency was assessed through the application of an indicator threshold for the mean daily mixed layer irradiance as well as the ratio of this irradiance to the light saturation parameter derived from photosynthesis-irradiance curves. Short-term physiological assays and long-term compositional nutrient status indicators were applied to assess the in situ nutrient deficiency of the epilimnetic phytoplankton communities. We observed regional differences in light conditions and nutrient chemistry related to incoming flow and nutrient load from the South Saskatchewan River. Light deficiency was detected 59% of the times measured during the open-water season of 2013. Both GPP and quantum efficiency of photosystem II (ϕPSII) were relatively high (108 ± 101 mmol O2 m− 2 day− 1 and 0.57 ± 0.07, respectively) across all regions and months. During conditions of light sufficiency, there was no evidence of N deficiency, and P was the limiting nutrient to phytoplankton communities 90% of the times measured. Nutrient use efficiency and GPP rates were significantly higher under light sufficient conditions. The asynchrony of light and nutrient supply influences production and biomass accrual in this reservoir.

A comparison of epilimnetic versus metalimnetic phytoplankton assemblages in two mesotrophic lakes

Abstract The epilimnetic and metalimnetic phytoplankton assemblages were compared in two mesotrophic lakes (L. Hańcza and L. Dejguny, northeastern Poland) and a “metalimnetic niche” is described. Phytoplankton analysis conducted during July and August 2006, 2007, and 2008 indicated that the total phytoplankton biomass in both lakes was relatively small (from 0.6 to 5.2 mg dm−3), with maximum values mainly occurring in the metalimnion. The lowest number of species and diversity indices values were usually observed in the metalimnetic layers. Lake Hańcza was dominated by diatoms (e.g. Cyclotella spp.) whereas Lake Dejguny was mainly dominated by cyanoprokaryotes (e.g. Planktothrix agardhii), with a distinct predominance of diatoms (e.g. Tabellaria flocculosa) in 2007. The common features of the phytoplankton assemblages in both lakes were a tolerance to nutrient and light deficiencies. A cluster analysis of the taxonomic patterns indicated large dissimilarities between the lakes. The phytoplankton assemblages developed under the influence of seasonal conditions, especially in Lake Hańcza. The distinction between metalimnetic and epilimnetic phytoplankton assemblages only appeared in Lake Dejguny.

Deep chlorophyll maxima and UVR acclimation by epilimnetic phytoplankton

1. It is well established that ultraviolet radiation (UVR) has many harmful effects on phytoplankton, but the factors controlling algal sensitivity to UVR are not fully understood. 2. We exposed phytoplankton communities from the epilimnia and deep chlorophyll maxima (DCM) of 2 Canadian lakes to 14 irradiance treatments of various spectral quality and monitored changes in the maximum quantum efficiency of Photosystem II photochemistry (Fv/Fm) using a pulse amplitude modulation fluorometer. 3. Phytoplankton from DCM did not show marked differences from epilimnetic communities in taxonomy or nutrient status, but exhibited substantially higher photosynthetic impairment under UVR exposure. 4. Our results suggest that epilimnetic phytoplankton acclimate to in situ light conditions in a spectrally-specific manner, and that ultraviolet-A radiation is a stronger stressor than ultraviolet-B or photosynthetically active radiation in the mixed layers of our study lakes. Model estimates of damage and recovery rate constants revealed that the phytoplankton of the two lakes relied upon different strategies of UVR-acclimation, in one lake minimising susceptibility to photodamage and in the other maximising recovery efficiency.

Estimates of phosphorus entrainment in Lake Mendota: a comparison of one‐dimensional and three‐dimensional approaches

Entrainment of phosphorus across the thermocline can be an important nutrient source for phytoplankton in stratified lakes. In eutrophic stratified lakes, seasonal entrainment can be responsible for delayed recovery following a decrease in external phosphorus load. We compared seasonal estimates of entrainment derived from single‐ and multilocation thermocline migration approaches. Entrainment estimates from these methods were similar. A sampling approach based on a single centralized location produced whole‐lake entrainment estimates in general agreement with the mean value from the multilocation approach (the single‐location approach reasonably reproduced patterns at a weekly and seasonal time scale). In this study, it was not essential to account for spatial variation in estimating annual rates of entrainment. We also estimated entrainment based on a three‐dimensional (3‐D) hydrodynamic model over a short period of significant thermocline migration. The 3‐D model was most useful in exposing spatial and temporal variation in temperature and phosphorus profiles that are otherwise difficult to observe. Spatial variation in phosphorus profiles was associated with upwelling of metalimnetic water represented by the 3‐D model. These transient dynamics, though a relatively small portion of an annual phosphorus budget, may supply nutrients to epilimnetic phytoplankton during periods of nutrient limitation. There is a need for further research that combines 3‐D hydrodynamic modeling with field collection of biological and chemical data.

Exploring ecological patterns with structural equation modeling and Bayesian analysis

Structural equation modeling is a multivariate statistical method that allows evaluation of a network of relationships between manifest and latent variables. In this statistical technique, preconceptualizations that reflect research questions or existing knowledge of system structure create the initial framework for model development, while both direct and indirect effects and measurement errors are considered. Given the interesting features of this method, it is quite surprising that the number of applications in ecology is limited, and even less common in aquatic ecosystems. This study presents two examples where structural equation modeling is used for exploring ecological structures; i.e., summer epilimnetic phytoplankton dynamics. Both eutrophic (Lake Mendota) and mesotrophic (Lake Washington) conditions were used to test an initial hypothesized model that considered the regulatory role of abiotic factors and biological interactions on lake phytoplankton dynamics and water clarity during the summer stratification period. Generally, the model gave plausible results, while a higher proportion of the observed variability was accounted for in the eutrophic environment. Most importantly, we show that structural equation modeling provided a convenient means for assessing the relative role of several ecological processes (e.g., vertical mixing, intrusions of the hypolimnetic nutrient stock, herbivory) known to determine the levels of water quality variables of management interest (e.g., water clarity, cyanobacteria). A Bayesian hierarchical methodology is also introduced to relax the classical identifiability restrictions and treat them as stochastic. Additional advantages of the Bayesian approach are the flexible incorporation of prior knowledge on parameters, the ability to get information on multimodality in marginal densities (undetectable by standard procedures), and the fact that the structural equation modeling process does not rely on asymptotic theory which is particularly important when the sample size is small (commonly experienced in environmental studies). Special emphasis is given on how this Bayesian methodological framework can be used for assessing eutrophic conditions and assisting water quality management. Structural equation modeling has several attractive features that can be particularly useful to researchers when exploring ecological patterns or disentangling complex environmental management issues.

Photosynthetic carbon allocation: Effects of planktivorous fish and nutrient enrichment

The proportions of lipid, polysaccharide and protein profoundly influence the nutritional value of food for planktonic organisms. Here, we investigated the effects of nutrients (N and P) and planktivorous fish (Phoxinus sp.) on the partitioning of photosynthetically fixed 14C-labelled bicarbonate into macromolecules (lipids, polysaccharides and proteins) and low molecular weight (LMW) metabolites of epilimnetic phytoplankton living in large (8 m diameter) enclosures from July to mid-October 1991. The enclosures were situated in an oligotrophic lake (north of Montreal, Canada) in both shallow (3.5 m) and deep (11 m) locations. The fractions were calculated using the sequential extraction of 14C-labeled phytoplankton samples incubated for short duration (∼2 hours) at maximum photosynthetic rates. Labelling patterns of all end products exhibited great temporal variations. Overall, the LMW, polysaccharide, lipid and protein fractions accounted for 32.1 ± 10.5%, 31.0 ± 8.8%, 19.2 ± 5.3% and 17.8 ± 6.1% of the total 14C fixed respectively. Percent 14C in the LMW fraction was, in general, lower in the nutrient-enriched enclosure and was usually lower in the deep than the shallow enclosures. The effect of fish on the labelling pattern of LMW varied throughout the sampling period but, overall, its impact was non-significant. The proportions of 14C into lipids and polysaccharides did not show the expected increase under nutrient deficiency. In contrast, the polysaccharide fraction increased, for the most part, with nutrient enrichment and was higher in the deep than the shallow enclosures with little changes in the presence of planktivorous fish. Incorporation of 14C into lipids remained relatively uniform throughout the study and averaged less than 20% of total fixed carbon. Nutrient addition increased the lipid fraction in early July but did not have a significant effect thereafter. In general, the presence of planktivorous fish decreased the lipid fraction especially in the deep N + F enclosures. With nutrient enrichment, the proportion of 14C incorporated into protein decreased in the shallow enclosures while it increased in the deep enclosures. During most of the study, there was no significant effect of fish on the distribution pattern of protein. Percent carbon protein averaged 20% of total carbon fixed during the summer and decreased to 10% in October with lower water temperatures. Any fish effects encountered throughout this study were probably mediated indirectly through changes in the species and cell size compositions of phytoplankton. An experiment where samples were incubated with depth (0.5 to 7 m) showed that, for all treatments, the protein fraction increased with depth at the expense of lipid and LMW metabolites syntheses. The polysaccharide fraction demonstrated no apparent relationships with depth. These changes with depth most likely reflect light attenuation rather than a decrease in temperature. Overall, our findings would suggest that the distribution patterns of photosynthetically incorporated carbon into LMW compounds and macromolecules in natural phytoplankton communities are controlled by a combination of both abiotic (e.g., nutrients, light and temperature) and biotic (species and cell size) environmental factors and the interactions between these factors will in essence influence the direction of carbon flow and the food quality of the phytoplankton. These observations are placed in context with a review of the literature on carbon allocation by natural populations of phytoplankton.

Century-long effects of forest harvest on the physical structure and autotrophic community of a small temperate lake

The effects of forest clearcut and regrowth on a small (8.1 ha) stained lake were studied using a 200-year record of fossil carotenoids and chlorophylls. Comparison of high-resolution cores recovered from exposed, sheltered, and reference lake basins showed that annual laminations of sediments ceased concomitant with logging (ca. 1870-1890), indicating a fundamental change in the lake mixing regime. Biological responses to forest harvest included elimination of deepwater populations of anaerobic photosynthetic bacteria and reduced abundance of metalimnetic chrysophytes. Photosynthetic bacteria remained absent for over 100 years, while sediment laminae and chrysophytes have only returned since around 1970. In contrast, populations of epilimnetic phytoplankton (cyanobacteria, chlorophytes, cryptophytes) were unaffected by the clearcut. Analysis of sediment profiles did not reveal evidence of eutrophication or of increased flux of organic matter following watershed disturbance. Instead, fossil records were most consistent with increased wind stress, leading to deeper water column mixing in fall and reduction in deepwater anoxia, the main factor promoting sediment laminations and populations of photosynthetic bacteria. Such century-long disturbance may be common in small, stratified boreal lakes that lack physical shelter due to landforms.

Century-long effects of forest harvest on the physical structure and autotrophic community of a small temperate lake

The effects of forest clearcut and regrowth on a small (8.1 ha) stained lake were studied using a 200-year record of fossil carotenoids and chlorophylls. Comparison of high-resolution cores recovered from exposed, sheltered, and reference lake basins showed that annual laminations of sediments ceased concomitant with logging (ca. 1870-1890), indicating a fundamental change in the lake mixing regime. Biological responses to forest harvest included elimination of deepwater populations of anaerobic photosynthetic bacteria and reduced abundance of metalimnetic chrysophytes. Photosynthetic bacteria remained absent for over 100 years, while sediment laminae and chrysophytes have only returned since around 1970. In contrast, populations of epilimnetic phytoplankton (cyanobacteria, chlorophytes, cryptophytes) were unaffected by the clearcut. Analysis of sediment profiles did not reveal evidence of eutrophication or of increased flux of organic matter following watershed disturbance. Instead, fossil records were m...

Responses of epilimnetic phytoplankton to experimental nutrient enrichment in three small seepage lakes

This paper describes the responses of three epilimnetic phytoplankton communities to experimental nitrogen and phosphorus enrichment as compared to the phytoplankton community in a fourth, unmanipulated, lake. Increased nutrient inputs increased total phytoplankton biomass, primary productivity, chlorophytes, cryptomonads and species turnover rates in all three enriched lakes; cyanobacteria increased in two of the three enriched lakes. However, nutrient addition also led to declines in previously dominant dinoflagellates and chrysophytes, and in species diversity. At the species level, there were large changes in community composition from year to year in both enriched and reference lakes, suggesting that phytoplankton community composition is highly dynamic even in the absence of enrichment. Overall, changes in total biomass, productivity and species diversity were consistent among the enriched lakes, while changes in species composition differed due to variation in the physical, chemical and biotic environment of each lake. This suggests that aggregated variates are more useful for quantitative prediction of nutrient effects, while species responses can be used to signal qualitative differences in environmental conditions among lakes.

Predicting chlorophyll vertical distribution in response to epilimnetic nutrient enrichment in small stratified lakes

Light-limited metalimnetic phytoplankton communities are thought to be negatively impacted by epilimnetic nutrient enrichment because of shading by increased epilimnetic phytoplankton biomass. We tested this expectation with a dynamic simulation model that was calibrated to three lakes undergoing whole-lake nutrient and food web manipulations. Total areal chlorophyll increased due to nutrient enrichment in each lake, but the magnitude of the response varied between lakes. Modeling experiments, which allowed analysis of separate components of each lake's response to nutrient enrichment, indicated that the response to enrichment depended on lake water color and food web structure. In weakly stained lakes (˜10 mg Pt 1−1, k4 = 0.4 m−1), metalimnetic chlorophyll was stimulated by nutrient enrichment up to moderate levels (1 μg Pt1−1 day−1). In more strongly colored lakes (25 mg Pt 1−1, k4 = 1.0), metalimnetic chlorophyll responded negatively to nutrient enrichment at all P loading rates. Food web structure, as expressed by rates of zooplanktivory, interacted with water color in two ways. One impact was through direct grazing losses on metalimnetic chlorophyll. The other process involved was indirect impact from grazing on epilimnetic phytoplankton, which reduced shading on metalimnetic chlorophyll. Vertical redistribution of chlorophyll between the epilimnion and the metalimnion led to little accumulation of areal chlorophyll with increased P loading over limited ranges of water color and nutrient input rates. Model predictions may be most effectively tested with whole-lake experiments contrasting food web structure, water color and nutrient loading.

Patterns in planktonic P:R ratios in lakes: Influence of lake trophy and dissolved organic carbon

A comparative analysis of planktonic metabolism in 20 southern Québec lakes was carried out to test the hypothesis that planktonic P : R ratios reflect gradients of both nutrient enrichment and dissolved organic carbon (DOC). Mean epilimnetic phytoplankton photosynthesis ranged from 8 to 377 mg C m−3 d−1, and the amount of C respired by the plankton in excess of phytoplankton photosynthesis ranged from 30 to 86 mg C m−3 d−1 . Plankton community respiration was 2–8 times greater than phytoplankton photosynthesis in all oligotrophic and mesotrophic lakes during the growing season, and this imbalance narrowed toward the more productive lakes. P : R ratios were positively related to chlorophyll and total P concentration, and inversely related to water color and DOC concentration. The strong influence of DOC on planktonic P : R ratios was almost exclusively due to its depressing effect on phytoplankton photosynthesis; DOC had no statistical effect on respiration. The calculated DOC loading for these lakes suggests that organic C loss through epilimnetic respiration in excess of phytoplankton photosynthesis is comparable to the estimated DOC loss within the lakes and that summer plankton metabolism could be supported by external DOC inputs in most lakes. The highly significant intercept of the respiration to production relationship, 27 mg C m−3 d−1, may indicate the baseline metabolism supported by external sources of C that occurs in these temperate lakes. Estimates of respiratory CO2 production from the pelagial of these lakes range from 11 to 60 mmol CO2 m−2 d−1, depending on lake trophy and DOC concentration. These estimates suggest that the planktonic metabolism of allochthonous DOC probably constitutes a major source of CO2 in lakes.

Trophic interactions and habitat segregation between competing Daphnia species.

Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.

Community structure in epilinmetic and metalimnetic phytoplankton assemblages

Epilimnetic and metalimnetic phytoplankton communities were compared to assess the relative importance of autogenic and allogenic factors regulating community composition and structure. In the epilimnion, opportunistic species were predominant, and as a result there was considerabletemporal change in the community as measured by the community coefficient. In the metalimnion, however, temporal changes in the community coefficient were small compared with changes along the vertical gradient. It appears that non-equilibrium dynamics caused by short term changes in physical processes might be regulating the phytoplankton community in the epilimnion, and vertical niche partitioning along the vertical gradient was minor compared with temporal changes of community structure. The metalimnion supported communities with a more persistent structure over time, although vertical resource partitioning between density layers resulted in considerable change of community structure with depths.

Phytoplankton Community Responses to Nutrient Addition in Lake 226, Experimental Lakes Area, Northwestern Ontario

Inclusion of phosphorus in fertilizer added to one of two basins of a small lake, between which water exchange was greatly reduced by a vinyl sea-curtain, significantly increased epilimnetic phytoplankton biomass, and altered species composition. Over an 8-yr period, the average biomass of phytoplankton in the basin receiving carbon, nitrogen, and phosphorus (in ratios of 10:5:1) increased 4 to 8 times and the biomass in the basin receiving only carbon and nitrogen (in ratios of 10:5) increased 2 to 4 times over non-fertilized years. The basin receiving all three nutrients consistently had blooms in late summer which were dominated by nitrogen fixing species of cyanophytes. In comparison with unfertilized reference lakes, the proportion of phytoplankton in fertilized basins which was available to zooplankton as food was 10 times greater in the basin receiving all three nutrients, and two times greater in the basin receiving only nitrogen and carbon. When fertilization of both basins was stopped, species composition and levels of biomass reverted within a year to the composition and biomass levels of phytoplankton observed in the reference lakes and remained at those levels for 2 further years of study.