A Framework for Understanding Variation in Pelagic Gross Primary Production of Lake Ecosystems

Light and nutrient availability are key physiological constraints for primary production. Widespread environmental changes are causing variability in loads of terrestrial dissolved organic carbon (DOC) and nutrients from watersheds to lakes, contributing to simultaneous changes in both light and nutrient supply. Experimental evidence highlights the potential for these watershed loads to create complex and context-dependent responses of within-lake primary production; however, the field lacks a predictive model to investigate these responses. We embedded a well-established physiological model of phytoplankton growth within an ecosystem model of nutrient and DOC supply to assess how simultaneous changes in DOC and nutrient loads could impact pelagic primary production in lakes. The model generated a unimodal relationship between GPP and DOC concentration when loads of DOC and nutrients were tightly correlated across space or time. In this unimodal relationship, the magnitude of the peak GPP was primarily determined by the DOC-to-nutrient ratio of the load, and the location of the peak along the DOC axis was primarily determined by lake area. Greater nutrient supply relative to DOC load contributed to greater productivity, and larger lake area increased light limitation for primary producers at a given DOC concentration, owing to the positive relationship between lake area and epilimnion depth. When loads of DOC and nutrients were not tightly correlated in space or time, the model generated a wedge-shaped pattern between GPP and DOC, consistent with spatial surveys from a global set of lakes. Our model is thus capable of unifying the diversity of empirically observed spatial and temporal responses of lake productivity to DOC and mineral nutrient supply presented in the literature, and provides qualitative predictions for how lake pelagic primary productivity may respond to widespread environmental changes.     

Nitrogen effects on the pelagic food web are modified by dissolved organic carbon

Global environmental change has altered the nitrogen (N) cycle and enhanced terrestrial dissolved organic carbon (DOC) loadings to northern boreal lakes. However, it is still unclear how enhanced N availability affects pelagic food web efficiency (FWE) and crustacean zooplankton growth in N limited boreal lakes. Here, we performed in situ mesocosm experiments in six unproductive boreal Swedish lakes, paired across a DOC gradient, with one lake in each pair fertilized with N (2011: reference year; 2012, 2013: impact years). We assessed how zooplankton growth and FWE were affected by changes in pelagic energy mobilization (PEM), food chain length (phytoplankton versus bacterial production based food chain, i.e. PP:BP), and food quality (seston stoichiometry) in response to N fertilization. Although PP, PEM and PP:BP increased in low and medium DOC lakes after N fertilization, consumer growth and FWE were reduced, especially at low DOC—potentially due to reduced phytoplankton food quality [increased C: phosphorus (P); N:P]. At high DOC, N fertilization caused modest increases in PP and PEM, with marginal changes in PP:BP and phytoplankton food quality, which, combined, led to a slight increase in zooplankton growth and FWE. Consequently, at low DOC (<12 mg L^?1), increased N availability lowers FWE due to mismatches in food quality demand and supply, whereas at high DOC this mismatch does not occur, and zooplankton production and FWE may increase. We conclude that the lake DOC level is critical for predicting the effects of enhanced inorganic N availability on pelagic productivity in boreal lakes.     

Unimodal response of fish yield to dissolved organic carbon

Here, we demonstrate a contrasting effect of terrestrial coloured dissolved organic material on the secondary production of boreal nutrient poor lakes. Using fish yield from standardised brown trout gill‐net catches as a proxy, we show a unimodal response of lake secondary productivity to dissolved organic carbon (DOC). This suggests a trade‐off between positive and negative effects, where the initial increase may hinge upon several factors such as energy subsidising, screening of UV‐radiation or P and N load being associated with organic carbon. The subsequent decline in production with further increase in DOC is likely associated with light limitations of primary production. We also show that shallow lakes switch from positive to negative effects at higher carbon loads than deeper lakes. These results underpin the major role of organic carbon for structuring productivity of boreal lake ecosystems.     

Light and nutrient control phytoplankton biomass responses to global change in northern lakes

Global change affects terrestrial loadings of colored dissolved organic carbon (DOC) and nutrients to northern lakes. Still, little is known about how phytoplankton respond to changes in light and nutrient availability across gradients in lake DOC. In this study, we used results from whole‐lake studies in northern Sweden to show that annual mean phytoplankton biomass expressed unimodal curved relationships across lake DOC gradients, peaking at threshold DOC levels of around 11 mg/L. Whole‐lake single nutrient enrichment in selected lakes caused elevated biomass, with most pronounced effect at the threshold DOC level. These patterns give support to the suggested dual control by DOC on phytoplankton via nutrient (positively) and light (negatively) availability and imply that the lakes' location along the DOC axis is critical in determining to what extent phytoplankton respond to changes in DOC and/or nutrient loadings. By using data from the large Swedish Lake Monitoring Survey, we further estimated that 80% of northern Swedish lakes are below the DOC threshold, potentially experiencing increased phytoplankton biomass with browning alone, and/or combined with nutrient enrichment. The results support the previous model results on effects of browning and eutrophication on lake phytoplankton, and provide important understanding of how northern lakes may respond to future global changes.     

Bottom‐up and top‐down effects of browning and warming on shallow lake food webs

Productivity and trophic structure of aquatic ecosystems result from a complex interplay of bottom‐up and top‐down forces that operate across benthic and pelagic food web compartments. Projected global changes urge the question how this interplay will be affected by browning (increasing input of terrestrial dissolved organic matter), nutrient enrichment and warming. We explored this with a process‐based model of a shallow lake food web consisting of benthic and pelagic components (abiotic resources, primary producers, grazers, carnivores), and compared model expectations with the results of a browning and warming experiment in nutrient‐poor ponds harboring a boreal lake community. Under low nutrient conditions, the model makes three major predictions. (a) Browning reduces light and increases nutrient supply; this decreases benthic and increases pelagic production, gradually shifting productivity from the benthic to the pelagic habitat. (b) Because of active habitat choice, fish exert top‐down control on grazers and benefit primary producers primarily in the more productive of the two habitats. (c) Warming relaxes top‐down control of grazers by fish and decreases primary producer biomass, but effects of warming are generally small compared to effects of browning and nutrient supply. Experimental results were consistent with most model predictions for browning: light penetration, benthic algal production, and zoobenthos biomass decreased, and pelagic nutrients and pelagic algal production increased with browning. Also consistent with expectations, warming had negative effects on benthic and pelagic algal biomass and weak effects on algal production and zoobenthos and zooplankton biomass. Inconsistent with expectations, browning had no effect on zooplankton and warming effects on fish depended on browning. The model is applicable also to nutrient‐rich systems, and we propose that it is a useful tool for the exploration of the consequences of different climate change scenarios for productivity and food web dynamics in shallow lakes, the worldwide most common lake type.     

Bacterioplankton in the littoral and pelagic zones of subtropical shallow lakes

We measured bacterioplankton (phylotypes detected by fluorescent in situ hybridisation, morphometric forms, abundance and production) in samples collected in summer in the littoral and pelagic zones of 10 subtropical shallow lakes of contrasting area (from 13 to 80,800 ha). Compared to the pelagic zones, the littoral zones were overall characterised by higher macrophyte dominance and lower concentrations of total phosphorus and alkalinity and higher concentrations of dissolved organic carbon (DOC) and humic substances. Similarities of bacterial production and biomass turnover and density of active phylotypes and morphotype proportions were related to similarities in a set of environmental variables (including nutrients, humic substances content, predator density and phytoplankton biomass), and some additionally to lake area. Horizontal heterogeneity in bacterioplankton variables (littoral versus pelagic) increased with lake area. Bacterioplankton biomass and production tended to be lower in the littoral zone than in the pelagic zone despite higher concentrations of DOC and humic substances. A likely explanation is higher predation on bacterioplankton in the littoral zone, although allelophatic effects exerted by macrophytes cannot be excluded. Our results indicate that organic cycling via bacterioplankton may be less efficient in the littoral zone than in the pelagic zone of shallow lakes.     

Influence of soil temperature and moisture on the dissolved carbon,nitrogen, and phosphorus in organic matter entering lake ecosystems

Concentrations of terrestrially derived dissolved organic matter (DOM) have been increasing in many north temperate and boreal lakes for over two decades. The concentration of DOM in lakes is influenced by a number of environmental factors, but there is still considerable debate about how the availability of terrestrial DOM, and associated dissolved nitrogen and phosphorus, may be affected by drivers of climatic change. Using experimental and observational methods, we considered how changes in soil temperature and moisture affected the composition of carbon, nitrogen, and phosphorus entering freshwater lakes. In our experiment, organic soil cores were collected from the wetland shoreline of a darkly-stained seepage lake in northern Wisconsin, USA and manipulated in laboratory with temperature and moisture treatments. During the 28-day study, soil leachate was sampled and analyzed for optical properties of DOM via UV/Vis absorbance, as well as concentrations of dissolved organic carbon (DOC), total dissolved nitrogen, and total dissolved phosphorus (TDP). DOM optical properties were particularly sensitive to moisture, with drier scenarios resulting in DOM of lower molecular weight and aromaticity. Warmer temperatures led to lower DOC and TDP concentrations. To consider long-term relationships between climate and lake chemical properties, we analyzed long-term water chemistry data from two additional Wisconsin lakes from the long term ecological research (LTER) project in a cross correlation analysis with Palmer drought severity index data. Analysis of the LTER data supported our experimental results that soil moisture has a significant effect on the quality of DOM entering lakes and that climate may significantly affect lake chemical properties. Although unexpected in terms of DOM loading for climate change scenarios, these results are consistent with patterns of decomposition in organic soils and may be attributed to an increase in soil DOM processing.     

FRESHWATER DIATOMS FROM THE CANADIAN ARCTIC TREELINE AND DEVELOPMENT OF PALEOLIMNOLOGICAL INFERENCE MODELS1

Relationships between surface sediment diatom assemblages and measured environmental variables from 77 lakes in the central Canadian arctic treeline region were examined using multivariate statistical methods. Lakes were distributed across the arctic treeline from boreal forest to arctic tundra ecozones, along steep climatic and environmental gradients. Forward selection in canonical correspondence analysis determined that dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), total nitrogen (TN), lake surface area, silica, lake‐water depth, and iron explained significant portions of diatom species variation. Weighted‐averaging (WA) regression and calibration techniques were used to develop inference models for DIC, DOC, and TN from the estimated optima of the diatom taxa to these environmental variables. Simple WA models with classical deshrinking produced models with the strongest predictive abilities for all three variables based on the bootstrapped root mean squared errors of prediction (RMSEP). WA partial least squares showed little improvement over the simpler WA models as judged by the jackknifed RMSEP. These models suggest that it is possible to infer trends in DIC, DOC, and TN from fossil diatom assemblages from suitably chosen lakes in the central Canadian arctic treeline region.     

Climate drivers of diatom distribution in shallow subarctic lakes

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Nutrient limitation of bacterioplankton and phytoplankton in humic lakes in northern Sweden

1. Two small humic lakes in northern Sweden with concentrations of dissolved organic carbon (DOC) between 15 and 20 mg L^–1 were fertilized with inorganic phosphorus (P) and inorganic nitrogen (N), respectively. A third lake was unfertilized and served as a control. In addition to this lake fertilization experiment, data from different regional surveys were used to assess the role of different limiting factors.
2. The P fertilization had no effects on bacterioplankton or phytoplankton, while phytoplankton were significantly stimulated by N fertilization. Inorganic nutrient limitation of bacterioplankton was a function of DOC concentration in water of the investigated region and nutrient-limited bacteria were found only in lakes with DOC concentrations less than around 15 mg L^–1
3. The fertilization experiments demonstrated that the DOC-rich experimental lakes contained a bioavailable pool of P that was not utilized to its full potential under natural conditions. The overall mobilization of energy (bacterioplankton plus phytoplankton) in the experimental lakes was restricted by lack of inorganic N.     

Scaling of Pelagic Metabolism to Size,Trophy and Forest Cover in Small Danish Lakes

ABSTRACT We tested whether pelagic light and nutrient availability, metabolism, organic pools and CO₂-supersaturation were related to lake size and surrounding forest cover in late summer–autumn measurements among 64 small (0.02–20 ha), shallow seepage lakes located in nutrient-rich, calcareous moraine soils in North Zealand, Denmark. We found a strong implicit scaling to lake size as light availability increased significantly with lake size while nutrient availability, phytoplankton biomass and dissolved organic matter declined. Forest lakes had significantly stronger net heterotrophic traits than open lakes as higher values were observed for light attenuation above and in the water, dissolved organic matter, pelagic community respiration (R) relative to maximum gross primary production (R/GPP) and CO₂-supersaturation. Total-phosphorus was the main predictor of phytoplankton biomass (Chl) despite a much weaker relationship than observed in previous studies of larger lakes. Maximum gross primary production increased with algal biomass and decreased with dissolved organic matter, whereas community respiration increased with dissolved organic matter and particularly with gross primary production. These results suggest that exogenous organic matter supplements primary production as an energy source to heterotrophs in these small lakes, and particularly so in forest lakes experiencing substantial shading from the forest and dissolved humic material. This suggestion is supported by 20–30-fold CO₂ supersaturation in the surface water of the smallest forest lakes and more than sixfold supersaturation in 75% of all measurements making these lakes among the most supersaturated temperate lakes examined so far.     

Climate and landscape influence on indicators of lake carbon cycling through spatial patterns in dissolved organic carbon

Freshwater ecosystems are strongly influenced by both climate and the surrounding landscape, yet the specific pathways connecting climatic and landscape drivers to the functioning of lake ecosystems are poorly understood. Here, we hypothesize that the links that exist between spatial patterns in climate and landscape properties and the spatial variation in lake carbon (C) cycling at regional scales are at least partly mediated by the movement of terrestrial dissolved organic carbon (DOC) in the aquatic component of the landscape. We assembled a set of indicators of lake C cycling (bacterial respiration and production, chlorophyll a, production to respiration ratio, and partial pressure of CO₂), DOC concentration and composition, and landscape and climate characteristics for 239 temperate and boreal lakes spanning large environmental and geographic gradients across seven regions. There were various degrees of spatial structure in climate and landscape features that were coherent with the regionally structured patterns observed in lake DOC and indicators of C cycling. These different regions aligned well, albeit nonlinearly along a mean annual temperature gradient; whereas there was a considerable statistical effect of climate and landscape properties on lake C cycling, the direct effect was small and the overall effect was almost entirely overlapping with that of DOC concentration and composition. Our results suggest that key climatic and landscape signals are conveyed to lakes in part via the movement of terrestrial DOC to lakes and that DOC acts both as a driver of lake C cycling and as a proxy for other external signals.     

Dissolved Organic Carbon Reduces Habitat Coupling by Top Predators in Lake Ecosystems

Increasing input of terrestrial dissolved organic carbon (DOC) has been identified as a widespread environmental phenomenon in many aquatic ecosystems. Terrestrial DOC influences basal trophic levels: it can subsidize pelagic bacterial production and impede benthic primary production via light attenuation. However, little is known about the impacts of elevated DOC concentrations on higher trophic levels, especially on top consumers. Here, we used Eurasian perch (Perca fluviatilis) to investigate the effects of increasing DOC concentrations on top predator populations. We applied stable isotope analysis and geometric morphometrics to estimate long-term resource and habitat utilization of perch. Habitat coupling, the ability to exploit littoral and pelagic resources, strongly decreased with increasing DOC concentrations due to a shift toward feeding predominantly on pelagic resources. Simultaneously, resource use and body morphology became increasingly alike for littoral and pelagic perch populations with increasing DOC, suggesting more intense competition in lakes with high DOC. Eye size of perch increased with increasing DOC concentrations, likely as a result of deteriorating visual conditions, suggesting a sensory response to environmental change. Increasing input of DOC to aquatic ecosystems is a common result of environmental change and might affect top predator populations in multiple and complex ways.     

Dynamic carbon budget of a large shallow lake assessed by a mass balance approach

To study the role of large and shallow hemiboreal lakes in carbon processing, we calculated a 3-year carbon mass balance for Lake Võrtsjärv (Estonia) based on in situ measurements. This balance took into account hydrological and biogeochemical processes affecting dissolved inorganic (DIC), dissolved organic (DOC) and particulate organic (POC) carbon species. Accumulation varied greatly on a seasonal and yearly basis. The lake exported carbon during most of the year except during spring floods and in late autumn. In-lake processes were responsible for exporting POC and storing DOC while DIC switched between storage and export. The carbon cycle was alternatively dominated in 2009 by biogeochemical processes and in 2011 by riverine fluxes, whereas in 2010 the two process types were of the same magnitude. These results suggest that the role of large shallow lakes like Võrtsjärv in the global C cycle is equally driven by hydrological factors, in particular seasonal water level changes, and by biogeochemical in-lake reactions.     

Sierra Nevada mountain lake microbial communities are structured by temperature,resources and geographic location

Warming, eutrophication (nutrient fertilization) and brownification (increased loading of allochthonous organic matter) are three global trends impacting lake ecosystems. However, the independent and synergistic effects of resource addition and warming on autotrophic and heterotrophic microorganisms are largely unknown. In this study, we investigate the independent and interactive effects of temperature, dissolved organic carbon (DOC, both allochthonous and autochthonous) and nitrogen (N) supply, in addition to the effect of spatial variables, on the composition, richness, and evenness of prokaryotic and eukaryotic microbial communities in lakes across elevation and N deposition gradients in the Sierra Nevada mountains of California, USA. We found that both prokaryotic and eukaryotic communities are structured by temperature, terrestrial (allochthonous) DOC and latitude. Prokaryotic communities are also influenced by total and aquatic (autochthonous) DOC, while eukaryotic communities are also structured by nitrate. Additionally, increasing N availability was associated with reduced richness of prokaryotic communities, and both lower richness and evenness of eukaryotes. We did not detect any synergistic or antagonistic effects as there were no interactions among temperature and resource variables. Together, our results suggest that (a) organic and inorganic resources, temperature, and geographic location (based on latitude and longitude) independently influence lake microbial communities; and (b) increasing N supply due to atmospheric N deposition may reduce richness of both prokaryotic and eukaryotic microbes, probably by reducing niche dimensionality. Our study provides insight into abiotic processes structuring microbial communities across environmental gradients and their potential roles in material and energy fluxes within and between ecosystems.     

Environmental determinants of chironomid communities in remote northern lakes across the treeline – Implications for climate change assessments

Chironomids (Diptera: Chironomidae) in northern lakes are especially sensitive to climate change impacts. In addition, environmental factors other than direct temperature increase might play an important role in functioning of these keystone aquatic communities. We examined 31 lakes at the treeline ecotone in subarctic Finnish Lapland for their surface sediment chironomid fauna to assess the influence of different environmental factors on the communities. We aim to improve understanding of the climate-driven catchment and limnological factors, for the assessment of climate change impacts. Our results indicated that organic content of the sediment, total nitrogen, water depth and pH that are all likely to change under global warming had statistically significant influence on the chironomid assemblages and associated indicator taxa were assigned for these variables. In addition, a dissolved organic carbon (DOC) threshold (4 mg l⁻¹) was observed that divided the study sites based on their chironomid composition. Sites with high DOC concentrations and benthic microbial mats had distinctive chironomid fauna from low-DOC sites without microbial mats indicating the significance of benthic versus planktonic productivity in the structure and functioning of polar lakes. The results provide important knowledge on chironomid-environmental relationships in climate-sensitive subarctic lakes and create basis for chironomid-based environmental change assessments in remote northern areas.     

Lake browning impacts community structure and essential fatty acid content of littoral invertebrates in boreal lakes

Hydrobiologia - Many lakes in the northern hemisphere are browning due to increasing concentrations of terrestrial dissolved organic carbon (DOC). The consequences of lake browning to littoral...     

Diatom assemblages and their relationship to environmental variables in lakes from the boreal forest-tundra ecotone near Yellowknife,Northwest Territories,Canada

The relationship between diatom (Bacillariophyceae) taxa preserved in surface lake sediments and measured limnological and environmental variables in 22 lakes near Yellowknife (N.W.T.) was explored using multivariate statistical methods. The study sites are distributed along a latitudinal gradient that includes a strong vegetational gradient of boreal forests in the south to arctic tundra conditions in the north. Canonical correspondence analysis (CCA) revealed that lakewater concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) each accounted for independent and statistically significant proportions of variation in the distribution of diatom taxa. Weighted-averaging (WA) models were developed to infer DIC and DOC from the relative abundances of the 76 most common diatom taxa. These models can now be used to infer past DIC and DOC concentrations from diatom assemblages preserved in sediment cores of lakes in the Yellowknife area, which may provide quantitative estimates of changes in lakewater chemistry related to past vegetational shifts at treeline.     

Bioclimatic influence on water chemistry and dissolved organic matter in shallow temperate lakes of Andean Patagonia: A gradient approach

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Comparative responses of phytoplankton during chemical recovery in atmospherically and experimentally acidified lakes1

Twenty lakes recovering from a century of atmospheric acid deposition over Northeastern Ontario were resurveyed for phytoplankton following a 20‐year period and were compared with a 23‐year study of an experimentally acidified lake, L302S (Experimental Lakes Area, ON, Canada). Phytoplankton species significantly tracked abiotic changes during both acidification and chemical recovery in all lakes based on concordance testing. However, ordination analyses showed that many phytoplankton communities had not returned to their preacidification state. Significant explanatory variables of taxonomic responses were pH, dissolved organic carbon (DOC), and inorganic nutrients (N, P), based on canonical correspondence analysis (CCA). Increases in DOC and pH influenced a significant taxonomic shift from acid‐tolerant dinoflagellates to a diverse assemblage of cyanobacteria, chlorophytes, and diatoms. Declining nitrogen levels defined a secondary environmental gradient, which was characterized by a decrease in filamentous green algal abundance. L302S remained remote in ordination space from the more chronically and heavily polluted lakes in Northeastern Ontario, indicating that experimental acidification provided a conservative estimate of the true damage to atmospherically polluted lakes. However, L302S did increasingly resemble lakes in Northeastern Ontario, suggesting that experimental acidification simulated the impacts of moderate levels of atmospheric pollution. Our findings demonstrate the importance of ecological history in understanding the responses by boreal lake ecosystems to environmental change.