Article

Effect of a nitrogen pulse on ecosystem N processing at different temperatures: A mesocosm experiment with 15NO3 addition

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Abstract

1. Shallow lakes may play an important role for the nitrogen (N) balance in drainage basins by processing, transferring and retaining N inputs. An increase in the frequency of storm-induced short-term N pulses and increased water temperatures are both likely outcomes of climate change, potentially affecting the N processing in lakes. 2. An experiment with a K 15 NO 3 À pulse addition (increase in NO 3 À concentration from c. 0.1 to 2 mg/L) was carried out in 12 mesocosms with relatively low (applies to Danish lakes) total N (TN) and total phosphorus (TP) concentrations (c. 0.3 mg N L -1 and 0.04 mg P L -1) to assess the effects of an N pulse on N processing and storage in shallow lake ecosystems. The mesocosms have a hydraulic retention time of approximately two and a half months, and at the time of the experiment, they had been adapted to contrasting temperatures for a period of 10 years: ambient, T3 (heat-ing according to the Intergovernmental Panel on Climate Change 2007 A2 scenario, +3.7–4.5°C, depending on season) and T5 (heating with A2 + 50%, +4.9–6.6°C). 3. Macrophytes and filamentous algae retained up to 40% and 30% of the added 15 N, respectively, reflecting their high biomass in the mesocosms. Macrophytes and filamentous algae constituted between 70% and 80% of the biomass of all primary producers during the experiment in the T3 and ambient treatments and between 20% and 40% in T5. By comparison, less than 1% of the added 15 N diffused to the sediment and less than 5% was lost to the atmosphere as N 2 gas. Snails represented the long-term storage of 15 N, retaining up to 6% of the tracer and with detectable enrichment 100 days after tracer addition. 4. We found no significant differences among the temperature treatments in the 15 N turnover after pulse dosing. However, a larger percentage of 15 N was stored in macrophytes in the ambient and T3 mesocosms, reflecting higher biomasses than in T5 where filamentous algae were more abundant. Macrophytes and filamentous algae rather than temperature were therefore key controllers of N processing during the summer N pulse in these shallow, relatively low TP lakes.

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Increased nitrogen loading may lead to changes in productivity or biodiversity in freshwater systems. Field surveys have shown reduced species richness of submerged and floating-leaved plant communities in shallow lakes as winter nitrate concentrations, a surrogate for nitrate loading, have risen above 1-2 mg NO3-N L−1. Experimental tank mesocosms, containing about 3 m3 of water and sediment from Hickling Broad, Norfolk, UK were initially planted with eleven submerged plant species from the lake and its connected waterway. Constant phosphorus loadings (designed to give added concentrations of 50 μg P L−1) were given to all tanks. Four nitrate loadings were given in a randomised block design with twelve-fold replication. Loadings were designed to increase the concentration in the water by 1, 2, 5 and 10 mg NO3-N L−1 (N1, N2, N5 and N10, respectively). Nitrate loading increased phytoplankton and periphyton chlorophyll a in the N2, N5 and N10 treatments compared with the N1. It complementarily decreased total plant volume and had varied effects on different species, with most species indifferent, a few (mostly charophytes) declining above the N1 treatment, and one (Elodea canadensis) performing best in N2 and N5 compared with N1 and N10. Species richness of submerged macrophytes declined with time in all treatments and with increasing nitrogen load in the first year. In the second year, species richness did not further decline in the N1 treatment but declined at increasing rates with increasing nitrogen load in the others. The rate of decline in the second year, plotted against nitrate load, fitted an exponential relationship, allowing calculation of a critical load associated with a stable species richness of 0.61-0.64 mg NO3-NL−1 expressed as concentration in inflow water, or of an empirically determined equivalent TN concentration in the lake water of about 1.50 mg N L−1. This value broadly corresponds with estimates from field data for concentrations associated with declining species richness and is much lower than values currently often found in lowland agricultural areas in Europe.
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1. Shallow lakes may switch from a state dominated by submerged macrophytes to a phytoplankton-dominated state when a critical nutrient concentration is exceeded. We explore how climate change may affect this critical nutrient concentration by linking a graphical model to data from 83 lakes along a large climate gradient in South America. 2. The data indicate that in warmer climates, submerged macrophytes may tolerate more underwater shade than in cooler lakes. By contrast, the relationship between phytoplankton biomass [approximated by chlorophyll-a (chl-a) or biovolume] and nutrient concentrations did not change consistently along the climate gradient. In warmer climates, the correlation between phytoplankton biomass and nutrient concentrations was overall weak, especially at low total phosphorus (TP) concentrations where the chl-a/ TP ratio could be either low or high. 3. Although the enhanced shade tolerance of submerged plants in warmer lakes might promote the stability of their dominance, the potentially high phytoplankton biomass at low nutrient concentrations suggests an overall low predictability of climate effects. 4. We found that near-bottom oxygen concentrations are lower in warm lakes than in cooler lakes, implying that anoxic P release from eutrophic sediment in warm lakes likely causes higher TP concentrations in the water column. Subsequently, this may lead to a higher phytoplankton biomass in warmer lakes than in cooler lakes with similar external nutrient loadings. 5. Our results indicate that climate effects on the competitive balance between submerged macrophytes and phytoplankton are not straightforward.
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1Stable isotope data are widely used to track the origins and transformations of materials in food webs. Reliable interpretation of these data requires knowledge of the factors influencing isotopic fractionation between diet and consumer. For practical reasons, isotopic fractionation is often assumed to be constant but, in reality, a range of factors may affect fractionation.2To investigate effects of temperature and feeding rate on fractionation of carbon and nitrogen stable isotopes in a marine predator, we reared European sea bass Dicentrarchus labrax on identical diets at 11 and 16 °C on three ration levels for 600 days.3Nitrogen trophic fractionation (Δδ15N) was affected by temperature. Bass Δδ15N was 4·41‰ at 11 °C and 3·78‰ at 16 °C.4Carbon fractionation (Δδ13C) was also affected by temperature. Bass Δδ13C was 1·18‰ at 11 °C and 1·64‰ at 16 °C. The higher lipid content in the tissues of bass reared at cooler temperatures accounted for the temperature effect on Δδ13C. When Δδ13C was determined using mathematically defatted values, there was a direct effect of ration size and Δδ13C was 2·51, 2·39 and 2·31‰ for high, medium and low rations, respectively.5Reported Δδ15N for all treatments exceeded the mean of 3·4‰ widely used in ecological studies of fish populations and communities. This would confound the interpretation of δ15N as an indicator of trophic level when comparing populations that are exposed to different temperatures.6The Δδ13C of 0–1‰ commonly applied in food web studies did not hold under any of the temperature or feeding regimes considered and a value of 2‰ would be more appropriate.
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Animals are important in nutrient cycling in freshwater ecosystems. Via excretory processes, animals can supply nutrients (nitrogen and phosphorus) at rates comparable to major nutrient sources, and nutrient cycling by animals can sup-port a substantial proportion of the nutrient demands of primary producers. In addition, animals may exert strong impacts on the species composition of primary producers via effects on nutrient supply rates and ratios. Animals can either recycle nutrients within a habitat, or translocate nutrients across habitats or ecosystems. Nutrient translocation by relatively large animals may be particularly important for stimulating new primary production and for increasing nutrient standing stocks in recipient habitats. Animals also have numerous indirect effects on nutrient fluxes via effects on their prey or by modification of the physical environment. Future studies must quantify how the impor-tance of animal-mediated nutrient cycling varies among taxa and along environmental gradients such as ecosystem size and productivity.
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The analysis of stable isotope ratios represents one of the most exciting new technical advances in environmental sciences. In this book, leading experts offer the first survey of applications of stable isotope analysis to ecological research. Central topics are - plant physiology studies - food webs and animal metabolism - biogeochemical fluxes. Extensive coverage is given to natural isotopes of carbon, hydrogen, oxygen, nitrogen, sulfur, and strontium in both terrestrial and marine ecosystems. Ecologists of diverse research interests, as well as agronomists, anthropologists, and geochemists will value this overview for its wealth of information on theoretical background, experimental approaches, and technical design of studies utilizing stable isotope ratios.
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Summary. Recent work by Reiss and Ogden provides a theoretical basis for sometimes preferring restricted maximum likelihood (REML) to generalized cross-validation (GCV) for smoothing parameter selection in semiparametric regression. However, existing REML or marginal likelihood (ML) based methods for semiparametric generalized linear models (GLMs) use iterative REML or ML estimation of the smoothing parameters of working linear approximations to the GLM. Such indirect schemes need not converge and fail to do so in a non-negligible proportion of practical analyses. By contrast, very reliable prediction error criteria smoothing parameter selection methods are available, based on direct optimization of GCV, or related criteria, for the GLM itself. Since such methods directly optimize properly defined functions of the smoothing parameters, they have much more reliable convergence properties. The paper develops the first such method for REML or ML estimation of smoothing parameters. A Laplace approximation is used to obtain an approximate REML or ML for any GLM, which is suitable for efficient direct optimization. This REML or ML criterion requires that Newton–Raphson iteration, rather than Fisher scoring, be used for GLM fitting, and a computationally stable approach to this is proposed. The REML or ML criterion itself is optimized by a Newton method, with the derivatives required obtained by a mixture of implicit differentiation and direct methods. The method will cope with numerical rank deficiency in the fitted model and in fact provides a slight improvement in numerical robustness on the earlier method of Wood for prediction error criteria based smoothness selection. Simulation results suggest that the new REML and ML methods offer some improvement in mean-square error performance relative to GCV or Akaike's information criterion in most cases, without the small number of severe undersmoothing failures to which Akaike's information criterion and GCV are prone. This is achieved at the same computational cost as GCV or Akaike's information criterion. The new approach also eliminates the convergence failures of previous REML- or ML-based approaches for penalized GLMs and usually has lower computational cost than these alternatives. Example applications are presented in adaptive smoothing, scalar on function regression and generalized additive model selection.
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Presenting the first continental-scale assessment of reactive nitrogen in the environment, this book sets the related environmental problems in context by providing a multidisciplinary introduction to the nitrogen cycle processes. Issues of upscaling from farm plot and city to national and continental scales are addressed in detail with emphasis on opportunities for better management at local to global levels. The five key societal threats posed by reactive nitrogen are assessed, providing a framework for joined-up management of the nitrogen cycle in Europe, including the first cost-benefit analysis for different reactive nitrogen forms and future scenarios. Incorporating comprehensive maps, a handy technical synopsis and a summary for policy makers, this landmark volume is an essential reference for academic researchers across a wide range of disciplines, as well as stakeholders and policy makers. It is also a valuable tool in communicating the key environmental issues and future challenges to the wider public.
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Individual animal species can impact ecosystem processes, but few exotic invaders have demonstrated ecosystem-scale impacts, even when population sizes are large. We combined whole-stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic freshwater snail, Potamopyrgus antipodarum, dominated these fluxes in a highly productive stream. The snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. Such large fluxes were due to high snail biomass rather than high rates of excretion or consumption. This exotic species may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport.
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1. To help improve our understanding of the nitrogen cycle in lakes, particularly in the context of climate change, we analysed total nitrogen (TN) and nitrate (NO � 3 -N) data from six mesocosm experiments (in Denmark, U.K., China and Turkey) covering different climatic regions. We assessed the effects of nitrogen (N) and phosphorus (P) loading, temperature, salinity and water level on N processing. 2. Water column N loss (defined as the nitrogen processed in and lost from the water column in units of net amount processed per unit area and per unit of time, or in relative terms as the percentage loss of the total pool in 2 weeks) was particularly sensitive to external nutrient loading to the mesocosms. Mean water column TN loss at high N loading varied from 111 to 250 mg m �2 day �1 and increased with N loading. High P loading resulted in increased water column N loss, possibly because of increased uptake into plants and attached algae and sedimentation of the increased algal crop. High salinity generally decreased water column TN loss; on average, 10% more TN was in the water column at 12& salinity than at 2& salinity, while no significant effect of water level was found. 3. Only weak relationships were observed between N processing and temperature, and mesocosms limited by P accumulated more nitrogen in their water columns than those with high P loadings. Our results suggest that N processing in lakes appears to be more sensitive to features of the catchment, such as hydrology and loading, than to climatic effects related to temperature, salinity and water level.
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(1) Enclosures (4.5 m$^3$) in a dense Potamogeton pectinatus community were enriched by weekly additions of NaNO$_3$ and KH$_2$PO$_4$ at four different rates (max. 1000 $\mu g$ N 1$^{-1}$ week$^{-1}$ and 100 $\mu g$ P 1$^{-1}$ week$^{-1}$) over a period of 15-23 weeks. (2) The most obvious effect of enrichment was the development of a dense filamentous algal population. There was no parallel development of planktonic algae however, and chlorophyll a levels remained low in all treatments. After 9 weeks at all levels of enrichment the community was able to remove all the added N and P within 1 day (3) $^{32}$P tracer experiments showed that most of the added P was absorbed, in the short term (2 h), by the filamentous algae associated with the macrophytes. Concentrations of N and P per unit dry mass in the algae, macrophytes and sediments were increased significantly only by the highest enrichment treatment. (4) Decomposing filamentous algae apparently provided the major input of N and P to the sediments in this treatment. (5) The data indicate that dense submerged macrophyte beds with their associated epiphytic algae may in some areas be useful nutrient filters.
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This new edition of an established textbook provides a comprehensive and stimulating introduction to rivers, lakes and wetlands, and was written as the basis for a complete course on freshwater ecology. Designed for undergraduate and early postgraduate students who wish to gain an overall view of this vast subject area, this accessible guide to freshwater ecosystems and man's activities will also be invaluable to anyone interested in the integrated management of freshwaters. The author maintains the tradition of clarity and conciseness set by previous editions, and the text is extensively illustrated with photographs and diagrams. Examples are drawn from the author's experience in many parts of the world, and the author continues to stress the human influence. The scientific content of the text has been fully revised and updated, making use of the wealth of data available since publication of the last edition. Professor Brian Moss is a lecturer in Applied Ecology at the University of Liverpool, and has written three previous editions of this well-established textbook.
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We used a stable isotope tracer to measure nitrogen (N) assimilation and transfer through Bull Trout Lake, a 0.3-km(2) mountain lake in Idaho, specifically to explore the relative importance of pelagic and benthic producers. (NO3-)-N-15 was added into the inflow stream above the lake during spring runoff and the resulting mass of tracer was measured within the various ecosystem compartments, including the outflow stream. Although a portion of the (NO3-)-N-15 moved through the lake quickly due to a low hydraulic residence time during the addition, the tracer was also assimilated rapidly by seston in the water column and at a slower rate by benthic primary producers. By the end of the 10-d injection, 10% of the tracer had left via outflow, 21% was within seston, and 17% was in epiphytes and macrophytes. However, 70 d after the termination of the injection, only similar to 1% of the tracer remained within seston, whereas 10% was within the benthic primary production compartment as N was recycled within the benthic zone. Quantitative transfer of N-15 to invertebrate and fish consumers was low, but turnover in these compartments was slow. A conservative water mass tracer (bromide) indicated that the turnover rate for lake water was 1.8% d(-1), whereas N-15 turnover for the whole lake was only 0.7% d(-1), demonstrating how lakes exert drag on nutrients as they move through the watershed. Due to uptake and storage of nutrients, Bull Trout Lake strongly influenced the timing and magnitude of nutrient export from its watershed.
Article
1. To help improve our understanding of the nitrogen cycle in lakes, particularly in the context of climate change, we analysed total nitrogen (TN) and nitrate (NO À 3 -N) data from six mesocosm experi-ments (in Denmark, U.K., China and Turkey) covering different climatic regions. We assessed the effects of nitrogen (N) and phosphorus (P) loading, temperature, salinity and water level on N processing. 2. Water column N loss (defined as the nitrogen processed in and lost from the water column in units of net amount processed per unit area and per unit of time, or in relative terms as the percentage loss of the total pool in 2 weeks) was particularly sensitive to external nutrient loading to the mesocosms. Mean water column TN loss at high N loading varied from 111 to 250 mg m À2 day À1 and increased with N loading. High P loading resulted in increased water column N loss, possibly because of increased uptake into plants and attached algae and sedimentation of the increased algal crop. High salinity generally decreased water column TN loss; on average, 10% more TN was in the water column at 12& salinity than at 2& salinity, while no significant effect of water level was found. 3. Only weak relationships were observed between N processing and temperature, and mesocosms limited by P accumulated more nitrogen in their water columns than those with high P loadings. Our results suggest that N processing in lakes appears to be more sensitive to features of the catchment, such as hydrology and loading, than to climatic effects related to temperature, salinity and water level.
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Resource pulses are infrequent, large-magnitude, and short-duration events of increased resource availability. They include a diverse set of extreme events in a wide range of ecosystems, but identifying general patterns among the diversity of pulsed resource phenomena in nature remains an important challenge. Here we present a meta-analysis of resource pulse–consumer interactions that addresses four key questions: (1) Which characteristics of pulsed resources best predict their effects on consumers? (2) Which characteristics of consumers best predict their responses to resource pulses? (3) How do the effects of resource pulses differ in different ecosystems? (4) What are the indirect effects of resource pulses in communities? To investigate these questions, we built a data set of diverse pulsed resource–consumer interactions from around the world, developed metrics to compare the effects of resource pulses across disparate systems, and conducted multilevel regression analyses to examine the manner in which variation in the characteristics of resource pulse–consumer interactions affects important aspects of consumer responses. Resource pulse magnitude, resource trophic level, resource pulse duration, ecosystem type and subtype, consumer response mechanisms, and consumer body mass were found to be key explanatory factors predicting the magnitude, duration, and timing of consumer responses. Larger consumers showed more persistent responses to resource pulses, and reproductive responses were more persistent than aggregative responses. Aquatic systems showed shorter temporal lags between peaks of resource availability and consumer response compared to terrestrial systems, and temporal lags were also shorter for smaller consumers compared to larger consumers. The magnitude of consumer responses relative to their resource pulses was generally smaller for the direct consumers of primary resource pulses, compared to consumers at greater trophic distances from the initial resource pulse. In specific systems, this data set showed both attenuating and amplifying indirect effects. We consider the mechanistic processes behind these patterns and their implications for the ecology of resource pulses.
Article
Shallow lakes are likely to be strongly impacted by climate changes and, in particular, by increased tempera- tures. To enable realistic experimental studies of the effects of higher temperatures on in-lake processes and dynamics, technologically advanced systems are required. This paper presents design details, operating charac- teristics, and background information on a currently operating experimental flow-through mesocosm system that allows investigation of the interactions between simulated climate warming and eutrophication and their impacts on biological structure and ecosystem processes in shallow lakes. We use 24 mesocosms to combine three temperature scenarios (one unheated and two heated relative to the Intergovernmental Panel on Climate Change climate scenario A2 and A2 + 50%, respectively) and two nutrient levels (enriched and nonenriched). Planktivorous fish (male sticklebacks, Gasterosteus aculeatus) are stocked in accordance with the nutrient level. The water residence time is regulated by the semicontinuous addition of water and is approximately 2.5 mo in each mesocosm. For heating, we use electrically powered heating elements. The heating system has performed well over 16 mo of continuous heating, and seasonal and diurnal temperature variations of the unheated refer- ence mesocosms were paralleled well by the heated mesocosms. The performance of the flow-through system and the heating technique are discussed with special emphasis on strengths, limitations, and potential improvements of the system. To illustrate the performance of the system and its potential, we present data for selected periods on total phosphorus retention in the mesocosms and system primary production and respiration.
Article
Under conditions of stress, shallow freshwater ecosystems can undergo a state change characterized by the rapid loss of macrophytes and subsequent dominance of phytoplankton. Elevated water temperature may promote such change. Here we report the impact of two warming regimes (continuous 38C above ambient and 38C above ambient during summer only), with two nutrient loadings and the presence or absence of fish, on 48 microcosm ecosystems created to mimic shallow pond environments. We found that warming did not significantly encourage phytoplankton blooms, even in combination with increased nutrients and fish. Instead, macrophyte communities remained dominant. Macrophyte-associated invertebrates (gastropods and ostracods) increased in numbers in the warmed microcosms, potentially helping to stabilize the macrophyte communities. Nevertheless, warming produced trends in water chem- istry that could be problematic. It increased phosphorus concentrations, total alkalinity, and conductivity. It decreased pH and oxygen saturation and increased the frequency of severe deoxygenation. These trends were largely inde- pendent of the other experimental treatments and support the suggestion that moderate warming has the potential to exacerbate existing eutrophication problems.
Article
Careful attention to pH, alkalinity, the neutralizing buffer, reaction vessels, and dilution factors have proved neccessary. This modified method is acceptable for samples from fresh to oceanic waters, is accurate for organic compounds tested, has a maximum capability of 40 micrometre nitrogen in undiluted samples, and has a mid-range precision of + or -2%. -from Authors
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Detailed gas exchange measurements from two circular and one linear wind/wave tunnels are presented. Heat, He, CH4, CO2, Kr, and Xe have been used as tracers. The experiments show the central importance of waves for the water-side transfer process. With the onset of waves the Schmidt number dependence of the transfer velocity k changes from k ~ Sc-2/3 to k ~ Sc-1/2 indicating a change in the boundary conditions at the surface. Moreover, energy put into the wave field by wind is transferred to near-surface turbulence enhancing gas transfer. The data show that the mean square slope of the waves is the best parameter to characterize the free wavy surface with respect to water-side transfer processes.
Article
Fates of nitrate (NO3-) reduction to nitrogen gas (N-2) and ammonium (NH4+) were measured in August and December 1999 on intact cores (Laguna Madre and Baffin Bay, Texas) using flowing seawater enriched with (NO3-)-N-15. The combination of membrane inlet mass spectrometry (MIMS) and high performance liquid chromatography (HPLC) allowed accurate and simple estimation of these 2 dissimilatory pathways of NO3- reduction. NO3- enrichment (similar to100 muM (NO3-)-N-15) did not stimulate denitrification (mean +/- SE = 55 +/- 16 and 69 +/- 15 [Aug 99], -11 +/- 16 and 11 +/- 18 [Dec 99] mumolN m(-2) h(-1) before and after (NO3-)-N-15 addition, respectively; n = 8). However, (NH4+)-N-15 production rates increased after the (NO3-)-N-15 addition (69 +/- 14 [Aug 99], 50 +/- 9 [Dec 99] mumolN m(-2) h(-1)), comprised about 1/3 of total NH4+ flux, and were comparable to denitrification rates. A larger portion of added (NO3-)-N-15 was converted to (NH4+)-N-15 (15 to 75%) than to N-2 (N-29+30(2); 5 to 29%) on both sampling dates. High dissimilatory NO3- reduction to NH4+ (DNRA) and low denitrification suggest that sulfide may influence the processes. High sulfide concentrations inhibit nitrification and denitrification but may enhance DNRA by providing an electron donor. Inhibited denitrification and enhanced DNRA may preserve available nitrogen in Laguna Madre/Baffin Bay, which has limited water exchange with other bodies of water.
Article
Seasonal cycles of delta(13)C and delta(15)N in dissolved organic carbon and size-fractionated plankton, ranging from bacteria to the jellyfish Aurelia aurita, were studied during a 1 yr cycle at a coastal station in the Baltic Sea. The observed isotopic changes were found with time lags in all size-fractions of plankton. The delta(13)C showed a bimodal cycle with 2 local maxima, the first coinciding with the spring bloom and the second with the autumn bloom. In delta(15)N, the annual cycle was trimodal with 3 local maxima. The first occurred in connection with the spring bloom, the second in mid-summer and the third was a broad autumn-to-winter maximum. The causes of these patterns are discussed in relation to measured oceanographic variables. In the summer, a depleted nitrogen isotopic signal was propagated through all size-classes of plankton, indicating direct or secondary utilisation of fixed nitrogen from cyanobacteria. The strength of the signal indicated that nitrogen-fixing cyanobacteria are more ecologically important as instantaneous nitrogen sources in the Baltic than previously assumed. Enrichment of delta(15)N in size-classes of plankton was found to be a linear function of logarithmic organism size from 20 to 500 mu m, reflecting size-related consumption patterns of marine plankton food-webs. The explanatory power of the Linear regression and the enrichment per unit size were stronger in spring and autumn than in the summer, reflecting time lags and diversity in the zooplankton community. The size-specific approach was found to be a simpler and more appropriate way of analysing trophic isotope enrichment in plankton food-webs than the assumption of a general enrichment factor per trophic level.
Article
Given the great potential value of stable isotopes in a variety of scientific investigations, surprisingly little attention has been paid to the underlying physiological and biochemical mechanisms that account for trophic in- creases in d 15 N values. This has lead to a general call for controlled studies investigating the relationship between organismal diet and corresponding isotopic composition. We conducted a series of laboratory studies varying dietary nitrogen content and measuring corresponding variations in organismal d 15 N values. Specifically, we investigated the relationship between the d 15 N values of the anomopod crustacean, Daphnia magna, and the C : N ratio of its food, the green algae, Scenedesmus acutus.Daphnids were raised to a standard life stage on three types of S. acutus as food, which ranged in C : N (atomic) from 7.3 to 24.8. The average C : N of the daphnids was 6.0. 15 N enrichment was found to be strongly linearly related to the C : N of the algae, ranging from nearly zero to approximately 6‰, which would normally be considered a span of almost two trophic levels. The d 15
Article
We experimentally warmed a series of shallow enclosures by 4.58C and measured responses of the epilithon (biofilm on rocky surfaces) and invertebrates. Maximum rates of net photosynthesis increased by 28-115% and rates of dark respiration increased by 29-103% as a result of warming. Long-term analyses using data from un- manipulated Lake 239 corroborated these findings, showing that rates of light-saturated photosynthesis and dark respiration were positively correlated with water temperature. Warming effects differed between communities (on natural and tile substrates, as well as well-developed and early successional communities). Warming consistently led to increased bacterial cell densities, but increases in total algal biovolume and diatom biovolume were seen only in an early successional tile community. Effects on the composition of the invertebrate community (studied only on well-developed tile biofilms) were small. We observed warming-related increases in carbon accrual within one community, and late in the experiment observed a change in carbon : phosphorus ratios of another community, possibly indicative of a degradation of food quality. Our study suggests that climate warming effects on epilithic community composition are likely to be heterogeneous and difficult to predict; however, the agreement between long-term and experimental results suggests that increased temperatures will increase metabolic rates of the epilithon.
Article
Denitrification occurs in essentially all river, lake, and coastal marine ecosystems that have been studied. In general, the range of denitrification rates measured in coastal marine sediments is greater than that measured in lake or river sediments. In various estuarine and coastal marine sediments, rates commonly range between 50 and 250 μmol N m-2h-1, with extremes from 0 to 1067. Rates of denitrification in lake sediments measured at near-ambient conditions range from 2 to 171 μmol N m-2h-1. Denitrification rates in river and stream sediments range from 0 to 345 μmol N m-2h-1. The higher rates are from systems that receive substantial amounts of anthropogenic nutrient input. In lakes, denitrification also occurs in low oxygen hypolimnetic waters, where rates generally range from 0.2 to 1.9 μmol N liter-1d-1. In lakes where denitrification rates in both the water and sediments have been measured, denitrification is greater in the sediments. -from Author
Article
The major aim of this study was to evaluate the relationships between the rates of microbial activities (phytoplankton primary production, bacterial secondary production, bacterial utilization of organic matter, enzymatic activities, protozoan grazing on bacteria), bacterial numbers, and dissolved organic carbon concentrations and the trophic state index (TSI) of lakes in the upper trophogenic water layer in the pelagial zone along the trophic gradient (from oligo/mesotrophy to hypereutrophy) in 19 lakes of the Mazurian Lake District (northeastern Poland). Multiple regression analysis (analysis of variance - ANOVA) on all collected data and the TSI along eutrophication gradient showed that all studied microbial processes and parameters were very tightly coupled to the trophic conditions of the studied lakes. All studied microbial processes involved in utilization and enzymatic degradation of organic matter were strongly positively dependent on the intensity and rates of photosynthetic organic matter production and exudation that markedly increased along the eutrophication gradient of lakes. Vmax of alkaline phosphatase, aminopeptidase, and nonspecific esterase showed significant correlation with the TSI of the studied lakes. Protozoans removed a significant portion of bacterial production, i.e., from ∼20% to 75-85% of newly produced bacterial biomass was simultaneously consumed by protozoans along the eutrophication gradient. These observations suggest that the importance of protozoan grazing on bacteria on regulation of bacterial production depends on lake productivity. The general working hypothesis that the intensity of microbial processes of organic matter can be tightly coupled to increasing eutrophication was proven in these studies. © 2006, by the American Society of Limnology and Oceanography, Inc.
Article
1. Climate warming is expected to change respiration in shallow lakes but to an extent that depends on nutrient state. 2. We measured sediment respiration (SR) over the season in the dark on intact sediment cores taken from a series of flow-through, heated and unheated, nutrient-enriched and unenriched mesocosms. The natural seasonal temperature cycle ranged from 2 to 20 °C in the unheated mesocosms. In the heated mesocosms, the temperature was raised 4–6 °C above ambient temperatures, depending on season, following the A2 climate change scenario downscaled to the local position of the mesocosms, but enlarged by 50%. We further measured ecosystem respiration (ER) in the mesocosms based on semi-continuous oxygen measurements. 3. SR changed over the season and was approximately ten times higher in summer than in winter. SR showed no clear response to warming in the nutrient-enriched treatment, while it increased with warming in the unenriched mesocosms which also had lower fish densities. 4. ER was not affected by artificial warming or nutrient enrichment, but it was ten times higher in summer than in winter. 5. SR contributed 24–32% to ER. The SR:ER ratio was generally stimulated by warming and was higher in winter than in summer, especially in the nutrient-enriched mesocosms. 6. Our results indicate that climate warming may lead to higher SR, especially in clear, macrophyte-dominated systems. Moreover, the contribution of SR to ER will increase with higher temperatures, but decrease as the winters get shorter.
Article
1. The effect of total nitrogen (TN) and phosphorus (TP) loading on trophic structure and water clarity was studied during summer in 24 field enclosures fixed in, and kept open to, the sediment in a shallow lake. The experiment involved a control treatment and five treatments to which nutrients were added: (i) high phosphorus, (ii) moderate nitrogen, (iii) high nitrogen, (iv) high phosphorus and moderate nitrogen and (v) high phosphorus and high nitrogen. To reduce zooplankton grazers, 1⁺ fish (Perca fluviatilis L.) were stocked in all enclosures at a density of 3.7 individuals m⁻².
Article
1. To examine how the vertical distribution of periphytic biomass and primary production in the upper 0–1 m of the water column changes along an inter-lake eutrophication gradient, artificial substrata (plastic strips) were introduced into the littoral zones of 13 lakes covering a total phosphorus (TP) summer mean range from 11 to 536 μg L−1. Periphyton was measured in July (after 8 weeks) and September (after 15 weeks) at three water depths (0.1, 0.5 and 0.9 m). 2. Periphyton chlorophyll a concentration and dry weight generally increased with time and the communities became more heterotrophic. Mean periphytic biomass was unimodally related to TP, reaching a peak between 60 and 200 μg L−1. 3. The proportion of diatoms in the periphyton decreased from July to September. A taxonomic shift occurred from dominance (by biovolume) of diatoms and cyanobacteria at low TP to dominance of chlorophytes at intermediate TP and of diatoms (Epithemia sp.) in the two most TP-rich lakes. 4. The grazer community in most lakes was dominated by chironomid larvae and the total biomass of grazers increased with periphyton biomass. 5. Community respiration (R), maximum light-saturated photosynthetic rate (Pmax), primary production and the biomass of macrograzers associated with periphyton were more closely related to periphyton biomass than to TP. Biomass-specific rates of R, Pmax and production declined with increasing biomass. 6. Mean net periphyton production (24 h) was positive in most lakes in July and negative in all lakes in September. Net production was not related to the TP gradient in July, but decreased in September with increasing TP. 7. The results indicate that nutrient concentrations alone are poor predictors of the standing biomass and production of periphyton in shallow lakes. However, because periphyton biomass reaches a peak in the range of phosphorus concentration in which alternative states occur in shallow lakes, recolonisation by submerged macrophytes after nutrient reduction may potentially be suppressed by periphyton growth.
Article
1. We studied the effects of increased water temperatures (0–4.5 °C) and nutrient enrichment on the stoichiometric composition of different primary producers (macrophytes, epiphytes, seston and sediment biofilm) and invertebrate consumers in 24 mesocosm ecosystems created to mimic shallow pond environments. The nutrient ratios of primary producers were used as indicative of relative nitrogen (N) or phosphorus (P) limitation. We further used carbon stable isotopic composition (δ¹³C) of the different primary producers to elucidate differences in the degree of CO2 limitation.
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We conducted continuous-flow experiments on intact sediment cores from Laguna Madre, Sabine Lake, East Matagorda Bay, and Nueces Estuary to evaluate internal nitrogen (N) sources, sinks, and retention mechanisms in Texas estuaries having different salinities. Mean ammonium (NH4+) flux ranged from slight uptake (negative values) to NH 4+ production rates of about 300 μmol m-2 h-1 (units used for all N rates) and increased with salinity (p = 0.10). Net nitrate (NO3-) flux (-20 to 32) and net N 2 flux (-70 to 100) did not relate to salinity. Mean net N 2 flux was positive but near zero, indicating that N2 sources and sinks are nearly balanced. Total denitrification, N fixation, and potential dissimilatory NO3- reduction to NH 4+ (DNRA) rates were estimated after inflow water was enriched with 15NO3- (100 μmol L -1). Total denitrification rates ranged from 0 to 90 versus N fixation rates ranging from 0 to 97. Potential DNRA, measured conservatively as 15NH4+ accumulation, ranged from 0 to 80 and related significantly to salinity (p < 0.01). Increases in total NH 4+ release after 15NO3- additions were higher but closely related (r = 0.9998) to 15NH 4+ accumulation, implying exchange reactions of DNRA-regenerated 15NH4+ with sediment-bound 14NH4+. The fate of NO3- was related to salinity, perhaps via sulfide effects on DNRA. Potential DNRA was high in southeastern Corpus Christi Bay in August during hypoxia when the sulfide transition zone was near the sediment surface. Nitrogen fixation and DNRA are important mechanisms that add and retain available N in Texas estuaries. © 2006, by the American Society of Limnology and Oceanography, Inc.
Article
Summary • Climate is changing. Predictions are for at least a 3 °C rise in mean temperature in northern Europe over the next century. Existing severe impacts of nutrients and inappropriate fish stocking in freshwater systems remain. • Effects of warming by 3 °C above ambient, nutrient addition and the presence or absence of sticklebacks Gasterosteus aculeatus were studied in experimental microcosms dominated by submerged plants, mimicking shallow lake ecosystems. • Warming had considerably smaller effects on the phytoplankton community than did fish and nutrients. It had very minor effects on chlorophyll a and total phytoplankton biovolume. However, it significantly decreased the biovolumes of Cryptophyceae (a major component in the controls) and Dinophyceae. Contrary to expectation, warming did not increase the abundance of blue-green algae (cyanophytes). Warming decreased the abundances of Cryptomonas erosa (Cryptophyceae) and Oocystis pusilla (Chlorophycota) and increased those of two other green algae, Tetraedron minimum and Micractinium pusillum. It had no effect on a further 17 species that were predominant in a community of about 90 species. • Fish and nutrients, either together or separately, generally increased the crops of most of the 21 abundant species and of the algal groups. Exceptions were for diatoms and chrysophytes, which were very minor components of the communities. Fish, but neither nutrients nor warming, increased the number of species of phytoplankton detected. This was probably through removal of zooplankton grazers, and parallels terrestrial studies where the presence of top predators, by controlling herbivores, leads to increased plant diversity. • There was no particular pattern in the taxonomy or biological characteristics of those species affected by the treatments. In particular, there was no link between organism size (a surrogate for many important biological features of phytoplankton species) and the effects of warming, nutrient addition or presence or absence of fish. However, all species were relatively small and potentially vulnerable to grazing. • Synthesis and applications. The results suggest that fears of an increasing abundance of cyanophytes with current projections of global warming may be unrealized, at least in shallow unstratified lakes still dominated by macrophytes. However, they emphasize that eutrophication and fish manipulations remain very important impact factors that determine the abundance of phytoplankton and subsequent problems caused by large growths.
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Summary 1. Temperature strongly affects virtually all biological rate processes, including many central to organismal fitness such as growth rate. A second factor related to growth rate is organismal chemical composition, especially C : N : P stoichiometry. This association arises because high rates of growth require disproportionate investment in N- and P-rich biosynthetic cellular structures. Here the extent to which these factors interact is examined - does acclimation temperature systematically affect organismal chemical composition? 2. A literature survey indicates that cold-acclimated poikilotherms contain on average 30-50% more nitrogen (N), phosphorus (P), protein and RNA than warm-exposed conspecifics. The primary exception was bacteria, which showed increases in RNA content but no change in protein content at cold temperatures. 3. Two processes - changes in nutrient content (or concentration) and in organism size - contribute to the overall result. Although qualitatively distinct, both kinds of change lead to increased total catalytic capacity in cold-exposed organisms. 4. Temperature-driven shifts in nutrient content of organisms are likely to resonate in diverse ecological patterns and processes, including latitudinal and altitudinal patterns of nutrient content, foraging decisions by organisms living in strong temperature gradients, and patterns of biodiversity.
Article
SUMMARY 1. Pelagic and epipelic microalgal production were measured over a year in a pre-defined area (depth 0.5 m) in each of two lakes, one turbid and one with clear water. Further estimates of epiphytic production within reed stands were obtained by measuring production of periphyton developed on artificial substrata. 2. Total annual production of phytoplankton and epipelon was 34% greater in the turbid lake (190 g C m−2 year−1) than in the clearwater lake (141 g C m−2 year−1). However, the ratio of total production to mean water column TP concentration was two fold greater in the clearwater lake. 3. Phytoplankton accounted for the majority of the annual production (96%) in the turbid lake, while epipelic microalgal production dominated (77%) in the clear lake. The relative contribution of epipelic algae varied over the year, however, and in the turbid lake was higher in winter (11–25%), when the water was relatively clear, than during summer (0.7–1.7%), when the water was more turbid. In the clearwater lake, the relative contribution of epipelon was high both in winter, when the water was most clear, and in mid-summer, when phytoplankton production was constrained either by nutrients or grazing. 4. Compared with pelagic and epipelic primary production, epiphytic production within a reed stand was low and did not vary significantly between the lakes. 5. The study supports the theory of a competitive and compensatory trade-off between primary producers in lakes with contrasting nutrient concentrations, resulting in relatively small differences in overall production between clear and turbid lakes when integrating over the season and over different habitats.