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# 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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... Denitrification permanently removes nitrate transferred from land to water and may potentially reduce the nitrogen supply to phytoplankton in these systems. Conversely, phytoplankton may outcompete denitrifying bacteria for new nitrate, inhibiting the natural denitrification process and exacerbating eutrophication symptoms in ICOLLs (Olsen et al. 2017). ...
... Colder temperatures in winter and reduced light conditions may also reduce phytoplankton productivity, but our in situ nitrate uptake measurements showed that the nitrate supplied to the water column of the enclosures was largely assimilated within 48 h, even in winter (Fig. 3). In summer, when denitrification occurred in our enclosures and nitrate uptake was more rapid, competition between denitrification and phytoplankton uptake was feasible (Olsen et al. 2017). In summer, sediment oxygen penetration depths were also shallower, reducing the depth into the sediment that nitrate must be transferred to the main zone of denitrification (the oxic-anoxic boundary). ...
... Although nitrate was depleted to low levels in the Lake Ellesmere and Tomahawk Lagoon enclosures, only a small proportion of nitrate was evolved as dinitrogen gas within 48 h. Olsen et al. (2017) showed that macrophytes and filamentous algae constitute a large nitrate uptake pathway (40 and 30%, respectively), and less than 1% of an added nitrate pulse was delivered to the sediments, and less than 5% was denitrified in their mesocosms. Although in situ denitrification rates in Tomahawk Lagoon were not significantly greater than in Lake Ellesmere, nitrate Fig. 11 Hierarchy of factors related to denitrification in the study ICOLLs. ...
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Intermittently closed and open lakes/lagoons (ICOLLs) can occur in alternate stable states: clear and turbid, with nitrogen inputs from high-intensity agricultural land use often fuelling phytoplankton growth in ICOLLs. Due to their limited water exchange, ICOLLs are particularly susceptible to eutrophication. In these environments, denitrification may remove a substantial proportion of the land-derived nitrogen load, reducing their vulnerability to eutrophication; however, the factors that influence denitrification in ICOLLs are poorly understood. In this study, we addressed the relative importance of physico-chemical and biotic factors related to nitrate-saturated denitrification rates (including temperature, nutrient/organic matter supply, oxygen conditions, sediment type and benthic macroinvertebrates) in two eutrophic ICOLL ecosystems: one supports some submerged macrophytes, while the other is in a persistent, turbid, phytoplankton-dominated system. Flexible in situ enclosures and denitrification enzyme assay measurements were employed to determine denitrification rates in response to new nitrate pulses, which are commonly observed in these systems. In situ denitrification rates were inhibited in both ICOLLs in winter, whereas in summer they were positively correlated with organic matter availability. Denitrification rates were greater in the shallow, marginal sediments of the ICOLLs. Bioturbating macrofauna significantly enhanced in situ sediment oxygenation and probably transported sediment organic carbon and nitrate simultaneously to sites of denitrification at the sediment oxic–anoxic interface. Our study found that nitrate-saturated sediment denitrification rates were controlled by a hierarchy of temporally and spatially structured physico-chemical and biotic factors in the following order of importance: temperature → organic matter availability → water depth → bioturbation.
... Periphyton is composed of algae, heterotrophic microbes, and detritus attached to submerged substrates. Periphytic algae are important primary producers in shallow lakes (Wetzel, 1964) and can shape the nitrogen turnover rate (Epstein et al., 2012;Olsen et al., 2017). Different periphyton are classified depending on the diverse natural substrates to which they are attached, including epilithon on stone, epiphyton on macrophytes, epipsammon on sand, and epipelon on sediment. ...
... All three macrophyte species in our study were perennial. As such, the senescent leaves started to decay and release nutrients into the water column by the end of the experiment, thereby contributing similar N concentrations in all mesocosms, as reported previously (Olsen et al., 2017). The low proportion of dissolved inorganic N (mainly N-NH 4 + N-NO 3 ) to TN in the water column also indicated a major contribution of organic N in the overlying water. ...
... Previous studies of the effects of extreme precipitation on submerged macrophytes have focused on either the effects of water level fluctuations (Evtimova and Donohue, 2016;Wang et al., 2016) or pulse nutrient loading Olsen et al., 2017). Major water level fluctuations ( ± 75 cm) have been revealed to adversely affect the growth of aquatic plants (Deegan et al., 2007;Yu and Yu, 2009;Zhang et al., 2012). ...
... Major water level fluctuations ( ± 75 cm) have been revealed to adversely affect the growth of aquatic plants (Deegan et al., 2007;Yu and Yu, 2009;Zhang et al., 2012). In contrast, pulses of nitrogen had no or only temporary effects on the biomass accumulation of submerged macrophytes under limited phosphorus conditions Olsen et al., 2017). Extreme precipitation causes, however, both pulsed nutrient inputs and sudden increases in water level with subsequent effects on submerged macrophytes, rendering precipitation an important element to be considered besides nutrient pulses and water level alterations. ...
... For dissolved nutrients (dissolved organic carbon, orthophosphate, nitrate, ammonium and silicate), water samples were filtered through pre-combusted (500°C) Whatman® glass microfiber filters grade GF/C (1.2 μm pore size). Although GF/F filters (0.6-0.7 μm pore size) are generally recommended in dissolved nutrient analyses (Wetzel and Likens, 2000), GF/C have also been commonly used, without considerable impact (Olsen et al., 2017;Trochine et al., 2017). Dissolved organic carbon (DOC) was measured in a TOC-L analyzer (Shimadzu Corporation, Japan). ...
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A mesocosm experiment was conducted in a temperate eutrophic lake with the hypotheses: 1) the addition of a labile form of DOC would trigger a more pronounced response in phytoplankton biomass and composition compared with a non-labile form; 2) DOC addition would increase phytoplankton biomass by co-inserting organic nutrients for phytoplankton growth; 3) DOC addition would change phytoplankton composition, in particular towards mixotrophic taxa due to higher DOC availability; and that 4) there would be differences in phytoplankton responses to DOC addition, depending on whether sediment was included or not. We used two types of mesocosms: pelagic mesocosms with closed bottom, and benthic mesocosms open to the sediment. The experiment ran for 29 days in total. The DOC addition occurred once, at Day 1. Besides the control, there were two treaments: HuminFeed® (non-labile DOC) at a concentration of 2 mg L−1 , and a combination of 2 mg L−1 HuminFeed® and 2 mg L−1 DOC from alder leaf leachate (labile). Responses were detected only in the treatment with alder leaf extract. Ecosystem processes responded immediately to DOC addition, with the fall in dissolved oxygen and pH indicating an increase in respiration, relative to primary production (Day 2). In contrast, there was a delay of a few days in structural responses in the phytoplankton community (Day 6). Phytoplankton biomass increased after DOC addition, probably boosted by the phosphorus released from alder leaf extract. Changes in phytoplankton composition towards mixotrophic taxa were not as strong as changes in biomass, and happened only in the pelagic mesocosms. With the DOC addition, diatoms prevailed in benthic mesocosms, while the contribution of colonial buoyant cyanobacteria increased in the pelagic ones. This study points towards the necessity to look in greater detail at specific responses of phytoplankton to DOC concentration increases considering lake-habitat and sediment influence-
... The analysis error was b0.1‰. During detection of the sediment index, three parallel tests were conducted for each sediment sample (Olsen et al., 2017). The data used in this study are the means of three parallel experiments. ...
Article
Nitrogen deposition in lake sediment is an important factor reflecting the evolution of lake environments. Over the past 150–200 years, lakes in China have been affected by natural factors and anthropogenic factors, and nitrogen deposition has increased. As a result, it is critical to reconstruct the spatiotemporal variation trend of nitrogen deposition and analyse the nitrogen source and driving factors. On a regional scale, based on the sediment TN, δ¹⁵N and C: N ratio variation trends, this study analysed the buried nitrogen variation trend in Yunnan-Guizhou Plateau lakes over the past 150–200 years. The effects of lake morphology on nitrogen deposition were also analysed by using natural lake parameters. At the watershed scale, the δ¹⁵N isotope in the sediment was used to distinguish the sediment sources. On this basis, this study analysed the relationship between nitrogen deposition in nine lakes and the socioeconomic conditions during 1949–2010. The results show that (1) during the last 150–200 years, the TN, δ¹⁵N and the C: N ratio in the sediments increased. (2) Lake depth and area are the main natural factors affecting the extent of nitrogen deposition. (3) Before 1950, the nitrogen in the lake sediments in the region was sourced mainly from natural sources such as precipitation, woodland, grassland and aquatic plants. After 1950, man-made sources such as sewage and farmland became the main sources of nitrogen. (4) Human social and economic activities have an increasingly significant influence on the lake water environment in the Yunnan-Guizhou Plateau and are also the main factors leading to the deterioration of the aquatic environment.
... Resource pulses can affect the physiological metabolism of plants, change their species pattern, and even influence the ecosystem function [4][5][6]. The effects of resource pulses have been widely explored in terrestrial ecosystems at some ecological scales, such as specific species [7,8]; wild population [9][10][11]; community [12][13][14][15][16]; and ecosystem [17,18]. However, few studies have focused on determining resource pulse effects on aquatic systems [3]. ...
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Ammonium pulse attributed to runoff of urban surface and agriculture following heavy rain is common in inland aquatic systems and can cause profoundly effects on the growth of macrophytes, especially when combined with low light. In this study, three patterns of NH4-N pulse (differing in magnitude and frequency) were applied to examine their effects on the growth of three submersed macrophytes, namely, Myriophyllum spicatum, Potamogeton maackianus, and Vallisneria natans, in terms of biomass, height, branch/ramet number, root length, leaf number, and total branch length under high and low light. Results showed that NH4-N pulse caused negative effects on the biomass of the submerged macrphytes even on the 13th day after releasing NH4-N pulse. The negative effects on M. spicatum were significantly greater than that on V. natans and P. maackianus. The effects of NH4-N pulse on specific species depended on the ammonium loading patterns. The negative effects of NH4-N pulse on P. maackianus were the strongest at high loading with low frequency, and on V. natans at moderate loading with moderate frequency. For M. spicatum, no significant differences were found among the three NH4-N pulse patterns. Low light availability did not significantly aggregate the negative effects of NH4-N pulse on the growth of the submersed macrophytes. Our study contributes to revealing the roles of NH4-N pulse on the growth of aquatic plants and its species specific effects on the dynamics of submerged macrophytes in lakes.
... Submerged macrophytes are considered as one of the most important primary producers in shallow oligotrophic freshwaters and strongly affect the nutrient turnover for freshwater ecosystem (Wetzel, 1964;Epstein et al., 2012;Olsen et al., 2017). In addition, submerged macrophytes can interact with other organisms, e.g., protecting the zooplankton from fish grazing or providing substrate for periphyton growth (Jeppesen et al., 1998;Cao et al., 2014Cao et al., , 2017. ...
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... However, d 15 N may not be a reliable indicator of nutrient utilisation when different nutrient sources with distinct isotopic signatures, such as NO 3 -N, NH 4 -N and dissolved organic N, are used by primary producers (Jones, King, Dent, Maberly, & Gibson, 2004;Kumar et al., 2011;Rau, Low, Pennington, Buck, & Chavez, 1998 (Kumar et al., 2011). A recent study by Olsen et al. (2017) shows that among the primary producers, macrophytes and filamentous algae are the main controllers of NO 3 -N processing during the summer in the unenriched mesocosms. Most likely, the algae used NH 4 + regenerated through zooplankton grazing or via microbial decomposition of organic N in the water or sediment (see Gu & Alexander, 1993). ...
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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.
Book
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.
Article
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
Article
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
Article
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.
Article
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.
Article
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.