No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Effect of a nitrogen pulse on ecosystem N processing at different temperatures: A mesocosm experiment with 15NO3 addition
Abstract
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.
An experiment with a K ¹⁵ NO 3 ⁻ pulse addition (increase in NO 3 ⁻ 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 (heating 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).
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.
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.
... 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. ...
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. ...
... Previous studies have shown that with sufficient light, nitrogen can stimulate periphyton growth, unlike phosphorus (Jones and Sayer 2003;Barker et al. 2008;Mosischet al. 2001). Since stable nitrogen isotope experiments have demonstrated that filamentous Chlorophyta can retain up 30% of added nitrogen, while macrophytes can absorb 40% (Olsen et al. 2017), nitrogen might promote the growth of metaphyte and Mougeotia simultaneously. However, our result didn't find a clear relationship between nitrogen and the biomass of submerged vegetation. ...
Periphyton plays a critical role in the progress of the regime shift between macrophytes and phytoplankton in shallow lakes, since its overgrowth could trigger metaphytic blooms and lead to the collapse of submerged macrophytes. Understanding the mechanisms of metaphytic blooms and the subsequent prediction are important for lake managers to prevent ecological disaster. In this study, a one-year field survey on periphyton was conducted in Lake Ulansuhai to explore the driving factors of metaphytic blooms. The result revealed that the filamentous chlorophyta Mougeotia was identified as the key genus involved in metaphytic blooms. Structural Equation Modeling showed that Mougeotia biomass was positively correlated with total nitrogen, temperature, and submerged vegetation density. While phytoplankton biomass was primarily positively correlated with total phosphorus and temperature. A logistic regression model indicated that when the biomass of Mougeotia reached 1.78 g m⁻² (95% CI 1.08–3.03 g m⁻²), the probability of metaphytic blooms exceeded 50%. These findings suggested that both nitrogen and phosphorus drive the regime shift, and a dual-nutrient-reduction strategy should be implemented during this hysteresis phase. Additionally, the biomass of submerged vegetation and water depth influence Mougeotia biomass. Therefore, controlling submerged vegetation and managing water levels are crucial in-lake strategies for mitigating metaphytic blooms.
... With the increasing intensity and frequency of precipitation in the future, it will cause considerable impacts on aquatic ecosystems. Keywords: Shallow lake; submerged macrophyte; water-level fluctuations; eutrophication; climate change; sexual reproduction 长江中下游地区浅水湖泊密集，是我国水生植物比较集中的一个区域 [1] 。然而，在全球气候变化、 人类活动加剧等多重因素影响下，以鄱阳湖、洞庭湖为典型代表的浅水湖泊湿地自然水文特征发生明显 变化，特别是近年来频繁受到极端洪涝和干旱气候的影响，严重威胁水生植物生存和生物多样性保护 [2] 。 相关研究表明，随着全球气温升高，极端洪水增加趋势显著 [3][4] ，使得浅水湖泊藻类暴发风险更大 [5] ，给 水生植物、水质以及生态系统结构带来了诸多影响 [6][7] 。沉水植物是水生生态系统中的初级生产者，稳定 的沉水植物群落是维持浅水湖泊生态系统健康的基石，因此在当前气候变化背景下，探究极端洪水对沉 水植物生长发育的作用，对深入理解浅水湖泊生态系统结构与功能的变化有重要意义。 极端洪水目前尚无通用的定义，其通常源于夏季极端降雨或温暖春季急剧融冰化雪引起的集水区下 游湖泊、河流水量迅速增加或水位迅猛上涨 [8] 。一方面，水位波动节律改变对沉水植物的生长、繁殖和 进化演替影响较大。洪水期间降水径流会导致水位短时间内显著增加，更高的水位可以迅速降低水体光 照条件，影响植物光合作用、气体交换和养分吸收，这些生理过程的变化会进一步影响植物形态和生长 [9][10] 。另一方面，极端洪水是引起湖泊内氮(N)、磷(P)等营养元素变化的重要因素，对湖泊湿地水 质以及生态功能具有深远影响。沉水植物与浮游植物及丝状藻类之间具有相互竞争关系，极端洪水可能 增加水体的外源营养物质输入，这些营养盐的波动性增加通常伴随浮游植物生物量的增加 [11][12] ，导致沉 水植物衰退 [13][14] [38] 。Wang 等 [39] 表明导致长江中下游湖泊沉水植物衰退 的水体 Chl.a 浓度临界阈值一般在 10 μg/L 以上。 本研究发现 E、 G 和 C+N 处理后 Chl.a 浓度为 29.9~40.23 μg/L。Zheng 等 [40] 在模拟实验研究中，发现蓝藻通过化感作用抑制了竹叶眼子菜幼苗的生物量、叶绿素 含量和光合作用。高浓度 N 还可能直接对沉水植物形成生理胁迫(如干扰碳氮代谢、造成氧化损失等)， 从而改变植物生长和形态特征 [41][42] 。另外，研究结果证明营养负荷增加显著抑制了黑藻雌性个体有性繁 殖器官的产生(即繁殖输出)，这可能意味着富营养化对植物繁殖的影响在以前的研究中可能被低估 [43] 。 大量研究表明，水体营养负荷增加导致浮游植物和附生植物大量繁殖，光衰减增加 [13] 。营养负荷增加会 改变底栖初级生产者竞争优势，附着藻类与沉水植物生物量呈显著负相关 [44] 。本研究结果与这些研究结 果一致。因此，极端洪水携带的高浓度营养盐会对沉水植物和水生态系统造成较大的不利影响 [18] ...
Extreme flooding significantly affects the habitat of aquatic plants. Its knock-on effects may be one of the reasons for a massive decline in submerged macrophytes. Increased nutrient loading and rising water level are major factors affecting the reproduction and growth of submerged macrophytes. Therefore, we used H. verticillata, a typical submerged macrophyte in the lake, as the target species. A simulation study was conducted to explore the effects of water-level fluctuations (an increase in water level within a short period of time) and nutrient pulses (an increase in nutrient concentration within a short period of time) caused by extreme flooding events on the submerged macrophyte biomass, growth and reproductive traits, water physicochemical properties and periphyton. Four water level and nutrient loading scenarios were set up, i.e., control (stable water level of 75 cm), gradual increase of water level from 75 cm to 150 cm + nitrogen (N) and phosphorus (P) inputs, sudden increase in water level from 75 cm to 150 cm + N and P inputs, and stable water level of 75 cm + N and P inputs. The latter three treatments had identical total N and P inputs. Phytoplankton, periphyton and N and P concentrations in the water column were monitored in a 90-d period. The results showed that the sizes of females were greater than those of males in the control, while the relative growth rate (RGR), root biomass and the reproductive organ number in males were higher than those in females in the extreme precipitation treatment (sudden change in water level). These findings demonstrated an evident sexual dimorphism in H. verticillata. Females responded more sensitively than males to water-level fluctuations (sudden and gradual water level change) and nutrient pulses. Both sudden and gradual increase in water level suppressed root biomass, RGR and the main branch number of plants, whereas plant height, aboveground and reproductive biomass remained unchanged. At the same time, a sudden rise in water level inhibited periphyton growth. Nutrient pulses increased the N concentration in the overlying water, promoted phytoplankton and periphyton growth, and significantly inhibited the biomass of shoots, roots and reproductive tissues. Within the designed range of water levels (75-150 cm), increased water level had no significant effect on the aboveground growth and reproduction of plants, while the combined effect of increased N and P inputs and water-level fluctuations had a negative effect on both aboveground biomass and reproductive output. Thus, the nutrient pulses aggravated the effect of water-level fluctuations on H. verticillata. In conclusion, the joint effect of water-level fluctuations superimposed on nutrient pulses caused by extreme flooding events will have a significant inhibitory effect on submerged macrophytes. With the increasing intensity and frequency of precipitation in the future, it will cause considerable impacts on aquatic ecosystems.
... Light limitation is the main mechanism for macrophyte decline and regime shifts (Scheffer et al., 1993;Le Bagousse-Pinguet et al., 2012) and treatment-related differences in light availability were observed in the replicated experiment. Various mechanisms on different scales could have contributed further: On community level the phytoplankton may have adapted to herbicide pollution through selection of tolerant species (Blanck, 2002;Christensen et al., 2006); on cellular level higher temperatures further increase nutrient uptake efficiency and detoxification rates (Chalifour and Juneau, 2011;Jensen and Andersen, 1992;Olsen et al., 2017). Remarkably, the negative effect on macrophytes at the end of the gradient experiment was found despite the crash of phytoplankton halfway during our experiment, indicating a long-lasting or time-delayed effect from phytoplankton blooms two weeks before. ...
In aquatic ecosystems, excessive nutrient loading is a global problem that can induce regime shifts from macrophyte- to phytoplankton-dominated states with severe consequences for ecosystem functions. Most agricultural landscapes are sites of nutrient and pesticide loading, which can interact with other stressors (e.g., warming) in additive, antagonistic, synergistic or reversed forms. The effects of multiple stressors on the resilience of macrophyte-dominated states and on critical thresholds for regime shifts are, however, unknown. We test the effects of individual and combined stressors of warming, nitrate, and various pesticides typically found in agricultural run-off (ARO) on the growth of macrophytes, periphyton, and phytoplankton in microcosms. We applied a one-level replicated design to test whether ARO induces a regime shift and a multifactorial dose–response design to model stressor thresholds and disentangle stressor interactions along a gradient. The individual stressors did not induce a regime shift, but the full ARO did. Nitrate and pesticides acted synergistically, inducing a shift with increasing phytoplankton biomass and decreasing macrophyte biomass. Warming amplified this effect and lowered critical thresholds for regime shifts. Shallow aquatic ecosystems in agricultural landscapes affected by global warming thus increasingly risk shifting to a turbid, phytoplankton-dominated state, and negatively impacting ecosystem service provisioning. Multiple stressor interactions must be considered when defining safe operating spaces for aquatic systems.
... 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). ...
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. ...
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]. ...
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. ...
Submerged macrophytes play a structuring role in the shallow freshwater ecosystem by increasing the heterogeneous state in freshwaters. The macrophytes in genus Ottelia were featured for their broad leaves, which might consequently produce specialized functions that differed from other submerged species. To explore the potential ecological role of Ottelia, a field investigation was conducted on leaf traits in eight populations of Ottelia ranging from the southwestern Yunnan–Guizhou plateau to the southern Hainan island in China covering a distance of >1,700 km. The eight populations included all the extant Ottelia species and varieties in China except the well-documented O. alismoides. Carbon-related traits [bicarbonate usage, photosynthetic characteristics, capability of Crassulacean acid metabolism (CAM)], pigment content and parameters of chlorophyll fluorescence, morphology and mass of the leaves were determined. The different populations showed distinct functional traits of mature leaves; O. acuminata var. songmingensis had the thickest and longest leaf with CaCO3 precipitation on the both sides of the leaf, and O. cordata showed putative CAM activity with the highest diel acidity changes 12.5 μequiv g⁻¹ FW. Our results indicated an important role of Ottelia populations in carbon cycling as the dominant species in karst freshwaters in China.
... 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). ...
Summary
1. Carbon (C) and nitrogen (N) stable isotope composition (15N:14N, d15N and
13C:12C, d13C) have been widely used to elucidate changes in aquatic ecosystem
dynamics created by eutrophication and climate warming, often, however, without
accounting for seasonal variation.
2. Here, we aim to determine the factors controlling the stable isotope composition
and C:N ratio of seston and periphyton in shallow lakes with contrasting nutrient
loadings and climate; for this purpose, we followed the monthly stable isotope
composition (c. 1 year) of seston (SES) and periphyton (PER) in 24 mesocosms
mimicking shallow lakes with two nutrient treatments (enriched and unenriched)
and three temperature scenarios (ambient, +3 and +5°C).
3. Nutrient enrichment and warming had a stronger impact on the d15N and d13C
values of seston than on periphyton, and the temporal isotopic variability in both
communities was large.
4. d15NPER did not differ markedly between nutrient treatments, whereas d15NSES
was lower in the enriched mesocosms, possibly reflecting higher N2-fixation by
cyanobacteria. d15NSES was higher in winter in the heated mesocosms and its
dynamics was linked with that of NH4-N, whereas d15NPER showed a stronger
association with NO3-N. d15NSES demonstrated a positive relationship with mean
monthly temperature, indicating less isotope fractionation among autotrophs
when production increased.
5. d13CSES was lowest in the enriched mesocosms during winter, whereas d13CPER
did not differ between nutrient treatments. d13CSES and d13CPER were positively
related to pH, likely reflecting a pH-induced differential access to dissolved
carbon species in the primary producers. The positive d13C-temperature relationship
suggested less fractionation of CO2 and HCO3
� and/or larger use of HCO3
�
at higher temperatures.
6. The C:N ratios varied seasonally and the differences between the enriched and
unenriched mesocosms were stronger for seston than for periphyton. Particularly,
the C:NSES ratios did not indicate deficiencies in N as opposed to the C:
NPER ratios, supporting the observed changes in d15N and suggesting that seston
and periphyton have access to different sources of nutrients. We did not
observe any clear effect of temperature warming on the C:N ratios.
7. Our study provides evidence of strong seasonality in the isotopic composition
and C:N ratios of seston and periphyton across nutrient and temperature levels;
also, we identified several factors that are likely to modulate the strength and
variability in stable isotopes values and stoichiometry of sestonic and periphytic
communities under these scenarios.
Global climate change models forecast an increase and intensification of extreme flood events in the future. Extreme flooding has profound impacts on functional traits and growth status of submerged macrophytes and the entire ecosystem. Studying the effects of extreme flooding on the growth and development of submerged macrophytes is important for understanding and predicting changes in aquatic ecosystems under the context of climate change. Here, we addressed the dual effects of extreme water level increase and enhanced nutrient loading induced by extreme flood events through setting up four treatments during a 90-day experimental period, including control (water level maintained at 75 cm), two extreme flood regimes (water level increased rapidly from 75 cm to 150 cm on the first day with N and P inputs; the water level increased gradually from 75 cm to 150 cm with N and P inputs), and water level maintained at 75 cm with N and P inputs (the total amount of nutrient input was identical among the latter three treatments). The effects of extreme flooding on the growth, reproductive strategies and biomass allocation of the submerged macrophyte Vallisneria natans were investigated with simultaneous monitoring of phytoplankton, periphyton and nutrient conditions (TN and TP) of the water column. Among the 17 measured indicators of V. natans, only root biomass allocation and sexual reproduction allocation did not change significantly among these four treatments. For the individual effect of water level, both of the two flood regimes (extreme and gradual water level increase) reduced ramet number, leave number, spacer number, total spacer length, maximum root length, number of flowers and fruits, biomass of each organ and the total plant biomass, and promoted the growth of periphyton on artificial stripes compared to the treatment with increased nutrient loading and constant water level. With sudden increase of the water level, the biomass investment in spacers tended to be the lowest, while plant height and the biomass investment in leaves tended to be the highest. For the individual effect of nutrient loading, the significant increase in water column N concentration in the treatment with increased nutrient loading and constant water level promoted the growth of phytoplankton and epiphyton on macrophytes compared to the control treatment. However, in terms of the concentrations of phytoplankton and epiphyton, the shading effect of phytoplankton was relatively greater, which inhibited the growth of plant leaves, roots and spacers, whereas the sexual reproduction biomass remained unchanged. For the pattern of the combined effect of increases in water level and nutrient loading, the combined effect of the two environmental factors would strengthen the effect of water level increase on the growth of submerged macrophytes due to the pulses of nutrient input. The combined effect of the two factors would substantially reduce the biomass of leaves, roots, spacers and sexual organs. Therefore, extreme flooding has a great detrimental effect on submerged macrophytes, including direct effect of water level increase, and indirect effects of the growth and development of submerged macrophytes through promoting phytoplankton biomass.
The global climate change may lead to more extreme climate events such as severe flooding creating excessive pulse-loading of nutrients, including nitrogen (N), to freshwaters. We conducted a 3-month mesocosm study to investigate the responses of phytoplankton, zooplankton and Vallisneria spinulosa to different N loading patterns using weekly and monthly additions of in total 14 g N m−2 month−1 during the first 2 months. The monthly additions led to higher phytoplankton chlorophyll a and total phytoplankton biomass than at ambient conditions as well as lower leaf biomass and a smaller ramet number of V. spinulosa. Moreover, the biomass of cyanobacteria was higher during summer (August) in the monthly treatments than those with weekly or no additions. However, the biomass of plankton and macrophytes did not differ among the N treatments at the end of the experiment, 1 month after the termination of N addition. We conclude that by stimulating the growth of phytoplankton (cyanobacteria) and reducing the growth of submerged macrophytes, short-term extreme N loading may have significant effects on shallow nutrient-rich lakes and that the lakes may show fast recovery if they are not close to the threshold of a regime shift from a clear to a turbid state.
The use of 15NO3 in experimental set ups with undisturbed sediment cores or waterlogged soils, provides detailed information on the complex microbial nitrogen cycle. Goering and Pamatmat (1970) were among the first to use the reduction of 15NO3 to 15N2 gas as an assay for denitrification in sediments; following this pioneering work, the method has been improved by many modifications (Nishio et al 1983; Enoksson and Samilsson 1987; Binnerup et al 1992; Rysgaard et al 1993, 1994; Risgaard-Petersen et 1994). Recently, a new method has been developed for simultaneous determination of coupled nitrification/denitrification, and denitrification of NO3 supplied from the water column (Nielsen 1992). The methods presented have mainly been developed and used for estimating nitrate reduction in sediment. After slight moidifications, these methods can also be applied to water logged soil.
Fresh waters make a disproportionately large contribution to greenhouse gas (GHG) emissions, with shallow lakes being particular hotspots. Given their global prevalence, how GHG fluxes from shallow lakes are altered by climate change may have profound implications for the global carbon cycle. Empirical evidence for the temperature dependence of the processes controlling GHG production in natural systems is largely based on the correlation between seasonal temperature variation and seasonal change in GHG fluxes. However, ecosystem-level GHG fluxes could be influenced by factors, which whilst varying seasonally with temperature are actually either indirectly related (e.g. primary producer biomass) or largely unrelated to temperature, for instance nutrient loading. Here, we present results from the longest running shallow-lake mesocosm experiment which demonstrate that nutrient concentrations override temperature as a control of both the total and individual GHG flux. Furthermore, testing for temperature treatment effects at low and high nutrient levels separately showed only one, rather weak, positive effect of temperature (CH4 flux at high nutrients). In contrast, at low nutrients, the CO2 efflux was lower in the elevated temperature treatments, with no significant effect on CH4 or N2 O fluxes. Further analysis identified possible indirect effects of temperature treatment. For example, at low nutrient levels increased macrophyte abundance was associated with significantly reduced fluxes of both CH4 and CO2 for both total annual flux and monthly observation data. As macrophyte abundance was positively related to temperature treatment, this suggests the possibility of indirect temperature effects, via macrophyte abundance, on CH4 and CO2 flux. These findings indicate that fluxes of GHGs from shallow lakes may be controlled more by factors indirectly related to temperature, in this case nutrient concentration and the abundance of primary producers. Thus, at ecosystem scale response to climate change may not follow predictions based on the temperature dependence of metabolic processes. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
1. Excess loading of phosphorus (P) and nitrogen (N) triggers a shift in the trophic structure of shallow lakes from a clear-water, macrophyte-dominated state to an algal-dominated turbid state. However , the role of N in the shift is debated, and experimental evidence is, with a few exceptions, based on short-term studies (days to a few months).
2. We studied the effect of N loading on macrophytes (dominated by Potamogeton lucens and Ca-bomba caroliniana), periphyton, filamentous algae and phytoplankton in mesocosms over 10 months (starting in October) in subtropical China (Wuhan). There were three N treatments: controls (CN) without nitrogen addition (mean TN = 1.9 mg L À1), low nitrogen (LN) addition (mean TN = 3.5 mg L À1) and high nitrogen (HN) addition (mean TN = 5.5 mg L À1). Total phosphorus (TP) concentration in the water column remained moderate (0.05–0.07 mg L À1) during the experiment in all treatments.
3. Macrophyte abundance declined in the LN and HN treatments in the first 6 months, but not in controls, followed by a partial recovery in the LN treatments. They disappeared completely in the HN treatments the following summer. Periphyton (biofilm on plastic) and phytoplankton biomass remained unaffected during the first 6 months but increased over the summer by two or three times, compared with controls, in low and high nitrogen treatments, respectively. By contrast, the abundance of filamentous algae increased over winter but declined during the summer with no obvious relationship to the N treatments. There was no difference in the TN or nitrate concentrations or soluble protein, soluble sugar and Chl-a content of P. lucens leaves and stems with increasing N load.
4. Macrophyte populations are partially resilient to abrupt increases in N loading at moderate TP concentrations, but, after prolonged exposure, a complete collapse occurs. Our results further indicate that macrophyte loss is exacerbated by shading by filamentous algae during the winter, and by phy-toplankton and periphyton in the summer, while there was no indication of direct N toxicity.
We present analysis of variations in relationships between nitrogen (N), phosphorus (P) and chlorophyll-a (chl-a) in lakes along a gradient of latitude inclusive of tropical, temperate and polar regions. Total nitrogen (TN), total phosphorus (TP), chl-a, latitude
and depth data were collated for 1316 lakes situated between 70 °S and 83 °N. Latitudinal variation was then analysed for three empirical measures of phytoplankton nutrient limitation and/or nutrient assimilation. Lastly, chl-a near-maxima conditional on TN and TP abundance
were empirically defined for this global dataset using quantile regression. Mean TN:TP increases with distance from the equator. This relationship is independent of variation in either lake depth or trophic state, reflecting latitudinal variation in nutrient cycling processes and/or nutrient
sources. There is a negative linear relationship between latitude and chl-a:TN which similarly suggests that N is less abundant relative to phytoplankton growth requirements at lower latitudes. Relative to temperate lakes, the statistical capability of TN and TP to predict chl-a
is poor for both tropical and polar lakes, reflecting latitudinal variation in lake ecosystem functioning and the subsequent potential unsuitability of applying relationships derived for temperate lakes elsewhere. Chl-a near-maxima correspond to chl-a:TN and chl-a:TP yields
of 0.046:1 and 0.87:1 respectively, although some observations greatly exceed near-maxima, suggesting possible physiologically plastic phytoplankton responses in these exceptional cases. Deficiencies in understanding the mechanisms that drive variation in macro-nutrient stoichiometry and phytoplankton
biomass-nutrient relationships across large spatial scales necessitates further landscape-scale research on this topic, particularly in the tropics.
Freshwater ecosystems and their biodiversity are presently seriously threatened by global development and population growth, leading to increases in nutrient inputs and intensification of eutrophication-induced problems in receiving fresh waters, particularly in lakes. Climate change constitutes another threat exacerbating the symptoms of eutrophication and species migration and loss. Unequivocal evidence of climate change impacts is still highly fragmented despite the intensive research, in part due to the variety and uncertainty of climate models and underlying emission scenarios but also due to the different approaches applied to study its effects.
We first describe the strengths and weaknesses of the multi-faceted approaches that are presently available for elucidating the effects of climate change in lakes, including space-for-time substitution, time series, experiments, palaeoecology and modelling. Reviewing combined results from studies based on the various approaches, we describe the likely effects of climate changes on biological communities, trophic dynamics and the ecological state of lakes. We further discuss potential mitigation and adaptation measures to counteract
the effects of climate change on lakes and, finally, we highlight some of the future challenges that we face to improve our capacity for successful prediction.
Nature of the problem
• Freshwater ecosystems play a key role in the European nitrogen (N) cycle, both as a reactive agent that transfers, stores and processes N loadings from the atmosphere and terrestrial ecosystems, and as a natural environment severely impacted by the increase of these loadings.
Approaches
• This chapter is a review of major processes and factors controlling N transport and transformations for running waters, standing waters, groundwaters and riparian wetlands.
Key findings/state of knowledge
• The major factor controlling N processes in freshwater ecosystems is the residence time of water, which varies widely both in space and in time, and which is sensitive to changes in climate, land use and management.
• The effects of increased N loadings to European freshwaters include acidifi cation in semi-natural environments, and eutrophication in more disturbed ecosystems, with associated loss of biodiversity in both cases.
• An important part of the nitrogen transferred by surface waters is in the form of organic N, as dissolved organic N (DON) and particulate organic N (PON). Th is part is dominant in semi-natural catchments throughout Europe and remains a signifi cant component of
the total N load even in nitrate enriched rivers.
• In eutrophicated standing freshwaters N can be a factor limiting or co-limiting biological production, and control of both N and phosphorus (P) loading is oft en needed in impacted areas, if ecological quality is to be restored.
Major uncertainties/challenges
• The importance of storage and denitrifi cation in aquifers is a major uncertainty in the global N cycle, and controls in part the response of catchments to land use or management changes. In some aquifers, the increase of N concentrations will continue for decades even if efficient mitigation measures are implemented now.
• Nitrate retention by riparian wetlands has oft en been highlighted. However, their use for mitigation must be treated with caution, since their effectiveness is difficult to predict, and side effects include increased DON emissions to adjacent open waters, N2O emissions to
the atmosphere, and loss of biodiversity.
• In fact, the character and specifi c spatial origins of DON are not fully understood, and similarly the quantitative importance of indirect N2O emissions from freshwater ecosystems as a result of N leaching losses from agricultural soils is still poorly known at the regional scale.
• These major uncertainties remain due to the lack of adequate monitoring (all forms of N at a relevant frequency), especially – but not only – in the southern and eastern EU countries.
Recommendations
• The great variability of transfer pathways, buffering capacity and sensitivity of the catchments and of the freshwater ecosystems calls for site specific mitigation measures rather than standard ones applied at regional to national scale.
• The spatial and temporal variations of the N forms, the processes controlling the transport and transformation of N within freshwaters, require further investigation if the role of N in
influencing freshwater ecosystem health is to be better understood, underpinning the implementation of the EU Water Framework Directive for European freshwaters.
Unlucky Lakes
The negative consequences of increased loading of nitrogen and phosphorus into aquatic ecosystems are well known. Management strategies aimed at reducing the sources of these excess nutrients, such as fertilizer runoff or sewage outflows, can largely mitigate the increases in nitrogen and phosphorus levels; however, it is unclear if these strategies are influencing other spects of these ecosystems. Using a global lake data set, Finlay et al. (p. 247 ; see the Perspective by Bernhardt ) found that reducing phosphorus inputs reduced a lake's ability to export reactive nitrogen, exacerbating nitrate pollution.
Acute biochemical responses of Potamogeton crispus L. to high external ammonium were investigated in an aquarium experiment. Shoots of P: crispus were incubated in aquaria for 24 h or 48 h at five treatments of ammonium-0, 1, 5, 10 and 20 mg/L NH4-N. Soluble sugar content of the shoots declined markedly with increasing ammonium levels, whereas soluble amino acid content increased dramatically. Responses of two antioxidant enzymes as well as soluble protein content fit a lognormal distribution with increasing ammonium levels. High ammonium levels (NH4-N≥5 mg/L) caused significant acute biochemical changes in P. crispus, which potentially could lead to significant biochemical damage.
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.
In the Baltic Sea, we tested how short nutrient pulses of different lengths and frequencies affect macroalgae, epiphytes, grazers and their interactions. We hypothesized that even small-scale variations in nutrient supply may have significant impacts by favoring fast-growing epiphytes which can cause large-scale declines of canopy-forming macro- algae. In a factorial field experiment single plants of the canopy-forming macroalga Fucus vesiculosus with and with- out epiphytes were exposed to pulses of elevated nutrients (N and P) over 25 d. Five 1 h pulses given every 5 d had no significant effects. A single 5 h pulse increased the epiphyte load but not F.vesiculosus growth rate. In contrast, increasing epiphyte load caused F. vesiculosus growth rate to decline and attracted higher densities of gastropod grazers. These results indicate that a single nutrient pulse can have rapid direct and indirect effects on macroalgae and their associated epiphytes and grazers. Temporal variability of nutrient supply (five 1 h vs one 5 h pulse) plays a significant role in determin- ing the response of primary producers and consumers to ele- vated nutrients.
From 1961–2004, surface water temperature in large and shallow Lakes Peipsi and V˜ortsj¨arv in Estonia
increased significantly in April and August; respectively 0.37–0.75 and 0.32–0.42 degrees per decade
reflecting the changes in air temperature. The average annual amount of precipitation in the catchment
increased significantly. Reflecting practices in agriculture and wastewater treatment, nutrient loadings to
the lakes increased rapidly in the 1980s and decreased again in the early 1990s. As total nitrogen (TN)
loading decreased faster than total phosphorus (TP) loading, the TN/TP ratio in the loadings decreased.
Both the increased temperature and low TN/TP ratio favoured the development of cyanobacteria blooms in
Lake Peipsi. In V˜ortsj¨arv, where the TN/TP mass ratio is about two times higher than in Peipsi, blooms did not occur. Recently, the TN/TP ratio has shown a tendency of increase in both lakes suggesting a certain reduction of blooms to be expected also in Lake Peipsi. Nutrient dynamics in the lakes followed the changes in loadings, showing the ability of shallow lake ecosystems to react sensitively to changes in catchment management as well as in climate.
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.
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.
Stable 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.
To 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.
Nitrogen trophic fractionation (Δδ ¹⁵ N) was affected by temperature. Bass Δδ ¹⁵ N was 4·41‰ at 11 °C and 3·78‰ at 16 °C.
Carbon fractionation (Δδ ¹³ C) was also affected by temperature. Bass Δδ ¹³ C 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 Δδ ¹³ C. When Δδ ¹³ C was determined using mathematically defatted values, there was a direct effect of ration size and Δδ ¹³ C was 2·51, 2·39 and 2·31‰ for high, medium and low rations, respectively.
Reported Δδ ¹⁵ N 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 δ ¹⁵ N as an indicator of trophic level when comparing populations that are exposed to different temperatures.
The Δδ ¹³ C 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.
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.
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 µ mol N m ⁻² h ⁻¹ , with extremes from 0 to 1,067. Rates of denitrification in lake sediments measured at near‐ambient conditions range from 2 to 171 µ mol N m ⁻² h ⁻¹ . Denitrification rates in river and stream sediments range from 0 to 345 µ mol N m ⁻² h ⁻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 µ mol N liter ⁻¹ d ⁻¹ . In lakes where denitrification rates in both the water and sediments have been measured, denitrification is greater in the sediments.
The major source of nitrate for denitrification in most river, lake, and coastal marine sediments underlying an aerobic water column is nitrate produced in the sediments, not nitrate diffusing into the sediments from the overlying water. During the mineralization of organic matter in sediments, a major portion of the mineralized nitrogen is lost from the ecosystem via denitrification. In freshwater sediments, denitrification appears to remove a larger percentage of the mineralized nitrogen. N 2 fluxes accounted for 76–100% of the sediment‐water nitrogen flux in rivers and lakes, but only 15–70% in estuarine and coastal marine sediments. Benthic N 2 O fluxes were always small compared to N, fluxes.
The loss of nitrogen via denitrification exceeds the input of nitrogen via N 2 fixation in almost all river, lake, and coastal marine ecosystems in which both processes have been measured.
Denitrification is also important relative to other inputs of fixed N in both freshwater and coastal marine ecosystems. In the two rivers where both denitrification measurements and N input data were available, denitrification removed an amount of nitrogen equivalent to 7 and 35% of the external nitrogen loading. In six lakes and six estuaries where data are available, denitrification is estimated to remove an amount of nitrogen equivalent to between 1 and 36% of the input to the lakes and between 20 and 50% of the input to the estuaries.
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.
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.
Executive summary
Nature of the problem
• Freshwater ecosystems play a key role in the European nitrogen (N) cycle, both as a reactive agent that transfers, stores and processes N loadings from the atmosphere and terrestrial ecosystems, and as a natural environment severely impacted by the increase of these loadings.
Approaches
• This chapter is a review of major processes and factors controlling N transport and transformations for running waters, standing waters, groundwaters and riparian wetlands.
Key findings/state of knowledge
• The major factor controlling N processes in freshwater ecosystems is the residence time of water, which varies widely both in space and in time, and which is sensitive to changes in climate, land use and management.
• The effects of increased N loadings to European freshwaters include acidification in semi-natural environments, and eutrophication in more disturbed ecosystems, with associated loss of biodiversity in both cases.
• An important part of the nitrogen transferred by surface waters is in the form of organic N, as dissolved organic N (DON) and particulate organic N (PON). This part is dominant in semi-natural catchments throughout Europe and remains a significant component of the total N load even in nitrate enriched rivers.
• In eutrophicated standing freshwaters N can be a factor limiting or co-limiting biological production, and control of both N and phosphorus (P) loading is often needed in impacted areas, if ecological quality is to be restored.
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.
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.
(1) Enclosures (4.5 m) in a dense Potamogeton pectinatus community were enriched by weekly additions of NaNO and KHPO at four different rates (max. 1000 N 1 week and 100 P 1 week) 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) 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.
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.
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.
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 ( ‐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.
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.
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.
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.
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.
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.
A procedure is presented for routine analysis of total nitrogen in solution which is similar to published methods using oxidation by potassium persulfate. Careful attention to pH, alkalinity, the neutralizing buffer, reaction vessels, and dilution factors have proved necessary. This modified method is acceptable for samples from fresh to oceanic waters, is accurate for organic compounds tested, has a maximum capability of 40 µ M nitrogen in undiluted samples, and has a mid‐range precision of ±2%.
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.
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.
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
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.
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
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.
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.
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 .
2. With the addition of phosphorus, chlorophyll a and the total biovolume of phytoplankton rose significantly at moderate and high nitrogen. Cyanobacteria or chlorophytes dominated in all enclosures to which we added phosphorus as well as in the high nitrogen treatment, while cryptophytes dominated in the moderate nitrogen enclosures and the controls.
3. At the end of the experiment, the biomass of the submerged macrophytes Elodea canadensis and Potamogeton sp. was significantly lower in the dual treatments (TN, TP) than in single nutrient treatments and controls and the water clarity declined. The shift to a turbid state with low plant coverage occurred at TN >2 mg N L ⁻¹ and TP >0.13–0.2 mg P L ⁻¹ . These results concur with a survey of Danish shallow lakes, showing that high macrophyte coverage occurred only when summer mean TN was below 2 mg N L ⁻¹ , irrespective of the concentration of TP, which ranged between 0.03 and 1.2 mg P L ⁻¹ .
4. Zooplankton biomass and the zooplankton : phytoplankton biomass ratio, and probably also the grazing pressure on phytoplankton, remained overall low in all treatments, reflecting the high fish abundance chosen for the experiment. We saw no response to nutrition addition in total zooplankton biomass, indicating that the loss of plants and a shift to the turbid state did not result from changes in zooplankton grazing. Shading by phytoplankton and periphyton was probably the key factor.
5. Nitrogen may play a far more important role than previously appreciated in the loss of submerged macrophytes at increased nutrient loading and for the delay in the re‐establishment of the nutrient loading reduction. We cannot yet specify, however, a threshold value for N that would cause a shift to a turbid state as it may vary with fish density and climatic conditions. However, the focus should be widened to use control of both N and P in the restoration of eutrophic shallow lakes.
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.
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 (δ ¹³ C) of the different primary producers to elucidate differences in the degree of CO 2 limitation.
2. Epiphytes were the only primary producer with significantly higher δ ¹³ C in the enriched mesocosms. No temperature effects were observed in δ ¹³ C composition of any primary producer. Independently of the treatment effects, the four primary producers had different δ ¹³ C signatures indicative of differences in CO 2 limitation. Seston had signatures indicating negligible or low CO 2 limitation, followed by epiphytes and sediment biofilm, with moderate CO 2 limitation, while macrophytes showed the strongest CO 2 limitation. CO 2 together with biomass of epiphytes were the key variables explaining between 50 and 70% of the variability in δ ¹³ C of the different primary producers, suggesting that epiphytes play an important role in carbon flow of temperate shallow lakes.
3. The ratio of carbon to chlorophyll a decreased with increasing temperature and enrichment in both epiphytes and seston. The effects of temperature were mainly attributed to changes in algal Chl a content, while the decrease with enrichment was probably a result of a higher proportion of algae in the seston and epiphytes.
4. Macrophytes, epiphytes and seston decreased their C : N with enrichment, probably as an adaptation to the different N availability levels. The C : N of epiphytes and Elodea canadensis decreased with increasing temperature in the control mesocosms. Sediment biofilm was the only primary producer with lower C : P and N : P with enrichment, probably as a result of higher P accumulation in the sediment.
5. Independently of nutrient level and increased temperature effects the four primary producers had significantly different stoichiometric compositions. Macrophytes had higher C : N and C : P and, together with epiphytes, also the highest N : P. Seston had no N or P limitation, while macrophytes and epiphytes may have been P limited in a few mesocosms. Sediment biofilm indicated strong N deficiency.
6. Consumers had strongly homeostatic stoichiometric compositions in comparison to primary producers, with weak or no significant treatment effects in any of the groups (insects, leeches, molluscs and crustaceans). Among consumers, predators had significantly higher N content and lower C : N than grazers.
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.
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.