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Factors influencing nitrogen processing in lakes: an experimental approach
Abstract
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.
... The key role played by phosphorus (P) in lake eutrophication has long been recognized [7]. Recently, however, besides the role of P, also the importance of nitrogen (N) limitation for lake eutrophication has been more intensively studied and discussed [8][9][10]. For example, Pearl et al. [11] and Müller & Mitrovic [12] pointed that ecosystem conservation efforts should take a balanced approach to N and P abatement and that the best strategy for lake managers to adopt would be to pursue a double focus on N and P. Thus, N has been also recognized as one of the most critical nutrients limiting lake productivity. ...
... Overall, the TN content in sediments in various study areas ranked from high to low as follows: B ≈ A > D ≈ C > E, with means of 2769, 2613, 1815, 1767, and 1523 mg·kg −1 , respectively, which is mainly related to the water quality and quantity of rivers flowing into the lake. For instance, before water was transferred from external drainage basins to Dianchi Lake, Panlong River was the river that supplied the largest quantity of (phosphorus in iron-manganese oxide fraction), when Eh in the environment declines, this N fraction becomes more easily release and is able to participates in the N cycle of the lake [9]. The OSF-N fraction refers to N that combines organic matters and sulfides. ...
... Nevertheless, in this study, IEF-N represents the lowest proportion; lower than that of CF-N and IMOF-N. The distribution of sediment N among these different N fractions is affected by multiple factors including pH and Eh values of the environment and the mineral composition of the sediments [9]. The final distribution of different N fractions within the sediment TN is a result of the synthetic action of multiple factors. ...
Columnar sediment samples were collected from five representative estuaries of Dianchi Lake, China. And the vertical distribution of each fraction of nitrogen (IEF-N, CF-N, IMOF-N, OSF-N) were tested. The results showed that the TN content in sediments from areas A, B, C, D and E gradually decreased with depth between 0 and 15 cm, then sharply decreased with depth between 15 and 30 cm and stabilized at depth below 30 cm, indicating the exogenous input of N in these areas has not been controlled effectively. The proportion of TN occupied by various N fractions in the sediments ranked as follows: OSF-N > IMOF > CF-N > IEF-N. Correlation analysis results showed both IEF-N and IMOF-N were significantly correlated with the content of TFe2O3 + MnO + Al2O3 in deeper sediments, while no correlation in superficial sediments. The areas A and B have extremely high release risks for N in superficial sediments. However, the N in the sediments of areas C, D and E were in relative equilibrium with the overlying water, indicating release potential risk was relatively low.
... In lake management, a reduction of P availability with the aim to decrease phytoplankton biomass is therefore often the first measure introduced to combat and reverse cultural eutrophication. discussed (Sterner, 2008;Conley et al., 2009;Moss et al., 2013;Olsen et al., 2015a). Worldwide, huge amounts are invested in reducing nutrient loading to aquatic ecosystems, and targeting eutrophication efficiently and cost effectively is of utmost importance (Schindler & Hecky, 2009). ...
We used data on nutrients, chlorophyll a (Chla) and submerged macrophyte cover from up to 817 Danish lakes to elucidate seasonal variations in nitrogen (N) and phosphorus (P) concentrations and to study the impact of N or its role in combination with P. In both deep and shallow lakes, we found marked seasonality in the ratio between total N and total P (TN:TP) and in the inorganic concentrations of nitrogen (DIN), indicating that N more easily becomes a limiting nutrient as summer proceeds. TN:TP reached its lowest values of <7 (by mass) in August in 25% of the shallow lakes. Chla generally related more strongly to TP than to TN, but at high TP concentrations TN explained more of the variability in Chla than TP. Macrophyte cover tended to decrease at increasing TN when TP was between 0.1 and 0.4 mg/l. At macrophyte cover above 20%, Chla was considerably lower compared with lakes with low macrophyte cover. We conclude that P is of key importance for the ecological quality of Danish lakes but that increased N concentrations, particularly in shallow lakes with moderate to high TP, may have significantly adverse effects on lake water quality and ecological status in summer.
... On a long-term scale (decades to centuries) changes in catchment properties will regulate the flux and fate of C, N, P, in catchments, while on a shorter time scale (seasonal, years to decades) changes in anthropogenic effects on the N cycle, temperature, precipitation patterns, and soil or root processes will be dominant. There are indications that the changes in hydrology and input of N from the watershed has a larger influence on the N budget of lakes than climatic effects related to temperature, salinity and water level (Olsen et al. 2015). For example In North Europe, the projected climate change in this century suggests, increased precipitation in winter months, and consequently a higher export of nutrients from catchments to surface waters (Andersen et al. 2006;Jeppesen et al. 2009Jeppesen et al. , 2011Trolle et al. 2014) enhancing eutrophication and risk of alga bloom. ...
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... Increasing nutrient load in combination with higher water temperatures and more intense and longer stratification as well as the fact that cyanobacteria have a large tolerance for water temperature and salinity ranges may lead to a dominance of these toxic bacteria (Paerl and Paul, 2012;Wagner and Adrian, 2009 and references therein). In some natural lakes, the changes in hydrology and input of N from the catchment have a larger influence on the N budget of lakes than climatic effects related to temperature, salinity and water level (Olsen et al., 2015). Hydrochemical or physical changes in lake water will most likely alter species and ecosystems. ...
1. Shallow lakes may play an important role for the nitrogen (N) balance in drainage basins by processing, transferring and retaining N inputs. An increase in the frequency of storm-induced short-term N pulses and increased water temperatures are both likely outcomes of climate change, potentially affecting the N processing in lakes. 2. An experiment with a K 15 NO 3 À pulse addition (increase in NO 3 À concentration from c. 0.1 to 2 mg/L) was carried out in 12 mesocosms with relatively low (applies to Danish lakes) total N (TN) and total phosphorus (TP) concentrations (c. 0.3 mg N L -1 and 0.04 mg P L -1) to assess the effects of an N pulse on N processing and storage in shallow lake ecosystems. The mesocosms have a hydraulic retention time of approximately two and a half months, and at the time of the experiment, they had been adapted to contrasting temperatures for a period of 10 years: ambient, T3 (heat-ing according to the Intergovernmental Panel on Climate Change 2007 A2 scenario, +3.7–4.5°C, depending on season) and T5 (heating with A2 + 50%, +4.9–6.6°C). 3. Macrophytes and filamentous algae retained up to 40% and 30% of the added 15 N, respectively, reflecting their high biomass in the mesocosms. Macrophytes and filamentous algae constituted between 70% and 80% of the biomass of all primary producers during the experiment in the T3 and ambient treatments and between 20% and 40% in T5. By comparison, less than 1% of the added 15 N diffused to the sediment and less than 5% was lost to the atmosphere as N 2 gas. Snails represented the long-term storage of 15 N, retaining up to 6% of the tracer and with detectable enrichment 100 days after tracer addition. 4. We found no significant differences among the temperature treatments in the 15 N turnover after pulse dosing. However, a larger percentage of 15 N was stored in macrophytes in the ambient and T3 mesocosms, reflecting higher biomasses than in T5 where filamentous algae were more abundant. Macrophytes and filamentous algae rather than temperature were therefore key controllers of N processing during the summer N pulse in these shallow, relatively low TP lakes.
Nitrogen dynamics and microbial food web structure were characterized in subtropical, eutrophic, large (2,338 km2), shallow (1.9 m mean depth), and polymictic Lake Taihu (China) in Sept–Oct 2002 during a cyanobacterial bloom. Population growth and industrialization are factors in trophic status deterioration in Lake Taihu. Sites for investigation were selected along a transect from the Liangxihe River discharge into Meiliang Bay to the main lake. Water column nitrogen and microbial food web measurements were combined with sediment-water interface incubations to characterize and identify important processes related to system nitrogen dynamics. Results indicate a gradient from strong phosphorus limitation at the river discharge to nitrogen limitation or co-limitation in the main lake. Denitrification in Meiliang Bay may drive main lake nitrogen limitation by removing excess nitrogen before physical transport to the main lake. Five times higher nutrient mineralization rates in the water column versus sediments indicate that sediment nutrient transformations were not as important as water column processes for fueling primary production. However, sediments provide a site for denitrification, which, along with nitrogen fixation and other processes, can determine available nutrient ratios. Dissimilatory nitrate reduction to ammonium (DNRA) was important, relative to denitrification, only at the river discharge site, and nitrogen fixation was observed only in the main lake. Reflecting nitrogen cycling patterns, microbial food web structure shifted from autotrophic (phytoplankton dominated) at the river discharge to heterotrophic (bacteria dominated) in and near the main lake.
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.
Nearly all freshwaters and coastal zones of the US are degraded from inputs of excess reactive nitrogen (Nr), sources of which are runoff, atmospheric N deposition, and imported food and feed. Some major adverse effects include harmful algal blooms, hypoxia of fresh and coastal waters, ocean acidification, long-term harm to human health, and increased emissions of greenhouse gases. Nitrogen fluxes to coastal areas and emissions of nitrous oxide from waters have increased in response to N inputs. Denitrification and sedimentation of organic N to sediments are important processes that divert N from downstream transport. Aquatic ecosystems are particularly important denitrification hotspots. Carbon storage in sediments is enhanced by Nr, but whether carbon is permanently buried is unknown. The effect of climate change on N transport and processing in fresh and coastal waters will be felt most strongly through changes to the hydrologic cycle, whereas N loading is mostly climate-independent. Alterations in precipitation amount and dynamics will alter runoff, thereby influencing both rates of Nr inputs to aquatic ecosystems and groundwater and the water residence times that affect Nr removal within aquatic systems. Both infrastructure and climate change alter the landscape connectivity and hydrologic residence time that are essential to denitrification. While Nr inputs to and removal rates from aquatic systems are influenced by climate and management, reduction of N inputs from their source will be the most effective means to prevent or to minimize environmental and economic impacts of excess Nr to the nation’s water resources.
Detailed studies of stream N uptake were conducted in a prairie reach and gallery forest reach of Kings Creek on the Konza Prairie Biological Station. Nutrient uptake rates were measured with multiple short-term enrichments of NO3 2 and NH4 1 at constant addition rates in the spring and summer of 1998. NH4
Although experimental ecosystems are basic and versatile tools widely used in coastal research, periphytic growth on container walls is an intrinsic artifact that must be considered when interpreting results. To better understand how this artifact may confound extrapolation of results from controlled experiments to conditions in natural estuarine ecosystems, we examined how wall periphyton varied with container size and shape in summer and autumn experiments. Replicate (n = 3) cylindrical mesocosms of 3 volumes (0.1, 1.0, 10 m(3)) were established in both constant-depth (depth = 1 m) and constant-shape (radius/depth = 0.56) series. Mesocosms were initiated with unfiltered estuarine water and homogenized sediments. Periphyton biomass and gross primary production (GPP) per unit of wall area increased with increasing radius (r) or decreasing ratio of wall area (A(W)) to water volume (V) for mesocosms in both series (A(W)/V = 2/r). As a consequence, periphyton biomass and metabolism expressed per unit of water volume increased as a quadratic function of increasing A(W)/V ratio. Results also suggest a secondary scaling effect, whereby wall periphyton growth may be directly related to mesocosm depth, although mechanisms for this effect remain unclear. Significant correlations between periphyton biomass (per m(2) wall area) and 3 environmental factors (light attenuation coefficient, nutrient concentration, and zooplankton abundance) suggest that these factors may have played important roles in regulating wall growth. Additionally, effects of wall periphyton growth on plankton community dynamics were also indicated by the significant negative relations between periphyton biomass and measures of both phytoplankton and zooplankton abundance. The overall effect of periphyton on the experimental ecosystems was evident in the fact that periphyton accounted or over 50% of total ecosystem GPP and biomass after 2 to 4 wk of these experiments. For mesocosm experiments designed to examine dynamics of planktonic-benthic ecosystems, our results imply that growth of wall periphyton, which is controlled by factors scaling to the radius of experimental ecosystems, tends to dominate major biotic pools and rates within weeks.
The role of light and nutrient availability in controlling the abundance and structure of phytoplankton populations was studied in Lake Okeechobee, a large eutrophic lake in south Florida, USA. Measurements of selected environmental parameters at sampling sites within four ecologically distinct regions of the lake were combined with direct experimental determinations of limiting levels of light and nutrients for phytoplankton growth to determine spatial and temporal variations in the relative roles of these limiting factors. Estimated mean light availability in the mixed layer, I-m, was significantly lower in the turbid central region of the lake than in other regions. Correlations between I-m and phytoplankton standing crops led to the conclusion that low light availability in the central region of the lake, and to a lesser extent in other areas, restricts phytoplankton standing crops to levels below the potential provided by the nutrients available. The results of the irradiance-growth experiments confirmed the conclusions of the correlation analyses that phytoplankton growth is restricted by the levels of Light availability experienced during the winter and spring in the central region of the lake. Bioassays indicated that nitrogen was the most frequently limiting nutrient for phytoplankton growth. High rates of nitrogen fixation were frequently observed in the lake, along with correspondingly high abundances of nitrogen-fixing cyanobacteria and nitrogen fixation activity. Elevated concentrations of soluble inorganic nitrogen appeared to suppress both nitrogen fixation and the relative abundance of nitrogen-fixing cyanobacteria.
Substrata samples were collected from Kings Creek on Konza Prairie Biological Station (Manhattan, Kansas) and incubated with varying levels of ammonium (NH ), nitrate (NO ), and dissolved oxygen (O2) to examine the 12 43 response of nitrogen (N) uptake and transformation rates. Substrata collected were fine benthic organic matter (FBOM), coarse benthic organic matter, filamentous green algae, bryophytes, suspended particulate organic matter, and epilithic diatoms. Nitrification and denitrification were estimated by use of the nitrapyrin and acetylene inhibition methods, respectively. Ammonium uptake demonstrated Michaelis-Menten kinetics, with the highest maximum rates (Vmax) associated with filamentous green algae (5.90 mg N gdm 21 d 21 ) and epilithic diatoms (4.96 mg N gdm 21 1 4 low O2 conditions, being undetectable when NO was added but not when NH was added. Denitrification increased 21 34 linearly with NO concentration when associated with epilithic diatoms and FBOM but became saturated at ;20 2 3 m gNL 21 above ambient concentrations when associated with filamentous green algae. Samples purged with N 2 gas had the highest rates of denitrification. We predicted stream ecosystem rates using equations derived from the experimental data and substrata mass estimates measured in the field. Substantial temporal variability was predicted in uptake (0-1,300 mg NH -N m 22 d 21 ; 0-5.2 mg NO -N m 22 d 21
This paper presents a new dynamic mass-balance model for suspended particulate matter (SPM) and sedimentation in coastal areas handling all important fluxes of SPM to, from and within coastal areas, as such areas can be defined according to the topographical bottleneck method. The model is based on ordinary differential equations and the calculation time (dt) is one month to reflect seasonal variations. An important demand, related to the practical utility of the model, is that it should be driven by variables readily accessed from standard monitoring programs or maps. Added to the dynamic core model are several (static) empirical regressions for standard operational effect variables used in coastal management, such as the Secchi depth, the oxygen saturation in the deep water, and chlorophyll-a concentrations. The obligatory driving variables include four morphometric parameters (coastal area, section area, mean and maximum depth), latitude (to predict surface water and deep water temperatures, stratification and mixing) and Secchi depth or SPM-concentrations in the sea outside the given coastal area. The model is based on four compartments: two water compartments (surface water and deep water; the separation between these two compartments is done not in the traditional manner from temperatures but from sedimentological criteria, as the water depth separating transportation areas from accumulation areas) and two sediment compartments (ET-areas, i.e., erosion and transportation areas where fine sediments are discontinuously being deposited, and A-areas, i.e., accumulation areas where fine sediments are continuously being deposited). The processes accounted for include inflow and outflow via surface and deep water, input from point sources, from primary production, from land uplift, sedimentation, burial (the transport of matter from surficial A-sediments to underlying sediments), resuspension, mixing and mineralization. The model has been validated with good results (the predictions of sedimentation are within the 95% confidence limits of the empirical data used to validate the model) against data collected by sediment traps placed in 17 Baltic coastal areas of different character. The paper also presents sensitivity and uncertainty tests of the model. The weakest part of the model concerns the sub-model to predict the ET-areas. Many of the structures in the model are general and have also been used with similar success for other types of aquatic systems (mainly lakes) and for other substances (mainly phosphorus, radionuclides and metals). We also present approaches to indicate how the model could be modified for coastal areas other than those included in this study, e.g., for open coasts, estuaries or areas influenced by tidal variations.
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.
In shallow lakes, environmental warming and nutrient loading are important influences
on the likelihood of a shift between clear and turbid ecosystem states. With
temperatures and nutrient runoff predicted to increase within the next decades,
climate change poses a threat to lake communities. However, current predictions on
the effect of these environmental factors on the abundance and timing of peak zooplankton
numbers are based on correlations rather than on experimental isolation of
thermal from other confounding effects. We present results of warming (48C above
ambient) and increased nutrient loading on plankton communities in 48 outdoor
mesocosms, simulating fishless and hypertrophic ponds. The timing of the chlorophyll
a peak and crustacean zooplankton peak abundance, dominated by Daphnia
pulex, responded strongly to temperature and nutrient addition. Daphnia numbers
reached peaks 22–24 days earlier in heated than in unheated mesocosms. The chlorophyll
a peak abundance advanced by 15–19 days with heating. Phytoplankton,
total zooplankton and D. pulex reached peak abundance 12–19 days later when doses
of nitrogen and phosphorus were added; this finding contradicts predicted earlier
phytoplankton and zooplankton spring peak abundances with nutrient enrichment.
Peak zooplankton and D. pulex abundances did not differ with temperature treatment,
contrary to our expectations, but peak abundances occurred at similar actual temperatures.
Nutrient additions had no effect on the peak zooplankton and D. pulex
abundances in our mesocosms. Overall, climate warming is likely to advance plankton
phenology in fishless ponds; however, this advance could be dampened in
systems with high nutrient concentration. We found very high zooplankton abundances
with warming and high nutrient loadings inducing a clear water state in all
our tanks owing to heavy zooplankton grazing despite high nutrient concentrations.
The observation that the relative importance of picophytoplankton is greatest in warm and nutrient-poor waters was tested here based on a comprehensive review of the data available in the literature from oceanic and coastal estuarine areas. Results show that picophytoplankton dominate (50%) the biomass and production in oligotrophic (chlorophyll a [Chl a] 0.3 mg m 3), nutrient poor (NO 3 NO 2 1 M), and warm (26C) waters, but represent 10% of autotrophic biomass and production in rich (Chl a 5 mg m 3) and cold (3C) waters. There is, however, a strong covariation between temperature and nutrient concentration (r 0.95, P 0.001), but the number of observations where both temperature and nutrient concentrations are available is too small to allow attempts to statistically separate their effects. The results of mesocosm nutrient addition experiments during summer in the Mediterranean Sea allowed the dissociation of the effects of temperature from those of nutrients on pico-phytoplankton production and biomass and validated the magnitude at which picoplankton dominates (50%) autotrophic biomass and production obtained in the comparative analysis. The fraction contributed by picoplankton significantly declined (r 2 0.76 and 0.90, respectively, P 0.001) as total autotrophic production and biomass increased. These results support the increasing importance of picophytoplankton in warm, oligotrophic waters. The reduced contribution of picophytoplankton in warm productive waters is hypothesized here to be due to increased loss rates, whereas the dominance of picophytoplankton in warm, oligotrophic waters is attributable to the differential capacity to use nutrients as a function of differences in size and capacity of intrinsic growth of picophyto-plankton and larger phytoplankton cells.
This article explores the effects of nitrogen loadings to freshwater and marine recipients in two large watersheds of southern Norway. Changes in nitrogen retention over larger areas would severely affect nitrogen concentrations in freshwaters, and subsequently increase the N load to marine recipients. Increased atmospheric deposition, changed hydrology, reduced nitrogen uptake by vegetation due to nitrogen 'saturation' and/or root damage caused by acidification, could contribute to this leakage. Climatic fluctuations strongly affect the seasonal runoff patterns, and a more long-term climate change could mobilize the huge stores of organic N following increased mineralization. The bioavailability and seasonal patterns combined will be major determinants to responses in the recipients, as will the nutrient ratios. For the sparsely populated heathland dominated Bjerkreim watershed, the stoichiometry of nutrient elements are strongly skewed towards high N:P ratios. P concentrations are far too low to support freshwater eutrophication, and a further increase in N would only strengthen the prevailing P limitation. This would also hold for the marine recipient where excess of N relative to P or Si in riverine runoff could drive the nearshore areas towards temporal P or Si limitation. In the Auli watershed, intense agricultural activity yields high outputs of both N and P, but very scattered and unpredictable runoff peaks with variable N:P ratio. The low concentrations of mineral nutrients in the Bjerkreim river imply a diluting effect relative to seawater concentrations of N and P, while the Auli river may support eutrophication both in the freshwaters and the inner fjord recipient. Due to a pronounced seasonality in nutrient concentrations and fluxes in the coastal current, the relative contribution of riverine sources would peak during summer. Data from both watersheds with their subcatchments underline the overall importance of agricultural activity for nitrogen runoff.
A drought-induced drastic decrease in mean depth and volume brought about several effects in the ecosystem of L. Vortsjarv (270 km2). Differences in alkalinity, Si and inorganic N in two consecutive winters can be attributed to the concentration effect resulting from different ice/water volume ratios in the lake. Smaller initial amount of oxygen that remained under the ice in winter 1995/96 accounted largely for the observed anoxia increase in phytoplankton biovolume represented another concentration effect, as a similar areal primary production in 1995 and 1996 resulted in about twice a higher density of algal crop in the summer of 1996, when the average amount of water in the lake was 1.7 times smaller. Due to a better mixing of shallow water, integral oxygen concentration during the ice-free period in 1996 was systematically higher than in 1995. Stronger sediment resuspension in 1996 enriched the water with suspended solids and nutrients, resulting in a slight increase in the trophic state. Lower water level in 1996 improved light conditions by 'cutting off' less illuminated and less productive deep layers. This was probably one of the main factors which caused the collapse of the common dominants Limnothrix redekei and L. planctonica and initiated mass development (max. 36 g m-3) of Cyanonephron styloides. The observed changes in the phytoplankton composition in 1996 could be caused also by the selective grazing of larger zooplankton.
Nutrient supply 'bulk deposition' and 'throughfall deposition' to open areas and the forest floor, respectively, was monitored in the Lake Gardsjon watershed. Deposition rates were 0.08 kg P ha- 1 yr-1 and 12.8 kg N ha-1 yr-1 for 'bulk' and 0.25 kg P ha-1 yr-1 and 12.3 kg N ha-1 yr-1 for 'throughfall' deposition. Monitoring of nutrient and DOC (dissolved organic carbon) losses from the land catchment showed low losses (0.03 kg P ha-1 yr-1, 1.9 kg N ha-1 yr-1 and 61 kg DOC-C ha-1 yr-1). Phosphorus retention upon water passage through podzol soil profiles was followed. An adjustment to the low catchment effluent concentrations was found in the B-horizon, where the raised Al concentrations would tend to precipitate phosphate. The low trophic status of the lake (P-regulated) was not due to excessive retention of P in the lake basin. Retention was normal or low (20-25% of input) compared to circumneutral lakes of similar water renewal time (1 yr). Lake denitrification was high and DOC behaved independently of P in several ways, which did not support the idea of DOC as a P carrier. Within-lake processes of P retention, including storage in benthic flora and detritus, and sudden benthic releases of P are discussed. The low external supply was the most probable main regulator of ambient lake P level. -Authors
Nitrogen retention in waterbodies from the mesotrophic/eutrophic Eikeren watercourse and the oligotrophic Bjerkreim watercourse was estimated by mass-balance studies over a period of three years. Studies of lakes of different trophic status revealed that the total annual nitrogen retention was higher in eutrophic lakes (20-30%) than in oligotrophic lakes (< 5%). In contrary to other studies, the N retention seemed not to be associated with water residence time. These findings indicate that N retention is bound to biological processes and not to passive sedimentation processes. A high N:P ratio in most of the lakes indicates that only part of the N pool in the lakes is involved in the biological processes leading to N retention. Thus, we suggest that N retention in Norwegian lakes seems to be phosphorus limited, i.e. high N retention is not to be expected unless the N:P ratio is close to the proportional needs for the aquatic plants.
Artificial substrata are increasingly used to study periphyton, but their ability to reproduce natural substrata remains controversial. Although many studies have made contemporaneous comparisons of periphyton assemblages on artificial and natural substrata at one or a few sites, no broadly based comparison exists. We therefore surveyed the literature to establish conditions under which artificial substrata satisfactorily mimic both the quantity and the quality of natural periphyton assemblages. In general, epilithon was underestimated by the artificial substrata; epiphyton was overestimated, but less severely. These trends were significantly affected by the time available for colonization of the artificial substrata before sampling, site trophy, ambient temperature, and whether the study was conducted in a lake or in running water. Neither the composition of the substratum nor its orientation appeared important. Natural diatom assemblages were usually well simulated by those on artificial substrata, whereas both epilithic and epiphytic green and bluegreen algae were severely underrepresented on the artificial substrata. Since artificial substrata often misrepresent both the quantity and the quality of natural periphyton, they should be used with more caution, especially in intersite and interseason surveys.
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.
Atmospheric deposition provides significant amounts of nutrients to the
continental and marine ecosystems. Using the mesoscale WRF/CMAQ modeling
system, the nitrogen (N) and sulfur (S) atmospheric deposition fluxes
over the Mediterranean and the Black seas and continental Europe are
evaluated for the year 2008. The annual N and S deposition fluxes are
calculated to be 4.89 Tg(N) yr-1 and
2.07 Tg(S) yr-1 over continental Europe,
0.92 Tg(N) yr-1 and
0.52 Tg(S) yr-1 over West Mediterranean,
1.10 Tg(N) yr-1 and
0.84 Tg(S) yr-1 over East Mediterranean and
0.36 Tg(N) yr-1 and
0.17 Tg(S) yr-1 over the Black Sea. Inorganic
N deposition fluxes are calculated to be about 3 times higher than
gaseous organic N deposition fluxes. Comparison to available
observations associates the annual mean model estimates with about
40 ± 30% of uncertainty depending on location. Dry
deposition dominates over wet deposition for both N and S in agreement
with the observations. Results suggest that an important fraction of the
N deposited over the Mediterranean basin can be attributed to
transported N species while S deposition is dependent more on the local
emissions. In Black Sea and West Mediterranean Sea waters the calculated
atmospheric N inputs are comparable to the N export measured by sediment
traps whereas in the East Mediterranean N input exceeds by a factor of
about 5 the N export. Our simulations show that the critical N load of
1 g(N) m-2 yr-1 is
exceeded over 84% of the European forested areas.
Atmospheric emissions of reactive nitrogen (N) species are at high levels in China in recent years, but few studies have employed N deposition monitoring techniques that measure both dry and wet deposition for comprehensive evaluation of the impacts of N deposition on ecosystems. In this study, to quantify the total N deposition, both dry and wet N depositions were monitored using denuder/filter pack systems, passive samplers and wet-only samplers at three sites with different land use types (forest, paddy field and tea field) in a 135-km2 catchment in subtropical central China from September 2010 to August 2011. At the three sampling sites, the annual mean concentrations of total N (the sum of +4NHNH4+, −3NONO3− and DON) in rainwater were 1.2–1.6 mg N L−1, showing small variation across sites. Annual mean concentrations of total N (the sum of NH3, NO2, HNO3, particulate +4NHNH4+ and −3NONO3−) in the air were 13–18 μg N m−3. High NH3 concentrations in the air were observed at the agricultural sites of tea and paddy fields, indicating significant NH3 emissions from N fertiliser application; and high NO2 concentrations were found at the upland sites of forest and tea field, suggesting high NO emissions from soils due to high N deposition or high N fertiliser input. The annual total N deposition for the three sites of paddy field, tea field and forest was estimated as 22, 34 and 55 kg N ha−1 yr−1, in which the dry N deposition components contributed to 21%, 36% and 63% of the annual total N deposition, respectively. The annual deposition of reduced N species was 1.1–1.8 times of the annual deposition of oxidised N species. To minimise the adverse effects of atmospheric N deposition on natural/semi-natural ecosystems, it is crucial to reduce the reactive N emissions from anthropogenic activities (e.g., N fertiliser application, animal production and fossil fuel combustion) in subtropical central China.
Twenty-five seepage meters were positioned in East Lake Tohopekaliga, Florida, to determine groundwater seepage contributions of water and nutrients to the lake in 1983. Seepage was found to be an important source of water to the lake, contributing 14.3% of the water sources, and rates decreased significantly (P < 0.01) with distance from shore. A comparison of the piezometer and seepage meter techniques for measuring nutrient loading to the lake indicates the direct seepage meter technique overestimated nutrient inputs due to the enclosure to the sediments, possibly resulting in anaerobic conditions and increased release rates of ammonium nitrogen and phosphate. These results suggest that past studies employing this technique may be in error. Nutrient loading, calculated from piezometer nutrient data and seepage meter flow data, show that the groundwater nutrient loading in the lake was significant, contributing 8.7 and 17.6% of the total phosphorus and total nitrogen inputs to the lake, respectively.
Mass-balance budgets linked among several materials are used to infer rates of processes affecting Lake Kinneret. Comparisons among budgets reveal the magnitudes of "internal" sources and sinks that cannot be directly inferred from individual budgets. A water budget indicates that - 180 x 1 O6 m3 of sublacustrine spring water plus ungauged surface flow enters the lake annually-about a fifth the total inflow and two-thirds as much water as is lost to evaporation. This total ungauged inflow delivers about 90,000 t of Cl yr-I, nine times the stream input. Ca input from total ungauged flow is about a third the stream input, and the net internal Ca sink in the system is sufficient to precipitate 60,000 t of CaCO, yr-I. Stream delivery of P, mainly as particulate material, is largely sequestered in the sediments (- 100 t yr- I). At least 1,100 t yr- I of N, primarily as N03- delivered by streams, are apparently lost to denitrification, while only 200 t yr-1 are sedimented. Cycling of N and P within the lake dominates over throughput in controlling standing stocks. Vertical mixing within the lake may play a dominating role in this cycling. Cycling of P in the lake can be regarded as "closed''-a sediment-water column turnover of materials with only minor hydrographic loss from the system. By contrast, N cycling is "open," with an important net loss to the atmosphere. Aquatic ecosystems for which hydro- graphic constraints are well defined provide an opportunity for mass-balance analysis of reactive materials. Biogeochemical patterns deduced in well-defined systems should have general application. In this paper, mass-bal- ance analyses are derived for water, Cl, P, N, and Ca in Lake Kinneret, Israel. Most mass-balance analyses of lakes treat indi- vidual elemental budgets independently of one another. We deliberately compare bud- gets to ask the following two questions: What
Budgets and dynamics of nitrogen and phosphorus in Lake Donghu were investigated from Oct. 1997 to Sept. 1999. The water residence time was estimated to be 89 days in 1997–1998 and 124 days in 1998–1999. The total external loadings were 53 g N m yr and 3.2 g P m yr in 1997–1998, and 42 g N m yr and 3.1 g P m yr in 1998–1999. On average. about 80% of nitrogen and phosphorus input was from sewage outlets, while the rest was from land runoff and precipitation. Ammonium ion was the most abundant form of inorganic nitrogen in the sewage. The nutrient output was mainly through water outflow and fish catch. The percentages of nutrients in fish were estimated to be 7.8%-11.2% for nitrogen and 47.6%-49.6% for phosphorus. Lake Donghu has a very high nutrient retention (63% for nitrogen and 79% for phosphorus) mainly due to its closure and long water residence time. Sedimentation is an important nutrient retention mechanism in this lake. Using mass balance method, we estimated that denitrification of Lake Donghu involves about 50% of the retained nitrogen. Lake Donghu is rich in inorganic nitrogen and phosphorus and showed great seasonal variation.
This study quantifies the role of lake morphometry and submerged macrophyte beds on the accumulation of sediments in the littoral zone. Stable Pb, a historical marker of lacustrine sediments in southern Quebec, was used to date the sediments (;110 yr) and to calculate three long-term sediment accumulation rates (SARs) in Lake Memphremagog (located in Quebec and Vermont). The anthropogenic Pb burden in the littoral zone of Lake Memphremagog was found to be two to eight times greater (per m 2 ) than the Pb burdens in the profundal zone of surrounding eastern township lakes. Pb concentrations were nearly fourfold higher than background Pb concentra- tions (115 and 31 m gg 21 , respectively), providing a reliable marker for littoral sediment core analysis. Lake morphometry is related to the three accumulation rates measured by providing distinct threshold limits (i.e., littoral slope, .10%; exposure, .10 km 2 ) where sediments are unable to accumulate. Macrophyte beds are shown to disproportionately accumulate sediments at rates 2 to 20 times greater (per m 2 ) than in the profundal zone. Linear regression models show that both the total SAR (mean, 1.7 mm yr 21
The available literature on the fate of nitrogen in waters and sediments is reviewed. Emphasis is placed on the importance of N to aquatic productivity, the pathways leading to N gains or losses in aquatic ecosystems, and the availability of N in sediments to the overlying waters. Important biological reactions include N mineralization and immobilization, nitrification and denitrification, and N fixation. The effect of sediment properties, lake morphology and environmental factors (pH, temperature, dissolved oxygen, oxidation-reduction potential) on the pathways and rates of turnover are considered. The mixing process in sediments appear to be the most important in releasing sediment-N to waters. Several facets of the N cycle in waters and sediments require further elucidation. Research needs are outlined.
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Quantifying periphyton (attached algal) contributions to autotrophic production in lakes is confounded by properties of substratum that affect community biomass (as chlorophyll content) and productivity. We compared chlorophyll content and productivity of natural algal communities (phytoplankton, epipelon, epilithon, epixylon, and epiphyton) experiencing high (>10%) incident radiation in lakes in the US, Greenland, and Quebec, Canada. Chlorophyll content and productivity differed significantly among regions, but they also differed consistently among communities independent of region. Chlorophyll content of periphyton on hard substrata (rocks and wood) was positively related to water-column total P (TP), whereas chlorophyll content of algae on sediment (epipelon) and TP were not significantly related. Chlorophyll content was up to 100× higher on sediments than on hard substrata. Within regions, chlorophyll-specific primary productivity was highest for phytoplankton and lowest for epipelon. Periphyton on hard substrata and on macrophytes (epiphyton) had similar rates of chlorophyll-specific productivity that were intermediate to those of epipelon and phytoplankton. Area-specific productivity of epipelon was 5 to 10× higher than area-specific productivity of periphyton on hard substrata. This broad geographic comparison indicates that, in low to moderately productive lakes under high-light conditions, algal communities have predictable differences in area-specific and chlorophyll-specific productivity based on substratum. As such, chlorophyll alone is an inadequate predictor of the relative contributions of different algal communities to total primary production. Our results highlight the importance of the relative abundance and spatial distributions of substrata in determining the role of the littoral zones in nutrient and energy cycles in lakes.
The importance of salinity and suspended minerogenic particles on the aggregation and sedimentation processes of iron, phosphorus and organic carbon was studied in experimental procedures with waters from the River Öre and from the Öre Estuary, northern Sweden. The aggregation of dissolved iron and phosphorus reached at least 40–90% and 30–60%, respectively, at salinities between 4 and 6. Only a few percent, less than 10%, of the dissolved organic carbon aggregated. Rapid and nearly complete sedimentation of iron and phosphorus aggregates was found at high salinities (4–5) and in the presence of clay particles. Mineral particles increase sedimentation by sorption of formed iron and phosphorus aggregates. Only a small amount of the organic carbon sedimented.
Nitrogen gas flux was measured from sediments taken from Narragansett Bay, Rhode Island, Boston Harbour, Massachusetts, and the Pawcatuck River Estuary, Rhode Island. In addition to studies of field cores taken directly from these systems, intact sediments were taken from Narragansett Bay and maintained in control and nutrient enriched mesocosms. Sediment denitrification was measured as a flux of N2 gas from sediments in N2-free chambers. The advantages of this technique are that it allows for the direct measurement of denitrification in undisturbed sediment cores under ambient conditions of dissolved nutrients, oxygen, and temperature. The challenge of this technique has been to correctly distinguish between N2 fluxes produced by denitrification activity and fluxes of N2 caused by sediment porewater de-gassing. In this study, anoxic 'control' cores were used to provide continuous checks on the magnitude of porewater de-gassing rates, and allowed measured rates of total N2 flux to be corrected for this background flux. The use of anoxic control cores allowed measurements to begin soon after core collection, without the need for long pre-incubations.
Groundwater seepage was the largest annual flux of water into (58–76%) and out of (73–83%) Williams Lake during a 12-year study, during which the entire volume of the lake was replaced four times. The only other water fluxes to and from the lake, which has no surface-water inlet or outlet, were atmospheric precipitation and evaporation. Nearly all of the annual input of calcium, magnesium, sodium, potassium, chloride, sulfate, and silica was provided by groundwater. Although much of the calcium and most of the silica input was retained in the lake, this retention did not result in increased chemical mass in the lake water mass because biologically mediated removal of calcium and silica to the sediments equaled or exceeded loss by lake seepage to groundwater. Groundwater represented as much as one-half the annual hydrological input of phosphorus and nitrogen; the remainder was supplied by atmospheric precipitation. From about 70 to 90% of the annual input of phosphorus and nitrogen was retained in the lake. Although water and chemical fluxes varied from year to year, interaction of the lake with groundwater determined the hydrological and chemical characteristics of Williams Lake.
Inorganic phosphorus (P) fertilizer was added to a small, acidic lake in southernmost Norway to test the potential of this technique to increase the retention of total inorganic nitrogen (TIN) and thereby mitigate the effects of elevated leaching of atmospherically derived N. The experiment was conducted over 4 years (1 pretreatment year followed by 3 years with weekly P additions during the growing season). To avoid any undesirable eutrophication effects, the total P concentration was increased from 45 µg P·L1 to a moderate level of 1012 µg P·L1. Compared with the pretreatment year, the P additions increased the average TIN retention in the experimental lake by nearly 70% (from 53 to 88 mmol·m2·year1) during three growing seasons. However, when considering that the reference lake experienced a 55% decrease in TIN retention during the same period, the increase might have been even larger. This underlines the role of P (both natural and anthropogenic) as an important regulator of N retention in upland lake districts that in large parts of southernmost Norway contribute significantly to the N loading on coastal marine areas.
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
Phosphorus (P) is conventionally thought to limit production in freshwaters and nitrogen (N) that in the sea. Before much human activity, however, co-limitation by N and P was probably normal, with systems developing ratios of N to P tending to the Redfield ratio. Single-factor limitation may be a symptom of human activity in many cases. It is widely believed that N fixation should compensate for N shortage because N fixers are ubiquitous and versatile, but this is not always the case and the issue has hitherto been considered largely with respect to plankton communities. Effects of N on macrophyte communi-ties provide justification for control of both nutrients, at least in shallow lakes and estuaries. Increased N loading reduces plant biodiversity, changes the struc-ture, and is associated with eventual loss, of macro-phyte communities. P control alone may suffice in many deep lakes where denitrification is low and stratified conditions favour cyanobacterial develop-ment. Therein may lie a resolution to current controversies.
1. Water-level fluctuations are typical of lakes located in the semi-arid Mediterranean region, which is characterised by warm rainy winters and hot dry summers. Ongoing climate change may exacerbate fluctuations and lead to more severe episodes of drought, so information on the effects of water level on the functioning of lake ecosystems in such regions is crucial.
2. In eutrophic Lake Eymir, Turkey, we conducted a 4-month (summer) field experiment using cylindrical 0.8-m- (low-water-level) and 1.6-m-deep (high-water-level) mesocosms (kept open to the sediment and atmosphere). Fish (tench, Tinca tinca, and bleak, Alburnus escherichii) were added to half of the mesocosms, while the rest were kept fishless. Ten shoots of Potamogeton pectinatus were transplanted to each mesocosm.
3. Sampling for physicochemical variables, chlorophyll a (chl-a), zooplankton and per cent plant volume inhabited (PVI%) by macrophytes was conducted weekly during the first 5 weeks, and subsequently biweekly. Macrophytes were harvested on the last sampling date. During the course of the experiment, the water level decreased by 0.41 ± 0.06 m.
4. Throughout the experiment, fish affected zooplankton abundance (−), nutrient concentrations (+), chl-a (+) and water clarity (−) most strongly in the low-water-level mesocosms and the zooplankton community shifted towards dominance of small-sized forms. The fishless mesocosms had a higher zooplankton/phytoplankton ratio, suggesting higher grazing.
5. Greatest macrophyte growth was observed in the low-water-level fishless mesocosms. However, despite high nutrient concentrations and low water clarity, macrophytes were also abundant in the fish mesocosms and particularly increased following a water-level decrease from midsummer onwards. Macrophyte growth was poor in the high-water-level mesocosms, even in the fishless ones with high water clarity. This was ascribed to extensive periphyton development reducing light availability for the macrophytes.
6. Our results indicate that a reduction in water level during summer may help maintain the growth of macrophytes in Mediterranean eutrophic shallow lakes, despite a strong negative effect of fish predation on water clarity. It is therefore probable that an expected negative effect of global climate change on water clarity because of eutrophication and enhanced top-down control of fish may be, at least partly, counteracted by reduced water level, provided that physical disturbance is not severe.
Local agricultural emissions contribute significantly to the atmospheric reactive nitrogen loads of Danish terrestrial ecosystems. In the vicinity of the sources this may be up to 6–8 kg N ha−1 yr−1 depending on location and ecosystem type. This contribution arises from dry deposition of gas phase ammonia derived from local livestock production. Long-range transport, however, often constitutes the largest contribution to the overall atmospheric terrestrial reactive nitrogen loadings in Denmark. This is often in the range 10–15 kg N ha−1 yr−1 and consists mainly of aerosol phase nitrate and ammonium (reaction products of nitrogen oxides and ammonia), but also dry deposition of other reactive nitrogen compounds (mainly nitrogen oxides in the form of gas phase nitric acid and nitrogen dioxide). In Denmark’s environmental management of the sensitive terrestrial ecosystems modelling tools are required that account for both the local and the long-range transported contributions. This motivated development of the Danish Ammonia MOdelling System (DAMOS) that has been successfully applied to the assessment of atmospheric nitrogen loadings to sensitive Danish ecosystems. We present here three different examples of such assessments. Our results show that ecosystems located in Western Denmark (Case 1) receive the highest loads of atmospheric nitrogen depositions which generally exceed the critical load. This part of the country has the highest livestock density. In the Eastern part of the country, the atmospheric loadings are often below or close to the lower end of the interval for critical load values. These lower loads in Eastern Denmark (Case 2) are due to lower density of agricultural activities, as well as, lower precipitation rates, which leads to less wet deposition of reactive nitrogen. In general there is a gradient in atmospheric deposition over the country, with the highest depositions in the South-Western part of Denmark (Case 3) due to long-range transport contributions from North-Western Europe, but also due to local ammonia deposition associated with the high local emission from the high density livestock farming in this area.
In many aquatic ecosystems, increased nutrient loading has caused eutrophication, which is reflected in the trophic structure of the ecosystem. In Lake Mangueira, a large shallow subtropical lake in Brazil, nutrient loading has also increased, but it is still unclear what the effects of this increase will be and how this relates to climate change. To evaluate the effects of increased nutrient loadings in such large lake one would need to integrate hydrological and ecological processes into one model, an approach that has rarely been used before. Here, we apply different versions of a complex 3D ecological model, called IPH-TRIM3D-PCLake, which describes the integrated hydrodynamic, water-quality, and biological processes in the lake. First, the nutrient loadings from the watershed were estimated using a separate hydrological water quality model of the watershed based on field data. Second, we calibrated the 3D ecological model for a 6-year monitoring period in the lake using a simplified non-spatial version of the model. Finally, the calibrated ecological model was applied to evaluate the spatial explicit effects of different scenarios of land use, water pumping for irrigation, and climate change. On short term (1.5 year), the system seemed to be rather resilient, probably because of the lake size related to its high inertia. Our simulations indicated warming can increase water transparency in Lake Mangueira which may be related to two factors: (a) the current meso-oligotrophic state of the lake which may easily lead to nutrient limitation; and (b) submerged macrophytes grow during the whole season. The combined effect of climate change and increased nutrient loading, less strong than increased nutrient loading alone. The model can only be used for qualitative predictions of the effect of management scenarios, such as maintenance of water levels in the dry season, and water-pumping rules for irrigation in order to maintain the ecosystem structure and functions in the future under additional stress caused by increased use or climate change.Highlights► We model the effect of climate change and eutrophication on a subtropical shallow lake. ► The model was able to reproduce the seasonal patterns for most trophic components. ► Simulations indicated that eutrophication negatively affects water transparency. ► An opposite effect was found considering warming. ► Combined effects can promote significant changes in the trophic structure.
1. While phosphorus (P) is often considered the most important growth limiting factor for plants in lakes, recent studies of shallow lakes indicate that nitrogen (N) may be of greater importance than realized hitherto and that submerged macrophytes may be lost when the N concentration exceeds a certain threshold, as long as the concentration of P is sufficiently high.
2. We studied the effects of different loadings of NH4-N and NO3-N on chlorophyll a and on a macrophyte tolerant of eutrophication, Vallisneria spinulosa (Hydrocharitaceae). In outdoor mesocosms we used water from a pond as control and created four levels of NH4-N and NO3-N (approximately 2.5, 5, 7.5 and 10 mg L−1) by dosing with NH4Cl and NaNO3, respectively. After the experiment, the plants were transferred back to a holding pond to study their recovery. In contrast to previous research, we used a low background concentration of phosphorus (TP 0.024 ± 0.003 mg L−1) so we could judge whether any effects of N were apparent when P is in short supply.
3. Chlorophyll a increased significantly with N dosing for both forms of N, but the increase was highest in the NH4-N dosed mesocosms (maximum 58 μg L−1 versus 42 μg L−1 in the NO3-N mesocosms), probably due to a higher total inorganic N concentration (part of the added ammonia was converted to nitrate during the experiment).
4. Although the number of ramets of V. spinulosa was not affected by the N treatment, the biomass increased up to concentrations of 7.5 mg L−1, while biomass at 10 mg L−1 remained at the control level for both N ions treatments. A similar pattern was apparent for the content of N and soluble sugar of the plant, while there were no differences in the plant P content among treatments. Five months after transplantation back to the pond no difference was found in the number of ramets or in biomass, except that the biomass of plants grown at 10 mg N L−1 during the experiment was greater than that in the control, while the N and P contents of plants were similar to those of the controls.
5. Nitrogen concentration had little influence on the growth of the eutrophication tolerant submerged macrophyte at moderately low concentrations of phosphorus. Moreover, the two N ions showed no toxic effects, suggesting that loss of macrophytes observed in other studies, run at higher phosphorus concentrations, was probably related to enhanced shading by periphyton and/or phytoplankton rather than to any toxic effects of N.
1. Shallow lakes and their ectothermic inhabitants are particularly vulnerable to the effects of climatic warming. These impacts are likely to depend on nutrient loading, especially if the combination of warming and eutrophication leads to severe hypoxia.
2. To investigate effects of realistic warming and nutrient loading on a fish species with high tolerance of warming and hypoxia, we observed population changes and timing of reproduction of three-spined sticklebacks in 24 outdoor shallow freshwater ecosystems with combinations of temperature (ambient and ambient +4 °C) and three nutrient treatments over 16 months.
3. Warming reduced stickleback population biomass by 60% (population size by 76%) and nutrient-addition reduced biomass by about 80% (population size 95%). Nutrients and warming together resulted in extinction of the stickleback populations. These losses were mainly attributed to the increased likelihood of severe hypoxia in heated and nutrient-addition mesocosms.
4. Warming of nutrient-rich waters can thus have dire consequences for freshwater ectotherm populations. The loss even of a hardy fish suggests a precarious future for many less tolerant species in such eutrophic systems under current climate change predictions.
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.
Results from the pioneering research on the interactions between pH and denitrification in soil from the 1950s to the present are reviewed, the changing perceptions of this complex relationship are discussed, and the current status of the subject is assessed. Facets of this relationship that are analysed in detail include the direct or indirect influence of pH on overall denitrification rates in soils, changes in the composition of gaseous products that depend on pH, methods for measuring the process, the concept of an optimum pH for denitrification, and the adaptation of microbial denitrifying communities to acidic environments. The main conclusions to be drawn are as follows. Total gaseous emissions to the atmosphere (N2O, NO and N2) have repeatedly been shown to be less in acidic than in neutral or slightly alkaline soils. This may be attributable to smaller amounts of organic carbon and mineral nitrogen available to the denitrifying population under acid conditions rather than a direct effect of low pH on denitrification enzymes. Numerous laboratory and field studies have demonstrated that the ratio N2O:N2 is increased when the pH of soils is reduced. The relation between soil pH and potential denitrification as determined by various incubation methods remains unclear, results being influenced both by original conditions in soil samples and by unknown changes during incubation. The concept of an optimum pH for denitrification has been frequently proposed, but such a term has little or no meaning without reference to specific attributes of the process.
Summary • Shallow lakes are important components of the biosphere, but they are also highly vulnerable to damage from human activities in their catchments, such as nutrient pollution. They may also be particularly vulnerable to current warming trends. • Forty-eight tanks were used to create 3-m3 mesocosms of shallow lake communities, in which the effects of warming by 4 °C and regular nutrient loading at two levels relevant to current degrees of eutrophication were studied in the presence and absence of fish. • Warming changed concentrations of soluble phosphate, total nitrogen and conductivity, increased total plant biomass and decreased the amount of phytoplankton through shading by floating plants. Nutrient additions decreased total plant biomass but increased floating plant biomass. Nitrogen increase and warming increased floating plant biomass and decreased plant species richness. The plant community remained intact and did not switch to the turbid-water, phytoplankton-dominated community often predicted to be a consequence of global warming and eutrophication. • Synthesis and applications. Likely future temperature increase will exacerbate some, but not all symptoms of eutrophication in shallow lakes. Alone it will not cause a switch from plant-dominated to algal-dominated systems, but may result in nuisance growths of floating lemnids. Currently underplayed, nitrogen loading should be taken more seriously in the management of European freshwaters.
A detailed mass balance on nitrogen was carried out in shallow and hypertrophic Lake Søbygård during 4.5 years before through 2.5 years after a 36 % reduction in nitrogen loading. Annual mean loss rate of nitrogen was 159–229 mg N m−2 d−1 before the loading reduction and 125 mg N m−2 d−1 after. In spite of a short hydraulic retention time (18–27 days) the proportion of nitrogen loading lost in the lake was high (38–53 %) and not affected by changes in loading. Calculated denitrification accounted for 86–93% of the loss rate, while 7–14% was permanently buried. Marked seasonal variations in the loss percentage were found during the season, ranging from 23 % in first quarter to 65 % in third quarter. The seasonal variation in the loss percentage of nitrogen showed a hysteresis like relationship to temperature, with a high percentage in fourth quarter. This suggests that the amount of available substrate, which mainly consists of sedimentated phytoplankton, accumulated during summer, is an important regulating factor. The ability of various published input-output models to predict the observed changes in in-lake nitrogen concentration in Lake Søbygård was tested. This study has further confirmed that small lakes with short retention and high nitrogen loading may significantly reduce the nitrogen loading of downstream aquatic environments.
As human activities continue to alter the global nitrogen cycle, the ability to predict the impact of increased nitrogen loading to freshwater systems is becoming more and more important. Nitrogen retention is of particular interest because it is through its combined processes (denitrification, nitrogen sedimentation and uptake by aquatic plants) that local and downstream nitrogen concentrations are reduced. Here, we compare the magnitude of nitrogen retention and its components in wetlands, lakes and rivers. We show that wetlands retain the highest proportion of total nitrogen loading, followed by lakes and then rivers. The differences in the proportion of N retained among systems is explained almost entirely by differences in water discharge. Denitrification is the primary mechanism of nitrogen retention, followed by nitrogen sedimentation and uptake by aquatic plants.
Summary 1. Temperature strongly affects virtually all biological rate processes, including many central to organismal fitness such as growth rate. A second factor related to growth rate is organismal chemical composition, especially C : N : P stoichiometry. This association arises because high rates of growth require disproportionate investment in N- and P-rich biosynthetic cellular structures. Here the extent to which these factors interact is examined - does acclimation temperature systematically affect organismal chemical composition? 2. A literature survey indicates that cold-acclimated poikilotherms contain on average 30-50% more nitrogen (N), phosphorus (P), protein and RNA than warm-exposed conspecifics. The primary exception was bacteria, which showed increases in RNA content but no change in protein content at cold temperatures. 3. Two processes - changes in nutrient content (or concentration) and in organism size - contribute to the overall result. Although qualitatively distinct, both kinds of change lead to increased total catalytic capacity in cold-exposed organisms. 4. Temperature-driven shifts in nutrient content of organisms are likely to resonate in diverse ecological patterns and processes, including latitudinal and altitudinal patterns of nutrient content, foraging decisions by organisms living in strong temperature gradients, and patterns of biodiversity.
1. Recent experimental and field studies on temperate shallow lakes indicate that nitrogen may play a greater role in their functioning than previously thought. Several studies document that abundance and richness of submerged macrophytes, both central in shallow lake ecology, may decrease with increasing nitrogen loading, especially at high phosphorus levels. However, the role of nitrogen in warm lakes with fluctuating water regimes remains to be described in detail.
2. The effect of increasing nitrate and phosphate concentrations on submerged macrophyte growth was examined in a 3-month mesocosm experiment conducted in summer in a shallow freshwater lake on the north western coast of Turkey with a Mediterranean climate. Twenty four field mesocosms, open to the sediment and atmosphere, were stocked with Myriophyllum spicatum shoots and small cyprinid fish. Three nitrate loadings in combination with two phosphate loadings were applied in a fourfold replicated design.
3. Mean ± SD nutrient concentrations maintained throughout the experiment were 0.55 ± 0.17, 2.2 ± 0.97, 9.2 ± 5.45 mg L−1 total nitrogen and 55 ± 19.2, 73 ± 22.9 μg L−1 total phosphorus. Mean periphyton biomass increased with increasing nutrient concentrations and peaked at the highest nitrogen and phosphorus loadings, while the mean phytoplankton biomass remained relatively low in all treatments.
4. Percent volume inhabited (% PVI) by macrophytes throughout the experiment and total macrophyte biomass at the end of the experiment did not differ among treatments. In addition to stocked M. spicatum, Ceratophyllum demersum and Potamogeton crispus appeared in the majority of the mesocosms. The plants grew continuously up to 50% PVI throughout the experiment and remained resilient to shading provided by periphyton and phytoplankton.
5. The mean summer air temperature in 2007 was 2.2 °C higher than the average of the last 32 years, which resulted in a water level decrease of 0.3 m in the mesocosms over three months. This might have counteracted the shading of submerged macrophytes provided by phytoplankton and periphyton. The results of the experiment are consistent with observations of higher macrophyte resilience to nutrient loading in Mediterranean lakes compared with northern temperate lakes.