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In the 1980s and 1990s, it became increasingly clear that changes in external nutrient loads alone could not entirely explain the severe eutrophication of surface waters in the Netherlands. Nowadays, 'internal eutrophication' has become a widely accepted term in Dutch water management practice to describe the eutrophication of an ecosystem without additional external input of nutrients (N, P, K). This review surveys the principal mechanisms involved in this process. It also discusses possible remedies to combat internal eutrophication.
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Chemistry and Ecology
Vol. 22, No. 2, April 2006, 93–111
Internal eutrophication: How it works and what to do
about it – a review
A. J. P. SMOLDERS*, L. P. M. LAMERS, E. C. H. E. T. LUCASSEN,
G. VAN DER VELDE and J. G. M. ROELOFS
Department of Ecology, Institute for Water and Wetland Research, Radboud University Nijmegen,
Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
(Received 18 November 2005; in final form 16 January 2006)
In the 1980s and 1990s, it became increasingly clear that changes in external nutrient loads alone
could not entirely explain the severe eutrophication of surface waters in the Netherlands. Nowadays,
‘internal eutrophication’ has become a widely accepted term in Dutch water management practice to
describe the eutrophication of an ecosystem without additional external input of nutrients (N, P, K).
This review surveys the principal mechanisms involved in this process. It also discusses possible
remedies to combat internal eutrophication.
Keywords: Alkalinity; Anaerobic decomposition; Biomanipulation; Internal eutrophication; Nitrate;
Phosphate; Sulphate
1. Introduction
In the Netherlands, the term internal eutrophication (‘interne eutrofiëring in Dutch) has
become popular through publications by Roelofs and co-workers since the late 1980s [1–8].
Most of these publications were written in Dutch and described the eutrophication of surface
waters in Dutch peaty lowlands as a result of changes in water quality without additional
external supply of nutrients (N, P, K). Their ideas were based on the observation that water
quality had deteriorated considerably, even in nature reserves, although in many cases no
significant increase had occurred in the external nutrient loads [1, 3, 5]. Initially, this idea
was received with great scepticism. However, as it became increasingly clear that changes
in external nutrient loads alone could not possibly explain the eutrophication of the surface
waters in the Netherlands, even in areas where external nutrient loads had been considerable,
this idea gradually became accepted during the 1990s. Nowadays, internal eutrophication has
become an accepted concept in Dutch water-management practice, as well as among fellow
scientists, who have shown increasing interest in this theme.
*Corresponding author. Email: a.smolders@science.ru.nl
Chemistry and Ecology
ISSN 0275-7540 print/ISSN 1029-0370 online © 2006 Taylor & Francis
http://www.tandf.co.uk/journals
DOI: 10.1080/02757540600579730
94 A. J. P. Smolders et al.
This review presents a brief overview of the mechanisms involved in the process called
internal eutrophication, which we define as the eutrophication of an ecosystem with-
out increased external input of nutrients (N, P, K). Possible remedies to combat internal
eutrophication will also be discussed.
2. How it works
2.1 Redox equilibria and organic-matter breakdown
Organic-matter breakdown and the phosphorus-binding capacity of soil complexesare strongly
influenced by microbial processes affecting the redox state. Redox reactions can be regarded as
reactions in which transfer of electrons takes place and are generally mediated by microbes that
derive energy from electron transfer [9]. As free electrons cannot exist in aqueous solutions,
redox reactions always consist of a sub-reaction in which electrons are mobilized, and a
sub-reaction in which electrons are consumed by an electron acceptor (oxidant).
In well-oxygenated systems, oxygen serves as the primary electron acceptor:
O2+4H++4e−→ 2H2O.
As oxygen is thermodynamically the most favourable electron acceptor, this reaction will
prevail, and as long as oxygen is present it serves as the primary oxidant in the decay of
organic matter:
Corganic +2H2O−→ CO2+4H++4e.
In most wetlands, however, oxygen penetration into the sediment is limited and is generally
restricted to the upper 10 mm [9,10]. Only in lakes with very low organic matter contents can
oxygen penetrate deeper. Once all oxygen has been consumed or if oxygen is absent from the
start, the decay of organic matter continues by a series of reactions that represent successively
lower redox potentials (Eh).
Once oxygen levels havebecome sufficiently depleted, nitrate is used as the terminal electron
acceptor by bacteria, if it is available:
2NO
3+12H++10e−→ N2+6H2O
NO
3+9H++8e−→ NH+
4+2H2O+OH.
This reduction of nitrate to dinitrogen gas (denitrification) or ammonium (ammonification or
dissimilatory nitrate reduction to ammonium, DNRA) involves a multitude of electron transfer
steps [9, 11].
Alternatively, iron(hydr)oxides and sulphate can become involved at successively lower
redox potentials (Eh) [9, 10, 12]:
FeOOH +3H++e−→ Fe2++2H2O
SO2
4+10H++8e−→ H2S+4H2O.
The reduction of Fe and SO2
4leads to the formation of FeSxand may result in a strong
decrease in the P-binding capacity of the sediment, as FeSxhas fewer sorption sites for P than
iron(hydr)oxides (FeOOH) [5, 7, 13,14]. This is discussed in more detail below.
Under anaerobic conditions, the availability of these alternative electron acceptors strongly
affects the breakdown of organic matter. It should be realized that although the consumption
Internal eutrophication 95
sequence of oxidants depends largely on their relative oxidative strengths, the anaerobic respi-
ration pathways are not entirely mutually exclusive, and the different pathways normally show
considerable overlap. In general, anaerobic decomposition is complex and mediated by a con-
sortium of physiologically different micro-organisms [9, 10, 12]. Fermenting bacteria bring
about extracellular hydrolysis of high-molecular-weight polymers (polysaccharides, proteins,
etc.) and ferment the products (monomers such as sugars, amino acids, etc.) to CO2,H
2,
alcohols, acetate, and other organic acids. Acetogenic bacteria are involved in the cleavage of
alcohols and organic acids into acetate, CO2, and H2. Associated with the fermentative organ-
isms are organisms that derive energy from the products of these reactions. Consumption of
acetate, H2, and CO2generally occurs by sulphate-reducing bacteria (SRB) and methanogenic
bacteria, the latter converting CO2and acetate into methane gas. The presence of the SRB and
methanogenic bacteria is therefore essential to ensure that the end-products are consumed and
that the anaerobic decay process keeps going [10, 12]. In addition to the availability of elec-
tron acceptors, (anaerobic) decomposition is obviously strongly regulated by the availability
of degradable organic matter [15].
2.2 Role of alkalinity
Apart from the lack of alternative electron acceptors, acid or very poorly buffered conditions
favour the accumulation of peat. It is well known that decomposition of organic matter is
inhibited in acid waters compared with alkaline waters [5, 16–25]. Roelofs [5] showed that the
alkalinization of surface waters in the Dutch peaty lowlands, due to the practice of letting in
alkaline river water, was responsible for an increased mineralization of the peaty substrates.
The decomposition rate of organic matter appears to be strongly correlated with the internal pH
of the detritus, while bicarbonate neutralizes the decay-inhibiting acids [20, 26]. As a result,
the decay rate of detritus appears to be a function of the buffer capacity of the surrounding
water, rather than of pH [5, 20].
Support for this hypothesis was found in a study of 600 freshwater ecosystems in the
Netherlands by Van Katwijk and Roelofs [27]. They found that the ortho-phosphate (PO3
4,
in this review) concentrations and the bicarbonate alkalinity in surface water and sediment
pore water solutions changed in the same direction in ordination diagrams. Smolders [28] also
found a correlation between bicarbonate alkalinity and the PO3
4and NH+
4concentrations
in sediment pore water obtained from organic freshwater sediments in the Netherlands (see
also [29]).
The effect of alkalinity on PO3
4availability has been confirmed by an experiment in which
the alkalinity of the water layer above an undisturbed weakly buffered peat layer was increased
stepwise (figure 1). The PO3
4levels in the water layer gradually increased as alkalinity
rose. This increase might be attributed to an increased decay rate due to the more alkaline
conditions. Apart from enhancing organic matter breakdown, however, bicarbonate can also
mobilize PO3
4because of the competition between HCO
3and PO3
4for anion adsorption
sites. Although no distinction could be made between these two mechanisms, this experi-
ment does confirm that increased bicarbonate alkalinity can greatly increase the availability
of nutrients in surface waters with organic sediments.
Alkalinity may increase in response to a change in hydrology, such as letting in alkaline
surface water, or due to increased alkalinity of the groundwater [3,5, 24, 30]. However, alka-
linity may also be generated within the system by the reduction of oxidants such as nitrate,
iron(hydr)oxides or SO2
4. The reactions under the heading ‘redox equilibria’ show that all
reduction reactions result in a net generation of alkalinity. This process is called internal
alkalinization [5]. Internal alkalinization can be observed, for instance, at ‘De Venkoelen’,
96 A. J. P. Smolders et al.
Figure 1. Increased phosphate concentrations in the water layer as a reaction to artificially increased alkalinity
above a weakly buffered fen peat sediment derived from the ‘Venkoelen’nature reserve (see also figure 2). Alkalinity
was increased stepwise on the days marked with an arrow. The experiment was carried out in glass containers
(30 ×30 ×30 cm), with a peat layer of 10 cm and a water layer of 18cm. The containers were placed in a water bath
at a constant temperature of 17 C.
a minerotrophic fen in the south of the Netherlands (figure 2), where this process occurs due to
high sulphate reduction rates in summer, resulting in increased alkalinity and decreased SO2
4
concentration in the water layer. In winter, microbial reduction processes are greatly impaired
by low temperatures, while the system is supplied with moderately buffered, sulphate-rich
groundwater, resulting in increased SO2
4concentrations and decreased alkalinity of the water
layer (figure 2).
2.3 Increased sulphate loads
The increased availability of sulphate in ecosystems due to anthropogenic changes has received
much attention. The SO2
4concentrations in the River Rhine are nowadays twice those mea-
sured in 1900 [31]. In the Netherlands, water from the River Rhine is let into large areas of
the peaty lowlands to prevent desiccation in summer [1,5, 29, 30]. This is necessary because
water levels are kept artificially stable. In the past, or in more natural situations, water levels
fluctuated throughout the year, being generally high in winter and low in summer. Current
water level regimes in the Netherlands tend to be the opposite: they are kept low in winter to
enable rapid runoff of excess water from agricultural lands and relatively high and stable in
summer to provide water for growth and evapotranspiration [29]. The water of the river Rhine
is alkaline and has relatively high SO2
4concentrations [5, 7, 30].
In addition, SO2
4concentrations in groundwater have greatly increased over the last
decades, as a consequence of increased atmospheric sulphur (S) deposition and leaching of
SO2
4from agricultural land. Falling water tables and infiltration of nitrate (NO
3) from agricul-
tural land and forest soils into the groundwater also increase groundwater SO2
4concentrations
by favouring the oxidation of FeSxin the subsoil [32, 33].
The potential role of nitrate as a SO2
4mobilizing agent has so far been largely over-
looked. Under natural conditions, nitrate (NO
3) concentrations in groundwater are low
(<32 µmol l1). During the last 60 yr, however, groundwater NO
3concentrations have greatly
increased in many parts of Europe, due to increased pollution. Excessive use of manure and
synthetic fertilizers has resulted in leaching of NO
3from agricultural lands. In Europe, the
Internal eutrophication 97
Figure 2. Variation over time of alkalinity and sulphate concentrations in the surface water of the ‘Venkoelen’,
a minerotrophic fen lake near the city of Venlo, the Netherlands. Seasonal variation can be explained by different
groundwater discharge rates and biological sulphate reduction in summer.
largest net applications of nitrogen (N) on agricultural land occur in the Netherlands and
Belgium, with surpluses of 200 and 125 kg N ha1yr1, respectively [34]. In addition, leakage
of NO
3from forest soils to the groundwater has increased as gaseous ammonia and ammo-
nium sulphate aerosols are effectively filtered by tree crowns. This causes high ammonium
(NH+
4) deposition rates, especially in pine forests, and increased NO
3concentrations in the
groundwater, as NH+
4is rapidly nitrified in forest soils, even under acid conditions [35–37].
Once it has accumulated in groundwater, NO
3causes chemolithotrophic oxidation of pyrite
in the subsoil [38–41]. Van Steenwijk [42], for instance, observed the presence of a pyrite con-
taining layer which coincided with the disappearance of infiltrating NO
3and an increase in
SO2
4concentrations in the groundwater. Hence, it can be concluded that increased leaching of
nitrate from agricultural lands and forests can result in greatly increased SO2
4concentrations
in groundwater and, after discharge, in surface water.
2.4 Sulphate-mediated eutrophication
The increased input of SO2
4in aquatic ecosystems can lead to great changes in anaerobic
sediments. As we have seen, SO2
4serves as an alternative electron acceptor in reductive
sediments and stimulates the decomposition of organic matter. Sulphate reduction also results
in internal alkalinization, which further enhances decomposition. Large amounts of SO2
4may
also cause PO3
4release due to the competition between SO2
4and PO3
4for anion adsorption
sites [13, 43].
Apart from this, sulphide (produced by SO2
4reduction) also interferes with the iron-
phosphorus cycle, by reducing iron(III)(hydr)oxides and iron(III) phosphates [10, 44–48].
Subsequently, highly insoluble FeSxis formed, reducing the availability of Fe to bind PO3
4
and increasing PO3
4mobility [12, 14, 46–48].
98 A. J. P. Smolders et al.
This phenomenon occurs especially in soils in which a large part of the P content is bound
to Fe. Fe-rich sediments tend to retain P that becomes available from decomposition processes
and/or external P loading (figure 3). Such sediments can become loaded with P, which in itself
should not necessarily cause any problem as long as the SO2
4loads remain low. However,
increased SO2
4loads can easily mobilize Fe-bound P through sulphide-induced Fe binding,
which may lead to eutrophication of such systems [7, 29,45, 49]. Prolonged high SO2
4loads
to such sediments will ultimately result in sediments in which most of the Fe is bound to
reduced sulphur (FeSx). As a consequence, the capacity of the sediment to retain P will
greatly decrease (FeSxhas far fewer sorption sites for P), which may result in decreased
total P concentrations but also in a high mobility of P in the sediments. If SO2
4reduction
continues, toxic concentrations of free sulphide may accumulate in the sediment pore water
and cause serious problems for rooted aquatic macrophytes by inducing sulphide toxicity and
iron deficiency [7, 14, 29, 45, 49–51].
Normally, the classic iron cycle [52, 53] can very well explain the actual release of P from
the sediment. In the oxygenated boundary layer between sediment and water layer, dissolved
Fe becomes oxidized and PO3
4is effectively bound by iron(hydr)oxides. This mechanism
probably explains the positive relation between the P-release to the water layer and the
dissolved-P:dissolved-Fe ratios in sediment pore water described by Smolders et al. [47].
Under sulphur-rich reducing conditions, however, most Fe is present as FeSx, so this mecha-
nism may no longer function [7, 13, 47]. Furthermore, dissolved sulphide (and other reduced
compounds) consumes oxygen in the top sediment layer, thus decreasing the thickness of
the oxidized boundary layer. This may greatly boost the release of dissolved PO3
4from the
sediment. Increased methane production rates under highly reductive conditions may further
stimulate P-release by ebullition, especially in organic sediments [5, 10]. Once the sediment
becomes weak and less structured, bottom-feeding fish species such as bream (Abramis brama)
become more abundant and may further enhance the exchange of nutrients between the sedi-
ment and the water layer [54]. Increased activity of benthivorous fish may make the sediments
more susceptible to wind- and wave-induced disturbance [55], further increasing turbidity and
the release of nutrients from the sediment.
Figure 3. Relationship between total-iron content and iron-bound P concentrations in sediments derived from
different lakes in the province of Zuid-Holland (the Netherlands). P fractionation of the sediment was carried out
according to Golterman [108] and allowed a distinction between unstable (loosely bound) P, Ca-bound P, organic P,
and Fe/Al-bound P. The numbers 1–10 represent different locations.
Internal eutrophication 99
Figure 4. Soil pore water characteristics of monoliths from a minerotrophic fen meadow (‘De Bruuk’, a nature
reserve near the village of Groesbeek, the Netherlands) during 32 weeks of waterlogging with either 0 or 2 mmoll1
sulphate (white and black marks, respectively). The figure shows means with standard errors (n=6). See also [14].
Many experiments have confirmed that increased SO2
4loads indeed lead to increased
nutrient availability [5, 14, 15, 25, 47–49, 56–58]. Figure 4, for instance, shows the results
of an experiment in which the SO2
4loads of monoliths obtained from a minerotrophic fen
meadow were experimentally increased (see also [14]). A SO2
4load of 2 mmol l1led to a
clear increase in alkalinity as well as in the PO3
4,S
2, and NH+
4concentrations of the soil pore
water (figure 4). At the end of the experiment, biomass regrowth after harvesting was signifi-
cantly smaller on SO2
4-treated soils, especially for Carex species [59], though Juncus species
appeared to be more resistant to the changes. Additional experiments confirmed that sulphide
toxicity was the main cause of the decline of Carex species in the SO2
4treatments [59].
The decline of vegetation dominated by species such as Stratiotes aloides and Potamogeton
compressus in the Netherlands can be attributed to increased SO2
4loads and the resulting
increase in PO3
4and NH+
4levels in the water layer, as well as increased concentrations of S2
in sediment pore water [28, 60, 61]. Sulphide and ammonium toxicity, iron deficiency, and
increased competition by free floating macrophyte species may occur separately but frequently
also occur simultaneously (multiple stress), resulting in a considerable decrease in vitality and
finally in the complete decline of mesotrophic vegetation types (figure 5).
Vegetation shifts in aquatic systems have also been revealed by an analysis of the extensive
database collected by De Lyon and Roelofs [62]. This database includes the weighted means of
many physico-chemical variables for most of the aquatic and semi-aquatic macrophyte species
in the Netherlands. In an analysis of this database, Smolders et al. [61] tried to establish
a relationship between the weighted means of the SO2
4and PO3
4concentrations in the
water layer for aquatic plants in the Netherlands. For species from very reductive sediments
(Eh <150 mV), a very clear relationship was found between these weighted means [61].
As SO2
4is reduced in these sediments, SO2
4-induced eutrophication by the mechanisms
described above can occur, and higher surface water SO2
4levels may correspondingly result
in higher PO3
4levels.
Among the species of reductive sediments, one can distinguish a group of species of waters
with SO2
4concentrations <0.5 mmol l1and PO3
4concentrations <5µmol l1(figure 6).
100 A. J. P. Smolders et al.
Figure 5. Multiple environmental stress hypothesis explaining the decline of Stratiotes aloides in the Netherlands.
Sulphate enters via surface water,groundwater, or atmospheric deposition. The reduction of sulphate to sulphide causes
eutrophication as the sulphide interacts with P binding, and mineralization is stimulated by alkalinity generation. As
a result, the water layer becomes eutrophied, leading to the dominance of floating species and algae. Simultaneously,
sulphide and ammonium toxicity and iron deficiency may occur.
These are species such as S. aloides,Potamogeton compressus,Potamogeton acutifolius, and
Utricularia vulgaris, which were once highly characteristic of the shallow surface waters in
the minerotrophic regions of the Netherlands, and are typical species of mesotrophic waters.
Two further groups that can be distinguished (II and III in figure 6) are characteristic of waters
with higher SO2
4(weighted average between 0.7 and 1.2 mmol l1) and PO3
4concentrations
(weighted average between 5 and 10 µmol l1). These groups include species such as Spirodela
polyrhiza,Lemna trisulca, and Ceratophyllum demersum, which tend to replace S. aloides or
become co-dominant in Stratiotes vegetation that has gradually become eutrophied [30].
Species of brackish waters with reductive sediment also appear to fit very well into the
scheme shown in figure 6. These species occur in waters with naturally high SO2
4concentra-
tions and hence also high PO3
4concentrations. It is important to realize that species adapted
to brackish water conditions have a competitive advantage over species that are not adapted to
high salinity, and therefore probably do not suffer from competition by non-rooting freshwater
species. Compared with pristine conditions, waters with Ceratophyllum submersum,Lemna
gibba, and Azolla filiculoides also have increased SO2
4concentrations, but the weighted aver-
age of the PO3
4concentrations in the water layer is very high. We suggest that these species
become dominant in waters in which, owing to sediment characteristics [48], increased SO2
4
reduction leads to very strong PO3
4mobilization, or in waters where external PO3
4loads play
an important role.
2.5 Role of nitrate
On agricultural lands in particular, large amounts of NO
3may be leached from the top layer
to the deeper soil layers. In situations where agricultural lands are located on top of anaerobic
Internal eutrophication 101
Figure 6. Relationship between the weighted average of the phosphate and sulphate concentrations in the water
layer for aquatic plants preferring sediments with different reductive states (Eh).The positions of species characteristic
of reductive sediments (Eh <150 mV) are indicated in the figure by squares (I), triangles (II), diamonds (III), and
crosses. All data are derived from [62].
peaty soils, this might lead to increased decomposition of the peat, as NO
3functions as an alter-
native electron acceptor (oxidant). Drainage ditches in such polder areas in the Netherlands
are characterized by the presence of large amounts of organic mud which cannot be explained
by the external input via surface water inlet. The mud is very probably produced within the
systems, from the decomposition of the local peat layers. Apart from aerobic decomposition
due to falling water tables and anaerobic SO2
4-mediated breakdown, nitrate-mediated decom-
position probably plays an important role in these agricultural systems. This will be a subject
of further research in the near future.
However, NO
3does not necessarily enhance internal eutrophication. As NO
3is an ener-
getically more favourable electron acceptor in anaerobic sediments than Fe and SO2
4, high
NO
3loads may function as a redox buffer, limiting the reduction of Fe and SO2
4[9, 63]. In
addition, NO
3reducing bacteria have the capacity to grow anaerobically, with Fe(II) as an
electron donor, resulting in the production of Fe(III) [64–66]. It is indeed known that NO
3
reduction can lead to the oxidation of Fe(II) [39–41, 67,68] and metal sulphides [69–72], and
so may increase the PO3
4-binding capacity of sediments under anaerobic conditions.
The hypothesis that NO
3may actually prevent internal eutrophication was confirmed by
recent research in Dutch alder carr fens [33, 73]. During the last decades, alder carrs in the
Netherlands have become highly eutrophied [73, 74]. Enclosure experiments have shown that
high SO2
4reduction rates lead to PO3
4mobilization and subsequent eutrophication [58].
However, field observations also revealed that no eutrophication occurs if there is a constant
102 A. J. P. Smolders et al.
input of groundwater that contains not only SO2
4but also high concentrations of NO
3[33].
The alder carrs that were fed by groundwater with high NO
3concentrations were characterized
by the lowest PO3
4concentrations and the development of aquatic plants characteristic of clear
waters [33].
This role of NO
3as a redox buffer was confirmed by subsequent laboratory experi-
ments [33], in which a continuous flow of NO
3-rich medium led to a higher redox potential
and much lower methane concentrations in the alder carr sediment than the same medium
without NO
3. In addition, the release of Fe and S2, resulting from the oxidation of FeSx,
Fe2+, and S2was much lower in sediments receiving NO
3-rich medium than in those receiv-
ing medium without NO
3, while the release of SO2
4was higher as a result of nitrate-induced
oxidation of FeSx.
Based on these observations, we propose the following mechanism for the current situation
in the Netherlands. NO
3leaching from agricultural lands and forest soils leads to increased
NO
3concentrations in the groundwater. When NO
3reaches FeSx-containing subsoil layers,
it may oxidize FeSx, leading to the mobilization of SO2
4and a decrease in the NO
3concen-
tration. The resulting NO
3:SO2
4ratio strongly affects the quality of groundwater-fed fens. If
SO2
4concentrations are high, and NO
3concentrations are low, eutrophication may occur as a
result of SO2
4reduction-related processes such as PO3
4mobilization and Fe immobilization
(FeSxaccumulation). However, if NO
3concentrations are also high, SO2
4and Fe reduc-
tion are impaired, and mobilization of PO3
4from iron-phosphate complexes is prevented. In
addition, NO
3may oxidize reduced Fe compounds, increasing the amount of Fe3+capable
of binding PO3
4. Hence, NO
3leaching into the groundwater increases the risk of eutroph-
ication of the discharge areas by mobilizing SO2
4in FeSxcontaining aquifers. However,
if NO
3reaches a discharge area in sufficiently high concentrations, it may prevent actual
eutrophication (at least of the directly fed parts) by functioning as a redox buffer [33].
2.6 Effects of temporary oxidation
It is well known that some aquatic macrophytes, such as isoetid species, release large amounts
of oxygen from their roots and so maintain an oxidized state in the sediments [51, 75–77].As
a consequence, PO3
4is bound to iron(hydr)oxide and remains highly immobile [78]. Taking
away the oxygen source by experimental removal of the vegetation results in an increase
in iron and PO3
4availability due to a large decrease in redox potential [76]. These aquatic
macrophytes may therefore lower the PO3
4availability by maintaining Fe in an oxidized
state. They can also decrease the nitrogen (N) availability by stimulating coupled nitrification–
denitrification processes [51, 79]. In the root zones and the water layer, NH+
4is nitrified to
highly mobile NO
3, while NO
3diffuses to the deeper anoxic sediment where it is denitrified.
In this way, oligotrophic conditions can persist for as long as 10000 yr, e.g. in soft-water lakes.
Temporary desiccation of sediments can mimic these effects and lead to lower nutrient
concentrations, while permanent high water levels may have the opposite effect. During
desiccation, pore-water PO3
4concentrations tend to decrease as PO3
4binds to oxidized
Fe(III) compounds, such as iron phosphate, iron(hydr)oxide-phosphate, and humic-iron-
phosphate complexes [59, 78, 80, 81]. Nitrification is stimulated after desiccation, while
subsequent waterlogging suppresses nitrification and stimulates denitrification [14, 33].
During desiccation, bicarbonate is also consumed by the oxidation of FeSxand the absence
of alkalinity-generating processes like denitrification. This can result in more poorly buffered
conditions after rewetting and hence in a decreased decomposition of organic matter.
However, Lucassen et al. [82] found that desiccation may lead to severe acidification and
mobilization of heavy metals when the sediment S/(Ca +Mg) ratio is high (>0.7 in alder
Internal eutrophication 103
carr forest). Total Ca +Mg should be regarded as a measure of the buffer capacity due to car-
bonate dissolution and cation exchange. The solubility of PO3
4complexes, including apatite
(Ca5(PO4)3(OH,F,Cl)), strengite (FePO4), and variscite (AlPO4), is also strongly influenced by
pH. With decreasing pH, apatite is the first to dissolve, followed by variscite and strengite [83].
Hence, the present sulphur pollution in many freshwater wetlands forms a threat not only by
inducing SO2
4-mediated internal eutrophication, but also by increasing the sensitivity of wet-
lands to desiccation [59, 82]. The combination of increased sulphur loading and a growing
risk of the occurrence of desiccation is therefore a major cause for concern. Furthermore, the
high SO2
4concentrations generated by desiccation and the subsequent runoff following rehy-
dration inevitably cause internal eutrophication in reduced peaty soils and sediments where
the water is discharged.
At the same time, however, temporary desiccation of the top layer of the sediment appears to
be important in preventing undesirable vegetation development in well-buffered systems such
as alder carr forests [73, 84, 85]. Oxidation of insoluble FeSxleads to mobilization of SO2
4
and precipitation of Fe as FePO4and Fe(III) hydroxides. After reflooding, the mobile SO2
4is
removed from the system via the flowing water layer, while oxidized Fe is gradually reduced
again, also resulting in the release of immobilized P. Hence, one would expect only a minor
effect of temporary desiccation. However, in the presence of significant amounts of insoluble
reduced Fe, such as iron sulphides (FeSx) or siderite (FeCO3), oxidation of the sediment results
in a net production of Fe(III), which greatly increases the amount of reducible oxidized Fe
in the sediment capable of binding o-PO3
4and may thus produce a more lasting beneficial
effect of desiccation [84].
This effect is illustrated by the results of an experiment in which the effect of two water
regimes (with and without a 6 week desiccation period) were tested on two sediment types [84].
One sediment originated from a zone that is fed by SO2
4- and Fe-rich seepage during a large
part of the year and that temporarily dries out during the summer (T-sediment). This sediment
had a relatively high reduced Fe (FeSx) content. The other sediment was derived from a for-
merly dry zone without reduced Fe (D-sediment) (figure 7) and had a low reduced Fe content.
The results showed that P release from the sediment and the concomitant development of algae
and lemnids in the water layer depend on the sediment type and the interaction between sedi-
ment type and water-table fluctuations. In the T-sediment, total P concentrations in the water
layer differed greatly between the temporary desiccation treatment and the control treatment.
Desiccation of T-sediment greatly diminished the accumulation of total P in the water layer
and the concomitant biomass production of lemnids and algae in the water layer (figure 7).
Figure 7. Total P release and development of algae +Lemna spec. biomass above two different sediments (T and
D sediment, see text) from alder carr forests in the Netherlands. Values are for a 3 month period after a 6 week
desiccation period (removal of the water layer; white bars) or a control treatment with constant flooded conditions
(black bars). The water layer of the control treatment was replaced when the sediment of the desiccation treatment
was reflooded [84].
104 A. J. P. Smolders et al.
In sediments with a high reduced iron concentration, release of P to the water layer apparently
remained low for a considerable time after desiccation and subsequent re-flooding. This can
be explained by the effect that, after desiccation and re-flooding, the same amount of P is now
immobilized by a much larger pool of oxidized iron, and it thus takes a longer time before all
iron is reduced again, and the same amount of P is mobilized.
In the D-sediment, however, temporary desiccation did not lead to significant differences
in pore water P or in (total) P concentrations in the water layer after re-flooding (figure 7).
In both treatments, the water layer was extremely eutrophied, with a very high production of
filamentous algae and lemnids. Formerly dry sediments such as the D-sediment have not been
influenced by groundwater for a long time and do not possess a reduced Fe pool, which is why
desiccation did not result in an additional increase in the reducible Fe pool [84].
Oxidation of the top layer of sediment may also be achieved by increased light levels on the
sediment surface. Figure 8 shows the effects of increased light levels on the sediment surface
on the oxygen profiles in the sediment top layer and the PO3
4concentrations in the water layer.
The sediment used in this experiment was extremely reduced, with a high FeSxcontent and
a very low concentration of dissolved iron (figure 8). In this experiment, increased irradiance
resulted in the growth of benthic algae on the sediment surface. Oxygen produced by these
algae resulted in oxidation of the sediment top layer (figure 8) and hence in the oxidation of
iron sulphide compounds. As a result, reducible iron concentrations increased in the top layer,
and PO3
4was bound more effectively in this layer, resulting in a decreased release of PO3
4
to the water layer (figure 8).
Figure 8. Oxygen profiles at the water/sediment interface (A), phosphate concentrations in the water layer (B),
and iron (C) and phosphate (D) concentrations in sediment pore water of a reduced sulphur- rich sediment under dark
(black squares) and light conditions (white circles) during a 5 month experimental period. Experiments (n=3) were
carried out in glass containers (25 ×25 ×30 cm), with a 10 cm sediment layer and an 18cm water layer. Under light
conditions, benthic algae were growing on the sediment surface.
Internal eutrophication 105
3. What to do about it
3.1 How to prevent eutrophication
The release of PO3
4from eutrophic sediments strongly depends on the redox state of the
top layer. As light may stimulate the growth of water plants and benthic algae, which may
subsequently oxidize the top layer of the sediment, turbid water results in internal eutrophi-
cation, while internal eutrophication results in turbid waters. Natural water-level fluctuations
(temporary desiccation in summer) may be a prerequisite to prevent internal eutrophication
of wetlands [73, 84, 85]. This is likely to be especially true for marshes and moorland pools.
Another advantage of the oxidation of the top layer of the sediment is the fact that many
seeds in the seed beds, which do not geminate in very reductive sediments, germinate and
colonize the bottom very quickly after a short drought [28]. Ill-considered measures to pre-
vent desiccation at any cost may have disastrous consequences for such systems [73,74].
A recent strategy to minimize the use of allochthonous water in drainage ditches involves
the application of a more natural water-table management strategy, allowing higher tables in
winter and lower tables in summer. In addition, this is expected to decrease PO3
4concentra-
tions and stimulate germination of aquatic macrophytes. However, this strategy needs further
research. Artificial oxygenation of the water layer may also increase the redox state of the
sediment top layer and prevent the release of PO3
4to the water column [86]. It is especially
in relatively small, shallow lakes that this might be a plausible method to combat internal
eutrophication.
3.2 Biomanipulation
Clear water is essential for the development of submerged plant communities, often dom-
inated by Characeae. This condition may be achieved via a bottom-up approach, leading
to a significant reduction in the nutrient availability, or by a more top-down approach like
biomanipulation [54, 87–90]. Biomanipulation is defined as the deliberate exploitation of
the interactions between components of the aquatic ecosystem, in order to reduce the algal
biomass [87]. The main aim is to increase zooplankton (mainly Daphnia) grazing on phyto-
plankton, thereby changing from a state of turbid water to one of clear water, even though
the nutrient concentrations remain equal [89]. To achieve this, the system has to be ‘pushed’
through the hysteresis effect that prevents an easy transition from one state to the other. Once
submerged vegetation has established, it helps to maintain the clear-water equilibrium by pro-
viding a habitat and refuge for zooplankton, preventing resuspension of sediment particles,
competing with algae for nutrients and depressing algal growth by the excretion of allelopathic
substances [89, 91].
In many shallow lakes, the desired change to clear water has been achieved by a dras-
tic reduction of the zooplanktivorous and benthivorous fish stocks [54, 87, 88, 92–94]. This
involves a reduction to 10–15kg ha1and 15–25 kg ha1of zooplanktivorous and benthivo-
rous fish, respectively (generally corresponding to a minimum reduction of 75%). In addition,
piscivorous fish like Esox lucius (Northern pike) and Stizostedion lucioperca (Pikeperch)
may be introduced or reintroduced. In some fens, such biomanipulation turned out to be
very effective in re-establishing underwater light conditions that are favourable to submerged
macrophytes. In others, however, the water remained turbid or became turbid again quickly
after the measures were taken, and submerged plant communities failed to develop.
In hypertrophic lakes, turbidity is the only possible stable situation. Biomanipulation will
only work after PO3
4concentrations are reduced to a range in which two alternative stable
106 A. J. P. Smolders et al.
states are possible [89]. The actual threshold value seems to lie below about 7µmol total
Pl
1, although shallow lakes may be clear at much higher concentrations [88, 95]. This can
be attributed to the strong influence of submerged macrophytes on water quality, as explained
earlier. Poor results with biomanipulation in fens well within the range of two alternative
stable states are often caused by insufficient reduction of the fish stock or by massive fish
remigration from small interconnected ditches or through fish exclosures [96]. In most fen
waters, the best way of fighting eutrophication seems to lie in a combination of biomanipula-
tion (top-down control) and active reduction of the influx and internal mobilization of PO3
4
(bottom-up).
3.3 Phosphate inactivation
Phosphorus can be very effectively bound in the form of calcium phosphates. In south-west
Norway,for instance, P concentrations as high as 100 mmol g1DW are encountered in the sed-
iments, but the lakes can still be characterized as ultra-oligotrophic, since almost all P is present
in the form of highly insoluble fluorapatite (Ca5F(PO4)3). In hard-water lakes, lime additions
have shown some potential for controlling P availability [97, 98]. Lime application may super-
saturate the water column with Ca2+, which can result in precipitation of (hydroxy)apatite and
also cause precipitation of phytoplankton. Binding P as calcium phosphate precipitation might
have great advantages, as Ca-bound P is redox-insensitive. However, compared with Fe and
Al additions, liming seems to be less effective [86], and multiple treatments may be necessary
for long-term management of phytoplankton biomass [97, 98]. In soft-water lakes with a peaty
sediment, the addition of lime to bind PO3
4will not work, because of the strong stimulation
of P mineralization in fen peat (see above), overruling any possible effect of chemical binding
of P [5, 28, 29].
The addition of Fe or Al to sediments to bind PO3
4(PO3
4inactivation) has a strong de-
eutrophying effect in both lakes and fens [47, 49, 86, 98–102]. Burley et al. [86] concluded
that Fe(III) and Al addition were more effective than liming, although cost-effectiveness
considerations may make these methods less desirable [47, 98]. If there is a constant sup-
ply of SO2
4enriched water, the response is only transient, because Fe consumption is
extremely high [47, 49, 86]. In such cases, Fe addition has proved to be too laborious
and costly to be recommended as a general and large-scale restoration measure against
eutrophication [47].
Aluminium addition has the advantage that Al-bound P is redox-insensitive and indeed
seems to be able to bind P more effectively than Fe [103]. A special problem related to
the addition of aluminium salts, however, is the fact that Al may cause toxicity symptoms,
especially in the fauna component of the system. Acidification due to Alx(OH)3forma-
tion (hydrolization) may lead to high concentrations of highly toxic inorganic monomeric
Al (Al3+) [104], while subsequent polymerization reactions (formation of Al hydroxides)
can temporarily result in an even greater toxicity [105]. Although, in well-buffered (alka-
line) lakes, such highly toxic shock events will be both local and temporary, they may
still be considered highly undesirable, especially when valuable fauna components are
present.
The presence of dense macrophyte stands may cause problems, especially when Ca, Fe, or Al
is applied directly to the water layer [102]. In cases where P is bound to the top layer following
the addition of Ca/Fe/Al, sediment P may be taken up directly from below the treated top
layer. The nutrients taken up from the sediment by these macrophytes then become available in
the water layer due to senescence and decay. Finally, Ca/Fe/Al-treated sediment may become
buried if sedimentation rates remain relatively high and so lose their effectiveness [102].
Internal eutrophication 107
3.4 Sediment removal
Dredging of PO3
4-enriched and/or iron-depleted sediments is another option and can be
highly effective [106,107]. However, if no additional measures are taken, the effect of sed-
iment removal is also temporary. In eutrophied, poorly buffered systems, sediment removal
may lead to a strong decrease in nutrient levels in the water layer, but also to a severe decrease
in the base saturation of the sediment. As long as water rich in acid and/or ammonium
enters the system, acidification or re-acidification of these systems may occur. An adequate
additional measure to counteract re-acidification may be a controlled supply of calcareous
groundwater [106]. Problems linked with sediment removal, however, are not only the high
costs but also the fact that, according to the present environmental standards, the sediment in
many cases has to be technically classified as chemical waste because of accumulated toxic
compounds.
4. Final comments
It can be concluded that internal eutrophication may play an important role in the deterioration
of aquatic ecosystems. Increased breakdown of organic matter under anaerobic conditions due
to increased alkalinity and increased availability of oxidants (SO2
4and possibly nitrate) play
key roles in this process. In addition, the interference of SO2
4with the iron-phosphate cycle is
of paramount importance. The restoration of the original, semi-natural hydrology (high water
levels in winter and low water levels in summer), both at the landscape level and at the habitat
level, may reduce the negative effects or may even be a prerequisite for an acceptable water
quality. Such measures will decrease the SO2
4and alkalinity loads of the systems and might
also increase the P-binding capacity of the sediments if temporary desiccation should occur.
Biomanipulation is a costly but potentially effective method by which an alternative stable
state can be reached, which permits clear water even while nutrient concentrations in the water
layer remain high. Treatments of whole water bodies with lime, iron, or aluminium salts have
also proved to be potentially effective. However, such measures are also costly, while the
improvement of the water quality is often transient.
More generally, it is important to realize that agriculture on peaty substrates, which is
currently being practised on a large scale in the Netherlands, is not sustainable. A complete
hydrological isolation of large areas would probably be the best way to prevent internal eutroph-
ication processes. Allowing more rain water to accumulate in the wetlands would reduce not
only the alkalinity but also the SO2
4load. Next, old seepage systems might be restored, which
would result locally in a renewed supply of iron via the groundwater. Raising water tables
and decreasing nitrate loads from former agricultural lands could further diminish the SO2
4
concentrations in groundwater.This might allow large marshes to start re-developing, and peat
could build up again in areas where organic soil is currently broken down rapidly. This would
diminish soil subsidence as well as net carbon losses to the atmosphere. Given the agricul-
tural functions involved, such measures would not be a feasible option at present. In the long
run, however, these alternative options might become more attractive because agriculture is
economically under great pressure, especially in the wet lowland areas.
Acknowledgements
The authors wish to thank Mr Jan Klerkx and Dr Hilde Tomassen for critically reading the
manuscript.
108 A. J. P. Smolders et al.
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... Important removal pathways of aqueous phosphate from surface waters include sorption to minerals and uptake into organic matter, 6,13−16 followed by deposition and incorporation of the particulate phases in sediments. Early diagenetic remobilization of aqueous phosphate can occur as a result of mineral dissolution, desorption, and hydrolysis of particulate organic P. 6,17,18 In particular, the reductive dissolution of phosphate-containing ferric iron (hydr)oxides and the desorption of phosphate from mineral surfaces are generally considered major processes driving internal P loading in lakes. Adsorption and desorption of phosphate can further be modulated by other anionic species, including arsenate, bicarbonate, sulfate, and silicate, that compete with phosphate for mineral binding sites. ...
... Adsorption and desorption of phosphate can further be modulated by other anionic species, including arsenate, bicarbonate, sulfate, and silicate, that compete with phosphate for mineral binding sites. 17,19,20 Among phosphate's anionic competitors, dissolved silicate is ubiquitous in aquatic environments. It occurs predominately in the form of silicic acid (H 4 SiO 4 ) produced by the dissolution of detrital silicate minerals and various forms of biogenic silica. ...
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Release of sorbed phosphate from ferric iron oxyhydroxides can contribute to excessive algal growth in surface water bodies. Dissolved silicate has been hypothesized to facilitate phosphate desorption by competing for mineral surface sites. Here, we conducted phosphate and silicate adsorption experiments with goethite under a wide pH range (3–11), both individually (P or Si) and simultaneously (P plus Si). The entire experimental data set was successfully reproduced by the charge distribution multisite surface complexation (CD-MUSIC) model. Phosphate adsorption was highest under acidic conditions and gradually decreased from near-neutral to alkaline pH conditions. Maximum silicate adsorption, in contrast, occurred under alkaline conditions, peaking around pH 10. The competitive effect of silicate on phosphate adsorption was negligible under acidic conditions, becoming more pronounced under alkaline conditions and elevated molar Si:P ratios (>4). In a subsequent experiment, desorption of phosphate with increasing pH was monitored, in the presence or absence of dissolved silicate. While, as expected, desorption of phosphate was observed during the transition from acidic to alkaline conditions, a fraction of phosphate remained irreversibly bound to goethite. Even at high Si:P ratios and alkaline pH, dissolved silicate did not affect phosphate desorption, implying that kinetic factors prevented silicate from displacing phosphate from goethite binding sites.
... More stable coarse substrate is representative for higher flow velocities and lower nutrient contents, whereas finer substrate often goes along with lower flow velocities and higher nutrient contents. It can potentially indicate eutrophication processes including the release of nutrients to the surface water [52]. ...
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Riverine macrophytes form distinct species groups. Their occurrence is determined by environmental gradients, e.g. in terms of physico-chemistry and hydromorphology. However, the ranges of environmental variables discriminating between species groups ("discriminatory ranges") have rarely been quantified and mainly been based on expert judgement, thus limiting options for predicting and assessing ecosystem characteristics. We used a pan-European dataset of riverine macrophyte surveys obtained from 22 countries including data on total phosphorus, nitrate, alkalinity, flow velocity, depth, width and substrate type. Four macrophyte species groups were identified by cluster analysis based on species' co-occurrences. These comprised Group 1) mosses, such as Amblystegium fluviatile and Fontinalis antipyretica, Group 2) shorter and pioneer species such as Callitriche spp., Group 3) emergent and floating species such as Sagittaria sagittifolia and Lemna spp., and Group 4) eutraphent species such as Myriophyllum spicatum and Stuckenia pectinata. With Random Forest models, the ranges of environmental variables discriminating between these groups were estimated as follows: 100-150 μg L-1 total phosphorus, 0.5-20 mg L-1 nitrate, 1-2 meq L-1 alkalinity, 0.05-0.70 m s-1 flow velocity, 0.3-1.0 m depth and 20-80 m width. Mosses were strongly related to coarse substrate, while vascular plants were related to finer sediment. The four macrophyte groups and the discriminatory ranges of environmental variables fit well with those described in literature, but have now for the first time been quantitatively approximated with a large dataset, suggesting generalizable patterns applicable at regional and local scales.
... The pond is eutrophic, caused by water inflow from allotments nearby, as well as bird feces, leaf litter, and food remnants from feeding waterfowl, resulting in a variety of complex organics in the pond sediment (Hermitage of Braid and Blackford Hill Local Nature Reserve Management Plan 2011-2021Pringle and Beale, 1960;Boyd, 1995). Eutrophication of water bodies can cause hypoxia in the water column and anoxic conditions in the sediment (Smolders et al., 2006;Waajen et al., 2014). Samples were taken from the top layer of the sediment, as this layer usually contains the highest concentration of organic matter (Munsiri et al., 1995). ...
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Meteoritic material accumulated on the surface of the anoxic Early Earth during the Late Heavy Bombardment around 4.0 Gya. These meteorites may have provided the Earth with extra-terrestrial nutrients and energy sources for early life. How could the presence of meteorites have affected the origin and evolution of early life on Earth? And what is the influence of geothermal activity on the Earth’s surface? This research investigates the growth of an anaerobic microbial community from pond sediment on non-pyrolyzed (pristine) or pyrolyzed (heat-treated) carbonaceous chondrite ‘Cold Bokkeveld’. A microbial community was grown anaerobically in batch cultures containing a liquid environment and powdered non-pyrolyzed or pyrolyzed Cold Bokkeveld. Cell concentrations were measured by Colony-Forming Units on agar plates. The community composition in the presence of non-pyrolyzed meteorite was determined by 16S rRNA amplicon sequencing. Non-pyrolyzed Cold Bokkeveld supported the growth of a stable, anaerobic community containing mainly the Deltaproteobacteria Geobacteraceae and Desulfuromonadaceae. Members of these families are known to use elemental sulfur and ferric iron as electron acceptors, and organic compounds as electron donors. Pyrolyzed Cold Bokkeveld however, was inhibitory to the growth of the microbial community. These results show that carbonaceous chondrites can host an anaerobic microbial community, but that pyrolysis, e.g. by geothermal activity, can inhibit microbial growth and potentially toxify the material. This indicates that extraterrestrial meteoritic material and the environment on Early Earth could have shaped the nature of early microbial ecosystems by enhancing growth of microorganisms with metabolic capabilities favored in the presence of this material.
... There are ongoing discussions as to the possible value of iron phosphates as slow-release fertilisers (Chandra et al., 2009;Nieminen et al., 2011;Andelkovic et al., 2019;Wang et al., 2020), at least in iron-deficient soils. Indeed, plants and fungi use a wide variety of strategies to access iron from iron-P effectively (Bolan et al., 1987;Hinsinger, 2001;Smolders et al., 2006). Further research into P-iron interactions may help to develop methods to manipulate iron−P chemistry in wastewater treatment processes that support P recovery (Qiu et al., 2015;Wilfert et al., 2015). ...
Technical Report
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Unsustainable phosphorus use is at the heart of many societal challenges. Unsustainable phosphorus use affects food and water security, freshwater biodiversity and human health. Increasing demand for food to support a growing global population continues to drive increases in phosphorus inputs to the food–system, as well as losses from land-based sources to freshwater and coastal ecosystems. These losses cause ecological degradation through the proliferation of harmful algal blooms in fresh waters, contributing to alarmingly high rates of biodiversity decline, economic losses associated with clean-up, and large-scale human health risks from contaminated drinking water supplies. The pace of species extinction, climate change and the growing number of extreme weather events, combined with population growth and the economic impact of COVID-19, have further strengthened the need to invest in phosphorus sustainability. If the world is to meet climate change, biodiversity, and food security targets, and avoid building costs of predicted phosphorus impacts, positive action on phosphorus management is essential. The present report calls for the establishment of an intergovernmental coordination mechanism to catalyse integrated action on phosphorus sustainability. This should be supported by an international framework to consolidate the collective knowledge, quantify of the economic and societal benefits of improvements in phosphorus management and establish targets for time-bound improvements. The report identifies a clear opportunity to raise awareness on the need for sustainable phosphorus management through the United Nations Environment Assembly (UNEA) and calls for a UNEA resolution on sustainable phosphorus management or equivalent global commitment to act.
... There are ongoing discussions as to the possible value of iron phosphates as slow-release fertilisers (Chandra et al., 2009;Nieminen et al., 2011;Andelkovic et al., 2019;Wang et al., 2020), at least in iron-deficient soils. Indeed, plants and fungi use a wide variety of strategies to access iron from iron-P effectively (Bolan et al., 1987;Hinsinger, 2001;Smolders et al., 2006). Further research into P-iron interactions may help to develop methods to manipulate iron−P chemistry in wastewater treatment processes that support P recovery (Qiu et al., 2015;Wilfert et al., 2015). ...
Technical Report
Full-text available
Currently large amounts of phosphorus are lost in waste streams. A global commitment to recycling nutrients in wastes and residues is needed. Phosphorus recovery provides the opportunity to recover a contaminant free, high purity source of phosphorus that can be used to create customised products, and substitute effectively for phosphorus derived from phosphate rock. Phosphorus recovery and recycling will catalyse new circular economy opportunities in line with national and international policies and directives.
... Major sources of nitrogen and phosphorus in wetlands are urbanized and agricultural lands (Hemond & Benoit, 1988). Whereas internal eutrophication can occur in wetlands due to decomposition of organic matter under anaerobic condition with alkaline environment and abundance of SO 4 2- (Smolders et al., 2006). According to Lucassen et al. (2005) eutrophication is likely to occur in wetlands in concentration of nitrate is greater than sulfate or sulfate is mobilized by FeS x . ...
Chapter
Wetlands show a diversity of appearances like salt marshes, tidal wetlands, inland freshwater wetlands, riparian wetlands, peat lands, and many other types. Each of the types host diverse biotic communities of flora and fauna. This biodiversity changes according to the physical and chemical properties of wetlands, climate, and the geological location. This biodiversity regulates the local ecosystem, carbon sequestration, fuelwood supply, fishery-based industries, and on many other ecological and socioeconomic aspects. In addition, the wetlands have other ecological aspects like maintaining freshwater quality by sedimentation, nutrient conservation, etc. However, around the world, the wetlands are subjected to several types of threats like both anthropogenic and natural. This study is a short review work on some of the outcomes of the studies of researchers around the world to see the importance of different types of wetlands, the threats to them by anthropogenic or natural causes, and focus areas for management strategy development.
... The content of nutrients, i.e., nitrogen and phosphorus, which determines the quality of shallow groundwater, can be a potential indicator of non-point source pollution, including agricultural pollution [55][56][57]. ...
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... Effect van sulfaat in oppervlaktewater In sulfaatrijke plassen (> 20mg SO4/L) kan de nalevering van fosfaat tot een factor 2 hoger zijn dan in sulfaatarme plassen (< 20mg SO4/L). De belangrijkste oorzaak hiervan is de extra mobilisatie van ijzergebonden P door sulfaat (Lamers et al., 1998;Smolders et al., 2006;Poelen et al., 2012). ...
Technical Report
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... The pond is eutrophic, caused by water inflow from allotments nearby, as well as bird feces, leaf litter, and food remnants from feeding waterfowl, resulting in a variety of complex organics in the pond sediment (Hermitage of Braid and Blackford Hill Local Nature Reserve Management Plan 2011-2021Pringle and Beale, 1960;Boyd, 1995). Eutrophication of water bodies can cause hypoxia in the water column and anoxic conditions in the sediment (Smolders et al., 2006;Waajen et al., 2014). Samples were taken from the top layer of the sediment, as this layer usually contains the highest concentration of organic matter (Munsiri et al., 1995). ...
Article
Meteoritic material accumulated on the surface of the anoxic early Earth during the Late Heavy Bombardment around 4.0 Gya and may have provided Earth's surface with extraterrestrial nutrients and energy sources. This research investigates the growth of an anaerobic microbial community from pond sediment on native and pyrolyzed (heat-treated) carbonaceous chondrite Cold Bokkeveld. The community was grown anaerobically in liquid media. Native Cold Bokkeveld supported the growth of a phylogenetically clustered subset of the original pond community by habitat filtering. The anaerobic community on meteorite was dominated by the Deltaproteobacteria Geobacteraceae and Desulfuromonadaceae. Members of these taxa are known to use elemental sulfur and ferric iron as electron acceptors, and organic compounds as electron donors. Pyrolyzed Cold Bokkeveld, however, was inhibitory to the growth of the microbial community. These results show that carbonaceous chondrites can support and select for a specific anaerobic microbial community, but that pyrolysis, for example by geothermal activity, could inhibit microbial growth and toxify the material. This research shows that extraterrestrial meteoritic material can shape the abundance and composition of anaerobic microbial ecosystems with implications for early Earth. These results also provide a basis to design anaerobic material processing of asteroidal material for future human settlement.
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Lake Xingyun is a hypertrophic shallow lake on the Yunnan Plateau of China. Its water quality (WQ) has degraded severely during the past three decades with catchment development. To better understand the external nutrient loading impacts on WQ, we measured nutrient concentrations in the main tributaries during January 2010–April 2018 and modelled the monthly volume of all the tributaries for the same period. The results show annual inputs of total nitrogen (TN) had higher variability than total phosphorus (TP). The multi-year average load was 183.8 t/year for TN and 23.3 t/year for TP during 2010–2017. The average TN and TP loads for 2010–2017 were 36.6% higher and 63.8% lower, respectively, compared with observations in 1999. The seasonal patterns of TN and TP external loading showed some similarity, with the highest loading during the wet season and the lowest during the dry season. Loads in spring, summer, autumn, winter, and the wet season (May–October) accounted for 14.2%, 48.8%, 30.3%, 6.7%, and 84.9% of the annual TN load and 14.1%, 49.8%, 28.1%, 8%, and 84.0% of the annual TP load during 2010–2017. In-lake TN and TP concentrations followed a pattern similar to the external loading. The poor correlation between in-lake nutrient concentrations and tributary nutrient inputs at monthly and annual time scales suggests both external loading and internal loading were contributing to the lake eutrophication. Although effective lake restoration will require reducing nutrient losses from catchment agriculture, there may be a need to address a reduction of internal loads through sediment dredging or capping, geochemical engineering, or other effective measures. In addition, the method of producing monthly tributary inflows based on rainfall data in this paper might be useful for estimating runoff at other lakes.
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After acidification of shallow softwater lakes as a consequence of atmospheric sulphur and nitrogen deposition, the concentrations of nitrogen and carbon dioxide in the water layer are raised and the cation reserves in the sediment are depleted. Liming can counteract acidification, but can also lead to further nutrient mobilisation. Controlled supply of calcareous groundwater is an alternative method to restore the pH and alkalinity of the water layer. Sediment removal and subsequent restoration of pH leads to a reversal of the nutrient status towards pre-acidification levels. However, rapid re-acidification may occur as a consequence of supplying water from the catchment which is acid and rich in nitrogen. After three years of repeated groundwater supply, the base saturation of the sediment was higher than that of adjacent reference lakes where the sediment had been removed simultaneously. This base saturation is possibly a key factor in the prevention of re-acidification. During a five-year period after the start of groundwater supply, CO2 and nitrogen concentrations in the water layer decreased and were not higher compared to adjacent reference lakes. Characteristic softwater macrophytes returned, but not in the reference lakes.