Reducing phosphorus concentration in rivers: wetlands not always to the rescueBen Surridge, Catchment Science CentreLouise Heathwaite, Lancaster Environment CentreAndrew Baird, Queen Mary, University of London

Phosphorus: a life-support elementMacro-nutrient, 2-4% dry weight of most cells, mostly PO4

Constituent of DNA and RNA

Cell structure phospholipids

Cell energy ATP and ADP

Limiting primary productivityPhosphorus limitation or co-limitation of many freshwater environments

Phosphorus limitation of oceanic primary productivity?

Limiting primary productivityAt what concentration does P become limiting?

Autotrophic activity:

Individual algal species 0.001 to >0.30 mg l-1 PConfounding issues e.g. luxury uptake

Heterotrophic activity

Habitats Directive guideline 0.20 mg l-1 PUK TAG EQS under the WFD 0.12 mg l-1 P

Non-limited UK riversPhosphorus enrichment

Environment Agency (2005)

Enrichment costs you moreIncreased autotrophic growth rate and biomassShifts in community structure: macrophyte epiphytic algae benthic and filamentous algaeDamage costs ~100 million yr-1 in England and Wales (Pretty et al. 2003)

Contributors to phosphorus loads

Morse et al (2003)Defra (2004)Defra (2006)

Reducing phosphorus in rivers Range of statutory and non-statutory instruments

90% of costs of these instruments borne by water industry (Pretty et al. 2003)

UWWTD most significant discharge limits to sensitive areas of 1-2 mg l-1 P as total phosphorus

Capital expenditure: 50 million yr-1 between 2000-2005 on improved phosphorus removal

Justified water industry investment?

.butMacrophyte growth still affected by epiphytic and benthic algae

Because of compounding factors phosphorus is not the only factor affecting productivity

Because targeting WWTPs is not sufficient baseline and spikes in river phosphorus concentration

The diffuse problemEngagement changing nutrient management at source Defras CSF

Inducement nutrient management and targeted mitigation Environmental StewardshipEntry level 3.5 million hectaresHigher level 65,000 hectares

Wetlands at our service?Nutrient attenuation functionRiparian zone an effective sediment and P trap

Wetlands at our service?Drive to re-establish and create wetlands:UK BAP ~18,000 ha wetland50-year wetland vision 12% of Yorkshire and Humber study area has potential for restoring wetland habitat

A second nutrient time bomb?Riparian zones are productive agricultural land~30% of applied phosphorus removed in produce~70% remains in soil or is exported

UK floodplain sediments ~500 - >2500 mg kg-1 total phosphorus (Walling et al. 2000)

How stable is this phosphorus?Could chemical, and potentially ecological, status be affected?

Riparian wetlands in the Norfolk Broads

External nutrient loadsEnvironment Agency (2005)River YareLackford Run

Phosphorus retained in sediment

Chemical extraction of phosphorusMajority of TP present as organic P

Up to 30% of TP as inorganic P:

Ca/Mg-P pH sensitiveFe-P sensitive to redox conditions

During seasonal water table fluctuation both pH and redox change significantly

Laboratory mesocosm incubations

Simulate P release following reflooding

Surface water and pore water sampling

Analysis of sediment-P pools

MRP release to surface and subsurface

Subsurface MRP and Fe2+ release0.00.51.01.52.02.53.03.50.02.510.017.532.547.5Depth (cm)0.010.020.030.00.02.510.017.532.547.5Depth (cm)Fe2+ (mg l-1)

Stoichiometry of MRP and Fe2+ release

Comparing field and lab P concentrationLaboratoryField

P delivery to receiving waters3.923.964.004.044.080000120000001200000012000000Time (hours)Water level (mAAD)0.000.100.200.300.400.50MRP (mg l-1 P)Ditch5 mMRP

P delivery to receiving waters3.803.853.903.954.004.054.104.15319321323325327329331Julian DayWater level (m AAD)650.0750.0850.0950.01050.0MRP (mg l-1 P)Ditch5 m25 mMRP0.000.600.450.300.15

Concluding commentsWetlands may effectively remove and store phosphorus

Store is potentially soluble and therefore bioavailable

Soluble phosphorus may be delivered to adjacent aquatic ecosystems a second nutrient time bomb?

Not all wetland functions can be restored, and restoration may have negative consequences

Today, talk about the element phosphorus, one of the most significant water and land management issues facing the UK now and in the coming decades. Really a story about how we might act to reduce the amount of P reaching our rivers, and specifically about whether the service of nutrient attenuation in wetlands could help to reduce the concentration of P. To give the take home message at the start of the talk, the situation as ever is not quite as simple as we might like it to be. P a macro-nutrient, required in relatively large amounts by all organisms from primary producers including algae and higher plants, to organisms higher up the food web such as macroinvertebrates which feed on plant material and fish species which feed on the macroinvertebrates, and ultimately humans as well. About 2-4% of dry weight of cells made up of P.Why is P such an important element its a constituent of several important biomolecules that are essential for life.P essential to the sugar-phosphate backbone of DNA and RNA between which the bases sitDNA which codes for the proteins and RNA which carries the DNA messages to other parts of the cell.P also a component of phospholipids the basic building block of cell membranes. Simple representation of a phospholipid here with a hydrophobic, water hating tale composed of two fatty acid chains, and a hydophilic water loving head which includes a phosphate molecule. When in an aqueous solution phospholipids will organise themselves into a bilayer exposing the water loving heads this is the basic structure of cells creating a distinction between intra and extracellular environments. Finally, formation and destruction of high-energy ATP and ADP molecules is how cells conserve and produce energy.So P really is a fundamentally important life support element. Because it is so key to life, when there is not sufficient P available this may limit the growth rate and biomass of organisms. This particularly true for primary production.The availability of P is thought to limit or co-limit primary productivity in many freshwater environments. Simple conceptual model to explain this know that organisms as a general rule require N and P in a relatively constant ratio of 16:1. N and P may come from several sources, delivery from rivers, from sediment or from anthropogenic sources such as STW effluent. However, if the ratio in solution falls below 16:1 suggestive of N limitation, there is a further source that can rectify this drawing on the large atmospheric pool of N nitrogen fixers can increase n concentration and return the ratio to 16:1. There is no such atmospheric source for P, hence it is generally seen as the major limiting nutrient in freshwaters. N generally seen as limiting in estuarine and marine waters for various reasons dont want to get into discussion lack of N fixers, n fixation slower than in fw, denitrification more rapid. However, recently renewed debate about limitation of oceanic primary productivity climatic shifts and changes in availability of trace metals such as Fe, can increase N fixation leading to P limitation important as this suggests we should focus on decreasing P not N delivery from river to estuaries and oceans to control eutrophication in these environments.A key question for science and for management continues to be, what concentration do we need to reduce P to to being to limit primary productivity?Difficult question and not one I want to spend too long on, just to give a feeling for the type of concentrations we might be talking about.For different species of algae the limiting concentrations may vary between 0.001 and over 0.3 mg/l P as external bioavailable P concentration, but there are several complicating issues here including the retention time of water within a reach, the sediment size, grazing pressures, luxury uptake of phosphorus which can de-couple autotrophic activity from extracellular P concentration when organisms can draw on intra-cellular P stores to maintain activity even under low external P concentrations, and seasonality its not necessarily the annual average P concentration that is important in determining primary productivity, but more likely the concentration at biologically critical times of the year.Also evidence that heterotrophic activity may be limited by P - heterotrophic bacterial communities in wetlands recently shown as P limited Sundaweshar et al (2003), also nutrient enrichment experiments in streams have begun to show P limitation of heterotrophic activity including leaf degradation and bacterial activity and biomass Dodds (2006). May also in extreme cases promote a shift from autotrophic to heterotrophic dominated systems by enrichment come to this in a moment. Clearly difficult to set standards for P in light of the range of limiting P concentration and the above discussion, but indicative standards in the UK include those for special area of conservation rivers under the Habitats Directive, up to a maximum of 0.20 mg/l depending on the river typology, and proposed standards by the UK technical advisory group under the WFD of up to 0.12 mg/l again depending on river typology. What is clear though is that few rivers and streams in the UK are likely to be limited by the availability of P, or indeed of N. Most of our systems are significantly enriched with P.Long-term records of P concentration are relatively difficult to obtain but show one here for the Hampshire Avon in the south of the UK - ~45 year record showing mean annual orthophosphate concentration generally seen as the most bioavailable fraction of the total P pool, increasing from below 0.1 mg/l in the mid 1950s to over 0.3 mg/l in the late 1980sLooking at a national picture over the last 25 years, mean annual orthophosphate as an average for all rivers in various regions of england see up to the very late 1990s concentrations were above, and in some cases way above 0.5 mgl, some sign of this decreasing from 2000 onwards but still generally at or above 0.3 mg/l which if you remember from the previous slide was towards the upper limit of where algal species may be limited by the availability of P. So what are the consequences of this enrichment with P?By establishing non-limiting P concentrations in systems that were previously limited by P availability you are likely to increase autotrophic growth rate and biomass.This can lead to shifts in autotrophic community structure from macrophyte dominance through epiphytic algae to dominance by benthic and filamentous algae at high P concentrations. Change in structure can have significant consequences for higher trophic levels that depend directly on the habitat or on the trophic levels that do depend on the habitat.Significant financial consequences from nutrient enrichment here we have a blue-green or cyanobacterial bloom, which often linked to increased nutrient availability in lakes and rivers, leads to massively increased water treatment costs where PWS are affected, can also have knock-on effects following the death of the blooms and the release of their associated neurotoxins which act on nerve cells and can lead to significant fish killsThe combined effect of things such as increased water supply costs and the decreased aesthetic and recreational value of nutrient-enriched waters cost ~100 million per year in England and WalesSo if we are to address enrichment of receiving waters we must identify the sources of P which contribute. This is a major challenge in the mixed urban-rural catchments which dominate the UK.Methodological basis for conducting source apportionment is highly uncertain give 3 examples here of the variability in output from essentially the same method but using slightly different parameters in each particularly interesting is the variation between the two Defra estimates over a period of only 2 years, based solely on the parameters chosen in the source apportionment methodology doubled the contribution from domestic sources and halved that due to agriculture. In each we see two dominant sectors which contribute agriculture and domestic sources. Agriculture includes direct delivery of P due to poor land management practice see here unrestricted access for cattle to the stream, and also the removal of P in surface runoff from agricultural land either carrying P attached to soil particles or directly eroding organic and inorganic fertiliser that hasnt become incorporated into the soil matrix. Domestic includes that from sewage and from detergents essentially what comes from sewage treatment work effluent.

There are a range of instruments which have been introduced in an attempt to tackle enrichment of surface waters with P90% of costs associated with water industry interesting because depending on which source apportionment you believe the industry contributes a maximum of 60% and a minimum of 30% to the loads of P, not quite as simple of sounding sorry for the water industry because concentrations may be more significant than loads, and it generally though point sources contribute most to concentrations in biologically critical spring and summer low flow periods.UWWTD brought in in 1991, by 2000 to meet various demands in terms of reducing the P concentration in final effluent may like to link this to the decline seen in P concentration from rivers across England since 2000Operating expenditure on top of this.Is the expenditure in improved P removal at sewage treatment works justified by decreases in P concentration and in biological recovery of systems?First part of that question the answer may be yes we do often see significant decreases in P concentration following investment in STW one example to demonstrate this from the River Kennet just downstream of Marlborough STWs introduction of P stripping in the STW resulted in significant decreases in SRP concentration SRP another operationally defined form of bioavailable P.Baseline instead of being up around 0.5 mg/l was reduced to ~ 0.1 mg/l with some variability related to rainfall eventsGraph covers around 4 year period from june 1997 to june 2001However, what is frequently observed is that although significant improvements in average P concentrations are achieved through investment the recovery in biological status does not follow. So in the example of the River Kennet which is a chalk stream there was not the recovery of the macrophtyes that was hoped for, with knock-on effects for species dependent on these habitats and subsequently higher organisms. So there was not a return to a stream looking like this image, from the perspective of management for water quality there was some success in that P concentration declined substaintially, but if were managing for biological status, as in the WFD, then there remain questions about how successful measures based on more stringent consented discharges may be.Why is this occurring - So we move onto the second sector identified as contributing significantly to P loads in rivers agriculture. More difficult to tackle as there are no end-of-pipe discharges to focus on the problem is more diffuse.Looking at the type of strategies that have been implemented to try and address this problem, really two areas, firstly that we try to change nutrient management at the source of the problem moving from distributor spreading to injection of fertiliser into the subsurface to reduce export of fertiliser from the sediment surface during runoff. Defras CSF initiative aims to achieve this through engagement of the farming community, rather than through regulation or subsidy.Second strategy is one of plain old inducement, with a bit of engagement to improve uptake, which includes measure to improve nutrient management at source but also has a strong focus on targeted mitigation strategies.Umbrella concept is Environmental Stewardship and the aim is by the end of 2007 to have 60% of farmland under environmental management agreements. Range of aims of these stewardship schemes including biodiversity and rare breeds, but various parts aims at improving nutrient management on farms and reducing export from them.Two particular schemes entry level and higher level. Entry level you gain points for certain activities on the farm, such as implementing buffer strips to reduce direct input of fertiliser and pesticides to water, and if you acrue sufficient points to enter the scheme you get a flat payment of 30/hectare/yearWithin High Level Scheme payments relate to specific options chosen rather than a flat rate across the farm. Particular interest to us is that payments can be made for creating or re-establishing wetlands within agricultural landscapes restoration of wetlands gets you 60/hectare, creation gets you 380/hectare.Why are these payments being made? Based on the traditional conceptual model of wetlands as nutrient sinks, or as the so-called kidneys of the landscape.See that in these conceptual models, firstly where P in overland flow from agricultural land a major pathway for P delivery to surface water is retained within a riparian wetland due to reduced flow velocities and increased residence time, may involve sedimentation of particulate P, uptake by vegetation, and sorption of dissolved P to sediment. Also same processes have been shown to be effective in retained P from tile drains if they do not bypass the riparian zone but are re-directed to flow into the wetland.Obviously being in the riparian zone these systems also exposed to flooding from the channel where the same uptake, sedimentation and sorption processes may remove P and suspended sediment from channel water and store within the riparian wetland.Effectiveness of these processes illustrated in this example , showing up to 80 or 90 % of P and suspended sediment entering the riparian wetland may be retained within it in both summer and winter.

Partially as a result of this effectiveness of nutrient retention, large drive to re-establish and create wetlands in the UK and in Europe.Couple of examples of this UK BAP.Secondly, from EA, EN, RSPB wetland vision, map here of the potential for wetland creation and re-establishment in the Yorkshire and Humber area, showing up to 12% of this area may offer potential for restoring wetland habitat.Example here from European context, policy drive in Denmark.Make point not solely for nutrient retention purposes that drive for wetland recreation multiple services may be provided including flood risk reduction and increased biodiversity within the landscape.

Need to make point drive to restore for reason more than just WQ improvement habitat recreation a major driver BUT, many of the areas identified for wetland recreation and the areas under the Environmental Stewardship schemes that would be sites of wetland creation are and have been for many years productive agricultural land example here of the drained and intensively agricultural landscape that is true of many riparian areas particularly in the UK.Known that a P surplus builds up in agricultural soils only around 30% of the P applied in fertiliser is removed in produce meaning up to 70% is either exported or builds up in the soil.Survey of UK floodplain sediments shows typical levels between 500 and over 2500 mg kg TP. Although no real standards for sediment P quality, some evidence from work in Canada that over 2000 mg kg could be levels which cause environmental damage.So there are significant stores of P within areas which are also the focus for possible wetland recreation two questions.In other words are we facing a second nutrient time bomb to go alongside the movement of nitrate through the unsaturated zone of aquifers towards groundwater? Key questions addressed in this study.Study involved looking at P release and transport in a riparian wetland in the Norfolk broads, in the floodplain of the River Yare which runs through Norwich out to the North sea at Grt Yarmouth.See the system here, wetland complex with areas of open water, interconnected ditch network and areas of fen dominated by Phragmites.Uplands are predominantly agricultural land, wetland sits at interface between this and receiving surface waters.

Image looking downstream, few features to pick out.Lackford Run a tributary of the River Yare.Although embanked, over-bank flooding events frequently input water, sediment and solutes from Yare and Lackford Run.Finally an interconnected ditch network with a managed connection to the Yare allow ingress of river water into the wetland and drainage of wetland water back into the river.Strumpshaw is a predominantly natural system no significant drainage or intensive agricultural landuse. Why were we interested in P dynamics in this system what was the source of P that may become mobilised?Source come from both the Yare and Lackford Run have orthophosphate concentrations in these two systems over last 25 years or so.First for the Yare, high concentrations, consistently above 0.5 mg/l with peaks of 4 mg l. Note the seasonal pattern in orthophosphate concentration.Secondly for the LR, consistently over 2 mg/l until closure of Strumpshaw STW in 1997 after which concentrations have decreased dramatically. Have these high external P concentrations lead to a store of P within the sediments at Strumpshaw?Series of cores taken from across the wetland system, sectioned at 5cm intervals and analysed for TP. See here depth profiles showing average TP plus/minus 1 standard deviation.Enrichment of surface 15cm quite obvious concentrations up to 1400 mg/kg, concentrations decline steadily at greater depth.In most wetlands not exposed to external nutrient loading, concentrations up to 500 mg/kg common.

Further analysis of the forms of P held in the sediment on the basis of chemical solubility TP tells us very little about specific form in which P is held in the sediment and therefore about the likely stability of the PLarge amount stored as organic P most in stable non-bioavailable forms likely contained in residual organic matter.However, up to 30% of total P as inorganic P, dominated by Ca/Mg and Fe bound phases.Significant because we know that during the shifting water table conditions that exist in riparian zones pH and redox conditions may change significantly. Strumpshaw system WT varies between 50 cm below ground surface in late summer to 50 cm plus of standing surface water for 9 months of the year.Location of sediment cores taken for initial P analysis.From this zone took a further set of cores to examine P release under changing WT position. Large intact cores of surface 50 cm of peat, collected during summer drawdown period when WT below ground surface.Reflooded during laboratory incubations to simulate reflooding in the field, collected surface water and pore water samples following reflooding, and analysed changes in sediment-bound P pools2 sets of incubations flooding medium changes, DI in one, simulated river water in another.Figure shows release of P to surface water of triplicate cores flooded with DI, using operationally defined MRP term which for our purposes same as orthophosphate.Rapid and sustained release of P to surface water beginning immediately after inundation. End of 700 hour (28day) incubation concentrations around and above 0.8 mg/lThis type of release been seen before in studies of lake-bottom sediments and some wetland sediments. What not seen before is release to the subsurface environment shown in this figure depth profiles of MRP in pore water within the cores thing to note at this stage is the very high concentrations reached at the end of the incubation in these examples over 2.5 mg/lFigures show release of MRP and Fe to the pore water of the same core.First 24 hours there was only a small amount of MRP release to the shallow pore water, not matched by any release of Fe.Major releases of MRP began between 24 and 72 hours after inundation and occurred alongside release of ferrous iron Fe2+. Not just increased periods of time between samples, but the actual rates of release increased dramatically.Same pattern repeated in all other cores

Important to say rates of p release also increase not just more time between samples.Make the point just one example here, other 2 replicates very similar resultsFigure shows correlation between molar concentrations of MRP and Fe in all pore water samples from the core incubations.Dominant MRP to Fe ratio of 0.45 consistent with dominant P to Fe molar ratio of ferric hydroxide which is around 0.5. Suggested that if this ratio is greater than 0.33 then insufficient Fe present to remove all P during delivery of anoxic pore water to an oxic environment and precipitation of Fe3+ - important for potential of wetland pore water to influence the nutrient status of adjacent oxic rivers and streams.

i.e. fe cannot remove all p on delivery to oxic environmentTo corroborate our lab findings we looked at concentrations of P in pore water from piezometers installed in the field within the range of concentration you would expect given that field samples had been inundated for a period of months rather than 28 days there was good agreement in magnitude of P concentration and depth profiles.Final thing to look at was whether the P that was released to pore water was able to move from the sediment into adjacent surface water systems.Instrumented three transects running from a section of the ditch network into the fen shown here. Transects had water level probes to allow us to record changes in water table gradients that would drive flow between the fen and the ditch in this shallow aquifer, and in the ditch we had an automatic water sampler collection samples for analysis of MRP.Give a couple of examples of the type of exchange patterns we observedFigure here has water level on left hand axis standardised to being above an arbitrary datum, MRP concentration in the ditch samples on the right hand axis, and time along the x axis a 3 day period.First example covers a tidal pulse moving from the Yare through the ditch network. Ditch water level record 3 significant periods, firstly one where water level is falling, secondly the clear tidal pulse, and lastly a prolonged period of water level decline. Remember a managed system and only one pulse was allowed through the control structure where the ditch network joins the Yare.Secondly have water table in the fen 5 m away from the ditch, again 3 significant periods of change, firstly a drop in water table, secondly a peak slightly delayed and damped compared to that in the ditch, and thirdly a longer period of decline. In terms of water table gradients driving flow, where green line is above brown the gradient is driving pore water from the fen into the ditch and vice versa when brown is above green.Here is the record of MRP in the ditch during the very start of the record relatively constant concentration around 0.05 mg/l, first period of hydraulic gradients driving flow into the ditch MRP increases significantly reaching around 0.15 mg/l just prior to the tidal pulse. Pulse of MRP passes through the ditch network with the tidal pulse, reflects the presence of nutrient enriched water from the Yare which had a concentration around 0.5 mg/l at this time. Important to note this enriched river water was transported into the fen, thereby continuing enrichment of the near-surface deposits.During final period when we had gradients driving flow into the ditch MRP concentration again increased reaching 0.25 mg/lClear evidence here that Fe concentrations are not sufficient to precipitate all MRP on delivery from anoxic pore water to oxic ditch environment, and that MRP released from the wetland sediments can lead to changes in the equilibrium MRP concentration in adjacent aquatic ecosystems.

Evidence supports idea that insufficient Fe to precipitate all P when entering oxic environmentSecond example, one where ditch network was used as a management tool to lower water tables across the entire wetland. 7 to 8 day period when ditch water was allowed to drain into the river leading to falling water tables within the fen, but again to water table gradients driving flow from the fen into the ditch.MRP concentrations increased significantly in the ditch during this period, reaching around 0.5 mg/lSo to finish with some concluding comments, firstly its true that wetlands may effectively remove and store P the large store of P present at Strumpshaw Fen is most likely to have come from retention of P delivered from the Yare and the LR. In this respect wetlands may be the so-called kidneys of the landscape.However, our work has shown that at least some of this store may become soluble under inundated conditions, and this significantly increases its bioavailability.Also shown that this P may be transported to adjacent ditch networks and lead to increases in equilibrium P concentration given our apparent drive to re-create wetlands in many places maybe this should be seen as a second potential nutrient time bomb.Need to think carefully about what we trying to achieve by re-creation or restoration not all of the functions of wetlands can necessarily be recreated and there may be some negative consequences. Essentially we have to decide which functions are most important for us, and over what timescales because this nutrient release may be a short term phenomena but other work has shown P release to continue several years after restoration.