The relative importance of nitrogen and phosphorus
in eutrophication
Brian Moss, School of Biological Sciences, University of Liverpool.
Slide 1. Nitrogen, phosphorus & eutrophication
• Standing waters, northwest Europe
• Key issue - problems of large algal growths/reduced ecosystem diversity
• Water supply, toxic blooms, loss of conservation value. Costs 75-114 million
pounds/yr in England & Wales
• Implication for restoration of good ecological status (WFD)
Slide 2. Development of ideas
• Thienemann & Naumann, lake types, eutrophic and oligotrophic
• Pearsall, continua
• N and P naturally very scarce (retention by catchment vegetation)
• Demise of idea of natural eutrophication
• Associations between effluent discharge and algal growths
• Predictions from biogeochemistry (Hutchinson, supply to need ratio for
lithosphere)
Slide 3. Development of ideas
• Late 1960s, detergent controversy
• Much increase in research in North America
• Experimental Lake Area experiments (Canada)
• Dramatic success of effluent (P) control at Lake Washington, Alpine lakes, lately
L. Windermere
• Solid basis for importance of P in many areas
Slide 4. Development of ideas
• However, evidence of nitrogen control of phytoplankton - tropical lakes, meres,
even in 1960s, 1970s
• Increasing recognition of internal P loading in shallow lakes and of grazing
influences
• Failure to note that notable instances of P control all in upland/hard rock areas
Slide 5. Hegemony of P control
• Past emphasis mostly on water supply
• Reservoirs mostly upland. Severe problems in lowland reservoirs generally
handled by symptom treatment
• P control (UWWTD) easy at point sources, cost easily passed to community
• Arguments that N control unlikely to succeed (N fixation)
• N control very difficult - diffuse, affects mostly politically influential landowners
• Situation changing with increasing diffuse supply of P from land and with
increased emphasis on ecological quality
-1-
Slide 6. N and P and common sense
• P is biogeochemically scarce
• So is N in available form (denitrification)
• Both key elements for living organisms
• Inherently unlikely that features of natural situation of low N and P restored by
reducing P and leaving N high
• Both production and diversity to consider
Slide 7. Nitrogen - questions
• Phytoplankton - are there naturally N -limited lakes? Is N limitation increasing as
waters increasingly fertilised with P? Is the situation similar in deep cf shallow
(macrophyte-dominated) lakes?
• Macrophytes - lost conservation value usually means loss of plants - what limits
macrophyte production, diversity and community stability. Is it nitrogen?
Slide 8. Deep lake cases
• Deep lakes phytoplankton dominated
• Upland, hard rock, high water throughput: P limited, much evidence
• Lowland, drift geology, drier, often ground water fed: can be N limited
• Evidence: NW Midland meres, (correlation, bioassays, palaeolimnology
Slide 9. West Midland Meres
See ‘Relationships between N, P and chlorophyll in a set of deep meres’.
Reference Moss et al (1994). Determination of phytoplankton crops by top-down and
bottom-up mechanisms in a group of English lakes, the West Midland meres.
Limnology & Oceanography 39, 1020-1029.
Slide 10. Bioassays from Whitemere
See Table 6 ‘Summary of biomass bioassays of Whitemere phytoplankton’.
Reference Hameed et al (1999). Physiological tests and bioassays – aids or
superfluities to the diagnosis of phytoplankton nutrient limitation? A comparative
study in the Broads and Meres of England. European Phycological Journal 34, 253270.
Slide 11. Whitemere - palaeopigments
See Fig 2 ‘Palaeopigments in dated sediment core from Whitemere’.
Reference McGowan et al (1999). Ancient blue-green blooms. Limnology &
Oceanography 44, 436-439.
Slide 12. Current bioassays from West Midland Meres
See Table 4 ‘Results of bioassays for July 2000 in a group of West Midlands meres’.
Reference James et al (In press). Nitrogen driven lakes: the Shropshire and Cheshire
Meres? Archiv. Fur Hydrobiologie.
-2-
Slide 13. Shallow lakes – Alternative states hypothesis
Slide 14. Shallow lakes
• Plant dominance the key to conservation status
• Restoration strategy complex, involves nutrient control. P usually controlled
• Restorations often fail or are temporary with monospecific plant stands only
Slide 15. Shallow lake function
• Plant biomass very important
• Buffer mechanisms for clear water (denitrification, refuges, allelopathy)
• Nitrogen very scarce in summer, P often abundant through sediment release
• Terrestrial work suggests inverse correlation between N and diversity
• Macrophyte/sediment system closer to terrestrial plant/soil system than
phytoplankton/catchment water system
Slide 16. Shallow lakes - evidence of importance of N
See Figure re ‘Nutrient deficiency indicators from Ormesby & Lily Broads.
Reference Hameed et al (1999). Physiological tests and bioassays – aids or
superfluities to the diagnosis of phytoplankton nutrient limitation? A comparative
study in the Broads and Meres of England. European Phycological Journal 34, 253270.
-3-
Slide 17. Shallow lakes - evidence of N -restoration of Little Mere
Map of Meres
-4-
Slide 18. Shallow lakes - evidence of N -restoration of Little Mere
Phosphorus changes in Little Mere
Slide 19. Little Mere
• Significant fall in TP (1990-2002)
• TP = -115 (Yr) + 1231 (P<0.01, r2, 0.44)
• Post 1992 (inclusive), still significant fall (P< 0.05, r2, 0.39)
• Post 1993, no significant change
• Similar for SRP and DIN
• DIN now determined by inflow from MM
Slide 20. Little Mere
• Total Daphnia: no change with time
• Relations between total Daphnia and mean seasonal chlorophyll all inverse but
barely significant
• Strong correlation between winter DIN and growth season chlorophyll, post 1992
• Cph = 36.8 DIN -3.1, r2 = 0.36 P<0.01
• Phytoplankton sometimes controlled by grazing, sometimes by nitrogen
(mesocosm experiments)
-5-
Slide 21. Shallow lakes - evidence of importance of N
Relationships between total N and total P with macrophyte diversity in British lakes
Moss et al. Unpublished data.
-6-
Slide 22. Shallow lakes - evidence of importance of N
Relationships between nitrate and SRP and macrophyte diversity in British lakes
Moss et al. Unpublished data.
-7-
Slide 23. Changes in plant community in shallow lakes
See diagrams re Plant communities from ‘The Broads’ Harper Collins New Naturalist
2002 “You must not miss this book”
Slide 24. Indications & Conclusions
• Phosphorus remains important and needs to be controlled at least in the uplands
and in hard rock areas
• But nitrogen very important in controlling phytoplankton in lowland and drift
areas
• And nitrogen crucial in controlling plant diversity, therefore conservation status in
shallow lakes
Slide 25. Implications
• Diffuse source N control necessary in lowlands
• Good status (Water Framework Directive) unachievable without N control
• N control not possible on localised basis
• NVZ standards (10 mg/l N) much too liberal
• Zero plant diversity around 6-7 mg/l winter nitrate
• Good plant diversity needs <2 mg/l winter nitrate
-8-