Ozone :: Freshwater


Effects and Implications

  • Visible leaf injury and/or premature leaf die-back, which could subsequently result in reduced growth during a growing season;
  • Reduced growth of sensitive species;
  • Potential alterations in numbers and timing of flowering and seed production;
  • Potential enhanced susceptibility to pests and diseases.

Habitat structure (of the plant community) could be altered via changes in growth of species, particularly as the extent of growth reduction would vary between species. 

Overview: evidence, processes and main impacts

Effects of ozone on vegetation (vascular plants) have been studied using both individual species and plant communities, although many studies have been short-term (< 3 years). Ozone is very reactive and the reactive oxygen (molecular) species that form as ozone enters a leaf can quickly cause damage to cell components e.g. cell membranes. Reactive oxygen species are also produced by plant cells in many processes involved in growth and development and from abiotic stresses, e.g. high light. This is one of the reasons that some symptoms of ozone pollution are not specific to ozone.

Few studies have been carried out specifically using freshwater species. Although submerged plants are not anticipated to be directly affected by ozone pollution, vascular plants of freshwater habitats may be vulnerable. Ozone exposure studies have shown that ozone-specific visible leaf injury symptoms can occur for some species e.g. Eriophorum angustifolium (Mortensen, 1994, Hayes et al., 2006). In addition, early and enhanced leaf die-back has been observed in Anthoxanthum odoratum (Dawnay and Mills, 2009) and Juncus subnodulosus (Williamson, 2009). Studies from grasslands have shown that species can show reductions in biomass in response to elevated ozone and alterations in partitioning of biomass between different parts of the plant, for example leaf biomass can sometimes be maintained at the expense of roots, particularly if leaves are damaged following ozone exposure and new leaves are developed to replace these.

Elevated ozone has also been shown to have carry-over effects the following spring in species that did not respond to summer ozone exposure, with changes in growth and flower number observed (Hayes et al., 2006). Reduced flower numbers and an earlier date of peak flowering with increasing ozone exposure have been observed for some species (Hayes et al., 2012). In addition, ozone can affect seed production, viability and germination. These effects may also occur for plant species of freshwater habitats but evidence for freshwater species is lacking. There is a wide range in sensitivity to ozone of the component species of communities, indicating that elevated ozone conditions could contribute to changes in species composition. 

A concentration-based critical level was established for the protection of the vitality of pasture and fodder quality in productive perennial grasslands, and the vitality of natural species in perennial grasslands of high conservation value. This critical level is also considered to be applicable to other natural vegetation communities and it is considered that the critical level may also be appropriate for vascular plants in freshwater habitats (see critical levels table), although there is currently no specific information on ozone critical levels for this habitat.

More recently it has been shown that the impacts of ozone depend on the amount of the pollutant reaching the sites of damage within the leaf and stomatal (leaf pore) flux-based critical levels were developed to address this. The flux-based critical levels for effects on (semi-)natural vegetation are based on flux to the upper canopy leaves of individual species frequently found in grassland communities across Europe, and are also considered to be relevant to (vascular) vegetation of freshwater habitats (see critical levels table). 

Pollutant type and risk

Ozone is a naturally occurring chemical in the lowermost layer of the Earth’s atmosphere. Natural sources of the precursors of ozone such as oxides of nitrogen and non-methane volatile organic compounds ensure that there is always a background concentration of ozone.  Additional ozone is formed from complex photochemical reactions of precursors, which include NOx, carbon monoxide and non-methane volatile organic compounds, released due to anthropogenic emissions. Ozone concentrations are usually highest in rural and upland areas downwind of major conurbations.  However, some regions with more moderate ozone concentrations also have high risk of effects where climatic conditions favour ozone uptake by the plants.

Indicators of ozone impacts

These can be difficult to identify in ‘field’ conditions. Often the symptoms of ozone injury are those of a general stress response, and in addition a rapid turnover of damaged leaves can make attribution to ozone pollution difficult.  Some species exhibit ozone-specific visible leaf injury symptoms in controlled studies and these ‘typical’ symptoms of visible leaf injury attributed to ozone have occasionally been observed in natural conditions

Examples of species specific responses

Some examples of specific responses are given in the table below. This does not represent a comprehensive review of all species impacts.




Anthoxanthum odoratum

Premature leaf senescence

Dawnay and Mills, 2009

Juncus subnodulosus

Increased leaf senescence (dieback)

Williamson, 2009

Sphagnum (angustifolium, magellanicum and papillsum)

Decreased chloroplast area within cells of the capitulum

Rinnan et al., 2004

Eriophorum angustifolium

Visible leaf-injury

Mortensen, 1994, Hayes et al., 2006

What factors modify ozone impacts?

Climatic (e.g. temperature, humidity) and soil factors (e.g. water content) that influence stomatal uptake of ozone can influence ozone impacts in some species (see above section on stomatal fluxes). Vascular freshwater species may be more sensitive to ozone pollution in field conditions than some species of drier habitats because soil moisture is rarely limiting in these communities and therefore stomatal pores remain open, allowing uptake of more ozone.

Critical levels

Flux-based critical levels (Mills et al., 2011b, LRTAP Convention, 2011)



(per cent reduction)



Critical level

(mmol m-2 PLA4)

Conservation grasslands (based on clover)1

Biomass (10%)



Concentration-based critical levels (LRTAP Convention, 2011)






Critical level

(ppm h)

(Semi-)natural vegetation communities dominated by perennials1

Growth reduction



1 Considered to be applicable for vascular plants of freshwater habitats too.

2 POD1 = Phytotoxic Ozone Dose above a flux threshold of 1 nmol m-2 PLA s-1.

3 AOT40 = Accumulated ozone above a threshold of 40 ppb during daylight hours.

4 PLA = Projected leaf area.

Environmental limit: 

Habitat/ Ecosystem Type Critical Load/ Level Status Reliability Indication of exceedance Reference
Semi-natural vegetation

AOT40 3000ppb hours over 3 months or AOT40 5000ppb over 6 months

UNECE, 2010 expert judgement i.e. only limited or no data are avaliable for this type of receptor

AOT40 is the Accumulated concentration Over a Threshold of 40 ppb. If an hourly average ozone concentration exceeds 40 ppb the difference between the concentration and 40 ppb is added to a running total. The units are therefore ppb multiplied by hours. For natural vegetation, the AOT40 is summed for the daylight hours for a period of three months. Daylight hours are defined as when solar radiation exceeds 50 W m-2. The daylight hours are when plant stomata are normally open.

Flux-based critical levels, based on biomass reduction, are also available for local and regional assessment but are not yet incorporated into APIS. See critical levels chapter of the UNECE Mapping Manual.



Hayes, F.; Mills, G.; Williams, P.; Harmens, H. ; Buker, P. 2006 Impacts of summer ozone exposure on the growth and overwintering of UK upland vegetation. Atmospheric Environment 40 4088-4097
Hayes, F.; Williamson, J. ; Mills, G. 2012 Ozone pollution affects flower numbers and timing in a simulated BAP priority calcareous grassland community. Environmental Pollution 163 40-47
Mills, G.; Pleijel, H.; Braun, S.; Buker, P.; Bermejo, V. ; Calvo, E. ; Danielsson, H. ; Emberson, L. ; Fernandez, Gonzalez ; Grunhage, L.; Harmens, H. ; Hayes, F.; Karlsson, P.E.; Simpson, D. 2011 New stomatal flux-based critical levels for ozone effects on vegetation. Atmospheric Environment 45 5064-5068
Mortensen, L. 1994 The Influence of Carbon Dioxide or Ozone Concentration on Growth and Assimilate Partitioning in Seedlings of Nine Conifers. Acta Agriculturae Scandinavica, Section B — Soil & Plant Science 44 157-163