Impact Type: Deposition of pollutant
Key Concerns:
Phosphorus has generally been considered more important than nitrogen in determining the biomass of phytoplankton and the trophic state of the system as a whole (Schindler 1977, Schindler 1978, OECD 1982).
Schindler (1977) argued that nitrogen fixation by heterocystous cyanobacteria can augment the nitrogen supply, making nitrogen limitation an infrequent event. N-fixation is, however, very energy intensive, and also requires a number of other conditions favourable to heterocystous cyanobacteria to be met. It is, therefore, by no means a definite response to N-limitation.
Nitrogen limitation, or co-limitation with phosphorus, is now considered more widespread (Elser et al. 1990), particularly in two circumstances (Wetzel, 2001):
- In eutrophic conditions with high phosphorus loadings (rarely naturally due to P-rich apatite minerals in catchment, more commonly due to eutrophication).
- In oligotrophic waters, particularly in mountainous regions or high latitudes where both phosphorus and nitrogen are naturally in short supply.
The N:P ratio may also be critical in determining which phytoplankton or aquatic macrophyte species predominates (Kilham 1990, Prairie et al. 1989). In natural situations, phosphorus and nitrogen inputs from a catchment do generally increase together, but this is not necessarily so when pollution is a significant source of one, or both, nutrients (Forsberg & Ryding 1980, Prairie et al. 1989).
Additional Comments:
A critical load cannot be given for nitrogen, as quantitative relationships between biology and nitrogen concentrations are poorly understood. The nitrogen to phosphorus ratio can be important, with a molar ratio of around 16:1 (7:1 by weight) being the threshold between N- and P-limitation (Wetzel 2001). Impacts could be assessed by deviation from a 'natural' ratio for an individual site.
Habitat Specific Information
Eutrophic standing waters
Deposition of ammonia, nitrate and other forms of nitrogen from the atmosphere is unlikely to be the largest source of this nutrient to eutrophic standing waters (Gibson et al. 1992, Gibson et al. 1995, Jordan 1997) and, therefore, in general, N deposition is unlikely to be very harmful to eutrophic standing waters, even when close to sources.
Mesotrophic standing waters
Deposition of ammonia, nitrate and other forms of nitrogen from the atmosphere could be an important source of this nutrient in mesotrophic standing waters (Gibson et al. 1995). Detailed nitrogen budgets of mesotrophic lakes, however, do not exist, so the relative inputs from atmospheric deposition are unknown.
Oilgotrophic standing waters
Deposition of ammonia, nitrate and other forms of nitrogen from the atmosphere is likely to be an important source of nitrogen for oligotrophic standing waters. Detailed nitrogen budgets of oligotrophic lakes, however, do not exist, so the relative inputs from atmospheric deposition are unknown. Isoetid plant communities characteristic of oligotrophic lakes (Littorella uniflora, Lobelia dortmanna and species of Isoetes), have been shown to be sensitive to acidification associated with enhanced nitrogen deposition. These plants decline in abundance in very acid waters (below 4.5) due to competitive effects of epiphytes, J. bulbosus and/or Sphagnum species (Roelofs 1983). Because of its faster growth rate, J. bulbosus is particularly competitive in shallow waters when acidification associated with enhanced nitrogen deposition occurs, as has occurred in many sites in the Netherlands (Arts et al., 1990; Roelofs 1983).
Aquifer-fed waterbodies (including Turloughs)
Deposition of ammonia, nitrate and other forms of nitrogen from the atmosphere can be an important source of this nutrient in oligotrophic and mesotrophic standing waters (Gibson et al. 1995, Jordan 1997). If, therefore, the aquifer fed water is oligotrophic or mesotrophic, N deposition could be harmful to these waterbodies. However, risks to eutrophic aquifer fed waters from N deposition are likely to be very small. The distinguishing feature of a turlough is the seasonal changes in lake water level as a result of changes in the level of the groundwater table; they can completely dry-up in summer. The main impacts on communities in turloughs are from these seasonal changes in lake water level.
| Habitat/ Ecosystem Type | Eunis Code | Critical Load/ Level | Status | Reliability | Indication of exceedance | Reference |
|---|---|---|---|---|---|---|
| Permanent oligotrophic lakes, ponds and pools (including soft-water lakes) | C1.1 |
3-10 kg N ha-1 year-1 The lower end of the range is intended for boreal and alpine lakes, and the higher end of the range for Atlantic softwaters. Site specific advice should be sought from the conservation agencies as to which part of the range is relevant. Note that the critical load should only be applied to oligotrophic waters with low alkalinity with no significant agricultural or other human inputs. |
UNECE 2010 - Noordwijkerhout workshop | reliable |
Change in the species composition of macrophyte communities, increased algal productivity and a shift in nutrient limitation of phytoplankton from N to P |
472 |
| Permanent dystrophic lakes, ponds and pools | C1.4 |
3-10 kg N ha-1 year-1 The lower end of the range is intended for boreal and alpine lakes, and the higher end of the range for Atlantic softwaters. Site specific advice should be sought from the conservation agencies as to which part of the range is relevant. Note that the critical load should only be applied to oligotrophic waters with low alkalinity with no significant agricultural or other human inputs.” |
UNECE 2010 - Noordwijkerhout workshop | expert judgement |
Increased algal productivity and a shift in nutrient limitation of phytoplankton from N to P. |
472 |