APPENDIX G
IMPACTS ON THE AQUATIC ENVIRONMENT
D.G. George
Centre for Ecology and Hydrology, Windermere, Cumbria.
Introduction
The concentration of suspended sediment deposited in lakes and rivers has a significant effect on their physical, chemical and biological characteristics. Here the likely impact of the measured erosion rates on the dynamics of lakes and rivers in the UK is assessed. The overview is based on a review of the recent literature and pays particular attention to the effects of increased sediment loads on the dynamics of freshwater fish. In the 1970's, the Institute of Freshwater Ecology conducted a detailed study of the impact of upland erosion on a headwater stream in the Pennines (Crisp and Robson, 1979) but more recent studies have been centered on a catchment in the west of Ireland (George and Rouen, 2000).
The general effects of high sediment concentrations on lakes and rivers.
In lakes, the most important effects of high sediment concentrations are those that relate to the attenuation of light. High levels of suspended sediment can significantly reduce the photosynthetic rate of suspended plankton and the deposition of sediment in the shallow littoral can have an even more pronounced effect on the growth and survival of periphyton. Any increase in the amount of allochthonous material present in suspension will reduce the amount of light available for photosynthesis, but these effects only become important when the concentrations remain high for several weeks in the year. Very high sediment loads will also have an effect on the growth and survival of benthic animals and the storage capacity of any lakes used for water supply. The erosion recorded in the Welsh lakes were of no practical significance but the highest rates recorded in the Pennines will shorten the life expectancy of the reservoirs and increase the cost of filtering the water. White et al (1997) estimate that that the erosion of peat from a number of catchments managed by Yorkshire Water has cost £74 m for the construction of new reservoirs to compensate for the progressive loss in storage capacity. The chemical effects of increasing the amount of allochthonous material washed into lakes and reservoirs have received rather less attention than the more obvious physical effects. Some recent studies (Sharpley et al., 1995) have, however, quantified the effects of increased erosion on the leaching of phosphorus and concluded that the amount of phosphorus entering a water body can increase dramatically when land is cultivated. The quantities of phosphorus leached from agricultural land ARE primarily controlled by the relative balance of the surface and subsurface flow. The loss of phosphorus in subsurface flow is lower than that in overland flow so any factor that increases the rate of surface discharge will also increase the annual load of phosphorus to the receiving waters. Since most of the water bodies included in our analyses were located in upland areas the trophic effects of the calculated erosion rates will be very low. Tipping (this appendix) has, however, shown that over 80 % of the total P leached from an upland catchment in the Pennines was associated with the episodic transport of particulates.
In rivers, the most important effects of a high sediment load are those that relate to the behaviour, growth and survival of invertebrates and fish. A number of investigators have described the effects of short-term changes in the sediment load on a selected range of aquatic species but there is still some debate regarding the best methods of quantifying these impacts. Much of the historical data collated on the response of individual species is based on the concentration of material present in suspension. More recent studies have, however, shown that these responses are influenced by the duration of exposure and suggest that such effects should be measured by a stress index that is based on a concentration-duration response. A large number of papers have been published on the effects of suspended sediments on salmonid fish but much less is known about the long-term response of aquatic invertebrates. Early work on the influence of inorganic sediment on aquatic life has been reviewed by Cordone and Kelly (1961) whilst a more up-to date review has been produced by Newcombe and McDonald (1991). Some of the general information given in Hellawell (1986) is of greater relevance to the UK whilst Alabaster and Lloyd (1982) have produced a comprehensive account of the impact of suspended sediment The general conclusions reached by Alabaster and Lloyd are of particular interest to the present study since they deal with the potential effects of increased sediment loads on the maintenance of freshwater fisheries. They note that salmonid fisheries can be affected by inert sediment (1) killing the fish either directly by reducing their growth rate or indirectly by reducing their resistance to disease; (2) interfering with the development of eggs and larvae; (3) modifying the natural movements and behaviour of the fish and (4) reducing the abundance of food available to the fish. Different species of invertebrates and fish respond differently to increased loads of suspended solids and there is evidence that not all kinds of sediment are equally harmful. Unfortunately, the critical concentrations quoted in the literature often cover a very wide range and it is often not clear how the risk of damage has been defined or quantified. Table 1 summarises some of the critical suspended solids concentrations reported in the literature. The invertebrate limits quoted are exceptionally wide and are often based on the responses of the whole community rather than individual species. The fish values defined are also highly variable but the results demonstrate that eggs and young fish are particularly susceptible to the effects of increased sediment loads. Results of this kind led Alabaster and Lloyd to conclude that:
- Concentrations of suspended solids of less than 25 mg l-1 would have little effect on fish.
- Reasonable numbers of fish would survive and breed in water containing between 25 and 80 mg l-1 of suspended solids.
- Waters containing between 80 and 400 mg l-1 of suspended solids are unlikely to support sustainable populations of freshwater fish.
Much, of course, depends on the nature of the particles in suspension and the duration of any extreme events. Several hundred mg l-1 of solids might not kill fish over several hours but if this sediment load was deposited in key spawning areas the long-term survival of the population could be at risk. The sediment concentrations recorded at the sites sampled during this study were well below the critical levels suggested by Alabaster and Lloyd. The maximum concentration recorded in the Pennine streams was 50 mg l-1 which can be compared with the historical maximum of 100 mg l-1 quoted by Crisp and Robson (1979) in their long-term study of peat erosion in a Pennine headstream. Very few concentration / duration curves have been reported in the literature, but Alonso et al. (1996) have simulated the potential effects of siltation on the survival of salmonid eggs in the Columbia River (northwestern United States).
During base flow suspended sediment content in the water column of streams is minimal. However during spates, or events on the catchment surface generating sediment, the wash load of suspended silt (0.2-60 µm), clay(<0.2 µm), fine sand (60 µm-0.2 mm)-sized particles and aggregates, plus organic matter, increase dramatically. Turbid water intrudes into the interstices of bed gravels and any redds. When the spate abates velocity falls and there is insufficient energy to remove the intruded wash load, which then deposits. This obstructs subsequent flow, and therefore delivery of dissolved oxygen. Additionally the intruded material may include organic matter with an oxygen demand competing with that of the eggs or fry. Further events compound the intrusion of fine material and the threat to the salmonids. This process is insidious and not readily seen. To illustrate this process Theurer et al. (1998), used the Sediment Intrusion and Dissolved Oxygen (SIDO) computer model developed by Alonso et al. (1996) for chinook salmon in the Colombia basin. Applying their values, (with the reservation that they are based on North American data), is sobering. A suspended sediment load of 50 ppm, once intruded into spawning redds, would reduce the dissolved oxygen to critical levels in little over 10 days. Lethal levels would be reached in 140 days.
Table 1. Summary of critical suspended sediment concentrations noted in the literature. Most of the references are taken from a review published by Newcombe and MacDonald in 1991.
Species or Group / Critical Concentration(mg l-1) / Effect / Source
Zooplankton / 24 / Reduced capacity to assimilate food / McCabe and O'Brien (1983)
Cladocera / 82-392 / Survival and reproduction / Robertson (1957)
Zoobenthos / 10-15 / Reduction in standing crop / Rosenberg and Snow (1977)
Benthic invertebrates / 62 / 77% reduction in population size / Wagener and LaPerriere (1985)
Nett-spinning caddis / 21-250 / Reduction in standing crop / Gammon (1970)
Stream invertebrates / 130 / Reduction in species diversity / Nuttall and Bielby (1973)
Brown Trout / 110 / 98% mortality of eggs / Scullion and Edwards (1980)
Brown Trout / 1040 / 85% reduction in population size / Herbert and Merkens (1961)
Rainbow Trout / 50 / Reduction in growth rate / Herbert and Richards
(1963)
Rainbow Trout / 100 / Avoidance response / Suchanek et al (1984)
Rainbow Trout / 500 / Physiological ill effects / Redding and Shreck (1980)
Coho Salmon / 100 / 45% reduction in feeding rate / Noggle (1978)
Coho Salmon / 509 / 50% mortality of smolts / Stober et al. (1981)
Coho Salmon / 1200 / 50% mortality of juveniles / Noggle (1978)
References
Alabaster, J. S., and R. Lloyd. 1982. Finely divided solids. Pages 1-20 in J. S. Alabaster and R. Lloyd, editors. Water quality criteria for freshwater fish, 2nd edition. Butterworth, London.
Alonso, C.V., Theurer, F.D. and D.W. Zachmann 1996. Sediment intrusion and dissolved oxygen transport model-SIDO. Technical Report No. 5, USDA-ARS National Sedimentation Laboratory, Oxford, Mississippi, 269 pp.
Cordone, A. J., and D. W. Kelly. 1961. The influences of inorganic sediment on the aquatic life of streams. California Fish and Game, 47:189-228.
Crisp, D.T. and Robson, S. 1979. Some effects of discharge upon the transport of animals and peat in a North Pennine Headstream. Journal of Applied Ecology, 16, 721-736.
Gammon, J. R. 1970. The effect of inorganic sediment on stream biota. U.S. Environmental Protection Agency, Water Pollution Control Research Series 18050 DWC 12/70. U. S. .Government Printing Office, Washington, D.C.
George, D.G. and Rouen, M.A. 2000. The use of automatic monitoring and dynamic modelling for the active management of lakes and reservoirs. Report to the European Commission (LIFE Programme). 36 pp.
Hellawell, J.M. 1986. Biological indicators of freshwater pollution and environmental management. Elsevier, London, 546 pp.
Herbert, D. W., and J. C. Merkens. 1961. The effect of suspended mineral solids on the survival of trout. International Journal of Air and Water Pollution, 5, 46-55.
Herbert, D. W., and J. M. Richards. 1963. The growth and survival of fish in some suspensions of solids of industrial origin. Journal of Air and Water Pollution, 7, 297-302.
McCabe, G. D., and W. J. O'Brien. 1983. The effects of suspended silt on the feeding and reproduction of Daphnia pulex. American Midland Naturalist, 110, 324-337.
Newcombe, C.P. and D.D. MacDonald, 1991 Effects of suspended sediments on aquatic systems. North American Journal of Fisheries Management, 11, 72-82.
Noggle, C. C. 1978. Behavioral, physiological and lethal effects of suspended sediment on juvenile salmonids. Master's thesis. University of Washington, Seattle.
Nuttall, P. M., and G. H. Bielby. 1973. The effect of china-clay wastes on stream invertebrates. Environmental Pollution, 5,77-86.
Redding, J. M., and C. B. Schreck. 1980. Mount St. Helens ash causes sublethal stress responses in steelhead trout. In Symposium on Mount St. Helens: effects on water resources. Portland, Oregon. (Pages and publisher not known.)
Robertson, M. 1957. The effects of suspended material on the productive rate of Daphnia magna. Publications of the Institute of Marine Science, University of Texas, 4, 265277.
Rosenberg, D. M., and N. B. Snow. 1977. A design for environmental impact studies with special reference to sedimentation in aquatic systems of the Mackenzie and Porcupine river drainages. Pages 65-78 in Proceedings of the Circumpolar Conference on Northern Ecology. National Research Council, Ottawa.
Scullion, J. and R.W. Edwards. 1980. The effects of pollutants from the coal industry on fish fauna of a small river in the South Wales coal field. Environmental Pollution, Series A, 21, 141-153.
Sharpley, A.N., Hedley, M.J., Sibbesen, E., Hillbricht-Ilkowska, A., House, W.A. and L. Ryszkowski 1995. Phosphorus transfers from terrestrial to aquatic ecosystems. Pages 171-199 in H. Thiesen, ed. Phosphorus in the global environment, Wiley, Chichester.
Stober, Q. J., B. D. Ross, C. L. Melby, P. A. Dinnel, T.H. Jagielo, and E. O. Salo. 1981. Effects of suspended volcanic sediment on coho and Chinook salmon in the Toule and Cowlitz rivers. Fisheries Research Institute, University of Washington, Technical Completion Report, FRI-UW-8124, Seattle.
Suchanek, P. M., R. P. Marshall, S. S. Hale, and D. C. Schmidt. 1984a. Juvenile salmon rearing suitability criteria. Alaska Department of Fish and Game, Susitna Hydro Aquatic Studies, Report 2, Part 3, Anchorage.
Theurer, F.D., Harrod, T.R. and Theurer, M. (1998). Sedimentation and Salmonids in England and Wales. EA R&D report P194
Tipping,E., Simon B.M. and Lawlor, A.J. (This appendix) Stream suspended sediment data and reservoir sedimentation rates
White, P. Labadz, J.C. & Butcher, D.P. (1997) Reservoir sedimentation and catchment erosion in the Strines catchment, UK. Phys. Chem. Earth, 22, 321-328.
APPENDIX G - IMPACTS ON THE AQUATIC ENVIRONMENT