Wetlands Ecology and Management vol. 2 no. 1/2 pp. 51-53 (1992) SPB Acadermc Pubhshing by, The Hague

Potential effects of greenhouse warming on freshwater systems in the U.K. with special reference to coastal lowlands

Brian Moss Department of Environmental and Evolutionao' Biology, Universi~ of Liverpool, Liverpool L69 3BX, U.K.

Keywords: Greenhouse warming, sea level rise, coastal lowlands, freshwater systems, U.K.

Abstract

There are two major potential effects of rising atmospheric temperature on freshwater wetlands: direct effects, and indirect effects of rising sea level. Direct effects are varied, but muted by the high specific heat of water and the key dependence of freshwater production on nutrients rather than temperature. However, there may be effects on lake stratification, fish distribution and restoration of lakes by biomanipulation. Rising sea levels have the potential to alter lowland riverine lakes very considerably by saline influx. A holistic, in- tuitive view is more likely to produce realistic scenarios for the future than one based on reductionism.

Introduction

The high specific heat of water will mute seasonal fluctuations following the rise in temperature that inland water bodies wilt experience when air tem- peratures rise by perhaps several degrees over the next century. In one sense this makes it likely that ecosystem changes will be less marked in a given place then they might be for terrestrial communi- ties, but does not mean that such changes can be discounted. Conversely, the effects of rising tem- perature on sea level are likely to be very consid- erable for the wetlands at the heads of estuaries in lowlands, particularly in countries bordering the southern North Sea basin where land subsidence is already a problem. This paper reviews first the pos- sible effects of rising temperature per se, then those of rising sea level. Effects of changing precipita- tion could also be marked but will depend on local hydrology. The term 'wetlands' has been used to include lakes, because the two systems are really parts of a single whole.

Temperature effects

There are many processes in lakes and wetlands which are significantly affected by temperature, but for most of them other factors are normally limiting (e.g. light or nutrient availability) so that a small rise in temperature may have little direct ef- fect. However, temperature rise may have indirect effects.

For deep lakes in the U.K., summer thermal stratification is usually of the warm monomictic type (Moss 1988). The lakes stratify directly in summer but the thermocline is often unstable and may partly break down during cool weather. In winter, prolonged inverse stratification is rare, so that greenhouse warming will be unlikely to change the seasonal pattern of stratification. In summer, however, it could stabilize the direct strat- ification, minimizing turbulent upward mixing of hypolimnion water, and nutrients, and reducing summer phytoplankton production. It may also in- tensify deoxygenation of the hypolimnion. These

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effects are unlikely to have major importance ex- cept potentially in reducing the favourability of the habitat for cold water, high oxygen-demanding fish like Coregonus spp. and salmonids. Presently in Britain the coregonids are confined to large lakes in the English Lake District, Wales and Scot- land. There is a fishery for char (Salvelinus wil- loughbii) in Lake Windermere, England (Macan 1970) but other species are rare or of already re- duced distribution (Maitland 1974).

Fish recruitment and growth in general could be improved by a rise in temperature. Brown trout do not reach their maximum growth in Britain, largely as a result of sub-optimal temperatures (Edwards et al. 1979), and the size of year classes is often a function of temperature in the first few weeks after hatching. This may be favourable to anglers, but may have undesirable effects for the rest of the eco- system. Young fish are usually zooplanktivorous and selectively remove algal grazers such as Daph- nia spp. At present the husbandry of Daphnia spp. is being seriously pursued as a way of restoring, by biomanipulation, lakes so extensively altered by eutrophication as to be un-restorable by chemical means alone (Shapiro 1980). Daphnia spp. are se- lective grazers and cannot effectively filter certain groups of phytoplankton. These include large col- onies and filaments, and particularly blue-green al- gae (Cyanophyta). Cyanophyta in general often have high temperature optima and although their occurrence in lakes is probably controlled more by the water chemistry than the temperature, it is pos- sible that their proportional representation in the phytoplankton of some lakes could be increased (Reynolds 1984).

High phosphorus concentrations favour blue- green algae that can fix nitrogen. In shallow lakes, release of phosphate from sediments may be a ma- jor source of phosphorus in summer. Decomposi- tion is often strongly temperature linked (Osborne and Phillips 1978). Thus in three ways - through Daphnia spp. reduction, blue-green algae growth, and phosphate release, increased temperature may exacerbate eutrophication problems in shallow lakes and hinder attempts to solve them.

In deep lakes, which are generally less fertile than shallow ones, these effects need not occur.

Daphnia spp. find refuge from fish predators by day in deep water; the light climate in a deep and often quite transparent water body favours blue- green algae less than in more turbid shallow wa- ters; and the strengthening of summer stratification should minimise internal loading of phosphate from the sediment. The more frequent occurrence of deoxygenation in the hypolimnion may, how- ever, favour the blue-green algae.

I would predict therefore an increase in eutro- phication in shallow lakes and a decrease or very small effect in deep ones.

Effects of rising sea level

Britain will lose many areas of coastal-associated freshwater wetland as sea levels rise. Around the coast are areas of wetland at the heads of estuaries - Morecambe Bay, the Severn estuary, the Somer- set levels, Romney Marsh, the Norfolk Broadland, and the Wash fens. Some have been partly drained for agriculture, while others form nature reserve of international importance. The drained areas are of- ten closest to the sea where silt and clay deposits from former marine transgressions form drained soils which do not shrink as much as more inland freshwater peat soils. The soils do shrink to some extent, however, and the rivers and estuaries are usually be embanked to prevent flooding. In the Norfolk Broadland (Moss 1983) many of these banks are old and cannot be readily raised for struc- tural reasons. Elsewhere the costs of raising them or installing flood barriers such as those on the R. Thames will not be justifiable based on the agricul- tural benefits likely to be attained. It is thus likely that former coastal wetlands will be converted to shallow seas having marginal or more extensive sand bank and salt marsh. It is not possible to be precise about which wetland areas will be lost, be- cause of the present imprecision in values of pre- dicted sea level rise. A traditional grazing regime with a pleasing interlink between agriculture and conservation will be lost, but in general there will be major conservation gains.

These gains may be offset by losses further up valleys where present day undrained freshwater

wetlands will receive inputs of more saline tidal water. This will change their nature and extent, though they will probably remain of high wildlife interest. At present many have been eutrophicated, particularly in the Norfolk Broadland, by sewage effluent and agricultural drainage that have re- placed a clear water, submerged and rooted plant- dominated system with a phytoplankton-dominat- ed one. Increased tidal flushing may reduce reten- tion times of water, as may the reduced extent of the freshwater area. There may then be shorter times for phytoplankton to build up, favouring a return of rooted plant dominance with a more di- verse fish community and greater aesthetic and conservation interest. The switches between plant and phytoplankton dominance, however, are not so simple that the outcome is certain. In one part of the Norfolk Broadland (the Hickling area - see Moss 1983), now a brackish lake district near the sea with considerable importance for birds, it is likely that the sea will reinvade either directly through the thin barrier of coastal dunes or up- stream via the fiver which drains the area. It is im- possible to assess the potential benefits and losses because the Hickling system has been much al- tered recently by eutrophication, increased salin- ity, and fish kills caused by a toxic flagellate, and ways to restore it seem especially intractable. Reed (Phragmites australis) is cut over much of the Broadland area. Reed is relatively salt tolerant, so it is likely that new areas will become available for its harvest when the old areas are flooded too deep- ly. The quality of some of this reed (predisposition to rotting when thatched onto roofs) has apparently declined recently for reasons unknown, but reed subjected to salt water is much favoured. Thus there may be positive benefits of a sea level rise for the thatching industry, and nature conservation benefits from the maintenance of reed marshes.

In all of these areas, much will depend on the amount of land available for freshwater flooding as sea levels rise and on the absolute level of the rise. The valleys are often wide and much former wet- land has been drained. The present political cli- mate towards deintensification of agriculture may

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mean that these areas will be allowed to reflood rather than be embanked and pump-drained.

Approaches to prediction

The currently favoured approach in ecological re- search is highly reductionist. In this instance pre- diction of the effects of a rise in temperature through mathematical models based on growth rates, etc. of wetland plants and animals is likely to be proclaimed the best way to assess the problem. That may be so, but I doubt it very much. The best way would be replicated ecosystem experiments, although these would be costly and reliable results would take extensive teams of scientists several years to obtain. On the other hand, the limited data available for reductionist approaches mean there is simply not time to make syntheses of realistic com- plexity from such data. In the long run, an intuitive approach to constructing possible scenarios is like- ly to be cheapest, most realistic and most produc- tive.

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Moss, B 1988. Ecology of Freshwaters 2 "d Edn Man and Medmm.

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