The Current Crisis: Ecological Effects of Low Flows on Chalk Streams.

Abstract

The ecological effects of low flow on the communities of chalk stream plants and animals are, in general, poorly understood. In particular there is a distinct lack of information on the effects, positive or negative, on waterside birds and mammals. Clearly, if low flow conditions persist for the next few years or longer we need to know whether populations of animals such as kingfishers, dippers, herons, water shrews, water voles, otters, etc. will be adversely affected.
It is difficult to ascribe particular changes to single factors (such as low flow rate) because other environmental factors (such as temperature, dissolved oxygen concentration, increased sedimentation, blanket weed growth etc.) are usually linked to the effects of low flow, producing a complex system of interactions. Only detailed scientific studies can tease apart these effects. More details of the effects are given in the scientific summaries.
Ranunculus spp. (water crowfoot) beds and the associated invertebrate communities suffer greatly from reduced current speeds and have declined markedly in recent years. Ranunculus is often replaced by Nasturtium officinale (water-cress) in silted, sluggishly flowing stream sections. Growths of filamentous algae ('blanket weed') often carpet the stream bed under these conditions. When prolonged low flows allow emergent vegetation such as Sparganium spp. (bur-reed) to choke the channel the changes will be difficult to reverse.
It is clear from available studies that both trout and salmon populations are being adversely affected by low flow conditions. It is likely, however that cyprinid fish (e.g. dace, chub, roach) will benefit from the slower current speeds and higher water temperatures associated with low flows.
From the available evidence of damage to chalk stream fauna and flora it is clear that ecologically acceptable low flow regimes for chalk streams need to be both defined and implemented. Fortunately the National Rivers Authority (NRA) is currently studying this problem with a view to assessing the need for reducing abstraction levels from chalk aquifers and/or increasing compensatory flows to ameliorate the problem.