The environmental range and tolerance limits of British Stoneworts (Charophytes)

• Main Author: Lambert, Stephen John
• Published: University of East Anglia 2007
• Subjects: 579.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.502103

Charophytes playa significant role in ecological succession and the community structure of many water bodies. Thirty three species have been recorded in the UK; however the future existence of 17 species. is now considered threatened due to habitat degradation and loss. The aim of this thesis was to investigate limits of environmental tolerance for British charophytes with particular reference to water quality and rarer species. The work was carried out in four stages: 1. Field surveys at 49 major British charophyte sites (2004-2007) to record (a) the abundance of all macrophyte species, (b) charophyte perfonnance as indicated by the efficiency of photosystem IT - (Fv/Fm), and (c) the chemical and physical properties of waters and sediment. 2. Intensive studies at three nationally important charophyte sites: Slapton Ley (Devon), Orton Pits (Cambridgeshire) and Hickling Broad (Norfolk) to include analysis of interstitial water chemistry. 3. Probabilistic detennination of environmental ranges for charophytes as derived via pair wise and multivariate statistical analysis of collected field data. 4. Hypotheses generated from analysis ofthe field data to demonstrate that environmental concentrations of copper, inorganic phosphate and salinity were key limiting factors of charophyte survival were tested via a series of laboratory investigations. The survey results indicate that the probability of charophytes occupying a site is dependent upon minimum concentrations of filterable calcium, chloride, magnesium, sodium, and sulphate; it was also limited by maximum environmental concentrations of filterable cobalt (0.36 ,ugr1 ), copper (4.9 ,ugr1 ), manganese (16 mgr1 ), nitrate (2.6 mgr1 0.58 mgr1 N), inorganic phosphate (56 ,ugr1 , 18.3 ,ugr1 P) and silicate (6.2 mgr1 ) • The probability of finding charophytes decreased significantly in waters of high lightextinction coefficient (Kd > 4.4), in fluid sediments or under competitive canopies. The probability of charophyte presence at a site is greatly reduced by· competitive succession pressure from Phragmites australis. Charophyte species may be grouped according to tolerance of the conductivity of open and interstitial waters and also according to interstitial water Eh and pH. Charophytes are also sensitive to low open water Eh, which is reflected as photosystem-IT stress. Environmental nitrate concentrations pose the widest threat to charophyte habitats, whilst dissolved cobalt lowers probability of presence at the lowest concentrations; copper, manganese and inorganic phosphate were also shown to limit the probability of charophyte survival at concentrations commonly recorded in the field. Of three SSSI sites investigated, none was in favourable condition: Slapton Ley was in danger of succeeding to Carr and reed-swamp, Hickling Broad contained no charophytes by 2007 and Orton Pits (Cambridgeshire) was suffering from diffuse nitrate and phosphorus pollution, with point-source pollution from copper. The results of the laboratory experiments re-enforced the field survey findings showing that copper caused reduced growth at 50 ,ugrl in all species tested, with a reduction of photosynthetic function at 100 ,ugrl . This concentration also caused a loss of rhizoids in Chara intermedia, and is similar to the mean copper concentrations (97 ,ugrl ) recorded at 70 sample points in the interstitial waters ofHickling Broad in August 2006. Growth rates of Charophytes cultured in a phosphate rich solution were significantly reduced at 100 ,ugrl P04 (32.6 ,ugrl P), however photosynthetic function remained unstressed suggesting alternative mechanisms of interference. Changes in salinity were shown to affect growth and photosynthetic function in three species previously common to Hickling Broad in different ways; Chara intermedia performed best at 12.5-25% addition of seawater to the recent median salinities, however Chara connivens and Nitellopsis obtusa performed better at a 12.5% reduction from recent median salinities. The results suggest that should recovery ever take place, changes in salinity are likely to affect the charophyte species assemblage in Hickling Broad and that such a recovery is likely to be hindered by deep sediments, interstitial heavy metal concentrations, internal phosphorus reserves and saline ingress.