Studies of the water quality and algae of Llangorse Lake

✍ Scribed by Kathryn Benson-Evans; Rachel Antoine; Samir Antoine

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 144 KB
• Volume: 9
• Category: Article
• ISSN: 1052-7613
• DOI: 10.1002/(sici)1099-0755(199909/10)9:5<425::aid-aqc360>3.0.co;2-z

✦ Synopsis

1. Despite the importance of Llangorse Lake as the largest natural body of fresh water in South Wales, no long-term limnological project has ever been planned for the lake.

2. A mass of data is available from sampling carried out at intervals since 1961, but these visits were for various reasons and used widely differing levels of expertise. However, from these data some indications of seasonal variations, effects of climate and human interference on the lake system over 30 years have been inferred.

3. Depth soundings revealed the general shallowness of the lake with two deeper basins. Wind action oxygenated the water and stirred up sediments releasing nutrients and resting algal stages into the water column. Periods of thermal stratification were transient in summer. Secchi disc and light readings showed greatest light penetration in April/May. The lake water remained alkaline throughout. Dissolved ammonia values were low in the 1960s, increased in the 1970s and decreased again in the 1980s. Nitrite-and nitrate-nitrogen showed a converse pattern over the years. Phosphate (PO 4 P) and silica (SiO 2 ) values were highest in the 1960s, showed a decrease in the 1970s and early-1980s, and rose again towards the end of the decade, as did the chloride values. Seasonal fluctuations of all nutrients were observed.

4. Diatoms dominated the phytoplankton, with highest production in summer and early autumn. The Cyanobacteria (blue-green algae) produced blooms in the 1960s and 1970s, but Chlorophyta (green algae) increased over the years and dominated in the 1980s after a diversion of sewage from the lake. Diatom production followed a vernal and late summer periodicity common in temperate lakes. Results from diatom stratigraphy of the deep sediment cores indicated a change from epiphytic to planktonic species related to land-use change and a subsequent increase in soil erosion and turbidity.