The Effects of Stratification on Phyto-Planktonic Diatoms in a Small Body of Water

Abbot's Pool (Nat. Grid ref. ST 536732) is a small artificial body of water near Bristol, England. It is situated in mixed, deciduous woodland, surrounded by agricultural land in a valley of Old Red Sandstone most of which is overlain by Carboniferous Limestone. The pool itself is small (c. 100 x 40 m) and shallow; the majority of the water lies between 2 and 3 m depth and only 18% of the total volume is below 3 m, The inflow is small, becoming virtually non-existent in summer. Despite its size and depth, Abbot's Pool has been shown to stratify both thermally and chemically (Happey 1970a, c); these conditions are summarized in Fig. 1. Other details of the algae and limnology of the pool are given by Fritsch & Rich (1908), Karim (1965), Round (1965), Eaton & Moss (1966), Eaton (1967), Happey & Moss (1967), Moss (1967, 1969a, b, c), Moss & Karim (1969) and Eaton, Karim & Moss (in prep.). Abbot's Pool is a small body of eutrophic water, supporting large standing crops of phytoplankton. Each year these undergo a similar pattern with a large spring diatom crop of Asterionellaformosa Hass. In the summer of 1967 an unusual event was recorded when a large plankton population of diatoms occurred during stable water conditions of thermal stratification. This diatom, Stephanodiscus rotula (Ktitz.) Hendey (formerly S. astraea (Ehr. ex Kiitz.) Grun.), also produced a diatom crop in the autumn, after the autumn overturn. Spring and autumn diatom crops are common in Abbot's Pool, as in larger lakes, but a massive growth of this diatom in conditions of summer stratification has apparently not been reported previously here, or in other lakes (Talling, personal communication).