Coldingham Loch, S.E. Scotland. II. Phytoplankton succession and ecology in the year prior to mixer installation

SUMMARY. 1. Total phytoplankton biomass in the small (8 ha), intermittently stratifying, eutrophic Coldingham Loch exhibited spring and autumn maxima of about 35 μg chlorophyll a l¹. Late winter and early spring assemblages included Chlorococcales (dominated by Coetxochloris pyrenoidosa Korsch. and Dichotomococcus curvatus Korsch.), diatoms (unicellular Centrales and Asterionella formosa Hass.) and desmids ( Closterium acutum Brèb. and Staurastrum chaetoceras (Schrod.) G. M. Smith). Small populations of Coetastrum and Oocystis characterized the summer period. The commonest "blue-green alga", Aphanizomenon flos-aquae Morr. ex Born, et Flah. (Cyanobacteria) attained its maximum population density in September just after the loch had permanently de-stratified. It was replaced by another growth of Closterium acutum which reached its annual peak density of 900 cells ml⁻¹ in October.
2. It is likely that nitrogen in summer and silica at various times of the year limited the growth of certain species; it is also possible that phosphorus limited growth in spring.
3. When physical conditions and nutrient concentrations are resolved into their basic, simple, seasonal patterns, a number of species preferences for particular environmental requirements become apparent. Of special significance is the competitive advantage over other plankters of Aphanizomenon in being able to resist sinking losses under stratified conditions and thrive at low ambient nitrate levels. Equally, there are some species exhibiting adaptability to a wide range of conditions.
4. The observed periodicity is examined in relation to recently published schemes that summarize phytoplankton-environment interactions.