THE ECOLOGY OF PLANKTONIC DIATOMS, ESPECIALLY FRAGILARIA CROTONENSIS, ASSOCIATED WITH ARTIFICIAL MIXING OF A SMALL SCOTTISH LOCH IN SUMMER

Abstract

Asterionella formosa, Fragilaria crotonensis, F. intermedia and unicellular Centrales (dominated by Stephanodiscus parvus) were the main planktonic diatoms in Coldingham Loch from May to September 1984, when the loch was artificially mixed by aeration. Although diatoms comprised a minor component of the plankton, F. crotonensis was of particular interest in remaining at detectable levels throughout this period, having been recorded in only low numbers in the corresponding months of the previous year when the loch stratified under natural conditions. The paper thus concentrates on the ecology of this diatom under the experimental regime.

Correlations between Fragilaria population parameters and physical factors (water temperature and clarity) are discussed, but the study focusses mainly on the influence of nutrients, and silica in particular. Nitrate and phosphate showed contrasting responses to mixing, and the effects of these on the diatoms are explored. Nitrate was reduced to potentially growth-limiting levels (<0.05 mg N.1⁻¹) whilst seasonal shifts in phosphorus in 1984 were virtually identical to those recorded in 1983; levels of 10—225 μg P.1⁻¹ suggest that this nutrient was always present well in excess of phytoplankton growth requirements.

Although shifts in silica concentration (from <0.05 mg SiO₂.1⁻¹ to ca 3.6 mg SiO₂.1⁻¹ in 1984) appeared to be influenced strongly by observed changes in Fragilaria abundance, close examination of the plots of the nutrient and the diatom reveals major discrepancies; in the main these relate to measured increases in Fragilaria numbers that accounted for only a fraction of the observed decreases in silica, but there were also phases during which considerable net increases in dissolved silica occurred. Theoretical values for the intrinsic sinking rate of the diatom are calculated, following the approach of Reynolds and colleagues. The results suggest that settling losses could explain the discrepancies, even where uptake by diatoms of silica released from the sediments has to be added to the observed decreases in silica.

The influence of low silica concentrations on an increase in the numbers of cells per colony in the declining Fragilaria population is discussed. As nitrogen and silica were restored to higher levels, Fragilaria was succeeded by Asterionella.