THE EFFECTS OF FACTORIAL NUTRIENT ENRICHMENTS ON A PLANKTONIC UNICELLULAR CENTRALES-DOMINATED ASSEMBLAGE: CLUES TO THE FACTORS CONTROLLING BIOMASS IN LOCH LEVEN LATE WINTER—EARLY SPRING IN 1981

Abstract

A previous paper showed that in spite of short daylengths and low temperatures (2–4°C) during late winter-early spring 1981, Loch Leven produced a dense crop of unicellular centric diatoms (37,000 cells ml-¹) and a considerable population of Asterionella (ca 9,400 cells ml-¹). More recently acquired data suggest that these populations must also have suffered losses owing to occasional periods of rapid throughput of water, i.e. equivalent to 30% of the loch volume per month. The present study shows that samples taken from open water in February and subjected to ecologically representative, factorial additions of nitrate, phosphate and silica exhibited no major differences in total biomass after one week, to samples to which no nutrients were added. Moreover, the net change in biomass in the experimental bags was negligible. These results are in keeping with the field observations regarding low light and abundant nutrients at this time. The results of subsequent experiments also supported the conclusions drawn from the field data. With phosphate and silica being reduced to very low levels in the loch, phosphate enrichment in early March, and the addition of silicate during late March-early April, resulted in considerably greater yields in biomass compared to those measured in the control containers—and especially so as overall phytoplankton abundance in the loch decreased. Throughout the period of study nitrate remained at high concentrations in the loch, and the experimental addition of it alone was almost without exception, non-stimulatory. While losses of cells via sinking and washout are nil in the experimental containers but considerable in the field, changes in diatom concentrations in the control containers over each experimental period closely paralleled those in the loch.