THE DYNAMICS OF SILICA IN A SHALLOW DIATOM-RICH SCOTTISH LOCH II: THE INFLUENCE OF DIATOMS ON AN ANNUAL BUDGET

Abstract

Stream inputs of soluble reactive silica (SRS), changes in its concentration in the loch and its export via the outflow of Loch Leven, and in-loch shifts in diatom silica (opal—OS) were estimated from samples collected every 8 days from 11 January to 13 December 1985 (0.92 of the year). A silica budget was erected for the whole period and, by considering the data in fourteen 24-day blocks, seasonal changes in the various component terms were assessed. In the absence of direct chemical determinations, the opal in the diatom assemblages (which were dominated throughout by unicellular Centrales falling mainly into two size groups, 6–9 μm and 18–21 μm diameter, together with Asterionella formosa) was estimated as follows: for the discoid forms, from cell dimensions and published values suggesting a general ratio of cell volume to silica volume of 10:1 (Sicko-Goad, Schelske & Stoermer 1984, Limnol. Oceanogr., 29: 1170–1178), and for Asterionella (and the sparse populations of Fragilaria crotonensis, F. intermedia, Melosira ambigua and M. granulata var. angustissima) from other published values. Stream inputs of 953 t SRS over the 336 days are equivalent to 97% of the total increase in opaline silica (OS) of 985 t, including 70 t passing down the outflow. However, if the amounts of SRS present in the loch at the beginning and the end of the period (383 t and 178 t respectively) and that exported (282 t) are taken into account, only 876 t was made available. The short-fall of 109 t (i.e. 985—876) represents the sum of the imbalances not appearing as measured inputs and outputs; when calculated by subtracting the changes in SRS from the shifts in OS for each of the 24-day periods, it is seen to comprise a gain of 372 t by the water column, attributable to sediment release, and a loss of 263 t, representing diatom sedimentation and grazing. Although the flux of SRS to the sediment represents only 40% of the external loading over the whole period, the daily rates (equivalent to 34–362 mg SiO₂ m ⁻² of loch floor) over the 24-day blocks were commonly equivalent to 3 times the amounts of SRS arriving meanwhile from the feeder streams. Internal loading is especially important in supporting the post-spring diatom populations; by contrast, the late winter-early spring crop (which sequestered approximately 40% of the SRS required by diatoms throughout the whole period) is able to draw on the initial reservoir of SRS in the loch. This is the only time when concentrations there (8.0 mg SiO₂ l⁻¹) resemble the annual average flow-weighted concentration of 8.6 mg l⁻¹ in the feeder water; for most of the year concentrations are less than one-third of this, and occasionally, virtually no dissolved silica is measured. All aspects of the work are discussed in relation to the literature although very little work on silica budgets outside North America and UK has been published.