. Bulletin. Natural history; Natural history. Aug. 1981 Moran: Phytoplankton 337 a: Fig. 8,—The ratio ot nitrogen (nitrate N + ammonia N) to total phosphorus in Lake Sangchris, 5 September 1973-13 April 1976. The shaded band marks the optimal ratio of S 'O'N'Dl J'F 'M'A'M'J'J'A'S'0'N"D| J'F'M'A'M'J'J'A'S '0'n'o| J'F'M'A' 1973 1974 1975 1976 pulse are often not evident. However, instances of silica limitation upon diatom growth have been documented. Lund (1950) found evidence that Asterionella formosa decreases coincided with drops in dissolved silica concentrations to below mg Si0
. Bulletin. Natural history; Natural history. Aug. 1981 Moran: Phytoplankton 337 a: Fig. 8,—The ratio ot nitrogen (nitrate N + ammonia N) to total phosphorus in Lake Sangchris, 5 September 1973-13 April 1976. The shaded band marks the optimal ratio of S 'O'N'Dl J'F 'M'A'M'J'J'A'S'0'N"D| J'F'M'A'M'J'J'A'S '0'n'o| J'F'M'A' 1973 1974 1975 1976 pulse are often not evident. However, instances of silica limitation upon diatom growth have been documented. Lund (1950) found evidence that Asterionella formosa decreases coincided with drops in dissolved silica concentrations to below mg Si02/1. Pearsall (1932) also found that a silica concentration of about ^ mg Si02/1 was critical to diatom multiplication. Lewin & Guillard (1963) ' found the growth-limiting concentrations to be mg Si02/1. In Lake Sangchris dissolved silica I concentrations had a mean annual concentration of mg/1 and were generally in excess of 2 mg/1 with the I exceptions of November-December 1973 : and March 1974. when concentrations dropped to mg/1 (Fig. 9). Pulses of Melosira distans v. alpigena and M. ttalica occurred at those times, and the heavily silicified frustules of these species i may have drained the silica supply to nearly limiting levels. The decline of this ' pulse was followed by an increase in silica to levels of 3 mg/1 or more. In the following winter another pulse of diatoms, primarily Synedra tenera, occurred, and this pulse was accompanied by a drop in silica to about 2 mg/1. The decline of this pulse was accompanied by an increase of silica to more than 4 mg/1. In March-April 1976 a pulse of a minute diatom, Cyclotella pseudo- stelligera, was accompanied by a drop in silica, but water chemistry data acquisition terminated at this point; so the extent of the drop is not known. The frustules of this species were so small and lightly silicified, however, that it seems unlikely that silica concentrations were reduced to limiting levels. The ratio of
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