THE DISTRIBUTION OF SILICATE IN THE SEA 



INTRODUCTION 



The silicate dissolved in the sea is involved in a 

 number of biological and geological processes. Many 

 marine organisms utilize silicate for the construction of 

 their tests or shells, and many bottom sediments contain 

 a high percentage of silica. Of marine organisms, the 

 diatoms are the most important consumers of silicate. 

 To show the dependence of diatoms on dissolved silicon 

 compounds, culture experiments have been conducted by 



a number of workers; for example, Murray and Irvine 

 (1891), Richter (1906), Allen and Nelson (1910), and Cou- 

 pin (1922). As regards the form in which these com- 

 pounds can be utilized, these workers are not in agree- 

 ment, however, and the question of the nature of the 

 compounds utilized by the diatoms in natural waters is 

 yet to be solved. 



HISTORICAL 



In the sea a correlation between the abundance of 

 diatoms and the quantity of silicate in the water fre- 

 quently has been observed. Raben (1905a, 1905b, 1910, 

 1914) found a seasonal fluctuation in the silicate content 

 of the water of the North Sea and the Baltic correspond- 

 ing to the seasonal fluctuation in the abundance of phyto- 

 plankton. This is strikingly shown in a diagram by 

 Johnstone (1908). Over a period of years Raben found a 

 maximupi of silicate in November and February with a 

 minimum in April and May, the minimum following the 

 maximum abundance of diatoms. A similar correlation 

 was later noted by Brandt (1920) in the Baltic. 



In the Gulf of Maine, Wells (1922) found concentra- 

 tions of silicate during the winter nearly ten times high- 

 er than in May. In this region maximum diatom growth 

 occurs in March and April. Following this growth sili- 

 cate increases but is reduced again during the latter 

 part of August, when a second period of diatom growth 

 occurs. During a number of years Atkins (1923b, 1926a, 

 1928, 1930) has observed a marked seasonal variation in 

 silicate in the English Channel. For example, during 

 1923 to 1926 a winter maximum of 260 to 310^ mg Si02 

 per cubic meter fell to 50 and 100 mg in April and June. 

 Thompson and Johnson (1930), in Puget Sound, observed 

 a reduction in the silicate content in May and June; and, 

 in the Strait of Georgia, near the mouth of the Fraser 

 River, Hutchinson, Lucas, and McPhail (1929) noted that 

 the silicate content varied with the growth of diatoms. 

 It was found that the greatest consumption of silicate at 

 depths where diatoms were abundant occurred in August. 

 Conditions here, however, are complicated by the fresh- 

 water discharge from the Fraser River. According to 

 investigations carried out at the Scripps Institution, the 

 seasonal variation in silicate along the coast of southern 

 California is less pronounced than in the other localities 

 referred to, probably because of upwelling of subsurface 

 water. Considerable changes in the silicate content at 

 the surface may occur at any time of the year. 



Since diatoms can grow only in the photysynthetic 

 zone and since they are the chief consumers of silicate 

 in the sea, we should expect to find a lower concentration 

 of silicate in the upper water layers than in the deeper 

 water. That this is the case has been observed by sev- 

 eral workers. Atkins (1926a) presents data for vertical 

 series of samples taken off the south of Ireland, and off 

 Portugal, in the Bay of Biscay, and in the Faroe -Iceland 

 and Faroe-Shetland channels. All these show an increase 

 in the concentration of silicate with depth. The concen- 



trations increased with depth from 120^^ to 190 mg Si02 

 per cubic meter at the surface to about 400 mg per cubic 

 meter at 500 meters. At 1000 meters at the station off 

 Portugal a value of 580 mg Si02 per cubic meter was ob- 

 tained, but at the other stations the concentration at that 

 level was 520 mg Si02 per cubic meter or less. At 3000 

 meters at a station at latitude 30° north, longitude 15° 

 west, the concentration of silicate was 1560 mg Si02 per 

 cubic meter. In the English Channel a similar increase 

 of silicate with depth down to 60 meters was observed 

 (Atkins, 1930). 



A marked increase in the concentration of silicate 

 with depth was found by Moberg (1926b, 1928) off the coast 

 of southern California and by Bigelow and Leslie (1930) 

 in Monterey Bay. The results of these investigators will 

 be discussed later in a comparison of their data with 

 those of the Carnegie . Hutchinson (1928) studied the 

 silica-diatom relation in the upper 20 to 30 yards in the 

 Strait of Georgia. The decrease in the concentration of 

 silica at the depth where the maximum number of dia- 

 toms occurred was very striking. At one station in par- 

 ticular the concentration of silicate decreased from 

 about 4000 mg Si02 per cubic meterb at the surface to 

 less than 1000 mg Si02 per cubic meter at 4 yards be- 

 low the surface, the depth at which the maximum diatoms 

 were most numerous. Below this the silicate content in- 

 creased to 2000 mg Si02 per cubic meter at 10 and 20 

 yards. At two other stations a similar correspondence 

 between the abundance of diatoms and silicate and depth 

 was found. In this same locality Hutchinson, Lucas, and 

 McPhail (1929) later determined the silicate content to a 

 depth of 250 yards. The concentration of silicate was 

 1500 mg Si02 per cubic meter at the surface, only 500 

 mg at 6 yards, and reached a maximum of 2800 mg per 

 cubic meter at 100 yards. Below this there was a slight 

 decrease. Okada (1932) gives the results of the analyses 

 of sixty-eight vertical series of samples taken in the 

 southern part of the Japan Sea by the Svunpu Maru in the 

 summer of 1929. At one station, 37, which is represent- 

 ative of the other stations in that region, the silicate con 

 content at the surface was 187 mg Si02 per cubic meter. 



^ To be comparable with Carnegie and other data, 

 Atkins' figures are multiplied by the factor 1.3. See At- 

 kins (1930, p. 848). 



bOn page 301 Hutchinson states his results are ex- 

 pressed in "parts per thousand" but he obviously means 

 "parts per million' as shown by his graphs on the next 

 page where the results are given in mg per liter. 



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