SMALL-SCALE DISTRIBUTIONS OF OCEANIC DIATOMS 



E. L. Venrick^ 



ABSTRACT 



A sampling study was designed to investigate small-scale abundance fluctuations of 

 diatoms over a distance of 10 miles. It was carried out at three depths in each of two 

 oceanic environments of the North Pacific. Significant nonrandom distributions were 

 observed. The intensity of aggregation varied with species and with depth. 



An expression for the approximate confidence intervals for single observations was 

 derived from the 5th and 95th percentiles of the observed frequency distributions. 



Statistical analysis of the fluctuations of Nitzschia turgiduloides indicated a pattern 

 of distribution with a scale of 1 mile. This may be associated with internal waves in 

 the region of the thermocline. 



Knowledge of small-scale distributions of or- 

 ganisms in the ocean is important for evaluation 

 of data based on widely spaced samples, and, 

 hence, is essential for design of efficient sam- 

 pling programs. Moreover, abundance fluctua- 

 tions on even the smallest scale relate directly 

 to the ecology of the species, and an understand- 

 ing of the magnitude and scale of such fluctua- 

 tions is an important step toward the under- 

 standing of a species' relationship to its 

 environment and to other species within its 

 community. 



Evidence indicates that the distribution of 

 phytoplankton in the ocean may be highly aggre- 

 gated (Bainbridge, 1957) . A few attempts have 

 been made to sample small-scale aggregations 

 and to investigate quantitatively their density 

 and spacing and the environmental factors which 

 influence them (e.g., Hasle, 1954; Holmes and 

 Widrig, 1956; Barnes and Hasle, 1957; Cassie, 

 1959a, b, 1960; Bernhard and Rampi, 1965). 

 Although these studies applied a wide variety 

 of statistical procedures to a range of spatial 

 and temporal scales, all of the phytoplankton 

 species studied were reported to have aggre- 

 gated distributions. However, such studies have 

 all been conducted in the nearshore environ- 

 ments. If, as has been suggested (Cassie, 1957) , 

 the contagious distributions of plankton reflect 



^ Marine Life Research Group, Scripps Institution of 

 Oceanography, University of California, San Diego, La 

 Jolla, CA 92037. 



heterogeneities in the environment, then the re- 

 sults of such studies may not be applicable to 

 the more homogeneous environments of the open 

 ocean. 



The study described in this paper was carried 

 out in two oceanic environments of the North 

 Pacific. While it was primarily designed to give 

 a quantitative estimate of the precision of sam- 

 ples collected for an extensive study of oceanic 

 diatoms, the results have general interest. 



LOCATION OF STUDY 



Closely spaced samples were taken twice dur- 

 ing Scripps Institution of Oceanography Expe- 

 dition URSA MAJOR, August-September, 1964. 

 Station 23 (lat 49°07'N, long 155°31'W) was 

 located in the Central Subarctic Pacific and Sta- 

 tion 5 (lat 37°00'N, long 155°02'W) in the Cent- 

 ral Pacific (Dodimead, Favorite, and Hirano, 

 1963) ; both regions were removed from the ef- 

 fects of either neritic environments or the North 

 Pacific Transition Domain. The phytoplankton 

 of the Central Subarctic consisted primarily of 

 diatoms which reached densities in excess of 

 5,000 cells/100 ml. A total of 27 diatom species 

 was recorded, of which Nitzschia turgiduloides 

 comprised 68-92 9f of the population, and Dentic- 

 ula seminae an additional 9-20%. The maximum 

 density of diatoms at the Central Pacific station 

 was only 30 cells/100 ml. The dominant spe- 

 cies, Hemiauliis hauckii, contributed 20% of the 



Manuscript accepted September 1971. 



FISHERY BULLETIN: VOL. 70, NO. 2, 1972. 



368 



