Title: Bulletin
Identifier: bulletin7073sout (find matches)
Year: 1971- (1970s)
Authors: Southern California Academy of Sciences
Subjects: Science; Natural history; Natural history
Publisher: Los Angeles, Calif. : The Academy
Contributing Library: New York Botanical Garden, LuEsther T. Mertz Library
Digitizing Sponsor: The LuEsther T Mertz Library, the New York Botanical Garden

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1974 PACIFIC C0/IS1 11/mi n i/i 1/ I (X>2) and ill low tide much "I the bay floor is exposed or only sparsely covered by shallow pools. The depth and salinity of the water, per se, are not considered lo be significant factors in con- trolling the distribution of the Tubificidae in this study although the amount of exposure of Ihe habi- tal ;il low tide, the degree of evaporation, and (he wider retention characteristics of the sediment, probably are. Such data are nol available, but related sediment parameters, the mean particle size and soiling coefficient, are known for most of the bay stations. A textural analysis of the bottom sediments demonstrated that a continuous range of conditions exist in the buy, from eoarse, well-sorted sands, to poorly-sorted clays which dominate the north end of the bay; a complex mosaic of intermediate sediment types occur in the central regions, and sands predominate in the bay's south end (Gorsline and Stewart, 1962). The latter authors also showed that, in general, the organic content of the sediment increased with decreasing particle size, hence it is probable that the amount of food available to oligoehactes is inversely proportional to this parameter. There- fore, the mean particle size of the sediment is thought to be a direct or indirect measurement of some factors determining tubificid distributions in this study, and will be considered in more detail below. The numbers of each tubificid species identi- fied from each San Quintin station (each sample = 0.065 m-) and other relevant data are sum- marized in Table 1; for purposes of brevity, and to facilitate comparisons with other studies, mean sediment particle sizes are discussed in terms of units ( = logL> particle diameter in mm, hence diameters of4-8/U. = 8<£, S - 16 /x = 7 <£, 16 - 31 p = 6 0, 31 - 62 p = 5 $, 62 - 125 p = 4 , 125 - 250 p = 3 cf>). From this information, and the locations of the benthic stations (Fig. 6) the distribution and density of each species can be inferred. Abstracting and comparing pertinent data from table 1. it is evident that the three species of Limnodriloid.es bear a definite relationship to very fine sand and silt (4 to 6 <£ optimum) in terms of both their frequency of occurrence and population size. The dominant tubificid, L. bar- nardi, occurred at 48 stations comprising all sedi- ment classes, but was most frequently found in 4 and 5 <£ sediments (21 percent and 40 percent of its stations respectively); maximum population density of L. barnardi occurred in the 4 to 6 <£ range. L. monothecus occurred at 21 stations
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Figure 6. Locations of benthic sampling stations in Bahia de San Quintin: redrawn from Barnard. 1962. of which 52 percent had 5 sediments and 24 percent were 4 0. However, the population den- sity of this species was remarkably constant over its whole sediment range of 4 to 7 i (about 280/m2). Of the 28 stations occupied by L. verrucosus, half had sediments in the 5 <£ class, with the remainder distributed fairly evenly over the whole sediment range of 3 to 8 : however, the abundance of L. verrucosus was highest in 5 to 6 <(, and 8 substrates which may indicate that while it is capable of exploiting the finest sediments, it is partially excluded from them this situation by competition with other deposit feeders or by the effects of predation. The two remaining tubificids, Tu, capillatus and Thalassodrilus belli, a associated with the finest sediments which located mainly at the north end of the bay. Tubifex belli was present in only eight samples but half of these were in the 8 6 class in which

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