Date of Award


Document Type


Degree Name

Master of Science (M.S.)


Biological Sciences

First Advisor

Steven Obrebski

First Committee Member

Chris Kjeldsen

Second Committee Member

Edmund H. Smith


The taxonomic composition of the diatom assemblages inhabiting the substrate on the mudflat, and the effects of a cohabiting deposit-feeding invertebrate on the diatom assemblage structure are the subjects of this paper. While pelagic diatoms have received considerable attention in the literature to date, comparatively little is known about the ecology and distribution of benthic diatoms are known to be cosmopolitan in their distributions (Alee, 1950). These plants have been found to be opportunistic species in that they are quick to respond to relatively small perturbations in physical conditions. It has also been demonstrated that distinct associations of diatoms exist (McIntire and Overton, 1971). Generalizations concerning habitat preference and the assemblages that can be found within a specific biotope have also been made (Hendey, 1954; Round, 1960' Aleem, 1950).

Examination of the biotic interaction between a deposit-feeder and the diatoms (Division: Bacillarioohyceae) on Lawson's mudflat located in Tomales Bay, California was initiated October, 1975, and was concluded December, 1976. The site chosen for the research was a dense bed of the Maldanid Polychaete, Axiothella rubrocincta (Johnson). Control areas immediately adjacent to the north-eastern and south-western boundaries of the bed were also studied. The morphology and feeding behavior of this deposit-feeder has been investigated by Kudenov (1971) and Weinberg (in press). Kudenov suggested that A. rubrocincta is a non-selective deposit-feeder and he also showed that it habitually ingested large amounts of sediment. Hargrave (1976) states that the efficiency with which deposit-feeders assimilate organic matter in the sediments is related to both the composition of the organic matter and the size of the particles ingested. Perkins' (1960) observations indicate that the relationship between the organic content and particle size is a major controlling factor in determining diatom community structure.

Because Axiothella rubrocincta attains high densities and ingests sediment, this polychaete may affect the structure of diatom assemblages either directly by predation or indirectly by physically altering the environment upon which the diatoms exist. Therefore, it is hypothesized that by reworking the sediment and thereby altering the sediment composition, deposit-feeders may be creating and maintaining a niche wherein specific diatoms can establish themselves in distinct assemblages to the exclusion of other competing diatom species.