Sediment turnover by a deposit‐feeding polychaete1

Monthly determinations of the size frequency distribution of specimens in a population of Pectinaria caZiforniensis, a deposit-feeding polychacte from Puget Sound, Washington, and laboratory measurements of sediment turnover by the same species reveal seasonal patterns in the rate at which the population processes the sediment during feeding. The relation between numbers of specimens and mean specimen size results in the most intense scdimcnt turnover in late autumn or early winter ( 1 kg dry sediment mm2 month-l), when recently scttlcd animals dominate the biomass. Annual turnover was 8.6 kg dry sediment m-‘. This intense activity is potentially important in the transfer of dissolved materials from sediment to the water column. Rhoads (1973) has suggested that sediment turnover by benthic invertebrates (whereby subsurface material expelled as fecal or pseudo-fecal material into the water column is redeposited on the substrate surface) may be responsible for much of the turbidity of overlying water and perhaps for the recycling of nutritive material from sediment to the water column. Equally important, the activity of such organisms may result in the resuspension or solution of other tract substances (pollutants or otherwise) found in deposited scdimcnt. For example, Bothner ( 1973) suggested that resuspension of bottom sediments by benthic organisms may partly account for the observed dccrcase of mercury with time from contaminated estuarine sediments in Bcllingham Bay, Washington. Increased knowledge of the processes of sediment turnover by bottom dwelling organisms would help us to understand the mechanisms of the flux of trace substances at the scdimcnt-water interface. Several investigators (Fox et al. 1948; Rhoads 1963; Mangum 1964; Gordon 1966; Rhoads 1967) h avc mcasurcd rates of sediment turnover by a variety of intertidal and shallow subtidal invertebrate speciesthe amount of time required by species populations to rework completely all the sediment in a given area to the depth ac1 The research was supported in part by National Science Foundation Grant GB 13205. cessiblc to ( adult?) individuals. Estimates of population size (used to convert laboratory-determined turnover rates of individuals to annual rates on areal bases) were made by these workers primarily from data from one season, usually summer. The turnover or reworking rates of individual species so determined vary from a low of once every 15 years to a high of once every 10 weeks (disregarding species diffcrenccs and slight differences in the reworking depth used in the calculations). Most rates, however, fall in the range of 1 to 5 years for populations of individual species (Mangum 1964; Rhoads 1967). The investigation described here represcnts an extension of a study of the dynamics and energetics of three depositfeeding benthic invertebrates ( Nichols 1972), in which rates of sediment turnover by one of these, the polychaetous annelid Pectinaria calif orniensis Har tman, were measured. Watson (1927) provided detailed dcscriptions of feeding and tube-building processes of adult specimens of Pectinaria koreni. The animals burrow, head down, into the sediment with the small end of the cone-shaped tube protruding slightly from the sediment surface. They handle sand grains individually with tentacles, ingcsting some grains, cementing others of precise size and .texture onto the large end of the tube, and propelling unused material out through the upper end of the tube. UnLIMNOLOGY AND OCERNOGRAPIIY 945 NOVEMBER 1974, V. 19 (6)