F. Rossi ® C. Lardicci Role of the nutritive value of sediment in regulating population dynamics of the deposit-feeding polychaete Streblospio shrubsolii Received: 20 March 2001 / Accepted: 15 November 2001 / Published online: 14 March 2002 Ó Springer-Verlag 2002 Abstract The importance of changes in nutritive value of the sediment in regulating the dynamics of the small deposit-feeding polychaete Streblospio shrubsolii (Buchanam, 1890) was examined. First, fluctuations in the population dynamics of S. shrubsolii and in the nutritive content of the sediment were studied on a seasonal scale. Subsequently, a manipulative field experiment was used to test the hypothesis that, in summer, availability of food regulates aspects of the population dynamics of S. shrubsolii. In summer 1997, sharp increases in protein concentrations and in the number of individuals of S. shrubsolii were found com- pared to all the other seasons. Densities of S. shrubsolii also showed an increase in the subsequent season. Newly recruited juveniles (19–31 setigers) were abundant in summer and autumn 1997, whereas the number of juveniles (32–42 setigers) increased in autumn 1997. Large numbers of adults were found in summer 1997. Significant negative correlations occurred in summer 1997 between the concentrations of proteins and the total number of individuals, the number of newly recruited juveniles and the density of adults. No differ- ences in densities of juveniles, adults or total number of individuals were found among plots before starting the experiment. After 20 days of experimental enrichment, proteins showed a tendency to increase. After 40 days of enrichment, the total number of individuals was signif- icantly greater in the enriched plots. Large numbers of newly recruited larvae and postlarval adults were found in the enriched plots. The experiment corroborated the model that the growth of population was limited by the availability of food in summer. Thus, in this season, patches of food could have a fundamental role in de- termining the population dynamics of small deposit- feeders under natural conditions and over short time scales. Introduction One of the main goals in modern ecology is to under- stand processes that affect observed changes in the distribution of populations. Descriptions of patterns of variability in the population are required. Then, exper- imental studies are necessary to identify the processes affecting the observed patterns (Underwood et al. 2000). The concept of scale as a method for analysing patterns has recently received considerable attention, since pat- terns depend on the scale of observation and different processes may affect populations at different scales (i.e. Levin 1992; Schneider 1994; Azovsky 2000). In coastal areas, benthic assemblages often show great variability at different temporal and spatial scales, which have been related to many different processes, such as competition, predation, recruitment, as well as avail- ability of food, depending on the scale of observation (Hines et al. 1990; Marsh and Tenore 1990; Posey and Hines 1991; Zajac 1991; Hunter and Price 1992; Power 1992; Foreman et al. 1995; Heip et al. 1995; Posey et al. 1995; Seitz and Lipcius 2001). The interplay between predation and availability of food (bottom-up–top-down theory) has been proposed as a regulator of abundances in many different systems, from freshwater pelagic webs (McQueen et al. 1989; Brett and Goldman 1997) to ma- rine pelagic systems (Micheli 1999), and in rocky inter- tidal environments (Menge and Olson 1990; Power 1992). In muddy sediments of temperate intertidal habitats, such as lagoons and salt marshes, marked seasonal Marine Biology (2002) 140: 1129–1138 DOI 10.1007/s00227-001-0768-3 Communicated by R. Cattaneo-Vietti, Genova F. Rossi (&) ® C. Lardicci Dipartimento di Scienze dell’Uomo e dell’Ambiente, Via Volta 6, 56124 Pisa, Italy Present address: F. Rossi Centre for Research on Ecological Impacts of Coastal Cities, Marine Ecology Laboratories, Science Rd A11, University of Sydney, Sydney, Australia e-mail: frossi@bio.usyd.edu.au Tel.: +61-2-93514282 Fax: +61-2-93516713