Bivalve nurseries are important links between intensive hatchery productions of postlarvae and spat available for on-growing. Land-based raceway nurseries dependent on seawater natural production may be economically feasible for growing Pecten maximus spat during the summer season, but little is known regarding natural food conditions and scallop spat growth during autumn conditions. Scallop growth was monitored together with several water parameters from summer toward the end of the growth season in November. Three raceways were used for the experiment and were loaded with an increasing biomass of large scallops (26, 52, and 78 kg) to reveal the effect of different food concentrations being offered to the spat. Experimental scallop spat (12.94 ± 0.142 mm) were kept in stacked trays and placed after (downstream from) the biomass loads. Temperature, salinity, chlorophyll a, phaeopigments, particulate organic material, particulate inorganic material, algal species composition, and particle size distribution were monitored throughout the experimental period, but did not change systematically with the biomass loads. The temperature was 15°C at the start and 9°C by the end of the experimental period. Chlorophyll a values were low (<1.0 µg/L) except for the diatom bloom on September 13, when it reached 3.21 µg/L. Particulate organic material varied between 1.6–2.3 mg/ L. Shell height and live weight biomass differed significantly between the control group and the 3 experimental groups, whereas there was no significant difference among the experimental groups. Growth slowed after mid September and stopped in the experimental groups in October, whereas the control group continued growing until November. Growth was affected by increased biomass load in the raceways after the diatom bloom. We conclude that a raceway nursery could hold large biomass of scallop spat during early autumn conditions, but 4 wk after the autumn bloom the production was limited by the poorer quantity and quality of the particulate matter.