During community reassembly, consumers may express adaptive feeding behaviour in response to the presence of other species or according to their development. During the community reassembly after hypoxia of a temperate estuarine intertidal area, we quantified the microphytobenthos contribution to the diet of the three numerically dominant macrofauna consumers, using 13C-carbon tracing experiments. We then explored the relationships between their size and the microphytobenthos contribution to their diet. The polychaetes Hediste diversicolor and Pygospio elegans did not show a clear pattern of diet shift. Conversely, at a late stage of community reassembly, there was a dramatic decrease in the contribution of benthic microalgae to the diet of the clam Macoma balthica within the juvenile specimens (≤5 mm), which were recolonising the sediment. The contribution of microphytobenthos decreased with the size (r = −0.81, n = 18) and the largest juveniles incorporated benthic microalgal carbon similarly to their co-specific adults found in the undisturbed areas. Including both juveniles and adults, the size-diet relationship of M. balthica followed an inverse logarithmic curve during community reassembly. Such shape differed from the linear relationship based on the natural abundance of stable carbon isotope as previously collected in the undisturbed surroundings. Our study provides evidence of diet shift during community reassembly and suggests that such diet shift might follow both consumer development in the recolonising areas and other processes related to successional stages.