Macrofaunal density, biomass, and respiration size spectra, as well as density- and respiration-body size allometries, were investigated in a deltaic environment of the eastern Mediterranean (Maliakos Gulf, Greece). Four stations were sampled along a gradient of increasing depth and decreasing water transparency from the river mouth to the plume area in May, August, and November 2000, and February and May 2001. Density and biomass significantly decreased in winter and spring near the river mouth. The shape and peaks of size spectra were temporally variable, indicating that no single factor determines body size at all times. The slopes of the seasonally averaged normalized biomass size spectra gradually decreased from –0.47 at the river mouth to –1.03 in the plume area, clearly indicating a biomass increase with increasing body size towards the river mouth. A flat lower boundary in density/body size relationships at all stations precluded a strong negative regression slope and indicated that the density of small- and intermediate-sized macrofaunal organisms is constrained by perturbations. Regressions of calculated respiration rates against body size were close to 0.75 only at the river mouth during winter, thus indicating that increases in riverine sediment discharges during that period constrain the macrofaunal community. Overall, there was no evidence of energy supply constraints for macrofaunal body size in Maliakos Gulf. Irrespective of whether size spectra and body size allometries conform to global patterns or not, they proved to be sensitive and straightforward descriptors for understanding the macrofauna’s response to the deltaic environment.