68% (dark) and 90% (light) confidence regions for the characteristic strain h_c are shown for 2000 Universe realizations drawn from the HORIZON-AGN MBHB population, assuming either circular orbits (blue) or noncircular orbits (orange). We also plot their respective deterministic mean expectation values in dotted and dashed lines. For the circular population, we overlay as a dashed-dotted line the average spectrum, assuming the binary dynamics are driven by GW emission only. The inclusion of eccentricity introduces a significant signal reduction at frequencies lower than 10 nHz, due to the reduction of the MBHBs residence time at low orbital frequency.