Temporal system-mass-density fluctuation analysis was performed on liquid-water molecular dynamics simulations at ambient pressure and 200 and 300 K, in three increasingly-large systems. A prominent mode in system-density fluctuations was observed at molecular-librational frequencies of similar to 600-800 cm(-1) (with pronounced temperature dependence). This mode displayed marked system-size dependence, disappearing for larger systems. Persistent system-density fluctuations were clearly evident at 10-11 cm(-1) for all systems and temperatures, with lower-amplitude 'overtones' evident only in larger systems. It is conjectured that this reflects similar to 3 ps timescales observed in earlier studies for dissipation of local-density fluctuations in liquid water in this 200-300 K temperature range. (C) 2016 Elsevier B.V. All rights reserved.