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The isotope effect, namely the isotope dependence of plasma confinement, is still one of the principal scientific conundrums facing the magnetic fusion community. We have investigated the impact of isotope mass on multi-scale mechanisms, including the characterization of radial correlation lengths (L-r) and long-range correlations (LRC) of plasma fluctuations using multi array Langmuir probe system, in hydrogen (II) and deuterium (I)) plasmas in the ISTTOK tokamak. We found that when changing plasma composition from the H dominated to D dominated, the LRC amplitude increased markedly (10-30%) and the L, increased slightly (-10%), The particle confinement also improved by about 50%, The changes of LRC and are congruent with previous findings in the TEXTOR tokamak (Xu et al 2013 Phys. Rev, Lett. 110 265005), In addition, using biorthogonal decomposition, both geodesic acoustic modes and very low frequency (<5 kHz) coherent modes were found to be contributing to LRC.