Alginates are water-soluble polysaccharides that bind metal cations like Ca2+, producing hydrogels. Here, we have determined time-dependent fluorescence Stokes shift of a guest fluorophore to elucidate molecular length-scale local dynamics within alginate-based solutions and hydrogels. We find a major bulk water-like fast response emanating from large water pools interspersed between the polysaccharide chains, together with a minor but significant slow response. The possible origin of the latter is discussed in terms of either water molecules constituting the polysaccharide hydration shells or ion distribution and diffusion around the fluorophore dipole, or microscopic structural inhomogeneity inside the alginate-based media. © 2015 Elsevier B.V. All rights reserved.