Ca-alginate gels were studied by small-angle X-ray scattering and rheology to determine relations between chemical composition and concentrations of the alginate and the elasticity and structure of the gels. The gels were prepared by in situ release of Ca2+ from either Ca-EGTA or CaCO3 with total Ca2+ concentration in the range 5-30 mM. Alginates with low (39%), intermediate (50%), and high (68%) fractions of alpha-L-GulA originating from the brown algae Ascophyllum nodosum, Laminaria hyperborea leaf, and Laminaria hyperborea stipe, respectively, were employed. Two to three different degrees of polymerization for each chemical composition were used in the experiments. The excess small-angle X-ray scattering for the alginates in solution yielded linear cross-sectional Guinier plots, and the cross-sectional radius of gyration, R-g,R-c, was determined to be 3.1-4.6 Angstrom. The SAXS profiles of the alginate gels depended on the alginate concentration, Ca2+ concentration, and the alginate composition. The SAXS data suggested that dimerization of chain segments was the principal association mode at low fractional Ca2+ saturation of guluronic acid of the alginate. Increasing the fractional Ca2+ saturation of guluronic acid, either by the concentrations or selection of alginate source, yielded coexisting lateral association modes, as manifested in a curvature in the cross-sectional plots. The coexisting junction zone multiplicities occur because of a delicate balance between the block length distribution of the alpha-L-GulA residues, polymer concentration, and Ca2+. These results are quantitative extensions of the "egg-box" model used to describe ionotropic gelation of alginate and hence enhance the understanding of the structure-function relationship of alginate gels.