Nutrient concentrations in freshwater are highly variable over time, with changes driven by weather events, anthropogenic sources, modifications to catchment hydrology or habitats, and internal biogeochemical processes. Measuring infrequently collected grab samples is unlikely to adequately represent nutrient concentrations in such dynamic systems. In contrast, in situ passive sampling techniques, such as the “diffusive gradients in thin films” (DGT) technique, provide time-weighted average analyte concentrations over the entire deployment time. A pair of recently developed DGT techniques for nitrate (A520E-DGT) and ammonium (PrCH-DGT), as well as the Metsorb-DGT technique for phosphate, were used to monitor inorganic nutrients in different freshwater systems (i.e., streams and wetlands) with a range of environmental values and that were affected by different catchment types. Measurements of grab samples collected frequently (1–2 times daily, 8–10 a.m. and 2–4 p.m.) showed that concentrations of NH4–N and NO3–N changed dramatically in most of the studied freshwater systems over short time scales, while there were only relatively small fluctuations in PO4–P. The DGT measurements were highly representative in comparison with the average nutrient concentrations obtained from daily grab samples over short-term (24 h) and long-term (72 h) deployments. The ratios of DGT-labile concentrations to the average concentrations from grab samples were between 1.00 and 1.12 over the studied deployment periods. The results of this study confirmed that DGT measurements provided a reliable and robust method for monitoring NH4–N, NO3–N, and PO4–P in a diverse range of dynamic freshwater systems.