We used high‐resolution paleoecological records of environmental change to study the rate and magnitude of eutrophication over the last century in two contrasting coastal ecosystems. A multiproxy approach using geochemical and biological indicators and diatom‐based transfer functions provides a long‐term perspective on changes in nutrient concentrations and the corresponding biological and sedimentary responses. In Roskilde Fjord, Denmark, total nitrogen (TN) increased 85% during the last century, with the most rapid increase occurring after the 1950s, corresponding to the postwar increase in N fertilizer use. In Laajalahti Bay, an urban embayment near Helsinki, Finland, total dissolved nitrogen (TDN) increased with growing wastewater inputs and decreased with the remedial actions taken to reduce these discharges. These changes are small relative to the order of magnitude increases in nutrient loading that have occurred in northwestern Europe, where the dissolved inorganic nitrogen (DIN) load has increased more than threefold in certain areas.

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One of the primary challenges of the Water Framework Directive (WFD) of the European Union is to provide a guide for the recovery of surface waters from pollution. However, few studies deal with reference conditions according to the WFD in coastal waters. Using the urbanised Laajalahti Bay (Helsinki, Finland) as an example, reference conditions and pollution history were defined using the stratigraphy of diatoms, sediment geochemistry, stable isotopes, sedimentary pigments, long-term monitoring results of water quality, and nutrient-loading. Principal components analysis was used to generate a multidimensional index of water quality on the basis of the sediment data. We distinguished 5 phases: (1) a pre-industrial phase (before ca. 1815); (2) a phase of slight human disturbance (ca. 1815 to 1900); (3) an onset of anthropogenic impact (ca. 1900 to 1955); (4) a severe pollution phase (ca. 1955 to 1975); (5) basin recovery and a phase of internal loading (from ca. 1975 onwards). Phase 2 was used to define reference conditions. Phase 1 was not used as it occurred before the formation of the semi-enclosed bay by post-glacial isostatic land-uplift. There was good agreement between the sedimentary record and the water-quality data during Phase 4. Despite an improvement in water quality after the local municipal treatment plant closed in 1986, Laajalahti Bay is still far from reference conditions due to internal loading.

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