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The open collection “Aquatic transitions: Tracking the nature and trajectories of anthropogenically forced change in freshwater and coastal ecosystems” stems from a session of the same name at the ASLO (Association for the Sciences of Limnology and Oceanography) Aquatic Sciences Meeting 2017 in Honolulu, Hawai'i. The five papers gathered here reflect the focus of the special session on long‐term ecosystem research and monitoring (LTERM), and collectively make use of monitoring data, palaeolimnology, and historical and documentary records to explore the timing, extent, and causes of human‐related impacts on aquatic ecosystems. Collectively, they demonstrate that because timescales of ecological change often extend beyond contemporary monitoring, LTERM plays a crucial role in supporting evidence‐based conservation. In this introduction, we reflect on the role that LTERM has had in each of the ecosystems studied, and discuss the opportunities for LTERM work to inform future conservation.

The effective management and restoration of aquatic ecosystems rely on the establishment of “reference conditions,” defined as the conditions expected in the absence of anthropogenic impacts, and require a thorough understanding of the natural variability within a system. However, at least in Europe, most systematic surveys post‐date the onset of human‐induced pressures on aquatic ecosystems, and thus fail to capture earlier degradation to water chemistry and flora and fauna, which were already advanced. Paleolimnological methods can be used to assess a range of anthropogenic stressors, but variability within a system is often smoothed to give long‐term patterns. Here, we compile monitoring, documentary, and archival records from a range of sources to extend our understanding of centennial‐ scale lake ecosystem change and recovery from increasing salinity. We use a case study of the Thurne Broads shallow lake coastal wetland system (Broads National Park, UK), which has been subjected to multiple pressures of anthropogenic land drainage and North Sea storm surges (primarily in 1938 and 1953 CE) that have influenced salinity. Although there are still periods with significant data gaps and the interactions with eutrophication remain unclear, we demonstrate that historical data sources can be used in combination to observe seasonal patterns and extend knowledge on past salinity change and macrophyte community structure back to the 1800s. A demonstrable change in the ecosystem is observed after the sea flood of 1938 CE, when salinity levels in parts of the Thurne Broads were close, or equivalent, to seawater. With the added anthropogenic pressures of the late 1900s, the system has failed to fully recover. Future management, whilst balancing the needs of multiple users, should focus on the current large seasonal fluctuations in salinity and the vulnerability of the system to future large salinity increases.