Long-term changes in kelp forests in an inner basin of the Salish Sea

Abstract

Understanding the historical extent of biogenic habitats can provide insight into the nature of human impacts and inform restoration and conservation actions. Kelp forests form an important biogenic habitat that responds to natural and human drivers. Global concerns exist about threats to kelp forests, yet long term information is limited and research suggests that trends are geographically distinct. We examined distribution of the bull kelp Nereocystis luetkeana over 145 years in South Puget Sound (SPS), a semi-protected inner basin in a fjord estuary complex in the northeast Pacific Ocean. We synthesized 48 historical and modern Nereocystis surveys and examined presence/absence within 1-km shoreline segments along 452 km of shoreline. Over the last 145 years, Nereocystis has been documented in 26% of the shoreline segments. Its extent decreased 62% basin-wide between the 1870s and 2017, with extreme losses in the two out of three sub-basins (96% in Central and 83% in West). In recent years, almost all Nereocystis occurred in the East sub-basin. In the majority of segments where Nereocystis disappeared, the most recent observation was 4 decades ago, or earlier. Multiple natural and human factors that are known to impact kelp could have contributed to observed patterns, but limited data exist at the spatial and temporal scale of this study. In some areas, recent environmental conditions approached thresholds associated with decreased kelp performance. Longstanding Nereocystis losses occurred exclusively in areas with relatively low current velocities. Remaining Nereocystis predominantly occurred in areas where circulation is stronger. Exceptions to this pattern demonstrate that additional factors outside the scope of this study contributed to trajectories of Nereocystis persistence or loss.