A century of canopy kelp persistence and recovery in the Gulf of Alaska.

BACKGROUND AND AIMS: Coastal Alaska contains vast kelp habitat that supports diverse marine and human communities. Over the past century, the North Pacific Ocean has undergone oceanographic and ecological regime shifts which have the potential to strongly influence the structure and function of kelp ecosystems. However, the remoteness and complexity of the glacially carved region precludes regular monitoring efforts that would be necessary to detect such changes.

METHODS: To begin to fill this critical knowledge gap, we drew upon historical and modern surveys to analyze change in spatial coverage and species composition of canopy kelp between two time points (1913 and the early 2000s to 2010s). We also incorporated decadal surveys on sea otter range expansion following complete extirpation and reintroduction to assess the influence of sea otter recovery on the spatial extent of canopy kelp.

KEY RESULTS: We found increases in the spatial extent of canopy kelp throughout the Gulf of Alaska where there was coverage from both surveys. Kelp in Southcentral Alaska showed extensive recovery after the catastrophic Novarupta volcano. Kelp in Southeast Alaska showed persistence and spatial increase that closely matched range increases of sea otters. Observations of thermally tolerant kelp species increased more than observations of cold adapted species between the two surveys.

CONCLUSIONS: Contrary to trends observed at lower latitudes, the kelp forests that ring the Gulf of Alaska have been remarkably stable and even increased in the past century, despite oceanographic and ecosystem changes. To improve monitoring, we propose identifying sentinel kelp beds for regular monitoring to more readily detect changes to these iconic and foundational canopy kelp species.