Puget Sound Kelp Forests: A Review of Natural History and Potential Impacts From Small Overwater Structures (SOWS)

Kelp refers to any macroalgae species in the order Phaeophyta and family Laminariales.

• All kelp species share similar environmental requirements and thresholds. Individual populations and life stages – rather than individual species - are the most important considerations for understanding environmental impacts to kelp.
• There is concern over the potential impacts of small overwater structures (SOWS), like residential docks, on kelp species and habitats in Washington state waters.
• Available information and expert opinion identify multiple potential positive and negative SOWS-related impacts to Washington kelp species.
• Kelp productivity is influenced by multiple factors including, but not limited to light, temperature, and nutrient availability, making it difficult to pinpoint specific environmental causes for the persistence, expansion, or decline of kelp populations.
• The following, non-exhaustive list summarizes the potential effects of SOWS on kelp species based on the available literature and expert opinion:
  • Shading: Unknown. Potential negative impact to kelp if light levels are reduced to below required thresholds for persistence.
    • Only one study in WA waters suggested a negative correlation between shading from SOWS and non-floating kelp abundance but was also confounded by other potential non-shading disturbances. Requires more study.
  • Physical disturbance: Unknown. Potentially temporary negative impact from SOWS construction and installation activities.
    • No available information in our region.
  • Additional hard substrates: Unknown. Potentially positive over the long term by providing additional substrate for all Washington kelp species. SOWS-attached kelp, like natural populations, may provide additional fish habitat.
    • There are no formal data on kelp attached to SOWS, interactions with natural populations, or its use by native fish species. However, anecdotal reports from experts and survey respondents document kelp attached to SOWS artificial structures.
  • Changes to water motion, adjacent benthic substrates, and nearshore sediment processes: Unknown and likely site-specific. Will vary based on regional SOWS density, tides, currents, shoreline geomorphology, and other physical oceanographic processes.
    • No available information in our region.
• Significant data gaps surrounding impacts and interactions of SOWS to and with nearshore physical and biological processes and the impacts of common human-associated stressors to Washington kelp resources hinder ongoing efforts at protection and recovery.
• There is no understanding of the cumulative, long-term impacts of SOWS and human activities associated with residential SOWS. Complicating this understanding are potential interactions with additional environmental variables such as ambient water clarity and nutrient availability that may compound or alleviate SOWS-related impacts.