Longstanding signals of marine community structuring by winter storm wave base

Background/Question/Methods

From the predicted increase in storm intensity with climate change to the developing technology for anthropogenic extraction of wave energy, wave energy and wave regimes are important areas for research. However, we lack understanding of the ecological implications on benthic marine communities for changing wave regimes and wave energy extraction. Waves can both directly and indirectly affect benthic marine communities through physical disturbance or through mixing resources like oxygen and nutrients throughout the water column. Are benthic communities spatially or ecologically structured by wave energy? Will climate change induced increases in storm frequency or anthropogenic wave energy extraction impact benthic communities? We investigated the impacts of wave energy on the structure of subtidal bivalve communities relative to more traditional drivers of marine community structure (ie. temperature, pH, dissolved oxygen, salinity, nutrients). We evaluated subtidal bivalve community structure using ordination methods and related this structure to wave and environmental data using non-linear correlations. We also tested a break-point regression of wave base data on subtidal bivalve community structure to look for a systematic response in the benthos to the direct impact of wave base. 

Results/Conclusions

Subtidal bivalve communities structured themselves along a depth gradient with a distinct shift in species’ rank abundance at 50 m (nMDS stress=0.076). The strongest correlate to this pattern is wave energy (r_s=-0.92, p<0.0001).  This result suggests that physical structuring of benthic habitat by waves more strongly influences biology than traditionally important resources such as dissolved oxygen, nutrients, or habitat characteristics like pH and temperature. The most important characteristic of wave energy that drives biological community structure is winter storm wave-base (WSWB). After WSWB, minimum seawater pH values are the only remaining environmental variable with a significant correlation to community data (r²=-0.278). These results indicate that changing storm regimes and large-scale wave energy harvest could substantially impact benthic communities by changing the WSWB. Changing WSWB would result in different bivalve communities across the continental shelf because communities above wave-base are taxonomically different from those below the wave base. Historically, the structure of bivalve communities matches those driven by modern day wave base, highlighting the longstanding nature of this relationship. Linking specific ecological outcomes with wave energy, as these data do, is invaluable for evaluating degrees of impact from increasing storm intensity with climate change or anthropogenic harvesting of offshore wave energy.