Hindcasting the cause of seagrass decline: A case study in the San Juan Archipelago region of the Salish Sea

Background/Question/Methods

The seagrass, Zostera marina (eelgrass) can form extensive underwater prairies that, in turn, provide key ecosystem services throughout the Northern Hemisphere. In 2003 and 2004, the relatively rapid decline of Z. marina prairies was observed at several sites in the San Juan Archipelago, located in the Salish Sea sub-region of the Pacific Northwest. The putative cause for decline remains unknown.  Our objective was to determine if disease symptoms would appear when seawater was inoculated with Labyrinthula zosterae, a pathogenic microorganism known to infect and kill adult ramets of Z. marina. To confirm that L. zosterae living in San Juan Archipelago waters could bring about an acute disease event we (a) executed a Koch's postulate test using L. zosterae isolated from the foliage leaves of Z. marina growing in our land based mesocosm (b) using genetic analysis to complete Koch’s postulate we are confirming the identity of the recovered isolate and (c) in a controlled environment, inoculated the seawater surrounding an adult Z. marinaramet with a liquid culture with the isolate used in the Koch’s postulate experiment.

Results/Conclusions

We observed that lesions, known to be symptomatic of disease presence, appeared within 5 days. These results stimulated additional tests to estimate the rate of leaf necropsy and whole plant death. We will present our findings and demonstrate the potential that this pathogen was a strong contributor to Z. marina declines observed in 2003 and 2004.