Masters Thesis

Tidal seed dispersal potential of Spartina densiflora In Humboldt Bay (Humboldt County, California)

The invasive salt marsh cordgrass Spartina densiflora degrades native salt marsh ecosystems by altering abiotic processes and reducing native biodiversity. The dispersal of S. densiflora seeds on tidal currents poses a threat to restoration sites and native coastal areas throughout the Pacific Northwest. This study examined the dispersal potential of S. densiflora seeds by measuring the movement of seeds into and out of tidal creeks of Humboldt Bay, California, and by determining how long seeds remained buoyant in simulated Humboldt Bay conditions. Nets set out to collect S. densiflora seeds in a tidal creek at an invaded site in Humboldt Bay collected significantly more seeds drifting away from the salt marsh than back in (p=0.0122). Nets collected fewer seeds at the restored salt marsh compared to the invaded marsh (p0.0001), with more seeds drifting into the restored site than out (p=0.0296) according to a zero-inflated negative binomial model. Laboratory tests of the buoyancy duration of S. densiflora seeds in still and agitated saltwater showed that the seeds could remain buoyant long enough to independently disperse long distances on surface currents. With minimal disturbance in laboratory conditions, approximately 90% of seeds in the unshaken treatment floated longer than a month. In the agitated treatment placed on an orbital shaker to simulate natural bay conditions, 99% (n=1193) sank within 10 days and less than 0.2% (2 out of 1193) remained buoyant for approximately one month. While only a small percentage of seeds remained buoyant in simulated bay conditions for an extended period, the invaded salt marshes in the Humboldt Bay and the Eel River are estimated to produce 36 to 40 billion viable seeds every year. Based on these figures, the Humboldt Bay region could be annually exporting an estimated 250,000 to 3.7 million S. densiflora seeds capable of independently drifting on surface currents for a month and invading the Pacific Northwest and central California coast. These findings underscore the importance of continuing eradiation of S. densiflora in Humboldt Bay and early detection and response efforts throughout the Pacific Northwest.

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