ARRIVAL AND EXPANSION OF THE INVASIVE FORAMINIFER TROCHAMMINA HADAI UCHIO IN PADILLA BAY, WASHINGTON: A NEW GEOLOGIC DATUM

The earliest records in Puget Sound of Trochammina hadai Uchio, a benthic foraminifer common in Japanese estuaries, is from surface sediment collected in 1971 from Cornet Bay, March Point, Drayton Harbor, and Padilla Bay (Penttila, pers. comm., 1998). The species, originally identified as Trochammina pacifica Cushman, was also recovered in nearly all of the 21 surface samples containing foraminifera obtained from Samish and Padilla bays in 1972-1973 where it occurred in high abundance, averaging 55% of the assemblage (Scott, 1974, Northwest Science, v. 48, p. 211-218).

A 2.06 m core collected in 2004 from the south-central portion of Padilla Bay (4.25 m water depth) provided us the opportunity to refine our estimation of the species' arrival and expansion in the region. Trochammina hadai is absent from the lower 135 cm of the core, then first appears at 71-69 cm, comprising <1% of the assemblage. Dating of these sediments by Pb-210 indicates that this first appearance occurred about 1958. The species' appearance occurs stratigraphically well above changes seen in the sedimentology of the core at 171 (from sand to silt; ~1890s) and 92 cm (from silt to sand; early 1940s) that likely reflect previous flooding or diversions of the Skagit River which impacted the deposition of sediments in the bay, indicating that its appearance is not associated with changes in sediment source. Trochammina hadai was still rare (1%) at 62-60 cm. By 52-50 cm, however, its abundance increased dramatically to 58%. Pb-210 dating indicates that this expansion occurred around 1972, consistent with the high abundances noted in the 1972-1973 surface samples collected by Scott (1974). The species continues to dominate the assemblage (32-88%) to the top of the core. This core provides evidence of the timing of the arrival (~1958) and proliferation (~1972) of T. hadai in Padilla Bay, a pattern consistent with, but considerably earlier than, the introduction of this invasive species in San Francisco Bay in 1981-1983. Trochammina hadai was probably transported from Japan to western North America in ship ballast sediment, in anchor mud, or in sediments associated with oysters imported for mariculture. It is not clear whether the species was introduced to the west coast of the United States through a single event or repeated innoculations.