Northwest Fisheries Science Center

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Evolution of the toxic April/May Pseudo-nitzschia bloom

By Xiuning Du
August 6, 2015

Domoic Acid (DA) toxin in shellfish
The Oregon Department of Agriculture made three shellfish closure announcements in May due to DA levels above the threshold of 20μg/100gm in shellfish tissue. The first announcement was on May 9, for the closure of the northern coast of Oregon. The second announcement occurred on May 14 and the entire Oregon coast was closed to razor clamming. The third closure announcement occurred on May 22, because the measured DA level was above dangerous levels on South Beach in Newport Oregon. We captured the initiation and evolution of this Harmful Algal Bloom (HAB) by the species Pseudo-nitzschia (PN) from five of our biweekly NH line cruises (Figure 1).

Wind and upwelling conditions
Cumulative alongshore wind stress indicated that the onset of upwelling (spring transition) began April 12, 2015 with light northerly winds. Upwelling favorable winds became much stronger from April 17 to April 22. Wind direction then reversed to the south (upwelling relaxation) on April 24 and remained calm until April 27. The northerly winds resumed from April 28 and dominated throughout May.

Pseudo-nitzschia (PN) bloom initiation and evolution off Newport
On April 7, prior to the onset of upwelling, low concentrations of PN cells were present in shelf waters. Following the end of the upwelling relaxation on April 27, it was clear that a PN bloom was in progress (Figure 2).

The May 4 cruise was conducted immediately following the first episode of strong northerly wind that occurred from 30 April to May 3. PN abundance decreased during this time (Figure 1) which might have been related to mixing of the water column from the strong upwelling event and the loss of PN cells offshore by dispersal. On May 7, at the beginning of the second strong upwelling episode (May 7-12), PN abundance had bounced back to above 105 cells L-1 level, and the bloom continued to thrive until at least May 19 (Figure 1 and 3). The percent increase of PN contribution to the total diatom community was dramatic from the 4th to 19th of May. During this time, the percent contribution of PN increased from 23.1% to 89.7%, eventually forming the monospecific bloom.


Pseudo-nitzschia PN bloom along the west coast
Spatially, this PN bloom was simultaneously observed along the west coast of North America in early May. The “warm blob” which has been present in the North Pacific since winter 2013-2014 has induced unusually warm ocean conditions, which is suspected to be the proximate driver of bloom formation and longevity. However, the ultimate drivers/factors that lead to the large and persistent PN blooms cannot be answered at the present time.

Tagged: HAB, Warm Blob

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