Northwest Fisheries Science Center

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Blog on ocean conditions along the Newport Line and the northern CA Current.

Pseudo-nitzschia blooms following the spring transition

By Xiuning Du
May 12, 2017

After a long and wet winter, sunny weather finally returned by the end of April. At the same time, north/northwest winds began to blow which signals upwelling favorable wind conditions. Wind stress measurements at Newport indicate that the spring transition occurred on April 26. Coastal upwelling brings high levels of nutrients from the deep water on to the continental shelf, which in turn fuels large diatom blooms during the spring and summer in the northern California Current.

Figure 1. Diatom bloom assemblage from a sample that was collected 5 nautical miles offshore Newport on April 27.

During our cruise on April 27, we saw an immediate response of phytoplankton species to the high nutrient concentrations and the relatively cloud-free days. The diatom bloom we observed (Figure 1) was comprised of a variety of dominant diatoms, species that typically occur offshore Newport, such as Skeletonema costatum.

During this spring transition period, we were concerned that the influx of nutrients would fuel a potentially toxic bloom of Pseudo-nitzschia (PN) species. Compared to the low PN cell abundance in early April, PN species grew fast and reached far beyond typical bloom thresholds (10,000 cells/L). Since upwelling favorable winds have been dominant, it is very likely that PN species have been abundant in the coastal waters since the onset of the spring transition.

We recently headed out on May 10 to sample the NH line, but unfortunately, the hydraulics on the boat malfunctioned after the first station, preventing us to sample offshore. However, we did collect a surface water sample very nearshore (1 nautical mile offshore Newport) which showed a decaying large diatom bloom. However, in that bloom, PN species were still active and dominated the phytoplankton assemblage, albeit in reduced cell abundance compared to April 27 (Figure 2).

Figure 2. A long-chain of Pseudo-nitzschia australis like species observed in a water sample that was collected 1 nautical mile offshore Newport on May 10.

However, this very nearshore sample may not be representative of the phytoplankton community and PN abundance in the mid-outer shelf off Newport. The strong north winds and offshore Ekman transport over the last four/five days might have advected the PN bloom center further offshore, leading to an underestimation of the bloom level from this nearshore sample. Whether the observed PN bloom is causing significant toxicity to the coastal shellfish and crab is still too early to be determined. We board the NOAA ship Bell Shimada on May 22 for a broad scale research cruise, and we will continue monitoring the PN bloom, so stay tuned for more updates in the following weeks.

Tagged: NH Line, HAB

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