Northwest Fisheries Science Center

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Blog Entries for May 2016

Spring upwelling phytoplankton bloom

By Xiuning Du
Posted on May 18, 2016

Upwelling conditions Prior to our biweekly Elakha cruise on May 12, upwelling favorable northerly winds were blowing since May 4 and our sampling directly followed this 9-day upwelling event. This upwelling event was the strongest one since late March in terms of its duration and magnitude and the nutrients from this event fueled a diatom bloom.

Figure 1. Diatom Skeletonema costatum in chains.

Overall phytoplankton assemblage The phytoplankton community from the May 12 sample was dominated by diatom species and the species richness (number of species) was moderately high. The predominant diatom species was Skeletonema costatum (Fig. 1) which is a neritic (coastal) species that is widely distributed in all seas. There were approximately a million cells per liter of this species in the nearshore waters off Newport Oregon. Other abundant diatoms included Thalassiosira nordenskioeldii (one of the dominant species in winter blooms), and several species from the genus Chaetoceros.


Figure 2. Pseudo-nitzschia cells (large cell form).

The phytoplankton community in nearshore waters was very similar to the community found 25 miles from shore indicating that Ekman transport from upwelling pushed nearshore blooms away from coast. Thus species rich water occurred over the continental shelf and shelf-break although there was a gradient in the abundance of the dominant species. For example, Chaetoceros lorenzianus, a species that prefers a bit warmer water, and Rhizosolenia and Probosia alata were found in higher abundance at the shelf-break compared to nearshore. These cross-shelf differences in species composition and abundance likely reflect differences in hydrography and nutrient conditions across the shelf.

Figure 3. Pseudo-nitzschia cells (medium size form).


HAB: Pseudo-nitzschia (PN) A significant increase in the abundance of the large-celled PN morph type was observed on May 12, although numbers were still below bloom level, about 1,000 cells/ L (Fig. 2 and 3). This increase in PN abundance was a clear response to the recent 9-day upwelling event which is common for PN. A mixture of both larger and smaller celled species (Fig. 4) were found in nearshore and offshore water, with the larger celled morph type in higher abundance compared to the smaller counterparts.

Figure 4. Pseudo-nitzschia cells (small size form).



A check on the live PN cells in seawater brought back to the lab revealed that they all appeared to be in good condition with large and bright chloroplasts. The question now is whether PN will soon become the most abundant diatom species in the phytoplankton community. Regardless, the presence and increase of the larger PN cells (the species that caused the toxic bloom in 2015) is cause for close monitoring of the phytoplankton community in the near future.

Tagged: HAB, NH Line

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