Northwest Fisheries Science Center

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Blog Entries for January 2017

Early winter diatom bloom and the continued presence of Pseudo-nitzschia off Newport

By Xiuning Du
Posted on January 17, 2017

Figure 1. The most dominant diatom Ditylum brightwelli from a phytoplankton net tow sample. You can see active cell dividing and a background presence of other diatom species.

On a beautiful sunny, but cold, afternoon January 12, the Peterson lab headed out for the first Elakha cruise of 2017 along the Newport Hydrographic line.

One of the surprising observations during the trip was that there was a prominent phytoplankton bloom occurring from nearshore out to 20 miles offshore. During this cruise, we collected a phytoplankton net tow at a mid-shelf station, located 5 miles offshore, for the qualitative examination of the micro-sized phytoplankton species in the upper water column. After taxonomic examination of the sample back on shore using a high-powered light microscope, we found out what species are actively growing in the ocean now.

This winter phytoplankton bloom was dominated by the diatom Ditylum brightwelli (see Figure 1, an image of phytoplankton net tow sample), a larger-celled diatom species. Besides this dominant species, there was a diverse array of other relatively abundant larger celled diatoms in the background, mostly of species that form chains such as Thalassiosira rotula, Chaetoceros spp. and Leptocylindrus danicus spp.

Figure 2. The sparkling presence of Pseudo-nitzschia australis (red arrow).

Larger-celled, and potentially toxic, Pseudo-nitzschia species were also seen in short-chain form, e.g. two cells (see Figure 2). This is the same species that caused the unprecedented toxic bloom in spring/summer 2015 and also in Sept-Oct 2016. Both of these harmful algal blooms led to harvest closures of razor clams for several months and the delayed opening of the Dungeness crab season for several weeks during both years.

An immediate question came to mind: Will those few cells grow into another large Pseudo-nitzschia bloom and become toxic again sometime in late winter and spring? Close monitoring of cells and toxin levels are needed from now on.

A high diversity of larger dinoflagellate species was also very noticeable, and those species were from the genus Cetatium (Figure 3) and Protoperidinium.

Figure 3. One of the dinoflagellate Ceratium spp. species (C. fusus).

In summary, it appears there is a well-balanced complex of species in the microplankton community in the shelf waters now, which should benefit the zooplankton that graze on these species. However, timing can be an issue. There were few large copepod species in this sample; instead many small copepods abound. The winter phytoplankton bloom occurs often, as we observed along the NH line in the last two years, but this usually occurs in February. At present, we can only guess as to whether a good phytoplankton prey field will actually fuel more secondary production in the following months as we continue to sample.

Tagged: HAB, NH Line

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