Northwest Fisheries Science Center

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

The winter phytoplankton bloom

By Xiuning Du
February 25, 2016

Figure 1. Diatom Thalassiosira nordenskioeldii (left) and Thalassiosira aestivalis (right).

Plankton samples collected during the recent research cruise aboard the NOAA ship Bell Shimada showed that a winter phytoplankton bloom was developing off the Oregon coast. Blooms do not always occur at this time of year and only occur following several sunny and cloud-free days; weather that is just perfect for the development of a winter phytoplankton bloom in the coastal waters of the northern California Current. We’ve observed a winter bloom in about half of our winter samples since 1996, and we last observed a winter bloom in 2015.

Water samples collected from a location 9 km from shore were filtered onto a glass fiber filter and the filter quickly became brownish due to the phytoplankton trapped on the filter. When a water sample was examined under a microscope, beautiful diatoms were found to be the dominant group in this phytoplankton assemblage. Some of the species are commonly seen off Oregon such as the dominant genus Thalassiosira, with two species being the most abundant (Figure 1). Species from two other common genera, Asterionellopsis and Chaetoceros, were also present (Figure 2), though numerically less abundant, as were a few other diatoms (Figure 3).

Figure 2. Diatom Asterionellopsis glacialis (left) and Chaetoceros contortus (right).

Compared to diatoms, dinoflagellate species were much less abundant. Only a few individuals were seen. I found a very rare dinoflagellate, a Ceratium species (Figure 4) slowly moving around, and this is the first time that I have seen this unusual dinoflagellate: it is an oceanic and warm-water species, indicating the water on the shelf at present originated from a farther offshore source.

Figure 3. Diatom Detonula pumilar with auxospores (in the middle of the chain) that are used for enlarging the cells of this species.

Although phytoplankton blooms are not common in winter, they are critical for the annual production cycle of many marine biota because they provide food for the principal predators, the zooplankton and krill, during winter. Zooplankton and krill are waiting for this bloom, because although these species have lipid reserves that help them overwinter, they are hungry for breakfast when they awake from their dormant over-wintering period. Zooplankton and krill species not only rely on this bloom for sustenance, they also rely on it for energy for spawning over a period of several weeks, thus producing a new cohort of individuals. This cohort grows up and matures by April or May.

Figure 4. Dinoflagellate Ceratium platycorne. This species is identified by its inflated hypothecal (open) horns that are almost symmetrical and have serrated margins.

Thus, a secondary benefit of winter phytoplankton blooms is a large biomass of zooplankton and krill which serves as prey for fish, birds and mammals which migrate to Oregon’s waters in spring to feed on this lipid-rich food web. Without a winter bloom, the biomass of plankton would be low during spring leading to poor feeding conditions for the small fishes upon which migrating salmon and seabirds feed. Thus although the magnitude of winter phytoplankton blooms are small, perhaps they are equally important as spring and summer blooms to the ecosystem productivity because they fuel productivity in winter and spring. This is one of many reasons why we track the abundance and production of plankton in waters off Oregon year around, not just during the productive upwelling season from May to September.

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