Northwest Fisheries Science Center

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

Fall transition in the phytoplankton community and harmful algae

By Xiuning Du
October 10, 2017

Large diatom blooms were common all summer and into early September at our long term continental shelf station NH-5 (five miles offshore of Newport). However, towards the end of September, a shift in the phytoplankton community occurred, changing from a diatom dominated community to one with more dinoflagellates. The largest change then was an increase of dinoflagellate species diversity and a decrease in diatom diversity. 

Figure 1. Pseudo-nitzschia species (4-5 cell chain) in the net tow sample from NH-5.

On a sunny morning on October 4th, we conducted a short cruise to NH-5. The clarity of the water from the boat and a clean zooplankton net after being hauled through the water indicated the disappearance of a phytoplankton bloom. Upon further analysis under the microscope, we actually saw a mixed composition of diatoms and dinoflagellates in the sample. A few species of diatoms, Cylindrotheca closterium, Proboscia alata and Leptocylindrus danicus, were relatively abundant compared to other diatom species. Also, smaller dinoflagellates were abundant and the diversity of larger dinoflagellates, such as Dinophysis and Protoperidinium, was relatively high. 


Harmful algae
Pseudo-nitzschia, the diatom that produces domoic acid, has been persistently present throughout the summer, however they were rare in this recent sample (Figure 1). 

Figure 2. Relatively abundant Alexandrium catenella (6 cell chain on the right side).

In contrast, harmful dinoflagellates were present in the net tow sample. Alexandrium catanella (Figure 2) is known to produce saxitoxins which can contaminate bivalves (e.g. mussels) and possibly lead to paralytic shellfish poisoning at low cell concentrations. We also observed the dinoflagellate species Dinophysis acuminata, D. fortii (Figure 3), D. rotundata and D. parva that occur offshore Newport. Some of these species produce okadaic acid and dinophysistoxins, which cause diarrhetic shellfish poisoning. Concentrations of Dinophysis species were low on Oct 4, but it will be important to monitor whether these concentrations increase.  


Figure 3. A single Dinophysis fortii cell.

Shifts in the physical conditions
These observed transitions of phytoplankton species composition and abundance, and harmful algae closely followed changes of the coastal wind direction, magnitude and persistence. North/northwest winds (upwelling favorable) significantly weakened after early September while southerly winds (downwelling favorable) occurred two to three times for 5 to 6 days in the last month. Phytoplankton species have in turn responded to these seasonal changes of the coastal winds and the underlying physical processes.

Tagged: NH Line, HAB

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