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This was just two diatoms starting a small chain. There are two in the first photo. The second photo shows a longer chain. I am not 100% sure they are the same species, but they both seemed to have a similar size and shape for each cell. Each cell has two spines on each valve (four total per individual), a characteristic of members of this genus. This was the most abundant genus in this sample, which makes sense as it is one of the largest genera of marine plankton.
A marine biologist on board skimmed some plankton using really fine mesh nets and we took a drop from that sample and placed it under a microscope. The sample was skimmed in Izhut Bay just off Afognak Island in somewhat shallow water (less than 200 feet).
Plankton from this genus can be damaging to fish, as the spines can cut up the gills of small fish, and during large blooms they can even clog up their gills. Spines also can introduce bacteria when they damage tissue and also stimulate mucus production, both of which can be damaging and sometimes even deadly to fish.