Taxonomic notes and preservation constraints

Most cells were poorly preserved, frequently appearing deformed, degraded, or lacking diagnostic protective structures such as frustules or thecae. Chain- and colony-forming taxa, including Thalassiosira and Chaetoceros, were often observed as solitary cells, which limited identification because colony organisation and chain structure are important taxonomic characters.

At some stations, Phaeocystis occurred in forms consistent with a transition between vegetative and flagellated stages. Relatively high abundances of ciliates and dinoflagellates were also recorded. Together with the degraded condition of many diatom cells, these features are consistent with a late- or post-spring-bloom assemblage, although this interpretation should be treated cautiously.

Specimens assigned to Chaetoceros sp. cf. convolutus or concavicornis were morphologically closer to C. convolutus, with some cells resembling forma trisetosa. However, most cells were solitary and had heavily damaged setae. Because setae arrangement and curvature are key diagnostic characters separating these taxa, species-level identification was not reliable.

Most Thalassiosira cells in the 20-40 um size range were probably attributable to T. gravida. However, poor preservation, cell swelling and possible early encystment prevented confident species-level assignment.

The two Chaetoceros size classes, 5-10 um and 10-20 um, should be interpreted as operational counting categories rather than distinct taxonomic units. Both classes likely contained overlapping species assemblages, potentially including C. gelidus, C. filiformis, C. tenuissimus and C. furcillatus. The same species may occur across both size ranges because of natural size variability and deformation during degradation. Size-based counting was therefore used for practical reasons, including size fractionation and future biomass estimation of dominant forms, but does not imply taxonomic separation.