We present ecological information regarding the diatom Pseudo-nitzschia allochrona and other P. delicatissima-like  species, collected in surface waters at long term ecological research (LTER) site, LTER-MC, in the Gulf of NAples. Cell density data were obtained for the group including P, allochrona, P. arenysensis, P. delicatissima and P. dolorosa, which are not distinguishable in light microscopy, and are presented along with temperature, salinity and nutrient data collected simultaneously. Based on molecular data from isolated strain and metabarcoding, Pseudo-nitzschia allochrona was shown to occur invariably only in summer and autumn, whereas the other three species were always detected in winter or spring. We arbitrarily attributed autumn-winter density data to P. allochrona and spring-summer ones to the other three species. This way we could investigate the ecological niche of P. allochrona compared to that of the closely related species.

For the collection and generation of cell abundance data of Pseudo-nitzschia, phytoplankton samples were collected in surface waters at the LTER-Mc sampling site (40°48.5’N, 14°15’E, Gulf of Naples, Mediterranean Sea) with Niskin bottles. Subsamples were placed in 500 ml dark glass bottles and immediately fixed with neutralised formaldehyde (0.8 % final concentration). A variable amount of fixed sample (3-50 ml) was allowed to settle in a cell count chamber. Cells from two transects were counted according to the Utermöhl method (Utermöhl, 1958; Andersen and Throndsen, 2003), using an inverted microscope (Axiovert 200, Zeiss, Germany) equipped with phase contrast at a final magnification of 400X. Records of Pseudo-nitzschia cf. delicatissima (including P. delicatissima, P. arenysensis, and P. dolorosa) were separated arbitrarily from those of P. allochrona based on the period of the year. Based on cultured strain genotyping and metabarcoding data, the former are found in winter-spring and the latter in autumn-winter.

For the ancillary environmental parameters,  collection and processing methods are found in Sabia, L., Costanzo, A., D’Alcalà, M. R., Saggiomo, V., Zingone, A., & Margiotta, F. (2019). Assessing the quality of biogeochemical coastal data: a step-wise procedure. Mediterranean Marine Science, 20(1), 56–73. https://doi.org/10.12681/mms.15935

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