\u3cem\u3eThalassiosira\u3c/em\u3e Community Composition and Diversity in Narragansett Bay

Diatoms contribute 40-45% of marine primary production. Understanding the contributions of individual species to diatom communities is important in light of warming ocean waters and changing food webs. Here, the diatom genus Thalassiosira, which exhibits irregular pulses in abundance in Narragansett Bay, is examined using data from the Narragansett Bay Long Term Phytoplankton Time Series. High-throughput sequencing (HTS) at the highly-variable V4 region of the 18S ribosomal gene identified fourteen Thalassiosira taxa in monthly samples over a six-year time period (December 2008-December 2014), seven of which had not previously been identified in this time series. During the same time period, only four Thalassiosira taxa were identified through light microscopy (LM) counts. Many of the newly identified species for this location are small in diameter and difficult to identify using LM. However, they appear to be very frequent and may have a significant ecological role. 18S gene copy number was shown to greatly effect sequence abundance for this locus and this genus, and therefore only presence data was analyzed. Thalassiosira species grouped into four seasonal assemblages, although only 25% of this pattern was explained by temperature, suggesting that other factors such as competition or prey selection may drive Thalassiosira community structure. Comparisons to historical records show possible shifts in frequency and abundance of Thalassiosira species