Morphologic comparisons of shallow and deepwater benthic marine diatoms of Onslow Bay, North Carolina

In October 2003, in-situ, living benthic marine diatoms were recovered from sandy sediments from 67 m to 191 m depth in Onslow Bay, North Carolina, by a remotely operated vehicle tethered to the R/V Cape Hatteras. Fourteen stations were sampled, with surface incident irradiance flux ranging from 3.740% of the surface irradiance at the shallowest station to 0.028% at the deepest. Attenuation coefficients fluctuated with depth, averaging between 0.0443 (4.43% loss per meter) and 0.0842 (8.42% loss per meter). Bottom temperatures decreased with depth from 25.3 °C to 14.9°C. One hundred twenty-six species of 29 genera were identified from cleaned sediments from all 14 stations. Eleven live species from six genera were documented as live and identified from the shallowest and deepest of the fourteen stations (Actinoptychus splendens, Amphora coffeaformis, Cocconeis disculus, Cocconeis distans, Cocconeis placentula, Diploneis chersonensis, Navicula Sp. a, Navicula digitoconvergens, Nitschia frustulum, Nitschia pellucida, and Nitschia panduriformis), and analyzed morphometrically based on apical and transapical axis lengths, degree of frustal ornamentation, and surface area to volume ratios. These data were then compared to those for shallow water samples previously collected from intertidal marsh sediments on Masonboro Island, North Carolina. Though apical axis lengths were significantly larger in the shallow water samples (p = 8.6 x 10-6), transapical axis lengths and degree of ornamentation did not differ significantly between deepwater and shallow water regions. Surface area to volume ratios of three out of five live species found in both shallow water and deepwater were significantly lower in the deepwater stations (p = 0.00045 to 0.0088), indicating that differences in shape may affect the efficiency of light collection. The results of this study indicate taxonomic diversity and variety of frustal morphology of benthic marine diatoms are greater shallow water; however, the presence of these microalgae in the deeper sediments of Onslow Bay may have far-reaching impacts on nutrient cycling and food web dynamics on the shelf and upper slope