MEPS 225:123-137 (2002)  -  doi:10.3354/meps225123

Thin layers and camouflage: hidden Pseudo-nitzschia spp. (Bacillariophyceae) populations in a fjord in the San Juan Islands, Washington, USA

J. E. B. Rines1,*, P. L. Donaghay1, M. M. Dekshenieks2, J. M. Sullivan1, M. S. Twardowski3

1Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, Rhode Island 02882-1197, USA
2Ocean Sciences Department, University of California Santa Cruz, Santa Cruz, California 95064, USA
3Department of Research, Western Environmental Technology Laboratories, Inc., 165 Dean Knauss Drive, Narragansett, Rhode Island 02882, USA

ABSTRACT: Two sets of observations were made on the distribution of Pseudo-nitzschia taxa in a fjord in the San Juan Islands, Washington, USA. From May 21 to 31, 1996, we observed the spatio-temporal distribution of a dense bloom of P. fraudulenta. Microscopic observations of live material were compared to physical-optical water-column structure, currents and wind. At the start of the study, dense concentrations of Pseudo-nitzschia spp. were observed directly at the surface. Optical profiles indicated that most cells were concentrated in a thin layer at ~5 m depth, which appeared to be contiguous throughout the sound. Several days later, sustained winds forced a plume of lighter water over the surface of the sound, displacing the original water mass, with its entrained flora, to depth. The resulting near-bottom thin layer persisted for several days, and contained >106 Pseudo-nitzschia spp. cells l-1. Microscopic examination of live cells from the deep layer revealed that colonies were alive and motile. In 1996 and again in 1998, we observed P. pseudodelicatissima living within colonies of Chaetoceros socialis. Water-column thin layers, near-bottom thin layers and populations of Pseudo-nitzschia spp. within C. socialis colonies could easily escape detection by routine monitoring procedures, and may be a potential source of unexplained toxicity events.

KEY WORDS: Pseudo-nitzschia · Chaetoceros socialis · Thin layers · Physical forcing

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