MEPS 233:1-12 (2002)  -  doi:10.3354/meps233001

Occurrence and mechanisms of formation of a dramatic thin layer of marine snow in a shallow Pacific fjord

Alice L. Alldredge1,*, Timothy J. Cowles3, Sally MacIntyre2, Jan E. B. Rines4, Percy L. Donaghay4, Charles F. Greenlaw5, D. V. Holliday5, Margaret M. Dekshenieks4,**, James M. Sullivan4, J. Ronald V. Zaneveld3

1Department of Ecology, Evolution and Marine Biology, and
2Marine Science Institute, University of California, Santa Barbara, California 93106, USA
3College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, USA
4Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882-1197, USA
5BAE Systems, 4669 Murphy Canyon Road, Suite 102, San Diego, California 92123-4333, USA
*E-mail: **Present address: Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA

ABSTRACT: Huge accumulations of diatom-dominated marine snow (aggregates >0.5 mm in diameter) were observed in a layer approximately 50 cm thick persisting over a 24 h period in a shallow fjord in the San Juan Islands, Washington, USA. The layer was associated with the 22.4 σt density surface. A second thin layer of elevated phytoplankton concentration located at a density discontinuity 1.5 to 2 m above the marine snow layer occurred within a dense diatom bloom near the surface. At the end of the study period, isopycnals shoaled and the 2 layers merged. More than 80% of the diatom bloom consisted of Thalassiosira spp. (50 to 59%), Odontella longicruris (5 to 14%), Asterionellopsis glacialis, and Thalassionema nitzschioides. A much higher proportion of O. longicruris occurred in marine snow (about 53%) than among suspended cells suggesting that this species differentially aggregated. Most zooplankton avoided the mucus-rich aggregate layer. The layer of marine snow was formed when sinking aggregated diatoms reached neutral buoyancy at the 22.4 isopycnal, probably due to the presence of low salinity mucus resistant to salt exchange in the interstices of the aggregates. Rates of turbulent kinetic energy dissipation throughout the water column rarely exceeded 10-8 m2 s-3 and aggregates below the thin layer were largely detrital in composition indicating that small-scale shears due to turbulence did not erode the layer of marine snow. The accumulation of marine snow and phytoplankton in persistent, discrete layers at density discontinuities results in habitat partitioning of the pelagic zone, impacts the distribution and interactions of planktonic organisms as well as the intensity and location of biological processes in the water column, and helps maintain species diversity.

KEY WORDS: Thin layers · Marine snow · Diatom bloom flocculation · Density discontinuity · Plankton patchiness · Plankton distribution

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