Physical, chemical, and microbial regimes in an anoxic fjord (Nitinat Lake)

Pawlowicz, Rich, Susan A. Baldwin, Annette Muttray, Jana Schmidtova, Bernard Laval, Grant Lamont

Limnol. Oceanogr., 52(3), 2007, 1002-1017 | DOI: 10.4319/lo.2007.52.3.1002

ABSTRACT: New observations of physical, biological, and chemical parameters were obtained between July 2003 and February 2005 in Nitinat Lake, British Columbia, a permanently anoxic seawater fjord. These observations include continuous profiles of nitrate and bisulfide using a new instrument for in situ ultraviolet spectrophotometry. Anoxic seawaters in this 200-m-deep basin can be found as shallow as 7 m. However, Nitinat Lake is surprisingly dynamic, with a deep-water renewal time of only about 5 yr and significant spatial and temporal variability. Exchange with the ocean is limited by the size of the linking channel, and during low tides the outflow is subject to an hydraulic control. This nonlinearity causes a relatively large fortnightly cycle in water level and explains flooding events associated with heavy rainstorms. In addition, the restricted flow makes deep-water renewal gradual so that anoxic conditions are preserved. Intermediate waters (5-40 m in depth) are renewed alternately by oxygenation from surface mixing in winter and by upwelling of anoxic water and horizontal inflow of subducted ocean water in summer. The horizontal inflow results in a subsurface region of suboxic water in mid-fjord during summer. No suboxic regime is evident at either the river end of Nitinat Lake (where oxygen and sulfide-rich regions adjoin in a nitrate-depleted water column) or at the ocean end of Nitinat Lake (where oxygen and nitrate-rich regions sometimes overlap with sulfide-rich regions). Maximum phytoplankton levels near the surface are high in both winter and summer. A second peak in biological activity appears near the anoxic boundary, possibly as a result of sulfur-oxidizing bacteria. Sulfate-reducing bacteria are also present in anoxic water in both seasons and just above the oxic/anoxic interface in winter and just above the suboxic/anoxic interface in summer.

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