, Alexander and Chapman,
1981) and subsequent ice-edge blooms (Niebauer
et al., 1995).
With cold (<-10°C) winds from the north, ice was blown over the southeastern shelf. The latent heat flux due to ice melt further reduced water temperatures (by ~2.0°C) and also reduced salinity (by ~0.5 psu). Although the latter provided a positive flux of buoyancy, it was not sufficient to prevent mixing. Wind (acting on ice or directly on water) and tidal stirring resulted in near-isothermal conditions over the middle shelf. Over the deeper outer shelf, winds provided the mixing energy to create a shallow (~30-m) mixed layer.
Tidal diffusion has been thought to dominate distribution of water properties over the middle shelf (Coachman, 1986). Our observations, however, show that advection of relatively warm, saline lower-layer water established the vertical structure over the middle shelf. Once instituted, this structure persisted. The ensuing ice melt and warming due to solar radiation enhanced the two-layer structure. Each year, nutrients are lost to the middle shelf and must be replenished by a combination of vertical mixing, release from sediments, nitrification, and horizontal fluxes (Whitledge et al., 1986). During winter (prior to formation of the two layers over the middle shelf), advective events like those observed in these records likely replenish the nutrients on the middle shelf, which are needed for blooms during the following spring. In the past, advection was thought to play no role in the nutrient and salt flux onto the middle shelf. The potential contribution for advection must now be considered as a mechanism for nutrient replenishment.
The high-resolution temporal evolution of an under-ice phytoplankton bloom
has not previously been measured. Although the sites were at similar latitudes,
the timing of initiation of the phytoplankton bloom and the ensuing chlorophyll
maximum differed markedly (~40 days), corresponding with the presence or absence
of melting ice. At mooring 2, over the middle shelf, the steady increase in
chlorophyll persisted through a 5-day period when active mixing extended to
>40 m. Chlorophyll-a increased under the ice at both locations. It continued
to increase for the duration of the ice cover, leveling off after ice retreat.
Levels of chlorophyll-a in this study were similar to those at ice algae
blooms (23 mg m, Alexander and Chapman,
1981) and subsequent ice-edge blooms (Niebauer
et al., 1995).
During years of minimal ice cover, a bloom does not begin until mid-April or early May (Sambrotto et al., 1986). During these years the trigger for the bloom is the occurrence of stratification, which allows the phytoplankton growth to exceed respiration loss. The presence of melting ice encourages an earlier bloom. Although chlorophyl-a concentrations increased during a short period when the water column was mixing, it is unknown whether this could continue if such conditions persisted.
Acknowledgments. We wish to thank L. Britt, C. DeWitt, D. Dougherty, D. Kachel, L. Long, and W. Parker for preparing the equipment and processing data, as well as S. Salo and the crew of the NOAA ship Miller Freeman for finding and recovering mooring 2. Special thanks goes to H. Milburn and the engineering group for the excellent design of the moorings, which permitted the gathering of the data presented in this paper. This research was sponsored by NOAA's Bering Sea FOCI Coastal Ocean Program. This is NOAA's Fisheries Oceanography Coordinated Investigations Contribution FOCI-B278 and NOAA's Pacific Marine Environmental Laboratory Contribution 1741.
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