Healy 02-03 (SBI Process Cruise 2) Service Team Report

Observations:

On this leg (Healy 02-03, 15 July – 26 August 2002), we completed 159 CTD casts and collected underway acoustic Doppler current profiler data. We have also analyzed 300 frozen nutrient samples from the Bering Sea collected during Healy cruise 02-02. All data have been edited and posted on the JOSS web site, and we have plotted the data from our sections in Bering Strait, Barrow Canyon, East Barrow Canyon, East Hanna Shoal, West Hanna Shoal, and a final cross shelf section. Our observations include continuous vertical profiles of temperature, salinity, dissolved oxygen, fluorometric “chlorophyll”, light transmission, Haardt fluorescence (an index of colored organic matter), and photosynthetically available radiation (PAR). Discrete sea water samples collected from our 30 liter rosette bottles have been analyzed for salinity, dissolved oxygen, ammonium, nitrate, nitrite, urea, phosphate, silicate, chlorophyll and phaeophytin concentrations.

Results:

In contrast with our June-July cruise (Healy 02-01) when several stations had surface nitrate concentrations in excess of 10 micromolar, surface nutrient concentrations in the Bering Strait inflow waters were low. In fact, nitrate was depleted or nearly so in the surface waters of all stations taken during Healy 02-03. During Healy 02-01, nitrate depletion was observed only in surface waters at our most offshore stations.

In the Bering Strait section taken during this leg, the highest fluorometric chlorophyll values were found over the bottom not at the surface, suggesting post-bloom conditions.

In our suite of Beaufort Sea stations subsurface chlorophyll maxima, dissolved oxygen supersaturations, and light transmission minima were common suggesting a downward penetration of the depth of maximum phytoplankton growth. This condition was made possible by the extreme stratification of the photic zone arising from melting ice, light winds and suppression of waves by the ice-cover. At some stations, measured surface salinities fell below 15, and one could see density currents in the upper few cm as the CTD/rosette was lowered into the water column. During these times when winds were weak and melting ice was present, large differences occurred between the CTD salinity and bottle salinity over a distance of only ~ 1m, and it is possible that the surface sea water sample was stratified within the Niskin bottle!

As was the case during Healy 02-01, nutrient distributions provided abundant evidence for shelf-basin interactions. Plumes of elevated silicate, phosphate, ammonium, urea and nitrite and N** concentrations extended from the shelf into deeper offshore waters at depths of ~50-200 meters on all cross-shelf sections except for the East Barrow Canyon Section, where the situation appeared to be a bit more complicated. Here, only the urea distribution showed a plume that seemed to have a clear origin on the shelf. Maxima in other variables had a tendency to peak offshore and to intersect the shelf break instead of the more shallow portions of the shelf. Our initial impression is that, in the East Barrow Canyon section, the silicate, phosphate, etc. maxima were under the influence of advection from the west and possibly complicated by the presence of an eddy. A Haardt fluorescence maximum that appears to have an offshore origin interacted with the plumes originating on the shelf during both Healy legs, and it will be interesting to use the offshore tracer provided by Haardt fluorescence and the data on the plumes emanating from the shelf to gain insight into the interactions of shelf and offshore waters.

A notable difference between Healy 02-01 and Healy 02-03 (when we compared data from the same sections) was an increase of 5 to 10 micromolar in maximum silicate concentrations in the plume originating over the shelf. This observation suggests fairly rapid settling and re-mineralization of diatoms produced by the spring bloom over the shelf. Indeed, this change may have begun during Healy 02-01 since our last section had the highest silicate concentrations in this plume, and light transmission data suggested an ongoing bloom producing rapidly sinking particles (presumably diatoms).

Analysis:

Our first cruise (Healy 02-01) during June-July appeared to begin early in the growing season with nutrient depletion evident only at our deepest (most offshore) stations in the Bering Sea. By the end of this leg, we began to see the beginning of a bloom over the shelf. It may also be worth noting that during Healy 02-01, we saw an abundance of ice-algae suggesting that the ice-algae bloom had peaked. By the time of Healy 02-03, the ice algae and surface layer blooms seemed to have ended, but subsurface chlorophyll maxima and associated oxygen supersaturations and light transmission minima suggested healthy rates of production deeper in the photic zone.

Consistent with the ending of the ice algae and surface layer blooms, silicate concentrations in the bottom waters over the shelf and in the high-silicate bloom extending offshore increased as the presumable consequence of the sinking and re-mineralization of diatoms. What was somewhat surprising to us was the rapidity of the increase in silicate concentrations between legs.

Despite a vigorous low pressure system that came by towards the end of the cruise with some wind gusts exceeding 40 knots, the stratification of the waters that we encountered was so strong, that we observed no significant interruption of the surface nitrate depletion.