USGS CMG 2010-051-FA Metadata

Trace metal sampling in the Copper River region of the Gulf of Alaska.

Recent work has suggested that high iron supply may contribute to a northward 
increase in phytoplankton biomass along the U.S. west coast, consistent with 
'bottom-up' control of these coastal ecosystems.
We examined this hypothesis in waters of the Copper River plume and nearby 
continental shelf in the northern Gulf of Alaska.  These are the first data we 
know of examining 'bottom up'
control of this coastal ecosystem by the supply of nitrate and iron.  These are 
also the first data we know of examining seasonal variability in the mechanisms 
that supply iron to this region, and in the iron concentrations.
Cruises were carried out in early April (late winter conditions), early May 
(peak bloom conditions) and late July (peak river discharge).
Sampling consisted in each case of bottle and CTD casts at five stations, and 
continuous surface-water sampling.
High concentrations (several hundred nM) of 'total dissolveable' Fe (unfiltered, 
pH ~2) were present in surface-waters spanning the continental shelf in
early April 2010, from resuspension of fine glacial flour.  Concentrations 
decreased dramatically beyond the shelf break.
This fine particulate matter represents a large source of 'dissolved' Fe to 
these waters.
Surface-water nitrate concentrations were fairly uniform (~15 uM) across the 
entire shelf at this time, due to deep winter mixing.  By late July this shelf 
particulate Fe source is greatly diminished, owing to strong stratification.
Yet there is abundant 'total dissolveable' Fe (several uM) at this time from the 
Copper River plume (largest single freshwater source to the GoA) and lower, 
significant concentrations in the AK coastal current (that reflect the 
cumulative impact of melting glaciers from further south).
By late July this abundant supply of iron in the Copper River vicinity, together 
with strong stratification, lead to complete consumption of surface-water 
nitrate across the entire shelf (and extending tens of km beyond the shelf).
These data are consistent with the idea that high primary productivity in this 
region is fueled by abundant wintertime surface-water nitrate, together with 
iron supply from fine, labile, glacier-derived particulate matter from 
seasonally variable sources.

Days at Sea: 24
 Degree of Certainty: 90%
 Report Date: 2010-06-24
Project Number: GX11LQ00E8KWH00

USGS BRD Global Change