The temporal and spatial distribution of dissolved and particulate iron over the Gulf of Alaska shelf

Abstract

The Gulf of Alaska (GOA) is a region with contrasting ecosystems where the availability of the essential micronutrient iron (Fe) contributes to the observed productivity. However, knowledge on the temporal and spatial variability of iron species over the GOA shelf is limited. The offshore GOA displays lower annual production and residual nitrate in surface waters throughout the year due to low Fe supply, while high spring production is observed over the shelf due to ample nitrate and Fe supply, but these waters become nitrate limited by mid-summer. Processes promoting the exchange of the Fe rich shelf waters with the nitrate rich offshore GOA waters create favorable conditions for phytoplankton to bloom. Mechanisms for Fe introduction and transport are seasonal freshwater input, alongshore advection from the Alaska Coastal Current eddies, deep wintertime mixing, downwelling, downwelling relaxation, and/or upwelling conditions. Additional Fe sources from subsurface waters and sediment re-suspension can impact Fe distributions. Highly seasonal glacial and river input bring in an abundance of both particulate and dissolved Fe species, which differ in their biological availability. For example, dissolved Fe (DFe) is much more readily available than particulate Fe (PFe). The PFe pool can be separated into a labile fraction, which is potentially transferable to the dissolved phase on time scales relevant to phytoplankton blooms, and a refractory fraction, which is considered biologically unavailable. Seawater samples to determine Fe speciation were collected in spring and early fall of 2013 during three GOA scientific cruises. Trace metal clean procedures were followed during sample collection, processing and analysis. Seawater samples were collected by two methods: 1) Vertical samples were obtained using custom-made samplers (UAF vanes) and filtered offline for PFe analysis; 2) surface samples were obtained by using a towed pump system ("the Fe fish") and filtered in-line for DFe analysis. The PFe fractions of suspended particles were further processed using chemical separation: a) 25 % acetic acid leach with a reducing agent to determine leachable particulate Fe; b) complete digestion of the filter using strong acids to determine refractory particulate Fe. Quantitative determination was by inductively coupled plasma mass spectrometry. Results indicate the broader Western GOA shelf displayed higher average concentrations of total particulate Fe (~121 nM on average) compared to the narrower Southeastern GOA shelf (~18 nM on average). Areas of high glacial input, such as in the vicinity of the Copper River discharge (western side of Kayak Island) and within Prince William Sound near Columbia Glacier, exhibited highly elevated concentrations of total particulate Fe (~430 nM to ~1100 nM). When comparing geographic location, the suspended leachable particulate Fe was higher (~ 22%) over the Southeastern shelf, while the Northern and Western shelf had lower percentage of leachable Fe (11 - 12 %). Over the Southeastern shelf, DFe concentrations were higher in late spring ranging (0.22 - 3.13 nM), while in early fall concentrations were lower (0.07 - 0. 84 nM). Surface water results indicate that there is a significant input of PFe and DFe that occurs in the early fall that extends over much of the Northern shelf and at the inner Western shelf. Variability in downwelling, downwelling relaxation, and upwelling conditions were observed to impact Fe distributions over the Southeastern shelf. These results highlight the impact that the intense environmental variability characteristic of the GOA has on the distribution of Fe species seasonally and geographically.