Primary production within the sea-ice zone west of the Antarctic Peninsula: I—Sea ice, summer mixed layer, and irradiance

Abstract

In shelf waters of the western Antarctic Peninsula (wAP), with abundant macro- and micronutrients, water-column stability has been suggested as the main factor controlling primary production; freshwater input from sea-ice melting stabilizes the upper water column by forming a shallow summer mixed layer. Retreating sea ice in the spring and summer thus defines the area of influence, the sea-ice zone (SIZ) and the marginal ice zone (MIZ). A 12-year time series (1995–2006) was analyzed to address two main questions: (1) what are the spatial and temporal patterns in primary production; and (2) to what extent and in what ways is primary production related to sea-ice dynamics. Data were collected on cruises performed during January of each year, at the height of the growth season, within the region bounded by 641S and 641W to the north and 681S and 661W to the south. Average daily integrated primary production varied by an order of magnitude, from � 250 to � 1100 mg C m � 2 d � 1 , with an average cruise primary production of 745 mg C m � 2 d � 1 . A strong onshore–offshore gradient was evident along the shelf with higher production observed inshore. Inter-annual regional production varied by a factor of 7: maximum rates were measured in 2006 (1788 mg C m � 2 d � 1 ) and minimum in 1999 (248 mg C m � 2 d � 1 ). The results support the hypothesis that primary production in the wAP shelf is related to sea-ice dynamics. To first order, shallower summer mixed-layer depths in the shelf correlated with late sea retreat and primary production. Principal component analysis showed that high primary production in January was associated with enhanced shelf production toward the coast and in the south, explaining 63% of the variability in space and time. This first mode captured the inter-annual variability in regional production. Temporal variability in primary production (time series of anomalies defined for each location) showed spatial dependence: higher primary production correlated with shallow mixedlayer depths only at mid-shelf; in coastal and offshore waters, primary production correlated with deeper mixed layers. Thus, coastal primary production can show a non-linear relationship with summer mixed layers. Under conditions of large biomass (420 mg chl a m � 3 ) and shallow mixed-layer depth (e.g., 5 m) phytoplankton production becomes light limited. This limitation is reduced with a deepening of the summer mixed layer (e.g., 20 m). Dominance of diatoms and the ability to adapt and photosynthesize at higher light levels characterized the large phytoplankton blooms. No significant regional trend in primary production was detected within the 12-year series. We conclude that the regional average primary production on the wAP shelf is associated with shallow summer mixed layers in conjunction with late sea-ice retreat. An opposite relationship is observed for the highest production rates in coastal waters, associated with large biomass, where a deepening of the summer mixed layer relieves light limitation.