Abstract
The West Antarctic Peninsula (WAP) has experienced significant change over the last 50 years. Using a 24 year spatial time series collected by the Palmer Long Term Ecological Research programme, we assessed long-term patterns in the sea ice, upper mixed layer depth (MLD) and phytoplankton productivity. The number of sea ice days steadily declined from the 1980s until a recent reversal that began in 2008. Results show regional differences between the northern and southern regions sampled during regional ship surveys conducted each austral summer. In the southern WAP, upper ocean MLD has shallowed by a factor of 2. Associated with the shallower mixed layer is enhanced phytoplankton carbon fixation. In the north, significant interannual variability resulted in the mixed layer showing no trended change over time and there was no significant increase in the phytoplankton productivity. Associated with the recent increases in sea ice there has been an increase in the photosynthetic efficiency (chlorophyll a-normalized carbon fixation) in the northern and southern regions of the WAP. We hypothesize the increase in sea ice results in increased micronutrient delivery to the continental shelf which in turn leads to enhanced photosynthetic performance.This article is part of the theme issue ‘The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change’.
Highlights
The West Antarctic Peninsula (WAP) is a highly productive marine ecosystem characterized by large phytoplankton blooms [1,2,3,4]
In the southern WAP, the upper ocean mixed layer depth (MLD) has declined by a factor of 2
Associated with the recent increases in sea ice over the last decade, there has been an increase in the photosynthetic efficiency in the WAP
Summary
The West Antarctic Peninsula (WAP) is a highly productive marine ecosystem characterized by large phytoplankton blooms [1,2,3,4]. Regional decadal studies of the dynamics in the upper mixed layer and its relationship to the phytoplankton in the Southern Ocean have been limited to local sampling from shore-based field stations [10,26,31,32]. Our results show that the upper MLD in the southern regions of the WAP has declined significantly over the last two decades These changes have 62° S. significant implications for the ecosystem and have resulted in an increase in biomass-normalized phytoplankton productivity. Following the thresholds set by Lorbacher et al [36], for the analyses presented in this study, a QI of 0.5 was used to reasonably warrant a calculation of MLD This determination of MLD is based on the principle that there is a near-surface layer characterized by quasi-homogeneous properties and where the standard deviation of the property within this layer is close to zero. The correlation slopes were 0.89 (p 0.01) and 1.05 (p 0.01) for the north and south respectively
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