Long-term (1993–2013) changes in macrozooplankton off the Western Antarctic Peninsula
Introduction
The Western Antarctic Peninsula (WAP) is a highly productive region of the Southern Ocean, supporting a high biomass of krill (Atkinson et al., 2009) and higher-level consumers such as penguins, seals, and whales (Smetacek and Nicol, 2005, Nowacek et al., 2011). The WAP region has undergone considerable change and rapid warming, with an increase in winter air temperatures of 6 °C from 1950–2001 (Vaughan et al., 2003), and increases in sea surface temperature, ocean heat content, and wind (Meredith and King, 2005, Martinson et al., 2008, Holland and Kwok, 2012). These changes are coupled to a rapidly shortening annual sea ice season caused by an earlier spring sea ice retreat and a later autumn sea ice advance (Stammerjohn et al., 2008a, Stammerjohn et al., 2012). A consequence of this regional warming is that a latitudinal climate gradient exists along the length of the WAP, with a warmer, moist, maritime climate in the north, and a colder, drier, continental climate in the south (Smith et al., 1999, Ducklow et al., 2012a, Ducklow et al., 2012b, Ducklow et al., 2013). A manifestation of this gradient is that the northern WAP near Palmer Station on Anvers Island is presently nearly ice-free (sea ice season duration ~3.5 months), while in the south (Marguerite Bay southward) perennial sea ice still persists (~7.5 months) – although sea ice has decreased in extent and duration throughout the WAP over time (Ducklow et al., 2013).
These environmental changes are affecting the WAP marine pelagic ecosystem differently along a north to south gradient. In the far northern WAP (tip of Peninsula to Anvers Island), the increase in winds and clouds, juxtaposed on a longer open water season has led to deeper mixed layers and more variable light conditions, resulting in decreased phytoplankton (chlorophyll a) biomass (Montes-Hugo et al., 2009) and shifts from larger diatoms to smaller flagellates (Montes-Hugo et al., 2010), compared to the south. Recent north vs. south comparisons within the Palmer Long Term Ecological Research (PAL LTER) mid-WAP study region also indicate smaller and less abundant microzooplankton in the north (Garzio and Steinberg, 2013), and lower lipid content and thus food quality of Antarctic krill (Euphausia superba) in the north (Ruck et al., 2014). There are distinct regional (Ross et al., 2008, Ross et al., 2014, Parker et al., 2011) and interannual (Ross et al., 2008, Ross et al., 2014) patterns of abundance in the major taxa of macrozooplankton in the mid-WAP. These differences in turn affect the relative impact of various macrozooplankton grazers, as illustrated in a two-year study showing shifts from dominant grazing by krill and pteropods in the south, to salps in the north (Bernard et al., 2012), and is suggested as a mechanism affecting WAP phytoplankton community composition (Garibotti et al., 2003). Long-term changes in the WAP food web, as characterized by an inverse model approach, indicate the north WAP is characterized by an increasingly dominant role of the microbial loop (as opposed to krill) in food web carbon flow, while in the south there is no detectable long-term trend toward dominance of either a microbial- or a krill-based food web (Sailley et al., 2013).
Both long-term (i.e., >10 years) changes in macrozooplankton abundance attributed to warming, and sub-decadal-scale shifts attributed to oscillations in atmospheric forcing, have been documented in the Antarctic Peninsula region. A long-term shift from a dominance of the Antarctic krill, Euphausia superba, to salps (mainly Salpa thompsoni) occurred in the SW Atlantic sector of the Southern Ocean during the period 1926–2003 (Atkinson et al., 2004). In the Elephant Island region just north of the Peninsula, a decline in sea ice during the period 1976–1996 was significantly correlated with a decrease in Antarctic krill and an increase in salps. In the WAP there was an increase in the range and frequency of occurrence of salps over the period 1993–2008 (Ross et al., 2008, Ross et al., 2014), with a negative correlation between salp abundance and both the timing of sea ice advance and duration of ice cover (Ross et al., 2008). Superimposed on these long-term warming trends are decadal-scale climate oscillations, such as the El Niño Southern Oscillation (ENSO), that affect sea ice dynamics (e.g., Yuan, 2004, Stammerjohn et al., 2008b) and sea surface temperature (Renwick, 2002). ENSO has been linked to variability in Antarctic krill reproductive and recruitment success along the WAP (Quetin and Ross, 2003, Loeb, 2007, Loeb et al., 2009, Ross et al., 2014), and to the abundance of other species. There was no long-term directional trend in the abundance of salps in the Elephant Island region during the period 1993–2009, but rather a strong correlation with ENSO cycles (Loeb and Santora, 2012). In the same region and time period, abundance peaks in pteropods were also related to ENSO (Loeb and Santora, 2013).
The purpose of this study is to characterize interannual variability and long-term trends in major taxa of macrozooplankton (euphausiids, salps, pteropods, chaetognaths, polychaetes, and amphipods) along the WAP over two decades (1993–2013), primarily in the context of the north-south climate gradient, but also considering the coastal-shelf- offshore (slope) gradient. Our study, part of the PAL LTER program, adds up to an additional decade of macrozooplankton abundance data to previous analyses of this region of the WAP (Ross et al., 2008, Ross et al., 2014), compliments time-series analyses of the far northern Elephant Island region of the Peninsula as surveyed by the Antarctic Living Marine Resources (AMLR) program (Loeb et al., 2009, Loeb and Santora, 2012, Loeb and Santora, 2013), and increases the breadth of the taxa considered. Further, we explore relationships between macrozooplankton abundance and a number of environmental parameters including: sea ice, atmospheric climate indices, sea surface temperature, and phytoplankton biomass (chl a) and productivity. Identification of long-term changes in zooplankton and the environmental factors affecting their abundance is key for predicting ecosystem change in this productive and sensitive polar ecosystem.
Section snippets
Study area
The PAL LTER study region is located mid-way down the western side of the Antarctic Peninsula, and extends from Palmer Station on Anvers Island (64.77°S, 64.05°W) in the north to approximately 700 km south near Charcot Island (69.45°S, 75.15°W), and from coastal to slope waters approximately 200 km offshore (Ducklow et al., 2007, Ducklow et al., 2012a, Ducklow et al., 2012b) (Fig. 1). A grid of stations was sampled on PAL LTER annual research cruises during austral summer (approximately 01
Overall abundance by region
The most abundant taxa overall were the euphausiids (krill) Thysanoessa macrura and Euphausia superba (mean 203 and 111 ind. 1000 m−3, respectively), followed by the pteropod Limacina helicina, salp Salpa thompsoni, and krill Euphausia crystallorophias (mean 65, 42, and 27 ind. 1000 m−3, respectively) (Table 1, Full Grid). Large chaetognaths, tomopterid polychaetes, and amphipods were several orders of magnitude lower in abundance and each generally averaged <10 ind. 1000 m−3 (Table 1). For most
Euphausiid patterns of abundance and links with primary production, ice, and climate
We did not find a long-term decrease in Antarctic krill, E. superba, unlike the decrease reported for the SW Atlantic sector of the Southern Ocean (Atkinson et al., 2004) and for the CCAMLR study region in the far northern WAP (Loeb et al., 1997). These long-term decreases in E. superba north of the PAL LTER survey grid have been attributed to trends in regional warming and decreasing ice cover. On a finer spatial scale in the 1993–2008 period, Ross et al. (2014) documented a~200 km shift
Conclusion
One of our predictions was that the north–south climate gradient along the WAP affects zooplankton abundance over space and time, and for several species we did find increases over time that were significant in one region but not another. For example, T. macrura increased in the North, and E. crystallorphias in the South. These long-term trends may ultimately be a result of increasing PP across the grid, especially in the South. We note that both our ‘North’ and ‘South’ subregions constitute
Acknowledgments
We thank the Captain, officers, and crew of the MV Polar Duke and ARSV Laurence M. Gould, and Raytheon Polar Services and Lockheed Martin personnel for their scientific and logistical support. We are grateful to the many student volunteers that assisted during the PAL LTER cruises and in the laboratory. We thank Doug Martinson and Rich Iannuzzi for consult on sea surface temperature data, Grace Saba for advice with climate index analyses, Josh Stone for assistance with figure preparation, and
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