Critical indirect effects of climate change on sub-A ntarctic ecosystem functioning
- Authors: Allan, E Louise , Froneman, P William , Durgadoo, Jonathan V , McQuaid, Christopher D , Ansorge, Isabelle J , Richoux, Nicole B
- Date: 2013
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/457934 , vital:75696 , xlink:href="https://doi.org/10.1002/ece3.678"
- Description: Sub‐Antarctic islands represent critical breeding habitats for land‐based top predators that dominate Southern Ocean food webs. Reproduction and molting incur high energetic demands that are sustained at the sub‐Antarctic Prince Edward Islands (PEIs) by both inshore (phytoplankton blooms; “island mass effect”; autochthonous) and offshore (allochthonous) productivity. As the relative contributions of these sustenance pathways are, in turn, affected by oceanographic conditions around the PEIs, we address the consequences of climatically driven changes in the physical environment on this island ecosystem. We show that there has been a measurable long‐term shift in the carbon isotope signatures of the benthos inhabiting the shallow shelf region of the PEIs, most likely reflecting a long‐term decline in enhanced phytoplankton productivity at the islands in response to a climate‐driven shift in the position of the sub‐Antarctic Front. Our results indicate that regional climate change has affected the balance between allochthonous and autochthonous productivity at the PEIs. Over the last three decades, inshore‐feeding top predators at the islands have shown a marked decrease in their population sizes. Conversely, population sizes of offshore‐feeding predators that forage over great distances from the islands have remained stable or increased, with one exception. Population decline of predators that rely heavily on organisms inhabiting the inshore region strongly suggest changes in prey availability, which are likely driven by factors such as fisheries impacts on some prey populations and shifts in competitive interactions among predators. In addition to these local factors, our analysis indicates that changes in prey availability may also result indirectly through regional climate change effects on the islands' marine ecosystem. Most importantly, our results indicate that a fundamental shift in the balance between allochthonous and autochthonous trophic pathways within this island ecosystem may be detected throughout the food web, demonstrating that the most powerful effects of climate change on marine systems may be indirect.
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- Date Issued: 2013
First oceanographic survey of the entire continental shelf adjacent to the northern Agulhas Current
- Authors: Lutjeharms, Johan R E , Durgadoo, Jonathan V , Schapira, Mathilde , McQuaid, Christopher D
- Date: 2010
- Language: English
- Type: Article
- Identifier: vital:6867 , http://hdl.handle.net/10962/d1011499 , http://www.sajs.co.za/index.php/SAJS/article/view/410
- Description: [from introduction] The Agulhas Current is by far the largest western boundary current of the southern hemisphere1 and carries about 70 × 106 m3/s of seawater past the eastern shores of South Africa.2 Being more than 2000 m deep, it follows the continental shelf edge quite closely. Its northern part, all the way downstream to Algoa Bay, has a very stable trajectory whereas the southern part meanders widely to either side of a mean geographical location,3 in the process creating shear edge eddies and attendant plumes of warm surface water over the shelf.4 However, the direct influence of the Agulhas Current on the waters and ecosystems of the adjacent shelf of South Africa remains largely unknown.
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- Date Issued: 2010
Physical and biological coupling in eddies in the lee of the South-West Indian Ridge
- Authors: Ansorge, Isabelle J , Pakhomov, Evgeny A , Kaehler, Sven , Lutjeharms, Johan R E , Durgadoo, Jonathan V
- Date: 2010
- Language: English
- Type: Article
- Identifier: vital:6493 , http://hdl.handle.net/10962/d1004477
- Description: Eddies have some decisive functions in the dynamics of the Southern Ocean ecosystems. This is particularly true in the Indian sector of the Southern Ocean, where a region of unusually high-mesoscale variability has been observed in the vicinity of the South-West Indian Ridge. In April 2003, three eddies were studied: eddy A, a recently spawned anticyclone south of the Antarctic Polar Front (APF),; eddy B, an anticyclone north of lying between the Subantarctic Front and the APF; and eddy C, a cyclone north of the APF west of the ridge. Elevated concentrations of total Chl-a coincided with the edges of the cyclonic eddy, whereas both anticyclonic eddies A and B were characterised by low total Chl-a concentrations. Biologically, the two anticyclonic eddies A and B were distinctly different in their biogeographic origin. The zooplankton community in the larger anticyclonic eddy A was similar in composition to the Antarctic Polar Frontal Zone (APFZ) community with an addition of some Antarctic species suggesting an origin just north of the APF. In contrast, the species composition within the second anticyclonic eddy B appeared to be more typical of the transitional nature of the APFZ, comprising species of both subantarctic and subtropical origin and thus influenced by intrusions of water masses from both north and south of the Subantarctic Front. Back-tracking of these features shows that the biological composition clearly demarcates the hydrographic origin of these features.
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- Date Issued: 2010