BackgroundSooty (Puffinus griseus) and short-tailed (P. tenuirostris) shearwaters are abundant seabirds that range widely across global oceans. Understanding the foraging ecology of these species in the Southern Ocean is important for monitoring and ecosystem conservation and management.Methodology/Principal FindingsTracking data from sooty and short-tailed shearwaters from three regions of New Zealand and Australia were combined with at-sea observations of shearwaters in the Southern Ocean, physical oceanography, near-surface copepod distributions, pelagic trawl data, and synoptic near-surface winds. Shearwaters from all three regions foraged in the Polar Front zone, and showed particular overlap in the region around 140°E. Short-tailed shearwaters from South Australia also foraged in Antarctic waters south of the Polar Front. The spatial distribution of shearwater foraging effort in the Polar Front zone was matched by patterns in large-scale upwelling, primary production, and abundances of copepods and myctophid fish. Oceanic winds were found to be broad determinants of foraging distribution, and of the flight paths taken by the birds on long foraging trips to Antarctic waters.Conclusions/SignificanceThe shearwaters displayed foraging site fidelity and overlap of foraging habitat between species and populations that may enhance their utility as indicators of Southern Ocean ecosystems. The results highlight the importance of upwellings due to interactions of the Antarctic Circumpolar Current with large-scale bottom topography, and the corresponding localised increases in the productivity of the Polar Front ecosystem.
Development in foraging behaviour and dietary intake of many vertebrates are age-structured.
Differences in feeding ecology may correlate with ontogenetic shifts in dispersal patterns, and
therefore affect foraging habitat and resource utilization. Such life-history traits have important
implications in interpreting tropho-dynamic linkages. Stable isotope ratios in the whiskers of
sub-yearling southern elephant seals (Mirounga leonina;
n = 12) were used, in conjunction with satellite telemetry and environmental
data, to examine their foraging habitat and diet during their first foraging migration. The trophic
position of seals from Macquarie Island (54°30′S, 158°57′E) was estimated using
stable carbon (δ1
3C) and nitrogen (δ15N) ratios along the
length of the whisker, which provided a temporal record of prey intake. Satellite-relayed data
loggers provided details on seal movement patterns, which were related to isotopic concentrations
along the whisker. Animals fed in waters south of the Polar Front (>60°S) or within
Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Statistical Subareas
88.1 and 88.2, as indicated by both their depleted δ1
3C
(<−20‰) values, and tracking data. They predominantly exploited varying proportions
of mesopelagic fish and squid, and crustaceans, such as euphausiids, which have not been reported as
a prey item for this species. Comparison of isotopic data between sub-yearlings, and 1, 2 and 3 yr
olds indicated that sub-yearlings, limited by their size, dive capabilities and prey capture skills
to feeding higher in the water column, fed at a lower trophic level than older seals. This is
consistent with the consumption of euphausiids and most probably, Antarctic krill (Euphausia
superba), which constitute an abundant, easily accessible source of prey in water masses
used by this age class of seals. Isotopic assessment and concurrent tracking of seals are
successfully used here to identify ontogenetic shifts in broad-scale foraging habitat use and diet
preferences in a highly migratory predator.
The Southern Ocean contributes substantially to the global biological carbon pump (BCP). Salps in the Southern Ocean, in particular Salpa thompsoni, are important grazers that produce large, fast-sinking fecal pellets. Here, we quantify the salp bloom impacts on microbial dynamics and the BCP, by contrasting locations differing in salp bloom presence/absence. Salp blooms coincide with phytoplankton dominated by diatoms or prymnesiophytes, depending on water mass characteristics. Their grazing is comparable to microzooplankton during their early bloom, resulting in a decrease of ~1/3 of primary production, and negative phytoplankton rates of change are associated with all salp locations. Particle export in salp waters is always higher, ranging 2- to 8- fold (average 5-fold), compared to non-salp locations, exporting up to 46% of primary production out of the euphotic zone. BCP efficiency increases from 5 to 28% in salp areas, which is among the highest recorded in the global ocean.
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