No abstract

The iron-limited Southern Ocean plays an important role in regulating atmospheric CO 2 levels. Marine mammal respiration has been proposed to decrease the efficiency of the Southern Ocean biological pump by returning photosynthetically fixed carbon to the atmosphere. Here, we show that by consuming prey at depth and defecating iron-rich liquid faeces into the photic zone, sperm whales ( Physeter macrocephalus ) instead stimulate new primary production and carbon export to the deep ocean. We estimate that Southern Ocean sperm whales defecate 50 tonnes of iron into the photic zone each year. Molar ratios of C export ∶Fe added determined during natural ocean fertilization events are used to estimate the amount of carbon exported to the deep ocean in response to the iron defecated by sperm whales. We find that Southern Ocean sperm whales stimulate the export of 4 × 10 5 tonnes of carbon per year to the deep ocean and respire only 2 × 10 5  tonnes of carbon per year. By enhancing new primary production, the populations of 12 000 sperm whales in the Southern Ocean act as a carbon sink, removing 2 × 10 5 tonnes more carbon from the atmosphere than they add during respiration. The ability of the Southern Ocean to act as a carbon sink may have been diminished by large-scale removal of sperm whales during industrial whaling.

Blue whales Balaenoptera musculus aggregate to feed in a regional upwelling system during November-May between the Great Australian Bight (GAB) and Bass Strait. We analysed sightings from aerial surveys over 6 upwelling seasons (2001-02 to 2006-07) to assess within-season patterns of blue whale habitat selection, distribution, and relative abundance. Habitat variables were modelled using a general linear model (GLM) that ranked sea surface temperature (SST) and sea surface chlorophyll (SSC) of equal importance, followed by depth, distance to shore, SSC gradient, distance to shelf break, and SST gradient. Further discrimination by hierarchical partitioning indicated that SST accounted for 84.4% of variation in blue whale presence explained by the model, and that probability of sightings increased with increasing SST. The large study area was resolved into 3 zones showing diversity of habitat from the shallow narrow shelf and associated surface upwelling of the central zone, to the relatively deep upper slope waters, broad shelf and variable upwelling of the western zone, and the intermediate features of the eastern zone. Density kernel estimation showed a trend in distribution from the west during November-December, spreading south-eastward along the shelf throughout the central and eastern zones during January-April, with the central zone most consistently utilised. Encounter rates in central and eastern zones peaked in February, coinciding with peak upwelling intensity and primary productivity. Blue whales avoided inshore upwelling centres, selecting SST ~1°C cooler than remotely sensed ambient SST. Whales selected significantly higher SSC in the central and eastern zones than the western zone, where relative abundance was extremely variable. Most animals departed from the feeding ground by late April.

Accurate identification of species that are consumed by vertebrate predators is necessary for understanding marine food webs. Morphological methods for identifying prey components after consumption often fail to make accurate identifications of invertebrates because prey morphology becomes damaged during capture, ingestion and digestion. Another disadvantage of morphological methods for prey identification is that they often involve sampling procedures that are disruptive for the predator, such as stomach flushing or lethal collection. We have developed a DNA-based method for identifying species of krill (Crustacea: Malacostraca), an enormously abundant group of invertebrates that are directly consumed by many groups of marine vertebrates. The DNA-based approach allows identification of krill species present in samples of vertebrate stomach contents, vomit, and, more importantly, faeces. Utilizing samples of faeces from vertebrate predators minimizes the impact of dietary studies on the subject animals. We demonstrate our method first on samples of Adelie penguin (Pygoscelis adeliae) stomach contents, where DNA-based species identification can be confirmed by prey morphology. We then apply the method to faeces of Adelie penguins and to faeces of the endangered pygmy blue whale (Balaenoptera musculus brevicauda). In each of these cases, krill species consumed by the predators could be identified from their DNA present in faeces or stomach contents.