Every autumn in the Southern Hemisphere, Magellanic penguins leave their coastal nesting sites in South America. For adults, their summer task — breeding, or at least trying to — is complete. Newly fledged chicks and adults gradually head out to sea to spend the winter feeding. They won’t return to land until spring.
In a paper published by UW Biology Professor Dee Boersma and Research Scientist Ginger Rebstock on Aug. 9 in the journal Marine Ecology Progress Series, they report that the Río de la Plata — which drains South America’s second-largest river system after the Amazon — strongly influences oceanographic conditions in the Magellanic penguins’ winter feeding waters. Those oceanographic features, they report, show up in the body conditions of Magellanic penguin females, but not males, when the penguins return to their nesting grounds in spring.
To understand the oceanographic dynamics in this region, Rebstock turned her attention to space. She analyzed 30 years of weekly sea-surface temperature data, which National Oceanic and Atmospheric Administration satellites collected for those South American coastal waters from 1982 to 2012. Data show that geographic features of coastal South America are responsible for key variations in ocean conditions.
For example, her analysis revealed that the Río de la Plata, which enters the ocean between Argentina and Uruguay, is the primary driver of oceanographic conditions in the penguins’ winter feeding waters. The river discharges silt, microbes and nutrients into the ocean as a plume, which disperses in different directions based on prevailing winds. Strong winds from the southwest, for example, can spread the plume north along hundreds of miles of Brazilian coastline. If winds are weaker, the plume stays near the mouth of the Río de la Plata.
Rebstock then analyzed whether variations in these oceanographic features — such as a strong, dispersed plume or a weak, localized plume — were associated with the body condition of penguins at the time of their arrival at Punta Tombo. Boersma and her team have collected information on the health and state of individual penguins when they arrive Punta Tombo for more than three decades. According to their data, the body conditions of male Magellanic penguins weren’t correlated with the extent of the plume. But Rebstock found that female penguins arrived back at Punta Tombo earlier, and in healthier body condition, if the Río de la Plata plume was weaker in winter. This may indicate that the plume affects how hard Magellanic penguins must work to find food.
“We believe that the Río de la Plata plume carries a great deal of nutrients into the coastal waters, making them very productive feeding grounds for the penguins,” said Rebstock. “But winds will affect where the plume is distributed and how far penguins will have to go to reach it.”
A weaker plume may keep the penguins’ prey closer together and closer to breeding colonies, increasing an individual penguin’s odds of catching fish. Magellanic penguins also are mainly visual hunters. A stronger plume that clings to the coast may obstruct visibility for the birds by making waters more turbid, said Rebstock.
The size and disposition of the plume may affect females more than males because male Magellanic penguins tend to be larger, which allows them to dive deeper. This may give males a slight edge in catching food, especially in difficult conditions, said Rebstock.
Read the original and full article by James Urton on UW News.