Learning from guano: In search of a paleo-seabird proxy

By Jessica Conroy (University of Illinois Urbana-Champaign, USA)

Take a vacation to the Galápagos Islands and you’re bound to see some of the archipelago’s most colorful denizens perched on guano-splashed basalt rocks, or a sweet ball of puffy white feathers sitting patiently in her nest, waiting for breakfast.

Seabirds like the red-footed and blue-footed booby are some of the unique inhabitants of the Galápagos. Apart from being photogenic, they are impressive animals, with amazing hunting abilities, sometimes flying over 100 km away from their nesting sites in search of food.

Seabirds are also harbingers of the large changes sweeping the world ocean. A study of long-term seabird populations shows many species are declining.  Recent research also points to longer-term changes in some seabird populations relative to the last several millennia, likely related to recent human exploitation of the marine ecosystem.

Natural climate and ocean variability can also affect seabird populations. In the tropical Pacific, many species of seabirds suffer the effects of interannual El Niño events, which influences the abundance of seabird prey by altering ocean properties such as temperature and nutrient availability. In the Galápagos, blue-footed booby populations are decreasing, perhaps due to shifts in climate that operate over decades.

It is important to define the natural baseline of these varying populations. In order to place recent changes in a long-term context, scientists must establish the range of natural ups and downs in seabird populations prior to recent changes. But unfortunately, there are very little long-term seabird population data.

Lake sediments have long offered a way to tell us something about past environments– how much it rained, or variations in past temperatures. But, in the right setting, could lake sediments tell us something about seabirds? We thought one such lake may have an interesting story to tell.

Genovesa Island is a small, uninhabited island in the northeastern part of the Galápagos. With no surface water, it is not a tempting site for human settlement; Darwin did not even make it here. But, it is home to thousands of red-footed boobies and lesser numbers of great frigatebirds. According to the only estimate of the island’s bird population in the scientific literature, it is home to the largest colony of red-footed boobies in the world. The birds nest across the island, including the steep crater walls at the center of the island, which shelter a very inaccessible (to humans) crater lake.

The Genovesa seabirds produce large quantities of guano. The crater is streaked in white, and the air has an acrid smell. Some of this guano makes it into the lake, either directly, or by washing in during rainy periods. We hypothesized that there would be a geochemical signal of this guano archived in the lake sediments. One candidate for a paleo-guano proxy was the stable isotopic composition of nitrogen in lake organic matter. The heavier, less abundant stable isotope of nitrogen, ¹⁵N, is preferentially sequestered in organisms, with animals higher on the food web containing more ¹⁵N relative to ¹⁴N. As seabirds sit high on the food web, they, and their guano, have a distinctly high ¹⁵N/¹⁴N ratio—much higher than the ¹⁵N/¹⁴N values related to other processes and organisms that are typically found in lake sediments.

We measured ¹⁵N/¹⁴N in organic matter in Genovesa lake sediments and guano samples. The data, expressed in delta notation, d¹⁵N, where the ¹⁵N/¹⁴N ratio is normalized to the ratio of ¹⁵N/¹⁴N in air, showed high values in both guano samples and in the sediments, supporting a geochemical signature of seabird presence in the sediment. Although pretty cool, this result has been observed before, in arctic seabird ponds. The most interesting aspect of our study was the variability in d¹⁵N values over time.

We found that higher d¹⁵N values, which we interpreted as indicating increased seabird activity on Genovesa, coincided with decadal changes in the counts of anchovies and sardines in the eastern Pacific. With more of these fish in the region, d¹⁵N was higher, suggesting more seabird activity on the island.  An abrupt increase in d¹⁵N also occurred around 1830 AD, right when ocean sediment cores off the coast of Peru and Chile showed increased ocean productivity and nutrients. Another abrupt shift occurred in the mid 1960s. Thus, seabird activity has been highly variable at Genovesa over the last 400 years, increasing and decreasing in concert with decadal and abrupt changes in the ocean environment.

What do these results imply about the seabirds living on Genovesa? They seem to be doing ok, at least in context of last 400 years. Although this year’s monster El Niño event may have a negative impact on seabirds elsewhere in the Galápagos, past observations and our guano proxy suggest Genovesa seabirds don’t seem to feel El Niño events the same way.  But a recent  increase in decade-to-decade ups and downs in the d¹⁵N guano proxy record suggests that perhaps Genovesa seabirds are becoming more sensitive to regional ocean and climate changes on the decadal timescale. Or, there may have been a strengthening of climate change on the decadal timescale in this region during the 20^th century. We hope to extend our lake sediment record deeper into the past and explore more potential guano indicators in order to find out.

About the author:

Jessica Conroy is an assistant professor in the Departments of Geology and Plant Biology at the University of Illinois Urbana-Champaign.