A groundbreaking study has revealed that seabird-derived nutrients can significantly boost the growth and calcification rates of corals, leading to enhanced reef-scale carbonate production. This discovery highlights the crucial role that natural nutrient pathways play in sustaining the health and resilience of coral reef ecosystems. The findings emphasize the importance of restoring natural nutrient flows, such as those from seabird colonies, as a priority for island and reef management. Coral reefs are vital ecosystems that provide habitat, food, and protection for countless marine species, as well as serve as natural barriers against coastal erosion and sea-level rise. Understanding the factors that influence their growth and productivity is essential for their conservation and management.
The Power of Seabird Poop
Coral reefs are typically found in nutrient-poor tropical waters, but they can thrive when they receive a steady supply of well-balanced nutrients. While excessive anthropogenic nutrient loads can be harmful to coral reefs, natural nutrient flows from sources like seabird colonies can actually benefit coral growth and overall reef functions.
The researchers, Ines D. Lange and Cassandra E. Benkwitt, investigated the effects of seabird-derived nutrient subsidies on the growth and calcification rates of two dominant coral species, Isopora palifera and Acropora vermiculata, in the remote Chagos Archipelago in the Indian Ocean. The Chagos Archipelago is an ideal natural laboratory for this study, as it contains islands with varying seabird densities due to the presence of invasive rats, which predate on bird eggs and chicks.
Coral Growth on Steroids
The researchers found that coral colonies located near the island with high seabird densities displayed significantly enhanced growth and calcification rates compared to those near the island with low seabird populations. Specifically, Isopora palifera colonies close to the seabird-rich island showed 1.4 to 2.1 times higher growth rates across various metrics, such as linear extension, planar area increase, surface area increase, and volume increase. For the Acropora vermiculata corals, the effects were even more pronounced, with 3.2 to 4.1 times faster growth rates.
Importantly, the researchers also found that the increased growth rates translated to higher colony-scale calcification rates, which are essential for the geo-ecological functions of coral reefs. Despite the fact that faster-growing corals typically have lower skeletal densities, the colonies near the seabird-rich island displayed 1.6 to 2.7 times higher calcification rates compared to those near the island with low seabird densities.
Boosting Reef-Scale Carbonate Production
The researchers then integrated the site-specific coral growth and calcification data into a comprehensive reef-scale carbonate production assessment using the ReefBudget methodology. They found that when using the same regional growth rates and skeletal densities, the total coral carbonate production was similar at both islands. However, when incorporating the site-specific growth rates and skeletal densities of Isopora and Acropora, the total coral carbonate production near the seabird-rich island was more than 2 times higher compared to the island with low seabird densities.
This finding highlights the importance of using site-specific data for accurately evaluating differences in reef ecosystem functions, as coral cover alone may not be a sufficient indicator. The researchers also showed that a simple multiplication factor could be used to integrate the effects of nutrient subsidies on carbonate production estimates if site-specific growth rates are not available.
Restoring Natural Nutrient Pathways
The study’s results emphasize that natural, well-balanced nutrient flows from seabirds can have very different and more beneficial effects on coral reefs compared to excessive anthropogenic nutrient inputs. The increased coral growth and calcification rates near the seabird-rich island not only support the reef’s framework production and vertical accretion but also enhance the ecosystem’s ability to provide habitat, protect coastlines, and keep up with sea-level rise.
The researchers conclude that restoring natural nutrient pathways, such as those from seabird colonies, should be a priority for island and reef management. By understanding and preserving these crucial nutrient subsidies, we can help ensure the long-term health and resilience of these vital ecosystems.
Author credit: This article is based on research by Ines D. Lange, Cassandra E. Benkwitt.
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