Seabirds may help the survival of coral reefs under environmental pressure. A new study showed that birds, along with natural ocean currents, deliver vital nutrients that help sustain some of the world's most threatened ecosystems.

Scientists from the Lancaster Environmental Centre examined coral reefs across various depths and locations to find that reefs receive nutrients through two distinct pathways, either passive delivery via ocean currents and/or active transport by seabirds, which deposit nutrient-rich guano near reef systems.

The research team, led by Dr Ruth Dunn, Visiting Researcher, measured carbon and nitrogen isotopes in reef samples to trace the origins of the nutrients. Their findings show coral reefs are supported by both oceanic and biological processes.

Deeper, seaward-facing reefs rely heavily on nutrients carried by ocean currents and upwelling from deeper parts of the ocean. These passive processes appear to be particularly important for reefs exposed to high-energy wave environments, where turbulent waters deliver phytoplankton and other nutrient sources.

However, the study's most interesting discovery concerns the role of seabirds as ‘mobile links’ between land and sea. Reefs off the coast of islands with breeding seabird colonies showed very different nutrient signatures compared to those with little or no birdlife.

The effects of seabirds were most noticeable in shallow, lagoon-based reefs, where their droppings create significant nutrient enrichment. Algae, hermit crabs, and gastropods near bird colonies were found to have higher nitrogen levels, suggesting they were directly benefiting from nutrients provided by seabirds. The influence of the seabirds reached beyond the shallows, with bird-sourced nutrients detected in deeper reef zones and as far as 200 metres from shore, supporting earlier findings that these nutrient effects can extend up to 300 metres offshore.

The findings emphasise the importance of natural nutrient pathways in sustaining coral reef ecosystems, something that is becoming increasingly critical as these habitats face increasing environmental pressures. The researchers stress that preserving and restoring these pathways should be a central focus of conservation efforts.

Dr Dunn said: “Our findings highlight seabirds are important components of the ecosystems they inhabit, supporting coral reefs by transporting nutrients from the sea to land, and then back again. Protecting and restoring seabird populations and their island habitats is therefore critical, not only for the birds themselves, but also for the reef habitats that they help to sustain.”

The study has relevance for ongoing conservation efforts across tropical islands, where habitat restoration and invasive species eradication programmes are being implemented to restore seabird populations. Such initiatives may deliver unexpected benefits for surrounding coral reefs through enhanced nutrient connectivity.

The research highlights how marine and terrestrial ecosystems are interconnected. Young fish that develop in nutrient-rich lagoon nurseries will migrate to other areas, likely spreading the benefits of seabird-derived nutrients across different reefs. As environmental pressures on coral reefs mount, this research highlights how healthy seabird populations play a vital role in supporting important and threatened marine ecosystems.

The full paper, published in the journal Coral Reefs, can be found here