Seabirds supply nitrogen to reef-building corals on remote Pacific islets

Abstract

Seabirds concentrate nutrients from large marine areas on their nesting islands playing an important ecological role in nutrient transfer between marine and terrestrial ecosystems. Here we investigate the role of guano on corals reefs across scales by analyzing the stable nitrogen isotopic (δ15N) values of the scleractinian coral Pocillopora damicornis on fringing reefs around two Pacific remote islets with large seabird colonies. Marine stations closest to the seabird colonies had higher nitrate + nitrite concentrations compared to more distant stations. Coral and zooxanthellae δ15N values were also higher at these sites, suggesting that guano-derived nitrogen is assimilated into corals and contributes to their nitrogen requirements. The spatial extent of guano influence was however restricted to a local scale. Our results demonstrate that seabird-derived nutrients not only spread across the terrestrial ecosystem, but also affect components of the adjacent marine ecosystem. Further studies are now needed to assess if this nutrient input has a positive or negative effect for corals. Such studies on remote islets also open fresh perspectives to understand how nutrients affect coral reefs isolated from other anthropogenic stressors.

Seabird nutrient subsidies alter patterns of algal abundance and fish biomass on coral reefs following a bleaching event

“…Corals assimilate nutrients from seabirds nesting on adjacent islands (Lorrain et al, 2017), and the ratio of nitrogen to phosphorous in seawater adjacent to seabird colonies is within the range considered optimal for increasing coral growth, photosynthetic efficiency and resistance to heat stress (N:P ratio of 13-32 in seawater compared to optimal ratio of 11-29; Allgeier et al, 2014;Savage, 2019;Wiedenmann et al, 2013). Corals assimilate nutrients from seabirds nesting on adjacent islands (Lorrain et al, 2017), and the ratio of nitrogen to phosphorous in seawater adjacent to seabird colonies is within the range considered optimal for increasing coral growth, photosynthetic efficiency and resistance to heat stress (N:P ratio of 13-32 in seawater compared to optimal ratio of 11-29; Allgeier et al, 2014;Savage, 2019;Wiedenmann et al, 2013).…”

“…Contrary to our prediction, hard corals declined regardless of whether they were adjacent to islands with seabirds or islands with invasive rats. Corals assimilate nutrients from seabirds nesting on adjacent islands (Lorrain et al, 2017), and the ratio of nitrogen to phosphorous in seawater adjacent to seabird colonies is within the range considered optimal for increasing coral growth, photosynthetic efficiency and resistance to heat stress (N:P ratio of 13-32 in seawater compared to optimal ratio of 11-29; Allgeier et al, 2014;Savage, 2019;Wiedenmann et al, 2013). Therefore, we expected the large inputs of nitrogen and phosphorous from seabird guano to enhance resistance of corals to bleaching (D'Angelo & Wiedenmann, 2014;Graham et al, 2018).…”

Effects of ultraviolet radiation and nutrient level on the physiological response and organic matter release of the scleractinian coral Pocillopora damicornis following thermal stress

“…Nutrient levels vary in nature with the water quality, the upwellings, the stratification of the water column, the seasonality or the presence of fish and bird colonies on the surrounding islands [ 57 , 58 , 59 ]. Reefs are generally oligotrophic, with ca.…”

Seabird colonies as relevant sources of pollutants in Antarctic ecosystems: Part 1 - Trace elements

“…It is estimated that global seabirds inputs of N and P are respectively of 591 Gg y -1 and 99 Gg y -1 , with the Antarctic and Southern Ocean coasts receiving the highest amounts, roughly 80% whereas these areas 26% of the global seabird population (Otero et al, 2018). Moreover, various authors (Ellis et al, 2006;Lorrain et al, 2017;Otero et al, 2015;Sutton et al, 2013) suggested that seabird colonies represent a positive geochemical anomaly (i.e., above background values) regarding the concentrations of N and P present in soils, sediments, and water, hence the term ornithoeutrophication (Otero et al, 2018). It is, therefore, reasonable to assume that seabird colonies enrich their ecosystems as a whole, in a mass balance perspective.…”