Fig. 1 | Nature Communications

Fig. 1

From: Seabird colonies as important global drivers in the nitrogen and phosphorus cycles

Fig. 1

Seabird ornitheutrophication coupling. Schematic summary of processes coupling local and regional environmental effects in seabird colonies. Colonies can be considered as nutrient hot spots, especially, for N and P. Nitrogen is the key nutrient in marine environments and phosphorous in continental waters. Both are found in high concentrations in seabird feces. Uric acid is the dominant N compound, and during its mineralization different N forms are produced: (1) ammonification produces NH3 and NH4+, and (2) nitrification produces NO3 by NH4+ oxidation. Under the alkaline conditions, typical of the seabird feces, the NH3 is rapidly volatized (3) and transformed to NH4+, which is transported out of the colony, and through wet-deposition exported to distant ecosystems, which are eutrophized (4). Similarly, nutrients in the colony can be leached and transported out through runoff and groundwater seepage (5), generating in cases (4) and (5) environmental impacts at the regional level. On the other hand, NH4+ in soil (ornithogenic soils) can be adsorbed by organominerals and remain as an exchangeable cation (panel a). The soil NH4+ in the colony can be oxidized to nitrate through nitrification processes, and rapidly washed to subterranean or superficial waters, eutrophizing nearby ecosystems (local impact, 5). In both cases, the NO3 and NH4+ can reach creeks and small lakes, eutrophizing them (regional impact). Phosphorus cycle is simpler and has a rather reduced mobility. This element is found in a number of chemical forms in the seabird fecal material, but the most mobile and bioavailable is orthophosphate (HPO4=), which can be lixiviated (solubilized) to subterranean or superficial waters (5). However, an important fraction of the P can be adsorbed by Fe/Al oxyhydroxides in acidic soils. Through erosion, these colloids can reach anoxic freshwater or estuarine sediments, where P is liberated to the water column by the reductive dissolution of Fe(III) oxyhydroxides (panel b). If the colloids reach oxic marine sediments, P can still be liberated to the water by alkaline desorption (panel c), a process that involves changes in the surface charge of Fe/Al oxyhydroxides. In both cases, water eutrophization is produced

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