Seabirds form large colonies during the reproductive period, producing substantial changes in coastal ecosystems. The present study quantifies the amount of N and P deposited in colonies of yellow-legged gull (Larus michahellis) in the Atlantic Islands of Galicia National Park (AINP). Based on the composition of droppings, the amount of total N (TN), total P (TP) and bioavailable P (Pbio) deposited directly on the area occupied by the colony was determined. In addition, the amount of NH3 released into the atmosphere was also estimated by applying a bioenergetic model. The results indicated that 5.35 t total N, 3.35 t total P and 1.24 t bioavailable P are deposited in the colony annually. The archipelagos that received the greatest amount of nutrients were the Cíes Islands (2.37 t TN y−1, 1.48 t TP y−1, 0.55 t Pbio y−1), Sálvora (1.94 t TN y−1, 1.22 t TP y−1, 0.55 t Pbio y−1) and Ons (1.04 t TN y−1, 0.65 TP y−1, 0.24 t Pbio y−1). Rainwater from the colonies showed higher values of nutrients than in the control plot, possibly also due to gull influence. Therefore, the yellow-legged gull colony seems to be the most important source of nutrients at a local level, exerting a clear influence on the N and P cycles in this National Park. Another aspect worth taking into consideration is that increased N and P bioavailability may have a negative effect on the conservation of rare or threatened habitats and species by promoting the expansion of non-native ruderal species.
Soil nutrient dynamics in colonies of the yellow-legged seagull (Larus michahellis) in different biogeographical zones
De la Peña-Lastra, S., Affre, L. and Otero, X.L., 2020. Soil nutrient dynamics in colonies of the yellow-legged seagull (Larus michahellis) in different biogeographical zones. Geoderma, 361, p.114109.
Seabirds drastically alter the ecosystems where they establish their colonies. However, previous studies have not considered how colonies of the same species affect their surroundings in different environmental contexts. The main objective of this study was to determine the degree to which environmental factors (particularly climate and lithology) modulate the impact of seabird colonies on soil nutrients. For this purpose, two breeding colonies of the yellow-legged gull were selected: one located in the Atlantic Islands National Park (AINP, Atlantic region) and the other in Calanques National Park (CNP, Mediterranean region). In both parks, samples of soil and excrement were obtained from colonies with different densities of birds and in control zones, without birds, in two different seasons (winter and summer). The samples were analysed to determine the concentrations of N-NO3–, N-NH4+, total P and different geochemical P forms, including bioavailable P. The colony soils were enriched in N and P relative to soils from the control zones. However, the annual nutrient dynamics were modulated by the environmental conditions in each park. In winter in CNP, NH4+ concentrations were low and similar to those in the control zones, while the concentrations of NO3– were highest at this time of year. By contrast, in AINP, the annual N cycling appeared to be less variable, although the NH4+ concentrations were lower than in the control zone in winter, while those of NO3– remained high, despite the high rainfall in this season. The concentrations of P (total and bioavailable) remained high in soils in both parks throughout the year. However, fractionation of the P forms revealed different geochemical behaviour at the two sites. In CNP, calcium phosphate and residual P were the dominant fractions. In AINP, the P was distributed more homogeneously between the different fractions, with a slight predominance of the P associated with Al hydroxides and clays. The findings clearly show alteration of the cycling of both nutrients in both parks, although the impact is modulated by the environmental conditions at each location.
Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)
Otero, X. L. et al. Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain). Science of The Total Environment 532, 383–397 (2015).
During the period 1980–2000, the yellow-legged gull population underwent exponential growth due to an increase in the availability of anthropogenic food resources. The aim of this study was to highlight the effect of the gull colonies on the P soil cycle and the associated effects on coastal ecosystems. Samples of soil, water and faecal material were collected in a colony of yellow-legged gulls (Cíes Islands) and in a control area. Four sampling plots were installed in the study areas, and samples were collected in summer and winter in 1997 and 2011. Sample analysis included soil characterization and determination of the total P content (TP), bioavailable-P and fractionated-P forms in the soils and faecal material. The 31P NMR technique was also used to determine organic P forms. Clear differences between the gull colony soils and the control soil were observed. The TP was 3 times higher in the gull colony soil, and the bioavailable P was 30 times higher than in the control soil. The P forms present at highest concentrations in the faecal material (P-apatite, P-residual and P-humic acid) were also present at high concentrations in the colony soil. The absence of any seasonal or annual differences in P concentration indicates that the P has remained stable in the soil over time, regardless of the changes in the gull population density. The degree of P saturation indicated that soils are saturated with P due to the low concentration of Fe/Al-hydroxides, which is consistent with a high P concentration in the run-off from the colonies. The P output from the colony soils to coastal waters may cause eutrophication of a nearby lagoon and the disappearance of a Zostera marina seagrass meadow. Similarly, the enrichment of P concentration in dune system of Muxieiro may induce irreversible changes in the plant communities.