Quantifying nutrient inputs by gulls to a fluctuating lake, aided by movement ecology methods

Abstract

Eutrophication of aquatic ecosystems is a global problem with major ecological and economic impacts. In many lakes and reservoirs, guanotrophication occurs when roosting waterbirds import nutrients (nitrogen and phosphorus) from surrounding terrestrial habitats. To date, nutrient loading by waterbirds has been estimated based on censuses in the absence of detailed information on their movements. We quantified nutrient importation by the lesser black‐backed gull (Larus fuscus) to Fuente de Piedra (1,350 ha) in Andalusia (south‐west Spain), where an average of 36,288 individuals are counted in January. During seven winters from 2010 to 2017, we used movement data from 20 individual gulls tagged with Global Positioning System trackers that foraged in four landfills. Together with monthly bird counts and measurements of total N and P content in faeces and pellet samples, movement data were used to quantify the total external loading effect for different winters. Movement data allowed us to quantify the proportion of time spent in the lake and the time spent at different foraging sites and enabled correction of censuses. According to tracking data, on average 69% of the birds had already left the lake to head for feeding sites when waterbird counts were carried out. Nutrient inputs to the lake depend partly on the proportion of the day that gulls spend there, which was higher in late winters and was reduced when lake depth went below or above 20–35 cm. An estimated average of 10.17 kg N ha−1 year−1 and 2.07 kg P ha−1 year−1 were imported to this closed‐basin lake by gulls each winter, with highest values recorded in winter 2016–2017. Gull guano is the most important winter source of nutrients to the lake. Regurgitated pellets have been ignored as a source of nutrients in other guanotrophy studies, but we found them to be a more important source of P than faeces. A movement ecology approach complements traditional censuses and facilitates the study of guanotrophication in multiple ways, including identification of sources of nutrients, correction of censuses, and measuring time spent at roost sites.

Comparing the diversity and composition of waterbird functional traits between natural, restored, and artificial wetlands

“…Thus, many wintering birds in our study area may be present at a given wetland, but not functionally active in it in terms of consumption, and thus not dependent on wetland characteristics other than quality of roosting sites. By contrast, these waterbirds remain functionally active in any wetland they visit in terms of nutrient transport and may contribute to eutrophication of wetlands (Martín‐Vélez et al., 2019). It is also possible that differences among wetland types were more pronounced during the breeding season owing to greater spatial heterogeneity in food supply, salinity and other environmental variables.…”

Spatial patterns of weed dispersal by wintering gulls within and beyond an agricultural landscape

“…We further assumed that seed ingestion occurred within ricefields between 07:00 and 17:00 hr UTC according to the timing of ‘active flight’ (Figure ). This time range excluded the main roosting periods (especially night time) when seed ingestion is improbable (see also Martín‐Vélez et al., 2019 for diel rhythms of L. fuscus ). The GPS points within the ricefields correspond to a total of 1,867 gull‐days, from a combination of eight different winters from 2010 to 2017 and 19 different individual gulls from five different breeding colonies (eight individuals from Zeebrugge, Belgium; five from Texel, the Netherlands; three from Skokholm and three from Walney, UK).…”

“…Median distances for seed dispersal by L. fuscus are likely to be lower in ricefields than in some other habitats, since L. fuscus have been found to move distances of 40-80 km on a daily basis to reach feeding sites in previous studies (Martín-Vélez et al, 2019;Thaxter et al, 2015).…”

Dynamic Space Use of Andalusian Rice Fields by Overwintering Lesser Black-Backed Gulls is Driven by Harvest-Related Flooding

“…Preference of LBBG for arti cial wetlands other than rice elds was also strong at the individual scale. Arti cial shponds were selected strongly by a subset of tracked gulls, whereas the natural marshes of 2018; Martín-Vélez et al 2019, 2020. Gulls are also likely to disperse genes for antimicrobial resistance into rice elds, both when feeding at land lls or when roosting in the nearby river, which is contaminated with urban wastewaters (Ahlstrom et al 2019).…”