Modelling water level influence on habitat choice and food availability for Zostera feeding brent geese Branta bernicla in non‐tidal areas

Abstract

Brent geese Branta bernicla spring fattening around Agerø, Denmark, alternate between feeding on saltmarshes and submerged Zostera beds in Limfjorden. It appeared from field observations that these alternations depended on the water level in Limfjorden. A model was developed to assess the impact of water level fluctuations on the habitat use. A second model was developed to estimate the impact of water level on Zostera availability. The first model was successful in demonstrating that fluctuations in water levels had considerable influence on habitat use by the brent geese, i.e. they fed on Zostera at low water levels and on saltmarshes during high water levels, particularly so in early spring, and that the switch between habitats occurred within a narrow water level span of ca 30 cm. The second model demonstrated that the switch between habitats could be explained by lowered availability of Zostera as water levels increased. By combining the output from the two models, differences between years could partly be explained by differences in Zostera availability in the early spring period (21 March ‐ 25 April), whereas a more complicated situation was detected later in spring (26 April ‐ 31 May). The models presented may be considered as tools in investigations of habitat use and carrying capacity of seagrass beds in non‐tidal areas, where birds' access to feeding areas regularly may be hindered by high water levels.

Long-term Patterns of Eelgrass (Zostera marina) Occurrence and Associated Herbivorous Waterbirds in a Danish Coastal Inlet

“…For non-diving herbivorous waterbirds such as mute swan and brent goose, and poorly diving species such as coot, the water level determines, which areas are available for foraging (Clausen et al, 1996;Clausen, 2000). The same is true for the non-diving wigeon, which often feed on spilled plant-materials from foraging swans or coot (Holm and Clausen, 2009 and references therein).…”

“…A mute swan can feed down to 1.15 meters below the surface (Clausen et al, 1996) and a brent goose to 40 cm (Clausen, 2000). Analysis of the fluctuations in water levels in NibeGjøl Bredning between August and December 1993 to 2013 showed that on average 50% of days had water levels below 0 (relative to normal), and another 30% in the range 0-0.20 m, whereas there were only 12.7 (range 3-24) days per autumn with water levels higher than 0.5 m. Assuming that eelgrass shoots typically are 0.5-1 m long (as found by Clausen et al, 1996;Clausen, 2000), this means that swans could reach the entire 0-1 m stratum most days, and even the deeper stratum out to at least 2 meters during some days.…”

Rise in human activities on the mudflats and Brent Geese (Branta bernicla) wintering distribution in relation to Zostera spp. beds: a 30-year study

“…The relative influence of the two Zostera species may in fact reflect the differences in their characteristics as food sources, in availability and in accessibility. This can be compared to the situation of staging Brent Geese in Denmark (Clausen 2000). There, they alternate between feeding on submerged Z. marina beds and salt-marshes, and the switch between habitats is partly determined by accessibility and availability of Z. marina.…”

Variation in Elevation and Sward Height Facilitate Coexistence of Goose Species through Allometric Responses in Wetlands

“…However, our results suggest that the two species could use the same food resource if they differ in their spatial distribution. The elevation gradient along the lake shore influences the spatial pattern of the food resources through a difference in the flooding regime, and thereby affects the forage quality and quantity and the vegetation heterogeneity (Clausen 2000). Grazers that differ in body size could apply different foraging strategies to satisfy their daily intake requirements (Wang et al 2013).…”