Repeat observations in June and July 2019, were not possible as geese were no longer present on the Zostera beds.Ĭanada geese samples were collected to investigate bird diet across a temporal scale from Kawhia between July to November 2019 (n = 33), and from Raglan between August to September 2020 (n = 26). Foraging was significantly reduced by disturbance events less than 30 m away and was also influenced by group size. Observations indicated that foraging incorporated a large proportion of their behavioural budget (> 85%), and birds utilised several destructive methods to forage on both above and below-ground Zostera biomass. Behaviours of Canada geese on Zostera beds were observed in January and February (2019), at two sites in Whāingaroa Harbour, with geese numbers varying between 8 to 200 at any one time. In order to better understand the grazing pressure placed on seagrass habitats, a three part investigation was conducted. In response to increase in Canada geese populations and use of estuaries along the West coast, the Waikato Regional Council commissioned this MSc (Research) study to investigate the consumption of Zostera by Canada geese in Kawhia and Whāingaroa (Raglan) harbours, West coast of the North Island, New Zealand. Canada geese (Branta canadensis) were introduced to New Zealand in 1905, and have since been increasing in numbers since a change in species management. Herbivory by waterfowl is a relatively unknown biotic disturbance that may cause additional stress to these vulnerable seagrass habitats. However, these habitats are globally in decline due to the impacts of multiple stressors including eutrophication, turbidity, coastal urbanisation, sedimentation, and sea level rise. Zostera muelleri is New Zealand’s single seagrass species, and occurs intertidally within several estuaries and sheltered harbours. Seagrass beds are highly biodiverse habitats delivering key ecosystem functions and services to mankind.
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Our results provide evidence for the multi-stressor effects of fine sediment on seagrasses, with substrate suitability for seagrass being detrimentally affected even where light exposure seems sufficient.
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This suggests that failure of seagrass to recolonize historical seagrass habitat reflects substrate muddiness and consequent unfavorable rhizosphere conditions. Historical seagrass substrate had significantly higher mud (35% average), bulk density (1.5 g cm−3), porewater ammonium concentration (65 µM), and a more reduced redox profile (negative redox at only 2 cm soil depth) as well as a lower light availability when submerged compared to other habitats, while total daily light exposure differed little between habitats. We tested these hypotheses in Pāuatahanui Inlet, New Zealand, by comparing seagrass presence, abundance, and health, together with light climate and substrate physico-chemistry at contrasting habitats where (1) seagrass used to thrive but no longer grows (historical seagrass), (2) seagrass still persists (existing seagrass) and (3) seagrass has been present recently, but not currently (potential seagrass). Here we tested two non-exclusive hypotheses, that mud particles (<63 µm) impact seagrasses through both (1) the light climate and (2) changes in substrate physico-chemistry. Seagrass meadows are vulnerable to fine sediment (mud) pollution, with impacts usually attributed to reduction in submerged light.