The assessed impacts of estuarine nuisance macroalgal blooms on over-wintering birds in temperate climates

Abstract Background and Aims Estuaries serve as vital foraging and resting grounds for over-wintering bird populations whilst providing provisioning, regulating and cultural ecosystem services. Macroalgal blooms, primarily from the Ulva genus, can grow superabundantly to create dense layers which smother intertidal mudflats in spring and summer months. During autumn and winter residual mats remain, potentially impacting over-wintering birds. This study assessed the impact of residual macroalgal mats on over-wintering birds in the Milford Haven inner waterbody, Wales, UK. Methods An unmanned aerial vehicle (UAV)-based normalised difference vegetative index (NDVI) imagery to classify algal density between May and September 2020. Subsequently, bird surveys were conducted from November 2020 to March 2021. Bird abundance, distribution, and behaviour across different areas of algal density (low, medium and high) were evaluated. Key Results Results indicated that golden plover (Pluvialis apricaria) and lapwing (Vanellus vanellus) preferred to rest in high and medium-density algal areas, while waterfowl and wading birds such as redshank (Tringa totanus) and Eurasian curlew (Numenius arquata) were more abundant in low-density macroalgal areas. There was, however, no effect of residual macroalgal mats on over-wintering bird abundance or diversity. Conclusions These findings indicate that while residual macroalgal mats may offer benefits for certain bird species, particularly for resting behaviours, the impact on the population metrics measured here had no effect, likely as a result of complex ecological interactions between overwintering birds and their environment. Macroalgal blooms, specifically Ulva sp., are expected to intensify under future climate and nutrient enrichment scenarios, which may provide ecotourism and recreational benefits through seasonal bird watching. Targeted monitoring is required to determine whether altered bird behaviour and foraging access translate into longer term impacts on population dynamics and estuarine functioning.