The impact of macroalgal blooms on the use of the intertidal area and feeding behaviour of waders (Charadrii) in the Mondego estuary (west Portugal)

The present paper examined the possible impact of macroalgal blooms, among other influential factors, on prey abundance and availability to waders, assessing the consequences for feeding behaviour and for the specific patterns of use of the intertidal areas by these birds when macroalgal mats become dense and contiguous covering large areas of the Mondego estuary (west Portugal). Three representative microhabitats were chosen in intertidal flats to control and evaluate the effects of various factors on waders. This study shows that foraging waders did not seem to be indifferent to the effects of some biological factors, sediment characteristics and epistructures when selecting feeding microhabitats. Overall, the results suggest that the predominant and constant negative effects on specific patterns of use of the intertidal sub-areas by the majority of waders were determined by two main factors: gull perturbation and macroalgal biomass. The macroalgal blooms assumed, by their persistence over a period of several months, a different magnitude of effect when compared with sporadic factors such as gull presence. Contrary to the effects on distribution, macroalgal blooms do not influence negatively the feeding behaviour parameters considered. Moreover, we were unable to prove that the presence of macroalgae on the sediment was pernicious to the wader prey organisms, perhaps because the total amount of the area of the estuary affected by weed during the study period never exceeded 36 % and was surrounded by important algal-free areas that ensured a healthy prey population. Nevertheless, on the long-term, an increase of dense and contiguous macroalgal mats due to the progressive eutrophication of the Mondego estuary, covering large areas of the intertidal flats, may affect directly or indirectly all wader species. (C) 1999 Editions scientifiques et medicales Elsevier SAS.