Responses of benthic invertebrates and their avian predators to the experimental removal of macroalgal mats Lesley J. Lewis*, John Davenport and Thomas C. Kelly Department of Zoology and Animal Ecology, National University of IrelandöCollege Cork, Lee Maltings, Prospect Row, Cork, Ireland. *Corresponding author, e-mail: l.lewis@ucc.ie A ¢eld experiment investigating the e¡ects of algal removal on benthic invertebrates and estuarine birds was carried out during September and October 2001. A randomized block experimental design was employed and three treatments were used: sites with algal cover maintained, when necessary by the addi- tion of algae, sites with algal cover removed and control sites that were not manipulated in any way. Total invertebrate numbers increased during the experiment, but species varied in their response to the di¡erent treatments. Although Hydrobia ulvae showed the greatest increase in cleared sites after two months, there was no signi¢cant di¡erence between treatments. Corophium volutator showed signi¢cant colonization of cleared sites during the experimental period, and numbers of Phyllodoce maculata also increased in the cleared sites. Results showed that while more sedentary benthic infauna are less capable of adjusting to the a¡ects of algal clearance, more mobile epifauna and polychaetes show an ability to disperse to cleared sites in a relatively short period of time.Wadingbirds, however, did not enter the study site in any abun- dance during the experiment. Black-headed gulls ( Larus ridibundus) were signi¢cantly more abundant in cleared sites than in algal cover-maintained or control sites when foraging. INTRODUCTION Increased nutrient inputs to marine coastal areas with resultant eutrophication have been recognized for many years (Rosenberg, 1985). Such enrichment, especially of the nutrients nitrogen and phosphorus, has been linked to an increased growth of macrophytes in many estuaries and coastal areas of the world, notably of fast-growing, mat-forming green algae such as Enteromorpha spp. In temperate regions algal mats may maintain high cover and biomass during the spring, summer and autumn before being broken down or buried in autumn and winter (Hull, 1987). While nutrients play a major role, other factors such as topography, local changes in hydro- graphy (Ra¡aelli et al., 1998), light attenuation (Lavery et al., 1991) and degree of exposure (Pihl et al., 1999) in£u- ence the spatial distribution and biomass of the algae. Many studies have documented the e¡ects of algal mats on the estuarine system. In general, dense algal cover reduces oxygen exchange between the sediment and water column, resulting in anoxic conditions, accentuated by enhanced bacterial activity during algal decay (Hull, 1987). In addition reduced water £ow under the mats and an increase in sedimentation rates (Hull, 1987), toge- ther with a possible accumulation of hydrogen sulphide may lead to signi¢cant e¡ects on macrofaunal commu- nities and the local ecology as a whole. Field studies have shown a reduction in invertebrate fauna under algal mats, for example sedentary sediment/ water interface feeders such as bivalve molluscs (Everett, 1994; Ra¡aelli et al., 1998) and species such as the burrowing amphipod Corophium volutator (Pallas) (Ra¡aelli et al., 1991). On the other hand, algal mats may provide a constant supply of detritus for burrowing and surface detrivores with species such as oligochaetes and capitellid worms increasing in number (Everett, 1994; Lopes et al., 2000). The mat itself may also serve as a refuge from pre- dators for some epi-benthic mobile species such as gam- marid amphipods. Algal mats are also known to e¡ect the feeding, abundance and distribution of other animals, particularly wading birds (Tubbs & Tubbs, 1980; Lewis & Kelly, 2001) and ¢sh (Isaksson et al., 1994). While a number of ¢eld studies undertaken to investigate the e¡ects of algal mats on invertebrates have adopted an experimental approach, most have quanti¢ed the e¡ects of the addition of algae to areas of mud£at which were devoid of algae at the start of the experiment (e.g. Lopes et al., 2000), or where there was no previous history of algal cover (e.g. Hull, 1987). Our study, undertaken at Clonakilty Bay, West Cork, Ireland, adopts a di¡erent approach in that it was designed to assess the e¡ects of removing algae from a mud£at area where extensive mats occur during the summer months. The experiment was designed to assess initial recolonization and recovery of invertebrates following algal mat removal, allowing com- parison of algal covered areas, clear areas and areas undergoing natural algal breakdown. Previous studies have shown that di¡erent species of wading birds have di¡erent responses to algal mats (Tubbs & Tubbs, 1980). For example, the black-tailed godwit ( Limosa limosa L.) when foraging, has been shown to probe through clear spaces amongst patchy algal mats especially when the mats are in the process of breaking up (Lewis & Kelly, 2001). Therefore this study was also designed to test whether wading birds would move into the study area and ‘sample’ newly created clear areas when foraging. J. Mar. Biol. Ass. U.K. (2003), 83, 31^36 Printed in the United Kingdom Journal of the Marine Biological Association of the United Kingdom (2003)