Effects of macroalgal mats on intertidal sandflats: an experimental study

The growth of green macroalgal mats is becoming increasingly common in many marine intertidal habitats. While the ecological effects of such growth has previously been experimentally investigated on mudflats, such experiments have rarely been performed on intertidal sandflats. This study investigated the ecological effects of macroalgal cover on a moderately exposed intertidal sandflat, Drum Sands, Firth of Forth, Scotland. Artificially implanted Enteromorpha prolifera (Müller) caused marked changes in the macrobenthos, together with significant changes in all the measured sediment variables. After 6 weeks, the weed significantly increased the macrofaunal diversity. The numbers of Pygospio elegans (Claparède) were significantly reduced under weed mats, while those of Capitella capitata (Fabricius), oligochaetes and gammarids increased. Percent water, organics and silt/clay contents, medium phi and sorting coefficient significantly increased in the sediments under weed mats which also became significantly more reduced between 1 and 8 cm depth. After 20 weeks, a macrofaunal community numerically dominated by C. capitata, with a significantly reduced diversity, was present under weed mats, while sediment variables were no longer significantly different from controls. The negative effect of E. prolifera on P. elegans was mainly due to larval filtering, suggesting that weed is likely to have detrimental effects on population maintenance of most species which rely on planktonic larval recruitment. These results are broadly similar to those obtained from algal manipulation experiments performed in much more sheltered, muddier environments. We suggest that a predictable deterioration in environmental quality results from the growth of macroalgal mats in soft-bottom habitats. However, the longer term effects of such algal growth are less predictable and depend upon the spatial distributions of the most abundant infaunal species and the spatial heterogeneity of weed mat establishment.