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 Ea s previously been experimentally investigated on mudflats, such experiments have rarely been performed on intertidal sandflats. This study investigate d the ecological effects of macroalgal cover on a moderately exposed intert idal sandflat, Drum Sands, Firth of Forth, Scotland. Artificially implanted Enteromorpha prolifera (Muller) 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 (Claparede) were significantly reduced unde r weed mats, while those of Capitella capitata (Fabricius), oligochaetes an d gammarids increased. Percent water, organics and silt/clay contents, medi um phi and sorting coefficient significantly increased in the sediments und er weed mats which also became significantly more reduced between 1 and 8 c m 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 eff ects on population maintenance of most species which rely on planktonic lar val recruitment. These results are broadly similar to those obtained from a lgal manipulation experiments performed in much more sheltered, muddier env ironments. We suggest that a predictable deterioration in environmental qua lity results from the growth of macroalgal mats in soft-bottom habitats. Ho wever, the longer term effects of such algal growth are less predictable an d depend upon the spatial distributions of the most abundant infaunal speci es and the spatial heterogeneity of weed mat establishment. (C) 2000 Elsevi er Science B.V. All rights reserved.