MEPS 298:179-188 (2005)  -  doi:10.3354/meps298179

The clam Macoma balthica prevents in situ growth of microalgal mats: implications for meiofaunal assemblages

Emil Ólafsson1,*, Jörgen Ullberg1, Nina Larissa Arroyo2

1Department of Zoology, Stockholm University, 106 91 Stockholm, Sweden
2Environmental and Marine Biology, Åbo Academy University, Akademigatan 1, 20500 Åbo, Finland

ABSTRACT: The tellinid clam Macoma balthica (L.), a key member of numerous marine temperate soft-bottom communities, was used in laboratory experiments designed to evaluate its impact on developing microalgal mats and meiobenthic assemblages. Experimental jars (100 ml, 33 cm2) were filled with azoic sediment, seeded with bivalves at various densities, placed in a large outdoor water tank with a constant flow of brackish water and left for 65 d. The bivalves efficiently kept the sediment surface clean of microalgal mats. At the end of the experiment, jars without clams were on average 99% covered by algae, while jars with clams (300 to 4800 ind. m–2) had less than 10% cover on average. There was a highly significant positive correlation between algal cover and the number of individuals belonging to the major meiofaunal taxa, i.e. Nematoda (r2 = 0.61, p < 0.001) and Copepoda (r2 = 0.79, p < 0.001). In containers with high clam densities (4800 ind. m–2), numbers of nematodes were significantly lower than in containers with low clam densities (300 and 600 ind. m–2), but no such difference was found for the copepods. Multidimensional scaling ordination indicated 3 distinct significantly different assemblages (ANOSIM, p < 0.01) of nematodes in jars with clam density of 0, 300 and 4800 ind. m–2, while assemblage structure of copepods was only significantly different between jars with or without clams. In a separate experiment, we tested if the size of the clam and an initial organic enrichment of the sediment would affect the colonizing meiofauna. The results indicated that small bivalves also effectively prevented algal formation and that the development of meiobenthic communities depended on initial organic matter in the sediment. We concluded that indirect effects of Macoma balthica on the assemblage structure of meiobenthos by hindering development of microalgal mats are much larger than any direct effects.

KEY WORDS: Microalgal mats · Soft-bottom · Macoma balthica · Colonization · Meiofauna

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