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Sedimentary coastal ecosystems like the European Wadden Sea in the northeastern Atlantic harbor large populations of burrowing infauna, such as arenicolide polychaetes. These 'ecosystem engineering' macrofaunal organisms destabilize sediments by reworking and irrigat- ing them, leading to a reorganization of sediment physicochemical state and bacterial communi- ties. Here, we tested the effects of the lugworm Arenicola marina on intertidal psammophilic pro- tistan community diversity and structure in a field experiment. We applied pyrosequencing and community structure analyses to samples in which we manipulated lugworm densities. Irrespec- tive of the presence or absence of the lugworm, protistan abundance and diversity (mostly diatoms and ciliates) was higher in oxic surface sediments compared to anoxic subsurface sediments. In the presence of A. marina, protistan abundances and diversity decreased decisively in surface as well as in subsurface bulk sediment. Also, the protistan community composition differed remark- ably, with only 28% of all phylotypes (ntotal = 855) shared between habitats with and without lug- worms. Twenty-seven percent of all taxa were detected exclusively in the presence of lugworms, and 62% of the taxa found in the oxic subsurface sediment surrounding the lugworm burrow were limited to this distinct microenvironment. This suggests that lugworm burrows provide an ecolog- ical niche potentially hosting protist taxa that are adapted to fluctuating oxygen supply. We con- clude that though the activities of A. marina reduce the overall abundance of protists, they select for very specific and well-adapted taxa. This adds to the overall protistan diversity in intertidal sandflats on larger spatial scales. Most likely, the observed effects of lugworms on protistan com- munity structure and composition are due to a combination of direct trophic and indirect sedi- ment- mediated effects, such as disturbance by reworking the sediment, oxygen supply by burrow ventilation and increased pore water exchange due to bioadvection.

Effects of the bioturbating lugworm Arenicola marina on the structure of benthic protistan communities