Re-colonization of anoxic/sulfidic sediments by marine nematodes after experimental removal of macroalgal cover

In shallow beaches of the southern Baltic Sea drifting macroalgal mats frequently accumulate in the summer months, inducing anoxic and sulfidic conditions with devastating effects on most members of the benthic fauna. Our study describes the meiobenthic nematode fauna under such a natural drifting algal mat and its short-term (30 days) response to the removal of the mat, simulating natural displacement. The sediment under algal cover was characterized by high concentrations of total organic matter (3.9%) and sulfide (10.0 mmol l(-1)). Sulfide was also detectable in the mat itself (2.2 mmol l(-1)) and in the overlying water column (0.3 mmol l(-1)). Thirty days after algal removal total organic matter had dropped to 1.7% and sulfide was present only in deeper sediment horizons. Changes in fauna composition of previously algal-covered sediments were compared with ambient reference stations by means of diversity indices and multidimensional scaling ordination. The nematode assemblage under the mat (prior to removal) was characterized by low diversity (H' = 0.42) and dominated by the deposit feeder Sabatieria pulchra, the only nematode present in algal-covered sediments in abundances similar to ambient reference stations. Significant changes in community composition in previously algal-covered sediments could be demonstrated after 30 days by means of multiple dimensional scaling techniques. At this time Daptonema sp. and Chromadorita tenuis dominated the nematode fauna and were present in significantly higher abundances than in the reference samples. While diversity indices showed no significant differences between treatments within 30 days, multidimensional scaling proved that differences between treatments were still significant, indicating that the recovery process had not finished. Reasons for the delayed re-colonization of nematodes are discussed.