Bacterial community structure and microbial activity were determined together with a large number of contextual environmental parameters over 2 years in subtidal sands of the German Wadden Sea in order to identify the main factors shaping microbial community structure and activity in this habitat. Seasonal changes in temperature were directly reflected in bacterial activities and total community respiration, but could not explain variations in the community structure. Strong sediment depthrelated patterns were observed for bacterial abundances, carbon production rates and extracellular enzymatic activities. Bacterial community structure also showed a clear vertical variation with higher operational taxonomic unit (OTU) numbers at 10-15 cm depth than in the top 10 cm, probably because of the decreasing disturbance by hydrodynamic forces with sediment depth. The depthrelated variations in bacterial community structure could be attributed to vertical changes in bacterial abundances, chlorophyll a and NO 3 À , indicating that spatial patterns of microbes are partially environmentally controlled. Time was the most important single factor affecting microbial community structure with an OTU replacement of up to 47% over 2 years and a contribution of 34% to the total variation. A large part of this variation was not related to any environmental parameters, suggesting that temporal variations in bacterial community structure are caused by yet unknown environmental drivers and/or by stochastic events in coastal sand habitats. Principal ecosystem functions such as benthic oxygen consumption and extracellular hydrolysis of organic matter were, however, at a high level at all times, indicating functional redundancy in the microbial communities.

No abstract

Large-scale experimental exclusion of lugworms (Arenicola marina) from 400 m 2 intertidal fine sand revealed significant effects of their bioturbation and bioirrigation on sediment characteristics, benthic infauna composition, and the dominant mineralization and benthic-pelagic exchange processes in the sediment. Absence of lugworms resulted in sediment clogging with organic-rich fine particles, an eightfold decrease in sediment permeability, low oxygen penetration depths, and accumulation of reduced mineralization products in the pore water. The sand flat inhabited by lugworms had low fine-particle and chlorophyll contents and low sulfide and nutrient concentrations in the pore water. The effects were not limited to the vicinity of lugworm burrows but extended throughout the entire sediment down to ,20-cm depth. Sediments with the lugworm shared the characteristic of low-organic, advection-driven permeable sand rather than of muddy, diffusion-dominated sediments in the absence of lugworms. Areal oxygen uptake rates depended strongly on hydrodynamic conditions: under calm conditions, sedimentary oxygen uptake was slightly higher at the exclusion site. Experimental flushing using controlled hydrodynamic conditions showed that oxygen uptake at the lugworm site would be higher during more dynamic conditions (e.g., storms) due to significantly deeper oxygen penetration permitted by higher sediment permeability. Our results indicate an interactive effect of bioturbating organisms and hydrodynamics on water-sediment exchange processes and highlight the importance of benthic infauna for sedimentary processes even in physically dominated systems.

No abstract

No abstract