Coastal sands filter and accumulate organic and inorganic materials from the terrestrial and marine environment, and thus provide a high diversity of microbial niches. Sands of temperate climate zones represent a temporally and spatially highly dynamic marine environment characterized by strong physical mixing and seasonal variation. Yet little is known about the temporal fluctuations of resident and rare members of bacterial communities in this environment. By combining community fingerprinting via pyrosequencing of ribosomal genes with the characterization of multiple environmental parameters, we disentangled the effects of seasonality, environmental heterogeneity, sediment depth and biogeochemical gradients on the fluctuations of bacterial communities of marine sands. Surprisingly, only 3–5% of all bacterial types of a given depth zone were present at all times, but 50–80% of them belonged to the most abundant types in the data set. About 60–70% of the bacterial types consisted of tag sequences occurring only once over a period of 1 year. Most members of the rare biosphere did not become abundant at any time or at any sediment depth, but varied significantly with environmental parameters associated with nutritional stress. Despite the large proportion and turnover of rare organisms, the overall community patterns were driven by deterministic relationships associated with seasonal fluctuations in key biogeochemical parameters related to primary productivity. The maintenance of major biogeochemical functions throughout the observation period suggests that the small proportion of resident bacterial types in sands perform the key biogeochemical processes, with minimal effects from the rare fraction of the communities.

We investigated the rates of the main microbiological processes (primary production, aerobic and anaerobic carbon degradation) and transport phenomena in an intertidal sand plate with a combination of in situ microsensor measurements and incubations. The sand was coarse, organically poor (0.6-1 mg of total organic carbon per gram dry weight of sediment), and highly permeable to water flow (k ϭ 1.5-7 ϫ 10 Ϫ11 m 2 ). Aerobic respiration rates ranged from 105 to 175 mmol m Ϫ2 d Ϫ1, sulfate reduction rates from 0.08 to 13.7 mmol m Ϫ2 d Ϫ1, and net primary production Ͻ35 mmol m Ϫ2 d Ϫ1 . In situ microsensor measurements showed large changes in oxygen and sulfide concentrations in the top 10 cm, depending on tides and waves. The observed dynamics and high aerobic degradation rates imply that pressure gradients drive advective influx of oxygen and organic material from the water column into the sediments. Our results show that intertidal porous sand plates have high aerobic degradation rates, despite having an organic matter content that is one to two orders of magnitude lower than that of fine-grained deposits with similar decomposition rates.

Among the increasing number of species introduced to coastal regions by man, only a few are able to establish themselves and spread in their new environments. We will show that the Pacific oyster (Crassostrea gigas) took 17 years before a large population of several million oysters became established on natural mussel beds in the vicinity of an oyster farm near the island of Sylt (northern Wadden Sea, eastern North Sea). The first oyster, which had dispersed as a larva and settled on a mussel bed, was discovered 5 years after oyster farming had commenced. Data on abundance and size-frequency distribution of oysters on intertidal mussel beds around the island indicate that recruitment was patchy and occurred only in 6 out of 18 years. Significant proportions of these cohorts survived for at least 5 years. The population slowly expanded its range from intertidal to subtidal locations as well as from Sylt north-and southwards along the coastline. Abundances of more than 300 oysters m À2 on mussel beds were observed in 2003, only after two consecutive spatfalls in 2001 and 2002. Analyses of mean monthly water temperatures indicate that recruitment coincided with above-average temperatures in July and August when spawning and planktonic dispersal occurs. We conclude that the further invasion of C. gigas in the northern Wadden Sea will depend on high late-summer water temperatures.

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