MEPS 249:69-78 (2003)  -  doi:10.3354/meps249069

Microbial dynamics on diatom aggregates in Øresund, Denmark

Hans Peter Grossart1,*, Susanna Hietanen2, Helle Ploug3,4

1Institute of Freshwater Ecology and Inland Fisheries (IGB), Dept. of Stratified Lakes, Alte Fischerhütte 2, 16775 Neuglobsow, Germany
2Finnish Institute of Marine Research, PO Box 33, 00931 Helsinki, Finland
3Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark
4Present address: Max Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany

ABSTRACT: Size, chemical composition, bacteria, flagellate, and ciliate abundance, bacterial production and growth rates as well as community respiration rates were measured on natural diatom aggregates of different sizes collected by SCUBA divers on 5 subsequent days offshore from northern Zealand, Denmark. Aggregate size was highly variable (0.16 to 524 mm3) throughout the sampling period, whereas aggregate (agg.) dry mass (70 to 390 µg agg.-1) and organic carbon content (48 to 130 µg agg.-1) varied less. The composition of particulate organic amino acids on aggregates was very different from that of total dissolved amino acids in the matrix water of the aggregates. Bacteria, flagellates, and ciliates were 10- to 10000-fold enriched on aggregates compared to their abundance in the surrounding water. Enrichment factors of bacteria, flagellates, and ciliates decreased significantly with increasing aggregate size. Ciliates showed the highest and flagellates the lowest enrichment on aggregates. Absolute rate of bacterial production was significantly correlated with aggregate size, and it constantly increased on aggregates of similar size throughout the sampling period. Cell-specific production rates were consistently higher for aggregate-associated bacteria than for free-living bacteria, and 3 to 20% of all bacteria were produced on aggregates. High release of dissolved organic matter and bacteria into the surrounding water was indicated by the release of amino acids into the surrounding water and high calculated detachment rates of aggregate-associated bacteria. Marine snow, thus, should be regarded as comprising integral components rather than isolated microenvironments of the pelagic zone. Its nature has important consequences for organic matter cycling from small scales up to the global scale.

KEY WORDS: Aggregates · Bacteria · Heterotrophic nanoflagellates · Bacterial production · Respiration · Grazing · C:N ratio · Particulate combined amino acids

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