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Relations between sediment properties and pools of organic carbon and microbial bio- mass were investigated in sediments of shallow water coastal inlets of the southern Baltic Sea (Ras- sower Strom, Nordrugensche Bodden). According to their mud content (percentage of the fraction <63 µm to total sediment dry weight), sediments were grouped into sand, slightly muddy sand, and muddy sand. Phospholipids and ATP were analysed to estimate total and 'active' microbial biomass. From measurements of chlorophyll a (chl a) and total bacterial numbers, photoautotrophic microbial and bacterial biomasses were estimated. Carbon pools were determined by total organic carbon (TOC), dissolved organic and inorganic carbon (DOC, DIC), and available dissolved organic carbon (ADOC). The latter was measured by means of a biosensor consisting of an oxygen microelectrode with bacteria attached to the tip. Nitrogen and phosphorus were also analysed. Size and composition of carbon pools and microbial biomass revealed pronounced differences related to the mud content of the sediments. Sandy sediments were characterized by a high contribution of DOC and ADOC to TOC. Microbial biomass could be equally estimated by phospholipid-, ATP-, and chl-a-based carbon. Bacterial carbon constituted 5% of the total microbial biomass carbon. With increasing mud content from sandy and slightly muddy sand towards muddy sand sediments, TOC and DOC increased, whereas ADOC decreased. In the same way, total microbial (phospholipid based) and bacterial bio- mass increased; whereas the contribution of photoautotrophic (chl a based) and active (ATP based) biomass to total biomass declined to 31 and 18%, respectively. As demonstrated by increasing ratios of carbon to nitrogen, nitrogen to phosphorus, and decreasing ratios of chl a to phaeopigments, the organic matter undergoes increasing aging from sandy to muddy sand sediments. Pronounced rela- tionships between phospholipid and bacterial based biomass on the one hand and between ATP and chl a on the other suggested similar structural and physiological features of corresponding biomass indicators. Total phospholipid based microbial biomass and bacterial biomass were closely related to mud content as well as to organic matter (carbon, nitrogen), whereas photoautotrophic and 'active' microbial biomass measured as chl a and ATP seemed to be inversely related to mud content. From the direct relation between photoautotrophic biomass and ADOC, it is suggested that easily decom- posable carbon for microbial utilization was supported by exudation during photosynthesis.

Characterization of carbon and microbial biomass pools in shallow water coastal sediments of the southern Baltic Sea (Nordrügensche Bodden)