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

Relations between sediment properties and pools of organic carbon and micro bial biomass were investigated in sediments of shallow water coastal inlets of the southern Baltic Sea (Rassower Strom, Nordrugensche Bodden). Accordi ng to their mud content (percentage of the fraction < 63 mum to total sedim ent dry weight), sediments were grouped into sand, slightly muddy sand, and muddy sand. Phospholipids and ATP were analysed to estimate total and 'act ive' microbial biomass. From measurements of chlorophyll a (chl a) and tota l bacterial numbers, photoautotrophic microbial and bacterial biomasses wer e estimated. Carbon pools were determined by total organic carbon (TOC), di ssolved organic and inorganic carbon (DOC, DIC), and available dissolved or ganic carbon (ADOC). The latter was measured by means of a biosensor consis ting of an oxygen microelectrode with bacteria attached to the tip. Nitroge n and phosphorus were also analysed. Size and composition of carbon pools a nd microbial biomass revealed pronounced differences related to the mud con tent of the sediments. Sandy sediments were characterized by a high contrib ution of DOC and ADOC to TOC. Microbial biomass could be equally estimated by phospholipid-, ATP-, and chi-a-based carbon. Bacterial carbon constitute d 5 % of the total microbial biomass carbon. With increasing mud content fr om sandy and slightly muddy sand towards muddy sand sediments, TOC and DOC increased, whereas ADOC decreased. In the same way, total microbial (phosph olipid based) and bacterial biomass increased; whereas the contribution of photoautotrophic(chl a based) and active (ATP based) biomass to total bioma ss declined to 31 and 18%, respectively. As demonstrated by increasing rati os of carbon to nitrogen, nitrogen to phosphorus, and decreasing ratios of chi a to phaeopigments, the organic matter undergoes increasing aging from sandy to muddy sand sediments. Pronounced relationships between phospholipi d and bacterial based biomass on the one hand and between ATP and chi a on the other suggested similar structural and physiological features of corres ponding 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 chi a and ATP seemed to be inversely related to mud co ntent. From the direct relation between photoautotrophic biomass and ADOC, it is suggested that easily decomposable carbon for microbial utilization w as supported by exudation during photosynthesis.