AME 39:69-83 (2005)  -  doi:10.3354/ame039069

ABSTRACT: This study investigated the microbial community structure and functioning in relation to properties of dominant sediment types (sand, mud) of hypertrophic inner coastal waters. A transect in the Großer Jasmunder Bodden (inner part of the Nordrügensche Bodden, Southern Baltic Sea), with 6 stations from the shore to the central part was sampled in June 1999. In light-exposed nearshore sandy sediments, microphytobenthos was the driving source for benthic metabolism. Positive relationships between photoautotrophic biomass and available dissolved organic carbon (measured with a microbial biosensor) suggested that the pool of available dissolved organic nutrients was supplied by photosynthesis products of the benthic microalgae. Organic matter produced by these algae was rapidly recycled and led to low standing stocks of organic matter. In contrast, muddy organic-rich accumulation sites in greater water depths with limited or no light were heterotrophically dominated. These sediments retained large amounts of deposited organic matter, especially organic carbon and phosphorus. Intensive microbial degradation of deposited organic matter resulted in enhanced enzymatic activity, and in enhanced concentrations of ammonium and phosphate in porewaters. Our findings revealed that sedimentary bulk parameters such as concentration and degradability of particulate organic carbon as well as microbial biomass and activity depend on mud content in pure sandy sediments (with a mud content <10%), while they remain unchanged in sediments with higher mud content. Carbon-normalized enzymatic activity rates revealed that substrate turnover rates in muddy sediments of hypertrophic waters were relatively low compared to sediments with comparable total organic carbon concentrations in eutrophic waters. These findings imply that the availability of degradable organic matter (accounting only for <1% of total organic matter) and probably inorganic nutrients might be of much greater importance in regulating microbial biomass and activity than total organic carbon.

KEY WORDS: Sediment characteristics · Sand · Mud · Organic matter · Available dissolved organic carbon · Benthic microbial colonization · Enzymatic degradation activity