Decomposition and significance of sea-grass leaf litter (Cymodocea nodosa) for the microbial food web in coastal waters (Gulf of Trieste, Northern Adriatic Sea)

Decomposition and microbial utilization of leaf material was investigated in micro-and mesocosm experiments using freshly sloughed leaves of a common Mediterranean seagrass species (Cymodocea nodosa).In the initial phase of decon~position, dssolved monomeric carbohydrates (MCHO) leached out of the material provolung a rapid (within 40 h) response of the free-living bacterial community.When ablotic leaching had ceased after ca 180 h, 13 mg MCHO-C g-' (leaf dry wt) had been lost from the material, 92 % of which was taken up by microorganisms.At the advanced stage of decomposition, a nch nanoflagellate community had developed and after 14 d of incubation a freeliving flagellate standing stock of 210 pg C per g (leaf dry wt) was still Living at the expense of decomposing leaf material.In a long-term experiment uslng htter bags, the microbial colonization pattern and the breakdown of leaf tissue were followed under reduced influence of macrofaunal shredders and physical forces.The initially uniform bacterial coverage on leaf surfaces, as followed by SEM, changed within 14 d to a 7 times more abundant and heterogenous bacterial assemblage.After more than 3 wk, leaf surfaces began to break down as indicated by crevices densely surrounded by bacteria.At the same time a protozoan community, mainly consisting of monads and choanoflagellates.developed on leaf blades, reaching a maximum density of 2.4 x 105 cells cm-' after 2 mo of degradation.Specific loss rates of weight, particulate organic carbon and nitrogen from litterbags were highest during the first 2 mo and declined thereafter to rates 5 times lower.Only 50 % of the original dry wt and organic carbon remained after 7 to 8 mo of incubation.Measurements of the O2 consumption associated with decaying leaf material indicated that 40 O/ O of the calculated C-mineralization rate can b e attributed to the decreasing organic C-concentration in the plant litter during the first 2 mo.In a later stage of decomposition, only 4 % of the O2 consumption could b e matched by the organic C-loss of the decomposing material; dissolved organic material also in the surroundmg water is proposed to be utilized by the attached microbes.It is concluded that the soluble fraction leaching out of the material in the early phase of decay is rapidly used by both the free-living and the attached bacterial comn~unity, thus supporting a microbial food web up to the protozoan level.It is suggested that most of the residual fraction of organic carbon of leaf debris is released more slowly into the water after hydrolysis by attached microbes thus indicating a loose hydrolysis-uptake coupling.Under exclusion of shredders and reduction of physical forces degradation was not complete after 231 d of incubation.Leaf debris of C. nodosa is suggested to have a small but significant impact on microbial secondary production in the study area largely vla its leachates.