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Decomposition and microbial utilization of leaf material was investigated in microand 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 be 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 be 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. INTRODUCTION reported to be abundant in many coastal ecosystems and is processed largely via detrital food webs based on Although vascular plants are thought to account for a bacterial decomposition (Mann 1972, Mann 1988, Valsignificant fraction of the primary production in coastal iela et al. 1985, Benner et al. 1988, Moran & Hodson marine waters (Mann 1982, Stevenson 1988) only a few 1989a). aquatic macrophytes are extensively used by grazers Among the seagrass species present in the Mediter(Mann 1988, and references therein). Therefore, litter ranean Sea, Cyrnodocea nodosa is one of the most and particulate detritus derived from vascular plants is important macrophytic primary producers (Den Hartog G Inter-Research/Printed in Germany 0171-8630/91/0071/0163/$ 03.00 164 Mar. Ecol. Prog. SE 1970, Peduzzi & VukoviE 1990). In the Gulf of Trieste (Northern Adriatic Sea) C, nodosa is the most frequent phanerogam (Simonetti 1973) and, due to its high productivity, a significant contributor to the overall production of organic material in this area (Peduzzi & VukoviE 1990). Bacteria are reported to utilize both the soluble fraction of plant debris, which leaches rapidly after plant death, and the more refractory compounds (Valiela et al. 1985, Benner et al. 1986, Findlay et al. 1986, Moran & Hodson 1989a, b). The conversion efficiency within the detritus-associated microbial community is one of the factors determining whether the degrading material can significantly support a microbial food web and/or higher trophic levels. Whereas only few specialized macrofaunal species might use this detritus with an efficiency up to 40 % (Peduzzi 1987, Mann 19881, most studies dealing with bacterial growth on particulate detritus from a variety of aquatic macrophytes suggest low (around 10%) bacterial growth efficiencies (Stuart et al. 1981, Newel1 et al. 1983, Linley & Newel1 1984). A more recent study reports carbon conversion efficiencies for bacterial growth on lignocellulose of up to 45 %; these high values, however, were obtained like many others with ground and processed material (Benner et al. 1988). In the present study it was asked whether Cymodocea nodosa leaf litter can support a microheterotrophic food web either via the soluble fraction or the utilization of the refractory structural components of unprocessed (i.e. unground) sloughed leaves. Furthermore, the significance of the leaf-attached microbial community for the utilization of this material is questioned. In the light of the high metabolic costs for the production of extracellular enzymes by bacteria (Gottschalk 1986) it was asked whether the material can b e processed completely and utilized immediately by surface attached microbes alone. Finally, we tried to assess the significance of C. nodosa primary production for overall microbial secondary production within the Gulf of Trieste. MATERIAL AND METHODS Leaf material of the seagrass Cymodocea nodosa was collected in fall in the Gulf of Trieste off the Laboratorio di Biologia Marina at Trieste-Aurisina (Italy) in depths of 5 to 7 m. We selected leaves which were recently deposited within the seagrass bed, as indicated by a yellow-brownish, senescent stage with ca 50 % yellow coloration (compare Newel1 et al. 1989). The leaf blades were used in all our different incubations as fresh, unground and undried material. In the study area C. nodosa is scarcely colonized by epiphytes (Peduzzi & Vukovii: 1990); only leaves with no visible epigrowth were selected (Robertson et al. 1982). Several authors using dried and ground material report on shortcomings arising from this method such as the difficulties in extrapolating DOC-release measured on leaves dried and powdered prior to incubation in seawater to actual in situ rates (Robertson et al. 1982), or the immediate release of high quantities of soluble components after rehydration (Pellikaan 1984). To avoid such severe alterations when processing the material prior to the incubation, we established a dry weight-to-fresh weight ratio for C. nodosa leaves (Robertson et al. 1982, Benner et al. 1988, Robertson 1988). This allowed us to calculate the initial amount added to the incubation jars on a dry wt basis. Initial leaching phase experiments. To investigate leaching of Cymodocea nodosa leaf litter and to evaluate the significance of this process for the microbial dynamics of the surrounding water, incubation experiments were performed a t 20 'C in the dark. The early course of dissolved monomeric carbohydrate carbon (MCHO-C) concentration and bacterial abundance was followed in an 80 h experiment; the development of the nanoflagellate population as well as of MCHO-C concentration was monitored in a 370 h experiment. In order to reduce bacterial predators, the seawater collected from the same site as the leaf material was filtered through 1.0 ym pore-size Nuclepore filters (Benner et al. 1986, Benner et al. 1988, Herndl 1988). To each precombusted (470 'C) glass jar containing 800 m1 of water, approximately equal amounts of leaf material were added. Each experiment was established in triplicate. Two separate jars were incubated with leaves as described above but the incubation media were poisoned with HgC12 (20 mg 1-l) to inhibit microbial uptake of abiotically leaching compounds (Boto et al. 1989). A possible effect of poisoning on the condition of leaf tissue cannot be ruled out; however even at the end of the experiments the tissue was of the same coloration and appearance in both poisoned and unpoisoned treatments. Two chambers were prepared with only filtered seawater to serve as a control for endogenous microbial growth. All jars were covered with parafilm and gently aerated with 0.45 Fm filtered air (Herndl & Peduzzi 1989). Water samples (12 ml) were taken at various time intervals. Subsamples were withdrawn for bacteria or nanoflagellate enumeration and for analyses of monomeric carbohydrates (MCHO). This type of compound was selected as a model component of the easily utilizable, dissolved organic matter (DOM) fraction of leaf material (compare also Pakulski 1986). Water samples for MCHO-analysis were filtered through precombusted glass fiber filters (Whatman GF/F) within 30 min after sampling and stored frozen (-20 "C). MCHO contents were determined Peduzzi & Herndl: Microbial seagrass decomposihon 165 according to the spectrophotometric method of Johnson & Sieburth (1977) as described in more detail by

marine ecology. progress series

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