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Abundance and size classes of fecal pellets in marine snow as compared to the ambient water were investigated in the Northern Adriatic Sea during summer 1990 and 1991. Numbers of fecal pellets in marine snow varied with depth and kind of aggregates ranging from 31 to 353 000 fecal pellets I ' marine snow. Fecal pellets were found to be enriched in marine snow by a factor ranging from 0.3 to 3800. Small fecal pellets ( 1 m) amorphous aggregates. Besides low sinking velocities, resuspension and reingestion, trapping of fecal pellets by marine snow is an efficient mechanism to keep fecal pellet-derived carbon confined to the upper layers of stratified water colun~ns. Fecal pellets produced by zooplankton are known to be a major vehicle for vertical transport of particulate organic carbon (POC) in the open ocean (Pilskaln & Honjo 1987). Rapidly sinking fecal pellets contribute to the pelagic-benthic carbon flux (Emerson & Roff 1987) since large fecal pellets display high sinking rates and therefore settle out of the euphotic zone without being significantly decomposed (Small et al. 1979, Suess 1980). On the other hand there is evidence for biodegradation, coprophagy, resuspension and mechanical breakage which increase the residence time of fecal pellet derived POC in a given depth stratum of the water column (Honjo & Roman 1978, Paffenhofer & Knowles 1979, Bathmann et al. 1987, Emerson & Roff 1987, Lampitt et al. 1990). Turbulence in the mixed layers and the development of pycnoclines in stratified waters can also account for a prolonged residence time of even large fecal pellets (Alldredge et al. 1987). Besides fecal pellets, marine snow is the major component of particulate organic fluxes in the ocean (AllPresent address: Ocean Sciences Centre, Memorial University, St. Johns, Newfoundland, Canada A l C 5S7 ' ' Addressee for reprint requests dredge & Silver 1988). Urrere & Knauer (1981) found that numerous small items (including fecal pellets) are associated with large amounts of flocculent material found in sediment traps in the northeast Pacific; often fecal pellets and marine snow flux cannot be treated separately since essentially all the material entering sediment traps arrive in association with marine snow (Asper 1987). In eutrophic marine ecosystems a large fraction of the primary production remains ungrazed and contributes to rapid POh4 fluxes to the bottom (Smetacek 1985, Lochte & Turley 1988) or provokes the formation of large buoyant mucous sheets (Stachowitsch et al. 1990, Herndl et al. in press) which does not necessarily lead to enhanced vertical transport (Puskaric et al. 1992). In this study we investigated the contribution of fecal pellets to the formation of marine snow and discuss their role in particle flux dynamics in a shallow, neritic environment based on recent findings on the characteristics of large mucoid aggregates in the Northern Adriatic Sea. Methods. The sampling locations were 1 to 3 km off the Laboratorio di Biologia Marina at Aurisina-Trieste (Italy) and 3 km off Rovinj (Croatia) in the Northern Adriatic Sea. Sampling was performed during summer 1990 and 1991. Mesozooplankton was collected by double-oblique net hauls between 10 m depth and the surface with a 300 pm plankton net equipped with a flowmeter. Marine snow and ambient water containing no visible aggregates were collected by SCUBA diving with 60 m1 disposable and 0.8 1 plexiglass syringes, respectively. Marine snow was sampled without inclusion of ambient water, thus the syringe volume of 60 m1 is also the actual volume of marine snow sampled including porewater. Samples were either examined immediately after collecting or stored in 4 % buffered formalin-seawater solutions. To estimate the flux of fecal pellets to the bottom, 4 sediment traps (63 mm O Inter-Research 1992 298 Mar. Ecol. Prog. Ser 89: 293-303, 1992 inner diameter; height : width ratio = 6.5 : 1) were deployed in 27 m ca 0.5 m above the bottom and exposed for 24 h on 3 and 5 September 1991, respectlvely. During the deployment of the sediment traps calm weather conditions led to reduced turbulence regimes in the water column; therefore resuspension of the sediment is expected to be negligible. Fecal pellets were counted and measured under a dissecting microscope (magnification: x90) in both marine snow and ambient water. Their volume was calculated by measuring length and diameter in order to calculate their volume assuming a cylindrical shape. The highly structured matrix of marine snow made it extremely difficult to detect fecal pellets smaller than those enumerated during this study. Oval-shaped fecal pellets were generally rare; since they are also hardly distinguishable from decay stages of certain algae like peridineans which are common in marine snow, we restricted counting and sizing to intact cylindrical fecal pellets known from fecal pellet production experiments with mixed zooplankton samples. For total organic carbon (TOC) analysis, 60 to 300 fecal pellets were collected by plexiglass tubes on which nets (mesh size 200 pm) were fixed at the lower end (Fig. 1). After removing excess seawater with a pipette they were rinsed twice with redistilled water, suspended in 1 m1 of redistilled water and stored frozen at -20°C in precombusted (480°C for 4 h) and sealed ampoules until analyses. The fecal pellets were hydrolyzed with 1 m1 of 12 N HCl for 24 h at llO°C, then evaporated to dryness and re-dissolved in 2 m1 double-distilled water. Two m1 of marine snow and clouds (for terminology see Stachowitsch et al. 1990) were collected with syringes centrifuged and resuspended in 5 m1 redistilled water to remove the salt. This procedure was repeated twice. Subsequently the samples were ultrasonicated to

Ecology of amorphous aggregations (marine snow) in the Northern Adriatic Sea. V. Role of fecal pellets in marine snow