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Glucuronidase activity of fecal coliforms was measured w ~ t h the fluorogenic substrate analog 4-methylumbelliferyl-P-D-glucuronide (MUG) in the northern Adriatic Sea during summer 1991 (1) to determine the distribution of MUG activity along a trophic gradient from oligotrophic waters to the eutrophic waters near the mouth of the PO River, and (2) to evaluate the role of manne snow as a scavenging agent of fecal coliforms in coastal waters. Along the trophic gradient, MUG activity was lowest in the most offshore central part of the northern Adriatic and increased towards the coastal stations by a factor of 2. MUG activity integrated over the entire water column increased almost 10-fold from May to August. Highest activity was generally detectable in the surface layers and declined with depth. MUG activity in marine snow was found to be up to 100 times higher than in ambient water and therefore corresponded with ennchment factors calculated for other autoand heterotrophlc microorganisms. Similarly to actively growing microbial communities, a n exponential increase in MUG activity in marine snow was detectable. Based on these findings we conclude that fecal coliforms in marine snow have higher survival rates accompanied by higher activity [measured here a s MUG activity) than in the ambient water. Fecal coliform bacteria are characterized by a specific enzyme which makes them distinguishable from other members of the bacterial community in aquatic systems on the basis of their enzymatic activity (Berg & Fiksdal 1988, Clark et al. 1991). Measurement of glucuronidase activity of fecal coliforms has been compared recently with the traditional membrane filtration fecal coliform (MFC) technique (Clark et al. 1991). Based on the experiments performed by these authors, glucuronidase activity as determined by specific substrate analogs corresponds well with the MFC technique if there are > 10 colony-forming units (CFU) of Escherichia coli 100ml-I (Clark et al. 1991). Thus, the use of fluorescent substrate analogs to estimate the abundance of fecal coliforms might be a powerful tool for rapidly evaluating the influence of fecal coliform contamination of nearshore waters. In the Adriatic Sea, counterclockwise currents isolate the northern water masses from the remaining Adriatic waters at the onset of summer stratification (Orlic 1987). Concurrently with this at least partial isolation of the water masses, human populations living along the coasts increase by about 2 orders of magnitude during summer months. In nearshore, coastal waters this situation might lead locally to elevated fecal coliform concentrations (Fuks 1980). Another pecularity of the northern Adriatic which is probably directly linked to the above-mentioned current pattern during summer is the development of marine snow and large marine clouds (Herndl & Peduzzi 1988, Stachowitsch et al. 1990, Herndl et al. in press). These amorphous aggregates have been shown to efficiently scavenge particles from the water column (Puskaric et al. 1992), leading to biomass accumulation in confined environments (marine snow) while concurrently depleting the ambient water of planktonic organisms (Herndl in press). The purpose of this study was 2-fold. Firstly, we wanted to determine the glucuronidase activity along a trophic gradient across the northern Adriatic Sea from the oligotrophic eastern part to the highly eutrophic western part off the PO River estuary during summer. We hypothesized that glucuronidase activity in the central parts of the northern Adriatic Sea would be undetectable as fecal coliform bacteria decay during transport from the coast. Secondly, we attempted to evaluate the role of marine snow as a scavenging agent for fecal coliforms. Sampling on a O Inter-Research 1992 Mar. Ecol. Prog. Ser. narrow temporal scale, we followed the development of glucuronidase activity in marine snow and ambient water by selectively collecting these amorphous aggregates. As has also been shown for other organisms (Bochdansky & Herndl 1992a, b, Kaltenbock & Herndl 1992), we expected h g h e r glucuronidase activity in marine snow as compared to ambient water. The study was carried out in the northern Adriatic Sea during summer 1991. Samples were collected on 3 cruises of the RV 'Vila Velebita' along a trophic gradient starting off Rovinj (Croatia) to the mouth of the PO River (Fig. 1, route 1) between May and September; on one occasion we also sampled along the more oligotrophic Istrian peninsula (Fig. 1, route 2). At each station samples were taken from 0.5, 5, 10 and 20 m depth. Additionally, marine snow and ambient water samples were collected by SCUBA divers ca 1.5 km off the Laboratorio di Biologia Marina di Trieste-Aurisina (Italy) in the Gulf of Trieste. (For a detailed description of the sampling procedure consult Herndl 1988.) Briefly, marine snow was collected using 60 m1 disposable syringes rinsed with ambient water prior to sampling. As marine snow aggregates were at least 0.5 cm in length and quite numerous (see also Herndl in press) it was possible to collect them without including the surrounding water. Ambient water was collected with hand-made 870 rnl syringes. Care was taken not to include visible aggregates in the ambient water sample. During the cruises the samples were analysed on board the ship while marine snow and ambient water samples collected by SCUBA divers were brought back to Fig. 1. Study location in the northern Adriatic Sea. Water samples were taken along the trophic gradient from Rovinj (Croatia) to the entrance of the PO River (Italy) (route 1) at 4 depths (0.5, 5, 10 and 20 m) at the stations indicated by circles; in July we also sampled along the Istrian Peninsula (route 2). Marine snow and ambient water were collected by SCUBA divers about 1.5 km off the Laboratorio d~ B~ologia Marina di Aurisina the shore-based laboratory. Generally, processing of samples began within 20 min after collecting the material. To determine glucuronidase activity, 4-methylumbelliferyl-P-D-glucuronide (MUG; Sigma Chemicals) was used as substrate. The increase in fluorescence over time (incubation period was usually less than 60 min) was measured with a Jasco 820-FP spectrofluorometer at in situ temperature. The increase in fluorescence over time was calibrated with known amounts of methylumbelliferone (Sigma Chemicals) and assuming a linear increase in fluorescence over a wide range of substrate released. This assumption had been verified in preliminary experiments (data not shown). We used the tracer approach rather than saturation kinetics to evaluate the actual activity of glucuronidase in the sea. A more detailed description on the differences between the 2 approaches (tracer vs kinetic studies) will be given elsewhere. In this study we used 2.5 pM final concentration of MUG. As shown in Fig. 2, MUG activity increased from May to early September by about 1 order of magnitude, with highest MUG activity generally found in the surface waters (Fig. 2). Only in August was higher MUG activity detectable in the 5 m depth horizon. Integrat~ng MUG activity over several depth horizons revealed an increase in activity towards the PO Rlver after an initial decline from the coastal station in front of Rovinj towards the oligotrophic Stn I1 (Fig. 3). The increase from the most oligotrophic station to the station in front of the PO River (Stn V) was generally less than 2-fold. This is a rather small increase considering the rapid decline of MUG activity with depth and time (Fig. 2). As shown in Fig. 1, Stn V1 is about glucuronidase activity (nmol MU released I-' h-') H route 2 Jul route l Sep

marine ecology. progress series

Activity of fecal coliform bacteria measured by 4-methylumbelliferyl-ß-D-glucuronide substrate in the northern Adriatic Sea with special reference to marine snow