Imbalance between phytoplankton production and bacterial carbon demand in relation to mucilage formation in the Northern Adriatic Sea

Spatial and temporal changes in phytoplankton production and bacterial C demand were investigated at four stations in the Northern Adriatic Sea over 3 years. The effect of the Po River plume was observed at the western stations; in particular, the northernmost one (B06) showed the highest values of primary production, both as hourly peaks (up to 14 mg C m(-3) h(-1)) and daily water column integrated values (up to 740 mg C m(-2) day(-1)), the southern station (C04) was only sporadically influenced and did not differ significantly from the easternmost ones (C12 and B13), where the lowest TPP values were recorded (around 1 mg C m(-3) h(-1)). In this study the first in situ data are reported on short-term phytoplankton C extra cellular release in the Northern Adriatic Sea. At every station a considerable percentage of primary production (PER>20% as an average, with peaks of up to 70%) was released as dissolved organic carbon. In particular, an association of fairly high PER (>10%) and specific production (Pb>10 mg C mg chl(-1) h(-1)) was observed from spring to summer, when the mucilage phenomenon usually starts. This result might suggest the presence of an uncoupling between photosynthesis and growth, probably related with nutrient availability, which would be responsible for a high production of extra cellular organic carbon. Phytoplankton primary production and bacterial carbon production were closely related and bacterial C production accounted, on average, for a higher percentage of primary production than the values typically reported in the literature on aquatic environments. The flow of organic matter from phytoplankton to bacteria seems to satisfy the bacterial carbon demand during most of the spring and summer, at least in the upper water layers. However, during the summer, there is evidence that BCD sometimes exceeds the amount of C produced by phytoplankton. Neither phytoplankton nor bacterial production showed significant differences over the relevant years, and their absolute values did not change when comparing periods with or without mucilage. However, there were indications of an uncoupling between phytoplankton photosynthesis and growth and of a shift from an autotrophic to a heterotrophic metabolism, especially during the spring and summer period when mucilage might occur.     

Relevance of the dinoflagellate Gonyaulax fragilis in mucilage formations of the Adriatic Sea

Oceanographic cruises were carried out monthly from June 1999 to July 2002 to follow the mucilage formation process in the Northern Adriatic Sea. Results show that in correspondence with these events the dinoflagellate Gonyaulax fragilis (Schütt) Kofoid was observed both in the water column and within mucilage aggregates. In the water column, increasing abundances were observed from May until July, with values never exceeding 8500 cells l(-1). Much higher densities were observed within superficial gelatinous aggregates (22800-3400000 cells l(-1)). In mucilage samples, a large number of decomposing cells were present, together with abundant alive cells, enveloped in exudates. G. fragilis isolated from mucilage samples was cultured in three different culture media; it was characterized by a low growth rate but it produced a high amount of polysaccharides. The highest yield both in terms of cell number and carbohydrate production was observed in the medium having the highest nitrogen and phosphorus content and the lowest N/P ratio. The monomeric composition of G. fragilis carbohydrates, compared with that of mucilage samples, showed that in both natural and cultured samples galactose was the most abundant sugar; in addition, an overall good correlation, especially between the monomeric carbohydrate composition of G. fragilis grown in f/2 medium and that of a mucilage sample in which this species was present in high density, was observed.     

Phytoplankton size-distribution and community structure in relation to mucilage occurrence in the northern Adriatic Sea

The spatial and temporal variations of pico-, nano- and microphytoplankton abundance and composition were investigated over a 37 month period, focusing on the ecological role of different size classes of phytoplankton, and on the changes of the community structure that might occur during periods when large mucilage macroaggregates appear. Samples were collected monthly from June 1999 to July 2002 at 11 stations, along three transects covering the northern Adriatic basin. Highest abundances were observed in late-winter/spring for microphytoplankton (mainly diatoms), in spring-summer for nanophytoplankton, and in summer for picophytoplankton. The autotrophic component was more abundant in the summers of 2000 and 2002 (when large mucilage aggregates occurred) than in the summers of 1999 and 2001 (when a massive phenomenon was not observed). This increase was statistically significant for pico-, nano- and, among microphytoplankton, only for dinoflagellates. Blooms of picophytoplankton were often observed at the bottom layer during mucilage summers. The microphytoplankton community during mucilage phenomena was characterized by a species composition (Chaetoceros spp., Cerataulina pelagica, Pseudo-nitzschia delicatissima, P. pseudodelicatissima, Cylindrotheca closterium, Dactyliosolen fragilissimus) comparable to that observed in summers without extensive mucilage occurrence. However, some species appeared with significantly higher densities in the summers of 2000 and 2002: Ceratium furca, C. closterium, Oxytoxum spp., Hemiaulus hauckii and Gonyaulax fragilis. Microscopic observation of aggregates revealed that the microphytoplankton species composition inside the aggregates was comparable to that observed in the water column, with an enrichment of opportunistic species such as C. closterium and P. delicatissima. The presence of mucilage aggregates affects the phytoplankton populations in the water column, even when aggregates are at early stages. It seems that there is a mutual relationship between phytoplankton and aggregates, i.e., several diatom and dinoflagellate species may contribute to the aggregate formation and enlargement, but mucilage aggregates themselves may also affect the phytoplankton populations, allowing the development of a rich diatom community and in general enhancing nanophytoplankton growth.     

Relationships between heterotrophic bacteria and cyanobacteria in the northern Adriatic in relation to the mucilage phenomenon

High variability of heterotrophic bacterial (HB; 0.1.10(9)-6.10(9) cells L(-1)), nanoflagellates (HNF; 0.02.10(6)-2.4.10(6) cells L(-1)) and cyanobacterial (CB; 10(6)-700.10(6) cells L(-1)) abundances were observed during approximately monthly measurements at six stations along the transect Po Delta-Rovinj from March 1999 to August 2002. Substantially higher values were observed during the stratification period (June-September) in the surface layer of the western stations that were under more direct influence of Po River discharges. Changes of the HB abundance were significantly correlated with temperature in the entire water column and chlorophyll a (Chla) concentration in the surface layer. The nutrients did not look to have directly influenced the HB growth, except orthophosphate in the eastern, more oligotrophic part of transect, where probably HB efficiently competed with phytoplankton for this nutrient. Temperature was also important for CB growth that, however, appeared to occur more intensively in waters with low nutrient concentrations. Probably, in these conditions CB can still develop, while larger autotrophs are strongly nutrient limited. In the upper water column of reduced salinity (35-37), the contributions of CB carbon biomass to the total picoplankton biomass (CB+HB) were mostly larger than 30% (up to 80%) in years with mucilage events (1991, 2000-2002) than in other years. For example, in 1999 only a few values were higher than 30% (up to 50%). In abundant presence of mucilaginous aggregates (e.g. in June 2000 and late June 2002) the chlorophyll a ascribed to CB accounted for much larger portions of total measured Chla. From these results it was concluded that cyanobacteria during mucilage events may play a substantially increased role both as primary producers and prey within the microbial loop of the northern Adriatic.     

Exo-enzymatic activities and dissolved organic pools in relation with mucilage development in the Northern Adriatic Sea

We tested the hypothesis that the appearance of mucilage in the Northern Adriatic Sea was related with the accumulation of dissolved organic compounds released by intensive enzymatic activities and not utilized as direct substrate for microbial growth. To do this enzymatic activities and dissolved organic and inorganic pools in periods characterized by the presence of mucilage and in the same seasons but in absence of mucilage were compared. Extracellular enzymatic activities (aminopeptidase, beta-glucosidase and alkaline phosphatase), nutrient pool concentrations (total dissolved nitrogen, dissolved organic nitrogen, total dissolved phosphorus, dissolved organic phosphorus) and the biochemical composition of particulate and dissolved organic matter (in terms of proteins and carbohydrates) were determined on a monthly basis over a period of 3 years. Aminopeptidase and alkaline phosphatase activities displayed higher values in springs preceding the appearance of mucilage than in spring when no mucilage was observed. Beta-Glucosidase activity showed significantly higher values in summer periods characterized by the massive production of mucilage than in summers without mucilage events. The months preceding mucilage events were also characterized by an increase of the alkaline phosphatase to aminopeptidase activity ratio and by a significant accumulation of dissolved proteins. These findings, together with the significant increase of the DON/DOP ratio, suggest that mucilage formation is favoured by the deficiency of organic P. The present study provides compelling evidences that mucilage formation is favoured by the unbalance between organic matter mobilization by enzymatic activities and the accumulation of labile dissolved organic-N compounds.     

Temporal dynamics of dissolved and particulate organic carbon in the northern Adriatic Sea in relation to the mucilage events

The accumulation of dissolved and particulate organic matter may play an important role in mucilage formation in the northern Adriatic. Distributions of dissolved and particulate organic carbon were therefore investigated during the period June 1999-July 2002, when massive mucilage events occurred: in the summer of 2000 and, to a greater extent, of 2002. The seasonal variations in dissolved organic carbon (DOC) concentrations were significant, doubling in summer (up to 150 micromol L(-1)) with respect to winter. The particulate organic carbon (POC) variations were also very large, with a less pronounced seasonal pattern compared to DOC, because the POC changes are much more dependent both on river discharges and on phytoplankton blooms. The comparison of the concentrations between the period before (March-May) and after the onset of mucilage events (June-August) showed that DOC, but particularly POC, were higher in the period before the event of 2002, more markedly in the surface waters of low salinity. The POC increased, reaching mean concentrations of up to 36 micromol L(-1) in March 2002 before the outbreak of the massive mucilage formation in June. This suggests that POC may have a more important role in the mucilage formation than DOC. The highest seasonal variations of organic matter concentrations took place in the upper layer of lower salinity, stressing the importance of stratification and pycnoclines for accumulation and aggregation of the organic matter in the northern Adriatic. The POC contribution to the total organic carbon was low in the oligotrophic waters (DOC/POC ratio >15) and increased with the phytoplankton biomass in the productive waters (DOC/POC ratio <10). Particulate organic carbon predominated over the dissolved inside the mucilage aggregate (DOC/POC ratio <1), probably because aggregation processes, in which colloidal organic carbon is involved, are important. The organic carbon within the aggregates reached a concentration of 13.6 mmol L(-1) which was about 100 times more than in the surrounding waters or in the waters when the mucilages were absent. This indicates that distributions of organic carbon in the northern Adriatic can be extremely patchy during mucilage events.     

Chemical characterization of different typologies of mucilaginous aggregates in the Northern Adriatic Sea

The chemical composition of mucilage aggregates found during summer 2000, 2001 and 2002 in the North Adriatic Sea depends on the nature of the organic matter during aggregation, on the environmental conditions of the site of formation and on the transformations during ageing. The mucilages were composed of organic matter, together with a significant inorganic fraction. Elemental analysis revealed 12.5-32.2% of organic carbon, 0-7.3% of inorganic carbon and 1.0-3.7% of nitrogen. The C(org)/N ratios of most aggregates were between 7.5 and 12.6, values close to those found in the suspended matter; higher ratios were found in large-size (>5 m) aggregates which are probably older. The content of carbohydrates and proteins determined in the aggregates, respectively, 15.4+/-8.9% and 7.9+/-4.8%, w/w, showed a prevalence of carbohydrates over proteins. Neutral carbohydrate analysis of purified polysaccharides from mucilage samples showed very similar signatures with high relative abundance of galactose and glucose. Humic, fulvic and humin substances extracted from the mucilages constitute an important fraction of the organic matter in the aggregates. The humin (a fraction insoluble in acidic and basic media) was present in all mucilage samples, indicating the refractory nature of a part of the organic matter in the mucilage. The iron and calcium could play a role during the aggregation process to form a complex with polysaccharides and humic fractions. The C(org)/N ratio 10+/-2 found in the humic acids extracted from the Adriatic aggregates disclosed a marine origin. The low phosphorus content and the high C(org)/P ratio found in the aggregates might depend from high bacteria activity or from the aggregation of organic fractions depleted of phosphorus. The principal inorganic species contained aluminium and silicon, part of which was of biogenic origin and was more significant in the offshore mucilage aggregates than in the coastal ones. The Si(biog)/C(org) ratio showed that diatoms were always present in the aggregates, although it cannot be established whether these are the producers or these develop within the aggregates.     

Meteorological and oceanographic conditions in the northern Adriatic Sea during the period June 1999-July 2002: influence on the mucilage phenomenon

Mucilage events (formation of very large organic aggregates and gelatinous surface layers) have been documented several times during the past two centuries in the northern Adriatic Sea (NA), while their frequency has significantly increased since 1988. In this work, meteorological and oceanographic conditions in the NA during the period June 1999-July 2002 are described and their relation to the outbreak and fate of the mucilage phenomenon was investigated. Salinity and temperature data were collected during approximately monthly cruises along three transects in the NA. Relevant meteorological situations (air temperature, rainfall, wind) were selected from large-scale ECMWF analyses and from the Local Analysis and Prediction System (LAPS; Emilia Romagna Meteorological Service), while sea conditions (waves) were analysed by means of the Wave Adriatic Model (WAM). Data for air temperature, rainfall, and wind from several meteorological stations in the region were used. Average seasonal cycles of sea temperature and salinity simulated with statistical models, based on historical data collected in the NA since 1972, were used to determine thermal and haline anomalies. The monthly anomaly variability of maximum and minimum air temperatures, rainfall amount and number of rainy days did not appear to be relevant for the mucilage phenomenon outbreak. In contrast, both vertical and horizontal thermohaline gradients in the region were more developed during late spring and summer of 2000 and particularly of 2002, when the mucilage events were of greatest extent in space and time, compared to 2001 (short-lived event) and 1999 (no event). These more pronounced gradients were due to a combination of several unusual conditions: sharp heating of the sea surface in May-June, domination of eastwards transport of freshened waters formed in the Po Delta area, and intrusion of very high salinity intermediate waters originating in the eastern Mediterranean. Moreover, in winter of both 2000 and 2002 very dense and cold water formed and remained in the bottom layer until spring, contributing to increase the stratification degree of the water column. The duration of the mucilage events and their spatial distribution in the region depend strongly on meteorological changes. Recurrent anticyclonic conditions, characterized by low wind and calm sea, favour extended events in time (up 2 months in 2002). In contrast, highly perturbed weather, particularly due to strong "bora" wind, can be determined in sharp decay of the event (e.g. in July 2000).     

Trends and interactions of physical and bio-geo-chemical features in the Adriatic Sea as derived from satellite observations

Time series of satellite data, generated by the AVHRR (1981-1999), CZCS (1979-1985) and SeaWiFS (1998-2002), have been used to assess trends and interactions of physical and bio-geo-chemical features in the Adriatic Sea. The images were processed to estimate Sea Surface Temperature (SST) and Chlorophyll-like Pigment Concentration (CPC). Long-term composites and climatologies were derived, using fixed geographical grids and projections. The AVHRR data show an apparent warming trend, when plotting the sequence of seasonal cycles (monthly mean SST, averaged over the whole basin) against time, due to a steady rise of summer values. Considering 3 regions (north, central and south), split into east and west sections, the northern Adriatic shows high SST fluctuations (possibly associated with the cycle of winter cooling and summer warming, typical of the relatively shallow sub-basin), while the southern Adriatic exhibits a lower variability (possibly influenced by the periodic water incoming from, and outflowing to the Ionian Sea). During summer, an east-west gradient prevails, while during winter only a general north-south gradient can be found. The SeaWiFS-derived CPC values, distributions and trends appear to be consistent with the historical CZCS record. Persistent differences in the quantitative assessment of CPC for coastal waters is due to the use of improved algorithms, less influenced by the presence of dissolved organics and suspended sediments in the water column, for the processing of SeaWiFS data. Apparent incongruities of the space and time patterns in the SeaWiFS record with respect to the reference climatology, obtained by CZCS more than a decade before, occur chiefly when considering the spring bloom in the southern Adriatic and the summer development of the north Adriatic front. The comparison of the long-term times series of satellite data shows that there is a high correlation between patterns in the thermal field and in the colour field. This suggests that different surface waters, identified by the SST index, are also traced by different ecological features, identified by the CPC index. Both indices also show a high correlation with the classical cyclonic circulation scheme of the Adriatic Sea, proposing once again an intimate relationship between the water dynamics and its bio-geo-chemistry.     

Cadmium and lead in selected tissues of two commercially important fish species from the Adriatic Sea

Baseline levels of cadmium and lead were determined in muscle tissue and liver of hake (Merluccius merluccius) and red mullet (Mullus barbatus), two commercially important fish species from the eastern Adriatic. Concentrations of trace metals in liver (Cd: 6–183 μg kg⁻¹ w. wt. ; Pb: 39–970 μg kg⁻¹ w. wt.) were within the range of recently published data for the Mediterranean. In the muscle tissue, cadmium concentrations (4.1–29 μg kg⁻¹ w. wt.) were among the lowest reported values for the Mediterranean, whereas lead levels (49–158 μg kg⁻¹ w. wt.) were within the range of values reported for various coastal areas of the Mediterranean. Presented data on cadmium and lead content in the studied fish species provide no proof of the general pollution of the Adriatic. Obtained data were tested in relation to fish length. Metal concentrations in liver decreased with the increase in fish size, whereas no significant correlation was found between trace metal levels in the muscle tissue and the length of both species. Relationships between metal concentrations and sex were also tested, but they gave no significant results. A comparison of contaminant concentrations in the edible tissue of hake and red mullet with the Croatian legislation shows that the consumption of their meat is not harmful for humans, not even for the most endangered population from the coastal region.     

Metallothionein and metal levels in cytosol of liver, kidney and brain in relation to growth parameters of Mullus surmuletus and Liza aurata from the Eastern Adriatic Sea

Two wild fish populations (Mullus surmuletus (striped red mullet) as a bottom, and Liza aurata (golden grey mullet) as a pelagic species, were sampled in Kastela Bay, Middle Dalmatia. Metallothionein (MT) and metal content (Cu, Zn, Cd) was determined in heat-treated supernatants (S50) of fish tissues. MT levels were statistically different in liver and kidney of sampled species, with highest value in kidney of M. surmuletus (31.1 +/- 7.96 micrograms mg-1 proteins). Metal levels are statistically different between all tissues. Highest metal concentrations are in liver cytosols, especially of Cu in L. aurata (6.46 +/- 3.45 micrograms ml-1 S50). High positive correlation of MT and cytosolic metals is established with Zn in liver of M. surmuletus (0.95) and with Cu in liver of L. aurata (0.69). Total length, body weight and age are mostly correlated biometric parameters with MT and soluble metal concentrations. MT and metal levels increase with age in liver cytosol of both species.     

Spatial and temporal variability of carbohydrates in the northern Adriatic--a possible link to mucilage events

Large-scale mucilage events in the northern Adriatic have become more frequent during the last few decades. Since carbohydrates (CHO) comprise one of the major constituents of the Adriatic mucilage, studying their seasonal and spatial patterns is one of the key prerequisites for the understanding of this phenomenon. This paper reports on the distribution of particulate (PTCHO) and dissolved carbohydrates (DTCHO) in the period from June 1999 to July 2002, with a special emphasis on the summer mucilage events. A particular attention was paid to the relationship between the phytoplankton crop as reflected by photosynthetic pigments and carbohydrate levels in different seasons. The concentration of PTCHO varied in a very wide range (1-72 micromol C/l). Their seasonal distribution revealed that enhanced levels of PTCHO occurred only in the surface layer (0-5 m) and were associated with major phytoplankton blooms. However, the main pool of CHO was in the dissolved fraction, comprising on average 84+/-10% of the total CHO. In contrast to PTCHO, there was no seasonal coincidence between the enhanced phytoplankton biomass and concentrations of DTCHO. In fact, the concentration of DTCHO showed regularly a marked increase during summer, when the phytoplankton biomass remained relatively low. Nevertheless, the concentration of DTCHO in the productive surface layer, influenced by the Po River discharges, was significantly higher than that in the deeper layers. The detailed vertical distribution of carbohydrates indicated that phytoplankton populations in the bottom layer possessed a rather modest potential as a source of CHO. The spatial distribution in the surface layer along the transect Rovinj-Po River mouth revealed only a moderate increase of the total CHO concentration in the more eutrophic western part of the basin. Analysis of pluriannual variability of the total CHO levels showed that probability of a mucilage event in a given year depended on the accumulation of CHO during spring, but, even more, on the prevailing circulation pattern in the basin during the stratification period.     

Mercury and methylmercury in the Gulf of Trieste (northern Adriatic Sea)

The distribution, sources and fate of mercury (Hg) in the water column of the Gulf of Trieste (northern Adriatic Sea), affected by the Hg polluted river Soca/Isonzo for centuries draining the cinnabar-rich deposits of the Idrija mining district (NW Slovenia), were studied in terms of total and dissolved Hg, reactive Hg, total and dissolved methylmercury (MeHg), mesozooplankton Hg and MeHg, and sedimentation rates of particulate Hg. Higher total Hg concentrations in the surface layer were restricted to the area of the Gulf in front of the river plume expanding in a westerly direction. Higher concentrations in bottom water layers were the consequence of sediment resuspension. Dissolved Hg exhibited higher concentrations in the surface layer in the area in front of the river plume. Higher bottom concentrations of dissolved Hg observed at some stations were probably due to remobilization from sediments, including resuspension and benthic recycling. The relationship between dissolved Hg in the surface layer and salinity showed nonconservative mixing in June 1995 during higher riverine inflow and nearly conservative mixing in September 1995 during lower riverine inflow. Both mixing curves confirm the river Soca/Isonzo to be the most important source of total and dissolved Hg, which are significantly correlated, in the Gulf. Reactive Hg is significantly correlated with dissolved Hg, indicating that the majority of dissolved Hg is reactive and potentially involved in biogeochemical transformations. The higher total MeHg in the bottom layer is the result of remobilization of MeHg from sediments including benthic fluxes. Strong seasonal variation of sedimentation rates of particulate Hg was found during a 2-year study in the central part of the Gulf. These variations followed those of total sedimented matter, indicating that sedimented Hg is mostly associated with inorganic matter. About a 2.5-fold higher fluxes of particulate Hg were observed at the depth of 20 m relative to 10 m which is attributed to bottom sediment resuspension. Temporal variability of mesozooplankton Hg and MeHg is the consequence of biomass and species variations, and grazing behaviour. From the preliminary Hg mass balance it appears that the Gulf is an efficient trap for total Hg and a net source of MeHg.     

Characterization of cholinesterase activity in three bivalves inhabiting the North Adriatic sea and their possible use as sentinel organisms for biosurveillance programmes

Anticholinesterases constitute a major portion of modern synthetic insecticides and the assessment of cholinesterase (ChE) inhibition is widely used as a specific biomarker for evaluating the exposure of non-target organisms to these pollutants. To evaluate the possible exposure of the North Adriatic sea coastal environment to residues of agriculture practices, we ascertained whether the oyster, Ostrea edulis and the clam, Tapes philippinarum, highly valuable resources commercially harvested in the area, could be selected as sentinel species. We characterized the ChE biochemical properties in their gills, and for comparison the analysis was carried out also in gills of the mussel, Mytilus galloprovincialis, extensively used as a biological indicator of water quality. In the oysters and mussels, the ChE activity was a function of increasing concentrations of substrate in the range 0.01-0.5 mM. K(m) values were 93+/-15 and 77+/-8 microM for O. edulis and M. galloprovincialis, respectively. No detectable ChE activity was found in gills of T. philippinarum, at any tested condition. Significant inhibition of ChE activity by eserine, carbaryl, and ethyl-paraoxon was observed in gills of O. edulis and M. galloprovincialis. Although field validation is needed, the present study suggests that O. edulis may be employed as a biological indicator for assessing pesticide contamination, whilst T. philippinarum does not seem useful for this purpose.     

Extracellular enzyme activity and dynamics of bacterial community in mucilaginous aggregates of the northern Adriatic Sea

Bacterial degradation of mucilaginous aggregates (creamy layers, stringers and macroflocs) collected during two summer events (2001-2002) was tested. The objective was to describe the temporal trend of the bacterial activity, abundance and composition in the aggregated and dissolved organic matter under different trophic conditions. In the native aggregates proteins and organic phosphorous were actively hydrolyzed as aminopeptidase and alkaline phosphatase activities represented up to 87% and 25% of total activity, respectively; polysaccharides were less hydrolyzed and the highest activities were observed for beta-glucosidase (5% of the total). This hydrolysation pattern tends to a progressive accumulation of long persistent polysaccharides. During short term incubations nutrient addition (P, N and Glucose) differently stimulated bacterial growth in the seawater: P played the main role in stimulating bacterial production from 3 to 6 folds higher than in the control, whereas a secondary C-limitation was observed only for bacteria growing on seawater from macroflocs. This scarce dissolved organic carbon (DOC) bioavailability was confirmed by the lower DOC removal (13% macroflocs, 36% stringers). The total amount of carbon incorporated by bacteria living on aggregates was similar (0.58 mg C L(-1)) both in the control and under P enrichments showing a more balanced condition with respect to the seawater. Hence the well-known P limitation in the Northern Adriatic Sea affects only dissolved organic carbon uptake without influencing the uptake of aggregated organic matter. Organic matter limitation was observed only on stringers--total C incorporated raised to 0.96 mg C L(-1) after PNG addition. Macroflocs release of refractory compounds leads to DOC accumulation (73 microM DOC) contributing to inflate the pool of refractory DOC in the surrounding waters. Several evidences, including different monosaccharide composition of stringers and macroflocs (glucose 15% and 56% on stringers and macroflocs, respectively), bring to the conclusion that stringers are in an older stage in comparison with macroflocs. Community composition described by fluorescence in situ hybridization did not show significant differences between free-living and attached bacteria but it was modified by the different enrichment conditions: Cytophaga-Flavobacteria increased after inorganic nutrients enrichments while organics advantaged gamma-Proteobacteria.     

Populations of exopolysaccharide-producing cyanobacteria and diatoms in the mucilaginous benthic aggregates of the Tyrrhenian Sea (Tuscan Archipelago)

The microalgal community of benthic mucilages sampled in various sites of the Tuscan Archipelago during the period May 1999-July 2002, was always dominated by diatoms. Synedra, Licmophora and Navicula were the most frequently found genera, their presence having been observed within all the period under study; other ten genera of diatoms were found in the benthic mucilages, but their presence was dependent on the month and the year of observation. As concerns the cyanobacterial community, the filamentous genera Leptolyngbya, Lyngbya and Rivularia were the most frequently found, independently on the period and the year of samplings, while other genera, namely Oscillatoria, Symploca, Aphanocapsa and Gloeocapsa, were less frequently observed. During the period under study, the highest number of different genera of diatoms and cyanobacteria in the mucilaginous aggregates was observed during summer periods, particularly in June and July. The microscopic observation of the samples demonstrated that only diatoms belonging to the genera Navicula, Amphora, Cylindrotheca and Pseudo-nitzschia possessed outermost polysaccharidic investments, the cells of the other genera being devoid of sheaths or capsules. On the other side, all the observed cyanobacteria, with the exception of Oscillatoria strains, were characterized by the presence of outermost polysaccharidic investments, most frequently in the form of sheaths. However, when the microalgal and cyanobacterial isolates were cultured under laboratory conditions, the release of polysaccharidic material into the culture medium was observed only in a few cases and always in very small amounts. The observations done on the natural samples and the results obtained with the cultures of the microalgae and cyanobacteria isolated from the benthic aggregates suggest that only diatoms have given some contribution to the formation of the polysaccharidic material constituting the matrix of the mucilages, whereas the role of cyanobacteria in the benthic aggregates seems to be related to their capability to create biofilms on solid surfaces, constituting the first step in the colonization of the benthic surfaces by the microbial community.     

Interannual variability of surface heat fluxes in the Adriatic Sea in the period 1998-2001 and comparison with observations

Surface heat fluxes of the Adriatic Sea are estimated for the period 1998-2001 through bulk formulae with the goal to assess the uncertainties related to their estimations and to describe their interannual variability. In addition a comparison to observations is conducted. We computed the components of the sea surface heat budget by using two different operational meteorological data sets as inputs: the ECMWF operational analysis and the regional limited area model LAMBO operational forecast. Both results are consistent with previous long-term climatology and short-term analyses present in the literature. In both cases we obtained that the Adriatic Sea loses 26 W/m2 on average, that is consistent with the assessments found in the literature. Then we conducted a comparison with observations of the radiative components of the heat budget collected on offshore platforms and one coastal station. In the case of shortwave radiation, results show a little overestimation on the annual basis. Values obtained in this case are 172 W/m2 when using ECMWF data and 169 W/m2 when using LAMBO data. The use of either Schiano's or Gilman's and Garrett's corrections help to get even closer values. More difficult is to assess the comparison in the case of longwave radiation, with relative errors of an order of 10-20%.     

Benthic fluxes of cadmium, lead, copper and nitrogen species in the northern Adriatic Sea in front of the River Po outflow, Italy

Trace heavy metal (Cd, Pb and Cu) and nitrogen species (N-NO3, N-NO2 and N-NH4) fluxes between sediment and water were examined for approximately 4 days, in a coastal marine station located in the northern Adriatic Sea in front of the River Po outflow. An in situ benthic chamber, equipped with electronic devices for monitoring and adjustment of oxygen and pH and with a temperature detector, was used. The benthic chamber experiment enabled study of the temporal trend of metals and nutrients when oxygen concentration varied in a controlled environment. Although particular care was devoted to chamber deposition and parameter control, sediment resuspension occurred at the beginning of the experiment and O2 fluctuations were observed during the course of the experiment. Pb concentration was affected by both resuspension and oxic conditions in bottom water, which prevented determination of any reasonable Pb flux value. Cd and Cu, not influenced by oxygen fluctuations, reached an equilibrium phase in a short period with initial positive fluxes from sediment of 0.68 (S.D. = 0.07) and 6.9 (S.D. = 5.6) pmol cm(-2) h(-1), respectively. With regard to nitrogen species, the highest positive flux was that of N-NH4 (10.5, S.D. = 2.4, nmol cm(-2) h(-1)) whose concentration increased in the chamber, while nitrate concentration (initial flux of -5.7, S.D. = 1.5, nmol cm(-2) h(-1)) immediately decreased after the beginning of the experiment. Nitrite concentration was almost constant throughout the experiment and its flux was generally low (initial flux 0.1, S.D. = 0.9, nmol cm(-2) h(-1)).     

Levels and long-term trends of polychlorinated biphenyls and DDT''s in mussels collected from the Eastern Adriatic coastal waters

It is well documented that synthetic chlorinated hydrocarbon residues are widespread through the oceanic ecosystem. The Adriatic Sea, as a semi-enclosed body of water, is of special interest for an evaluation of the entry, extent and fate of the pollution by chlorinated hydrocarbons. An investigation of levels of chlorinated hydrocarbons in mussels in the eastern Adriatic coastal waters, off Croatia was performed. Samples were collected between 1972 and 1992 at a large number of stations. All these samples were analyzed by a single analytical group (mostly by the same analyst), using a uniform methodology which inter calibrated during seven international inter-calibration exercises. Mass fractions of chlorinated insecticides and polychlorinated biphenyls in mussels ranged from ND (<0.1) to 107.3 for DDT's and from ND (<0.5) to 1072 × 10⁹ wet weight for PCBs and from 19 to 35,600 for DDT's and from 52 to 103.000 × 10⁹ on an extracted organic matter basis. Averages and ranges of chlorinated hydrocarbons mass fractions in mussels from the Adriatic are compared with the data of mussels obtained in other Mediterranean areas. In all four investigated areas of the Mediterranean (II, IV, VI, and VIII) DDT total mass fractions in mussel are higher than those for the Adriatic. In areas (II, IV, and VIII) the PCB values are higher compared to the Adriatic. Linear regression analysis of pollutant mass fractions in the mussel samples from the eastern Adriatic coastal waters collected during the 20-year period with the years of collection show that a statistically significant negative correlation coefficient is obtained by comparing the values of total DDT mass fractions. For PCBs, a slight negative correlation coefficient is obtained but it is not statistically significant. In the Pula area this correlation coefficient is positive, but it is also statistically non-significant.     

Introduction of cAMP and establishment of neutral lipids alterations as pollution biomarkers using the mussel Mytilus galloprovincialis. Correlation with a battery of biomarkers

The results of the present study suggest that the signaling molecule cAMP could constitute a reliable biomarker of pollution monitoring. Increased levels of cAMP were observed in tissues of mussels Mytilus galloprovincialis exposed to PAHs, 2,4-dichlorophenoxyacetic acid and lindane for 15 days. cAMP levels were higher in mantle/gonad complex, in relation to other tissues studied (digestive gland and gills). In support of our suggestion, cAMP was significantly correlated with established biomarkers, such as lysosomal membrane stability, AChE activity and metallothionein content. In addition, our results support the establishment of morphometrical alterations of neutral lipids as biomarker of organic pollution, since high accumulation of neutral lipids in the digestive gland of mussels exposed to organic pollutants in relation to non-exposed mussels was noted. In addition, correlation analysis between cellular and biochemical biomarkers showed that the volume density of neutral lipids negatively correlated with both lysosomal membrane stability and AChE activity. Regarding micronucleus test, our results do not support its use as a biomarker of organic pollution, since no statistical differences were found between control and exposed mussels. On the other hand, significant correlations were observed among the battery of biomarkers, such as lysosomal membrane stability, AChE activity and metallothionein content after the mussels' exposure to the specific organic compounds, thus supporting the important role of these biomarkers as reliable indicators of organic pollution. In conclusion, our results support cAMP and morphometrical alterations of neutral lipids as biomarkers of environmental pollution.