Circulation and horizontal fluxes in the northern Adriatic Sea in the period June 1999-July 2002. Part I: geostrophic circulation and current measurement

Seasonal and Interannual Trends of Oceanographic Parameters over 40 Years in the Northern Adriatic Sea in Relation to Nutrient Loadings Using the EMODnet Chemistry Data Portal

Long-term data series (1971–2015) of physical and biogeochemical parameters were analyzed in order to assess trends and variability of oceanographic conditions in the northern Adriatic Sea (NAS), a mid-latitude shallow continental shelf strongly impacted by river discharges, human activities and climate changes. Interpolation maps and statistical models were applied to investigate seasonal and spatial variability, as well as decadal trends of temperature, salinity, chlorophyll-a and nutrients. This analysis shows that sea surface temperature increased by +0.36% year−1 over four decades. Annual mean flow of the Po River markedly changed due to the occurrence of periods of persistent drought, whereas the frequency of flow rates higher than 3000 m3 s−1 decreased between 2006 and 2015. Moreover, we observed a long-term decrease in surface phosphate concentrations in Po River water (−1.34% year−1) and in seawater (in summer −2.56% year−1) coupled, however, to a significant increase in nitrate concentration in seawater (+3.80% year−1) in almost all seasons. These changes indicate that the nutrient concentrations in the NAS have been largely modulated, in the last forty years, by the evolution of environmental management practices and of the runoff. This implies that further alteration of the marine environment must be expected as a consequence of the climate changes.

Monitoring of marine mucilage formation in Italian seas investigated by infrared spectroscopy and independent component analysis

The aim of this study is to present and to discuss some characteristics of recalcitrant organic matter mechanism and formation. These aggregates called mucilages that are produced by the degradation reactions of several algae, have been investigated by infrared (FTIR) spectroscopy. FTIR spectra of macroaggregates produced by different algal samples have been daily collected in order to investigate the steps of aggregation. Afterwards, they have been elaborated by means of Independent Component Analysis (ICA). ICA investigation of FTIR spectra showed that the global aggregation process of marine mucilage always consisted of two different phases or independent components (ICs). One IC is related to the first degradation step of algal cells leading to the production of mono and oligosaccharides with aminoacids and oligopeptides. The second IC is related to the polymerization of oligosaccharides with aminoacids and oligopeptides and to their interaction with less polar compounds such as lipids thus producing supramolecular structures. The emerging mechanisms of anomalous size aggregates of organic matter match those of natural organic matter aggregation. The approach we suggest is to use synthetic mucilages which allows to monitor the macroaggregates formation because it can hardly be performed by means of natural marine macroaggregates.

Circulation and horizontal fluxes in the northern Adriatic Sea in the period June 1999-July 2002. Part II: nutrients transport

Nutrient concentrations (orthophosphate, orthosilicate and dissolved inorganic nitrogen-DIN) were measured on three transects in the northern Adriatic approximately monthly in the period June 1999-July 2002. The net nutrient transports across the three transects were estimated from these concentration values and calculated water fluxes [Grilli F, Paschini E, Russo A, Precali R, Supić N. Circulation and horizontal fluxes in the Northern Adriatic Sea in the period June 1999_July 2002. Part I: geostrophic circulation and current measurement. Sci Total Environ 2005-this issue], with the aim to verify their possible role in the mucilage phenomenon. The nutrient transports in the northern Adriatic were very variable, seasonally and from year to year, both in intensity and direction. Some differences were noticed also among the nutrient species. At the northernmost transect Po Delta-Rovinj minimal values and variable directions were recorded, probably due to the prevailing of eastward transversal transports, observed already in late winter, and generally dominant in spring and summer. Northward transport was often measured at the central transect Cesenatico-Cape Kamenjak, particularly in spring 2000 and 2002, and summer 2001, as well as for only DIN in summer 1999. In contrast, southward directions prevailed at the southernmost transect Senigallia-Susak Island. In October significant southward transports occurred at all transects and for all nutrients, particularly strong in 2000, when exceptionally high Po River discharges occurred, and the Western Adriatic Current (WAC) was well developed. However, the nutrient transports in the upper water column were low or northwards at both transect Cesenatico-Cape Kamenjak and Senigallia-Susak Island in June and July 2000, 2001, and 2002, when the mucilage phenomenon developed, but were opposite in 1999, a year without events. The results suggest that high variations of nutrient fluxes and their ratios, in conditions of reduced water dynamics, are essential for the development of the phenomenon, rather than the absolute amounts of the nutrient inputs.

Mucilaginous aggregates in the northern Adriatic in the period 1999-2002: typology and distribution

The spatial and temporal distributions of different types of visible mucilaginous aggregates were investigated monthly by underwater video cameras in the northern Adriatic (NA) from June 1999 to July 2002. Small aggregates (flocs, macroflocs and stringers) were observed in all seasons with higher values in autumn and winter. Larger aggregates (ribbons, cobwebs, clouds, and false bottoms) formed only during late-spring and summer, particularly in 2000 and 2002, when the seasonal thermohaline stratification was marked. In fact, the process of mucilage formation took place mainly in the upper water column, above the main pycnocline. The spatial distribution shows that higher concentrations of small aggregates occur in the more productive coastal waters. The larger aggregates form or accumulate mainly in coastal waters in the southern part of the NA and in the central part of the Po River delta-Rovinj transect, where a gyre greatly influences the aggregates distribution. The fronts between low salinity coastal waters and high salinity waters of southern origin play an important role for accumulation and aggregation of the mucilage during spring and summer. Most of the aggregates accumulate in correspondence with strong pycnoclines with differences in density anomaly of 2 kg m(-3) or higher. False bottoms formed in correspondence with strong haloclines (DeltaS congruent with 2), while cobwebs and ribbons also occurred when the halocline was less marked (DeltaS<0.5). Meteorological conditions greatly influenced the aggregate formation. Calm weather and weak winds favour aggregation processes, while strong north-easterly winds, causing intense mixing, result in the dispersion of the mucilaginous aggregates, as occurred during the Bora event in July 2000.

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.

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).

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.