Strategy of port baseline surveys (PBS) in the Adriatic Sea.

Port baseline surveys (PBS) provide species inventories in and around ports, with a focus on non-indigenous species that may have been introduced by vessels, primarily via ballast water. PBS are an essential tool to support effective management strategies for non-indigenous as well as native harmful aquatic organisms and pathogens (HAOP). This paper describes the methodology of PBS that were conducted in 12 Adriatic ports. The PBS employed existing protocols that were adapted to meet the characteristics of the Adriatic sites. Their results are reported in several papers included in this special issue, each of which is devoted to a specific community. An overview of existing surveys protocols - which provide valuable support to decision-making and to design effective monitoring of non-indigenous species - is also supplied.

Oceanographic characteristics of the Adriatic Sea - Support to secondary HAOP spread through natural dispersal.

Vessels, specifically ballast water and hull fouling, are a major vector for the introduction of non-indigenous species (NIS) in European seas. The Mediterranean is one of the world's marine regions where their invasion is heaviest. The shallow Adriatic basin is a highly sensitive area that is already experiencing its consequences. The secondary spread of NIS over a wider area through natural dispersion is a complex process that depends on a wide range of oceanographic factors. This work analysed the dataset of the BALMAS project, in whose framework twelve ports in the Adriatic Sea were subjected to a Port Baseline Survey (PBS), to estimate the natural spread of NIS organisms from their port of arrival to the wider Adriatic basin. Its findings indicate that the prevailing water circulation patterns facilitate the natural dispersal of harmful aquatic organisms and pathogens (HAOP).

Adaptation of marine plankton to environmental stress by glycolipid accumulation.

A systematic investigation of non-phosphorus containing glycolipids (GL) was conducted in the northern Adriatic Sea during two years at two stations with different nutrient loads. GL concentration varied both spatially and temporally, with values of 1.1-21.5 µg/L and 0.4-44.7 µg/L in the particulate and the dissolved fraction, respectively. The highest concentrations were measured during summer in surface waters and at the more oligotrophic station, where GL yields (% of total lipids) were often higher than 20% and 50% in the particulate and dissolved fractions, respectively. To obtain more insight into factors governing GL accumulation autotrophic plankton community structure (pico-, nano- and microplankton fractions), chlorophyll a, heterotrophic bacteria and nutrient concentrations were measured together with hydrographic parameters and sunlight intensity. During the investigated period smaller autotrophic plankton cells (pico- and followed by nanoplankton) prevailed in abundance over larger cells (microplankton), which were found in large numbers in freshened surface samples. Several major findings resulted from the study. Firstly, during PO4 limitation, particularly at the oligotrophic station, enhanced glycolipid instead of phospholipid accumulation takes place, representing an effective phosphate-conserving mechanism. Secondly, results suggest that at seawater temperatures >19 °C autotrophic plankton considerably accumulate GL, probably to achieve thermal stability. Thirdly, high sunlight intensities seem to influence increased GL accumulation; GL possibly plays a role in cell mechanisms that prevent/mitigate photooxidation. And finally, substantial accumulation of GL detected in the dissolved fraction could be related to the fact that GL do not contain biologically relevant elements, like phosphorus, which makes them an unattractive substrate for enzyme activity. Therefore, substantial portion of CO2 could be removed from the atmosphere in P-limited regions during summer via its capture by plankton and conversion to GL.

Phytoplankton and bacterial alkaline phosphatase activity in the northern Adriatic Sea.

The importance of bacterial, phytoplankton and dissolved alkaline phosphatase activity (APA) in the northern Adriatic was investigated during 2006. In surface waters total APA increased from early spring (0.07-0.08 micromol l(-1) h(-1)) to late spring (up to 4.64 micromol l(-1) h(-1)) and remained relatively high during the summer (0.46-0.71 micromol l(-1) h(-1)), due to an increase in specific phytoplankton (up to 30 nmol microg C(-1) h(-1)) and bacterial APA (up to 17.11 nmol microg C(-1) h(-1)). Activity of free enzymes was not important. During late spring and summer both communities exploited dissolved organic phosphorus although, taking into account biomass, phytoplankton activity usually dominated over bacterial activity. In autumn an extra P supply from deeper waters drastically reduced phytoplankton APA, though not bacterial APA, in upper waters. Probably in these months bacteria that were degrading phytoplankton produced organic matter were P limited. In deeper waters APA was low and mainly due to the activity of free enzymes.

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).

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.

Changes in nutrient concentrations and ratios during mucilage events in the period 1999-2002.

Nutrient and chlorophyll a concentrations and salinity were measured, approximately monthly, from June 1999 to July 2002 at 20 stations along three transects in the northern Adriatic Sea, north of the line Susak Island-Senigallia, with the aim of confirming or rejecting the hypothesis that changes in nutrient ratios may have an important role in the mucilage phenomenon. The data analyses were focused on the two major water types identified in the region: lower salinity (32-37) and oxygenated surface waters (type 1) in which the mucilage phenomenon primarily developed, and high salinity water originating from other parts of the Adriatic (type 4). Marked variability of dissolved inorganic nitrogen (DIN) in type 1 waters was roughly correlated with extreme fluctuations of the Po River flow rate during the investigated period. In contrast, the orthophosphate (PO4) concentration was primarily controlled by phytoplankton assimilation. The nutrient discharges and DIN/PO4 ratios (median 120) in the freshened surface layer were much higher and more variable in the period before the mucilage event in 2001 than in 2000 (median 75), and particularly in 2002 (median 30), although intensity and duration of the 2001 event were the lowest. However, in that period of 2000 and 2002 significant transversal transport of freshened waters occurred, despite the unusually low Po flow rates. In summer, in conditions of low freshwater discharge and the prevailing of semi-enclosed circulation in the region, more efficient DIN assimilation by phytoplankton occurred, probably due to a faster recycling of PO4. However, in 2002 this process appeared to have already started in March. Changes of the orthosilicate (SiO4)/DIN ratio were mainly dependent on DIN concentrations. In the more saline waters (type 4) the nutrient concentrations, particularly DIN, were much lower and no significant relationships were noticed among the studied parameters. Nutrient concentration and ratio changes do not trigger mucilage events, although very probably they have an essential role in combination with several other physical (pulsing freshwater discharge, marked stratification, minimal water exchange) and biological (e.g., increased plankton excretion, limited bacterial degradation) factors.

Inter-annual variations of planktonic food webs in the northern Adriatic Sea.

The temporal dynamics of microphytoplankton, microzooplankton and mesozooplankton were monitored over 37 months in the Adriatic Sea in order to identify alterations in the plankton structures, which can lead to, or enhance the production of macro-aggregates, that affected the entire northern basin in summers 2000 and 2002, and to assess any negative effects of mucilage on plankton temporal patterns. Samples were collected monthly, from June 1999 to July 2002, on three transects at 9 stations across the northern and central Adriatic Sea. Besides the high year-to-year variations in abundances and taxonomical composition, plankton communities only showed a clear seasonal succession during 2001, when since April a grazing food web developed and was able to control large sized phytoplankton increase. In spring-summer 2000 and 2002 consumer abundances remained quite low and the dominant mesozooplankton summer species (Penilia avirostris) did not reach its usual summer maximum. The lack of an efficient top control was more evident on the northernmost transect, where generally grazing food web prevails over the microbial one. A large part of the microphytoplankton blooms, although not particularly intense, was exported to the bottom in the particulate phase, where it was processed by bacteria, enhancing the production of refractory dissolved material.

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, Supic 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.

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

The dramatic increase in the occurrence of massive mucilage events in the northern Adriatic (NA) since their recent conspicuous reappearance in the late 1980s prompted a study of circulation and horizontal fluxes. Three transects with equidistant stations (10 km) were thus monitored monthly between June 1999 and July 2002. The geostrophic method was used to compute currents across the three transects from the CTD data, and dynamic heights provided a picture of the horizontal surface circulation. Currentmeter data records were used to adjust the reference surface and to validate the results for the southernmost and deeper (up to 70 m) transect (Senigallia-Susak Island). Geostrophic currents allowed estimation of monthly water fluxes across the transect. Different circulation regimes in the NA were observed, which may have affected mucilage events. When mucilage was absent (1999) or reduced (2001) in the western sector, the Western Adriatic Current (WAC, carrying water out of the NA) was found to be active, whilst the WAC was very weak or reversed when massive mucilage events occurred (2000 and 2002). Opposite behaviour has been observed for the Istrian Coastal Counter-Current (ICCC, retaining freshwater water in the NA) which was more intense during or after massive mucilage events and did not appear when mucilage was absent. Both WAC weakening and ICCC strengthening indicate a longer residence time of riverine waters in the NA, which favours mucilage development. Conclusively, WAC and ICCC result as key elements in controlling massive mucilage phenomena in the NA.