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SERIE TEMPORALI NELLA LAGUNA DI VENEZIA. ANALISI DEI POPOLAMENTI FITOPLANCTONICI DAL 1977 AL 2004 SOCAL G.1, ACRI F. 1, BERNARDI AUBRY F. 1, BIANCHI F. 1, CAPUZZO E. 1, COPPOLA J. 1, FACCA C.2, SFRISO A. 2 1 Istituto di Scienze Marine (ISMAR) CNR. Sezione di Venezia. Sistemi Marini e Costieri. Castello 1364/a Venezia giorgio.socal@ismar.cnr.it 2Dipartimento di Scienze Ambientali Università Cà Foscari Venezia Questo lavoro scaturisce dall'esigenza di iniziare a predisporre un quadro sintetico e realistico sullo stato delle comunità fitoplanctoniche nella laguna di Venezia, ottenuto da un cospicuo set di dati, raccolti dal 1977 ad oggi, valutato nell'ambito delle complesse distribuzioni spazio-temporali dei forzanti fisici e chimici, dai diversi impatti antropici e dalla grande eterogeneità morfologica lagunare. I dati raccolti implicano problematiche legate alle diverse dinamiche di scala temporale (da quella oraria, a quella mensile, a quella annuale, fino a quella pluriennale), in microhabitat molto eterogenei tra loro per le condizioni idrochimiche di base dell'acqua: dalle acque saline nelle bocche di porto e nelle loro aree limitrofe, alle acque salmastre, influenzate dai carichi portati del bacino scolante, nelle aree che lambiscono la terra e che sono lontane dal mare. I popolamenti fitoplanctonici subiscono inoltre l'influenza della batimetria lagunare, estremamente eterogenea, per la presenza di fosse e canali profondi, di canali meno profondi (< 10 m), di velme, di paludi, di zone intertidali soggette a periodiche emersioni ed immersioni, scandite dal ritmo della marea. Il set di dati consta di 2034 campioni da conteggio prelevati ed analizzati al microscopio invertito, cui corrispondono dati idrochimici raccolti contestualmente. Come primo passo si è rappresentato un andamento generale sull'intero periodo per descrivere la fisionomia stagionale dei popolamenti, rappresentata da una netta crescita da gennaio ai mesi primaverili, da un leggero decremento in maggio-giugno, poi da un picco a luglio, seguito da una caduta di tarda estate e da un progressivo decremento autunnale. Per quanto riguarda le frazioni dei gruppi tassonomici le diatomee rappresentano circa il 70 %, le nanoflagellate il 27%, le dinoflagellate soltanto l'1% del popolamento in termini di abbondanza. Durante l'anno dominano nettamente le diatomee nei periodi marzo-aprile e luglio-settembre, mentre si osserva un maggior bilanciamento tra quest'ultime e le nanoflagellate negli altri mesi dell'anno. Su un numero ridotto di taxa, ritenuti i più rappresentativi per frequenza ed abbondanza durante l'intero periodo, si è costruita una successione delle principali specie durante l'anno. La maggior parte dei taxa considerati ha dimostrato di seguire un trend generale, in fase con quello della temperatura. La diatomea pelagica Skeletonema costatum invece ha presentato il classico andamento tardo invernale primaverile, mentre la diatomea di origine bentonica Nitzschia frustulum ha mostrato una maggior distribuzione in estate- inizio autunno. Confrontando i dati raccolti nel periodo 1977-1980 con quelli recenti, ottenuti nel quinquennio 1997-2002, si è dimostrato, soprattutto nei canali lagunari, un aumento significativo dell'abbondanza fitoplanctonica imputabile a seguenti fattori: i) una maggior risospensione dei sedimenti lagunari legati all'aumento intensivo dell'attività di pesca sul bivalve Tapes, esercitato in tutta la laguna negli ultimi anni, avrebbe favorito la mobilizzazione delle diatomee bentoniche epipsammiche dai sedimenti, portandole ad uno stile di vita pelagico, in acque contenenti grandi quantità di ortosilicato; ii) la diminuzione della biomassa a macroalghe negli anni recenti avrebbe provocato una minor domanda di azoto inorganico disciolto, rendendo questo nutriente maggiormente disponibile per il fitoplancton e favorendone la crescita. Queste considerazioni dimostrano quanto lo studio delle strutture dei popolamenti fitoplanctonici possa divenire importante per indicare lo stato trofico di un ambiente, pur così complesso ed in continua evoluzione, come la laguna di Venezia.

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 and daily water column integrated valuesthe 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. 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 greater than 20% as an average, with peaks of up to 70%) was released as dissolved organic carbon. In particular, an association of fairly high PER (greater than 10%) and specific production 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.

A late-winter diatom bloom (Skeletonema costatum) typically occurs in the NW Adriatic Sea, triggered by nutrient inputs from the Po River and by increasing irradiance and temperature. In 2002, the onset of the bloom was delayed with respect to previous years, due to light limitation caused by cloud coverage in the form of fog. Therefore, phytoplankton could not efficiently use the available (and abundant) nutrients originating from the river out?ow until the weather conditions and the total amount of irradiance improved to the point of triggering cell growth. In fact, when at the end of February an increased rainfall boosted Po river discharge, dissolved nutrients became available in significant amount, but irradiance was still low due to the fog. Only in March, mutated climatic conditions allowed sufficient light availability, which, coupled with available nutrients, triggered the S. costatumbloom. The temporal dynamics of bloom initiation may be fundamental in determining species composition, which, in turn, has been shown to have a strong impact on consumers recruitment.

The distribution of phytoplankton abundance and species composition is described for coastal waters of the NW Adriatic Sea, for the period 1990–1999. Hydrochemical and biological properties were investigated every month, at 48 stations. This coastal system is quite unstable and heterogeneous, due to the influence of several rivers, intense water exchange with the Lagoon of Venice, and complex advective processes. In the vicinity of river plumes, dissolved nitrogen and dissolved silica appear to be in excess, rarely limiting phytoplankton growth. Examining the general trend of phytoplankton over the years, the main limiting factors for phytoplankton growth in the Adriatic coastal system are light, temperature, and the strong influence of meteorological events. The NW Adriatic coastal area was subdivided into three zones, each characterised by different trophic conditions. In each zone, phytoplankton was similar in terms of community structure and was different only in abundance. Three main periods of growth were observed: February, April and July. As expected in a nutrient-enriched system, the community structure was dominated by diatoms (both micro- and nanoplankton fractions), over most of the year. The importance of dinoflagellates in the communities was generally low, with significant abundances present only in June–July, once the spring bloom of diatoms had left ‘nutrient-poor’ conditions in the water. These low concentrations of nutrients favoured the growth of dinoflagellates, which have lower nutritional requirements. Over the 10-year study, a decrease in orthophosphate concentrations was observed. This pattern could be explained as a consequence of reduced quantities of phosphorus used in detergents, in the late 1980s. No significant change in any hydrochemical or biological property was observed.

A comparison between recent data (1997–2002) and older data (1975–1980) related to the Venice lagoon is presented here. These data have been focused on plankton and dissolved nutrient distribution obtained within the framework of differing research projects. Investigations were carried out in the central and northern areas of the Lagoon of Venice. Comparing the two data sets, a general increase in orthosilicates and in dissolved inorganic nitrogen (DIN) was observed. Conversely, phosphates showed a decrease in polyhaline waters (salinity between 18 and 30 PSU) because of the reduced amounts of phosphorous compounds in detergents, following Italian laws enforced in the 1980s. In the recent data set (1997–2002), phytoplankton abundance was higher than in the years 1975–1980 because of more frequent diatom blooms. The increased resuspension processes induced by dredging for molluscs probably favour a mobilisation of benthic epipsamic diatoms from sediments, resulting in a more pelagic life style. Regarding zooplankton, a decrease in the standing stock, more marked in winter, was revealed. These results indicate that the trophic conditions of the Lagoon of Venice are subjected to natural and anthropogenic temporal changes that can be understood only through the analysis of uninterrupted time series.

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Spatial and seasonal distribution patterns of suspended particulate matter, particulate organic carbon (POC), particulate nitrogen, chlorophyll a and phytoplankton abundance and biomass were analysed along two transects in the southern Adriatic and northern Ionian Seas. Moreover, 2 years of samples collected by sediment traps at one selected station for each transect are used to assess the variability of fluxes of biogenic (organic carbon, total nitrogen, carbonates, biogenic silica) and lithogenic constituents. According to the low suspended matter concentration, the two study areas can be considered as representative of oligotrophic systems. The availability of diverse nutrient forms (inorganic or organic), due to the different water column structure and circulation dynamics, affected the structure of phytoplankton communities. Along the Adriatic transect, the influence of waters coming from the northern Adriatic was evident at the surface of the western stations. The phytoplankton community was mainly represented by diatoms and nanoflagellates. In the northern Ionian, particulate matter concentration decreases with respect to the Adriatic stations (st. A1) and the community was dominated by nanoflagellates (54% in cell number), and coccolithophorids (26%). In the southern Adriatic, the vertical mixing convection related to the climatic variability, supplying inorganic nutrients in the upper layers from the deep, represents the dominant process, increasing primary production and downward fluxes of particulate matter in early spring. Higher mass fluxes measured at the bottom are likely due to horizontal and advective contributions of material originated in the shelf area and transported into the basin by mesoscale circulation. Vertical fluxes in the Ionian mainly depend on production processes occurring in the photic layer, such as spring and autumn blooms, controlled by the seasonal mixing and dynamics of the water column. Fluxes are characterised by discontinuous and short-time events correlated with the end phase of micro-phytoplankton blooms. At both stations, the low carbon export (less than 4% of primary productivity) indicates high carbon utilisation and/or high degradation efficiencies of particulate organic matter, i.e. fast recycling in the upper water column.

This work, carried out within the framework of the PRISMA II project, aims at evaluating the effects of the Po River flow on primary productivity, measured by C with in situ sample incubations. A total of four cruises was carried out in late winter and early summer (1996-1998) along transects from the coast offshore. In both seasons, the highest primary productivity was found in the most oligohaline stations. The effects of the Po River inputs were therefore clearly evident in the westernmost area, both in high productivity values and in the shape of productivity profiles. In late winter, extensive blooms of Skeletonema costatum and Pseudo-nitzschia delicatissima resulted in productivity values that were sometimes higher than in early summer. In early summer 1996, greatly reduced river flow clearly influenced productivity which was much lower than in 1997.

By comparison with measurements carried out in 1990–91, phytoplankton biomass in 1998–99 appears strongly reduced in the central basin of the Venice lagoon. Chlorophyll a concentrations,which in the past easily reached 100 _g dm_3,reached only 5 _g dm_3 during this investigation. Phaeopigment concentrations were close to the detection limit. The introduction of the bivalve Tapes philippinarum Adams and Reeve in the lagoon and its harvesting by hydraulic and mechanical dredges strongly increased the amount of sediment resuspension and settlement. This phenomenon caused a marked increase in water turbidity and the disruption of the benthic microlayer of the lagoon bottoms composed by bacteria, microphytobenthos and small macrophytes. Significant correlations between microphytobenthos abundance and the suspended and settled sediment, and with the underwater light availability were found. In the areas affected by the highest sedimentation fluxes significant negative correlations were also found between the abundance of the microphytobenthos and phytoplankton communities. Many benthic taxa such as Amphora, Cocconeis, Navicula, Nitzschia, Pleurosigma and Thalassiosira, were found in all the water column samples and they were more abundant than exclusively planktonic diatoms.

The aim of this paper was to improve the knowledge of the primary producers living in the Venice Lagoon by studying the composition and the distribution of the benthic diatom communities populating subtidal surface sediments. The upper 5 mm of sediment from four areas distributed between the Malamocco inlet and the mainland north of Venice were collected and analysed. Sampling was carried out monthly for one year (1998-99). Species composition, cell abundance and carbon content were determined by light microscopy after separation of cells from the sediment particles using hydrogen peroxide. A total of 147 taxa were identified, mostly pennate diatoms (Bacillariales) belonging to the genera Amphora, Cocconeis, Navicula, Nitzschia and Pleurosigma. Cell abundance ranged from 0.7 x 106 cells cm-3, near the Malamocco inlet (Station A), to 10.4 x 106 cells cm-3 near the mainland (Station C). Diatom carbon content was between 28 and 380 mg cm-3. Seasonal trends were slight, except at Stations D and C where the lowest abundance was found in October-November and the highest one in summer. Significant relationships between benthic diatoms and macrophyte biomass, sediment grain size, underwater light transmission, sediment fluxes and the salinity gradient were found.