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Fitoplancton e microfitobenthos come indicatori dello stato di trofia in ambienti costieri
No full textLa distribuzione delle comunità planctoniche microalgali è stata studiata in due lagune mediterranee, entrambe soggette ad un intenso
sfruttamento antropico. Le due aree di studio, sebbene abbiano alcune caratteristiche comuni (ambienti di transizione separati dal mare da un
cordone litoranei), simili variazioni stagionali di temperatura), presentano condizioni morfologiche ed idrodinamiche diverse. La laguna di
Venezia è caratterizzata da una profondità media di poco superiore al metro, da un forte riciclo mareale (60% del volume totale ogni 12 h) e da
varie tipologie d'impatto (industriale, urbano, agricolo, sfruttamento delle risorse alieutiche). La laguna di Thau, al contrario, oltre ad essere
mediamente più profonda (ca. 4.5 m), ha un lento ricambio delle acque (da 1 a 5 mesi) e riceve reflui di natura prevalentemente urbana ed
agricola. Le principali attività economiche dell'area sono l'allevamento dei molluschi bivalvi e il turismo.
In ciascuna laguna sono stati effettuati campionamenti per approfondire le conoscenze sulla distribuzione del fitoplancton a diverse scale
spaziali e temporali.
Nella laguna di Venezia, sono stati descritti gli andamenti stagionali in sei stazioni con condizioni ambientali rappresentative delle
caratteristiche dell'ecosistema lagunare. È stata, inoltre, tracciata una mappa della distribuzione spaziale, sulla base dei campioni raccolti in 65
siti nell'estate 2002. L'insieme di questi dati, unitamente ai risultati recuperati dalla letteratura, ha permesso di aggiornare le informazioni
relative alle abbondanze e alla composizione tassonomica della comunità fitoplanctonica. Sebbene le specie identificate coincidano con quelle
segnalate da precedenti autori, è stata notata una generale riduzione della densità microalgale nella colonna d'acqua. Solo nell'estate 2001 e alla
fine dell'inverno 2002, in alcune stazioni, sono state osservate abbondanze dell'ordine di grandezza (IO7 cells dm-3) di quelle registrate fino
all'inizio degli anni '90. La variabilità stagionale del fitoplancton era regolata, principalmente, dalle variazioni di temperatura, mentre la
distribuzione spaziale è dipesa, soprattutto, dalla concentrazione di nutrienti. Le aree con le abbondanze maggiori sono risultate, infatti, quelle a
ridosso degli scarichi urbani. Sebbene il fosforo abbia condizionato l'abbondanza delle specie, è la disponibilità di radiazione luminosa che
sembra aver assunto il ruolo di fattore limitante nella parte centrale della laguna di Venezia. La luce penetra, infatti, difficilmente fino al fondo
della colonna d'acqua ed, in molte zone dell'area indagata, il disco Secchi non è visibile oltre 20 cm di profondità. La causa principale
dell'elevata torbidità della colonna d'acqua è la frequente risospensione di sedimento, provocata dall'incontrollato sfruttamento dei banchi del
bivalve Tapesphilippinarum Adams & Reeve.
Nella laguna di Thau, sono state studiate le variazioni a breve scala temporale (settimanale e giornaliera) e sono state confrontate alcune
stazioni a diverso livello di trofia. Uno degli scopi del lavoro era, infatti, quello di comprendere se le comunità microalgali potessero fornire
informazioni attendibili sullo stato trofico dell'ecosistema. In entrambe le lagune, l'abbondanza e la composizione tassonomica del fitoplancton
hanno permesso una discreta distinzione qualitativa tra aree diversamente impattate dall'eccessivo carico di nutrienti.
Risultati migliori sono, tuttavia, stati ottenuti studiando la distribuzione delle diatomee che colonizzano i sedimenti superficiali. Le
comunità microfitobentoniche sono state prelevate, durante le stesse campagne di campionamento in cui è stato indagato il fitoplancton, anche se
difficoltà logistiche, per il momento, hanno permesso di operare solamente nella laguna di Venezia. Questi organismi hanno dimostrato una
minor variabilità stagionale e la loro distribuzione sembra dipendere, soprattutto, dalle caratteristiche del sito di prelevamento. Lo studio del
microfitobenthos deve essere sicuramente approfondito, perché può fornire informazioni più utili, rispetto al fitoplancton, per la messa a punto
d'indici trofici in grado di valutare la qualità degli ambienti costieri.
The phytoplankton communities have been studied in two Mediterranean lagoons, affected by an intense anthropic exploitation. Although
the studied areas are characterised by common features (they are both separated from the sea by a bar beach and temperature shows similar
variations), they have different morphological and hydrodynamic conditions. The Venice lagoon is characterised by shallow bottoms (on average
ca. 1 m), high water renewal (ca. 12 h to change ca. 60% of the basin water) and numerous human activities (industries, urban centres,
agriculture, clam fishery, tourism). The Thau lagoon, on the contrary, is deeper than Venice (on average ca. 4.5), the seawater renewal is very
low (ca. 1-5 months to change the entire water volume) and the basin receives mainly urban and agriculture waste waters. Shellfish farming and
tourism are the most important human activities.
In each lagoon water samples were collected to investigate phytoplankton distribution at different spatial and temporal scales.
In the Venice lagoon, six sites, with different environmental conditions, were chosen to describe microalgal seasonal trends. Moreover a
map of the spatial distribution was drawn according to data referred to samples collected in 65 sites during summer 2002. The present results
update the knowledge on the phytoplankton community abundance and taxonomic composition. Most of the identified species were the same
ones quoted in previous works. The abundance, on the contrary, showed a drastic reduction in the whole studied area. Summer 2001 and late
winter 2002 were the two exceptions (abundance up to 55 x 106 cells dm-3). Seasonal variations depended mainly on temperature, whereas spatial
distribution was related mainly to nutrient concentrations. The highest abundance was, in fact, recorded close to urban discharges. Although
phosphorus had a key role in controlling phytoplankton distribution, the limiting factor of the microalgal production was light. In the central part
of the Venice lagoon light radiation hardly penetrated to the bottom and the Secchi disk was not frequently visible below 20 cm. The main cause
of the water column turbidity was the frequent sediment resuspension due to clam-fishing activities (Tapes philippinarum Adams & Reeve).
In the Thau lagoon, small-scale variations (daily and weekly) were investigated and some sites characterised by different trophic levels
were compared. One of the main objectives was, in fact, to verify the ability of the microalgal communities to assess the ecosystem trophic state.
In both lagoons, phytoplankton abundance and taxonomic composition allow to distinguish qualitatively areas with different nutrient loads.
However, more precise information on the trophic conditions of the ecosystem was provided by benthic diatoms rather than by
phytoplankton. Microphytobenthos was collected in the central part of the Venice lagoon during the same sampling campaigns made to
investigate phytoplankton. At present logistic difficulties do not allow similar investigation in the Thau lagoon. The community living on the
surface sediment displayed a seasonal variability lower than the planktonic one and its distribution depended mainly on the local features. More
information on microphytobenthos is needed as these organisms allow a good assessment of the coastal ecosystem trophic conditions.
PHYTOPLANCTON ET MICROFITOBENTHOS COMME INDICATEURS DE L'ETAT TROPHIQUE EN MILIEU COTIER
RESUME
La communauté du phytoplancton a été analysée dans deux lagunes Méditerranéennes, qui sont le siège d'activités humaines diverses. Les
deux sites d'étude, ont des caractéristiques communes (lagunes séparées de la mer par un cordon littoral avec variations similaires de
température), mais une topographie et une hydrodynamique différentes. La lagune de Venise est caractérisée par une faible profondeur (1 m en
moyenne), par un rapide recyclage des eaux (60% du volume total toutes les 12 heures) et par des impacts anthropiques différents. L'étang de
Thau, au contraire, est plus profond (4.5 m en moyenne), ses échanges avec la mer sont réduits (le recyclage de tout l'étang s'effectue de un a
cinq mois); l'activité principale développée est la conchyliculture.
Nous avons procédé dans les deux lagunes, à des échantillonnages adaptés dans le but d'approfondir les connaissances relatives à la
distribution du phytoplancton.
Dans la lagune de Venise sont décrites les variations saisonnières, observées dans six stations caractéristiques choisies, tandis qu'une carte
visualise les données de 65 sites échantillonnés en été 2002. Ces donnés correspondent à une importante mise à jour des informations relatives à
la composition taxonomique du phytoplancton et à l'abondance relative de ses constituants: les espèces identifiées se retrouvent dans celles
signalées précédemment mais avec une nette réduction de leur abondance (à l'exception de deux stations, où étaient observées des floraisons,
allant jusqu'à 55 x 106cells dm-3). Les variations saisonnières des populations phytoplanctoniques dépendaient principalement de la température,
tandis que leur distribution spatiale était conditionnée par la concentration en sels nutritifs. Malgré l'impact du phosphore dans la production
algale, le facteur limitant le plus déterminant s'est révélé être l'intensité lumineuse (le coefficient d'extinction n'excédant parfois pas 20 cm,
conséquence d'une récolte abusive au niveau des sédiments de Tapesphilippinarum Adams & Reeve).
Dans l'étang de Thau l'étude des variations des populations phytoplanctoniques a été réalisée sur une échelle de temps plus réduite qu'à
Venise (journalière ou hebdomadaire) sur trois sites, caractérisés par des niveaux trophiques différents (en prenant en compte certains critères
chimiques et floristiques précédemment établis). Ceci a permis leur comparaison. Le but de cette analyse était de vérifier dans quelle mesure la
composition phyloplanctonique était représentative de l'état trophique d'un écosystème lagunaire donné et permettait l'identification de divers
niveaux trophiques préétablis.
Dans les deux lagunes, l'étude de la composition phytoplanctonique et celle de l'abondance relative de ses populations dominantes ont
ainsi permis d'identifier des populations répondant à un niveau trophique défini. Cette approche générale et l'étude qui en découle sont une étape
importante dans l'analyse de l'état trophique des milieux côtiers en général
Sedimentation rates and erosion processes in the lagoon of Venice
No full textSince the early 1990s in the lagoon of Venice, especially in the central basin, the surface sediment underwent high re-suspension and sedimentation changes and water turbidity increased both because of the disappearance of the macroalgal coverage and the harvesting of the Manila clam Tapes philippinarum Adams and Reeve, which had rapidly colonised the bottom free of macrophytes. Clams are harvested with hydraulic and mechanical dredges which remove and re-suspend surface sediments causing the transport and loss of the finest materials. Sediment transport and re-deposition in the lagoon have been monitored with sediment traps placed onto the bottoms near the Malamocco mouth (st. A), the Lido watershed (st. B) and the mainland (sts. C and D). From 1989–1993 to 1998–1999 sedimentationrates increased significantly at st. A (from 41 to 228 kg DW m-2 year-1), st. B (from 65 to 760 kg DW m-2 year-1) and st. C (from 140 to 721 kg DW m-2 year-1), while at st. D sedimentationrates increased only by ca. 20%. In parallel sediment grain-size changed with a loss of the finest fraction especially near the mainland.
The erosion or sedimentation status, acquired by utilizing sedimentation devices placed onto the bottoms, showed that sts. B, C, D were affected by sediment losses, while st. A, populated by seagrasses and characterised by seasonal variations which depend on the shoot development, did not show any significant bathymetric change on an annual basis. The highest sediment erosion was recorded at st. D (ca. 3.6 cm year-1) whereas a loss of ca. 1.5 and 0.5 cm year-1 was found at sts. C and B, respectively, which accounted for a mean loss in the central lagoon of ca. 1.2 million tonnes year-1. Those data agree with the previous indirect estimation of sediment loss which was based on the number of fishing boats operating in the lagoon on an annual basis
Phytoplankton composition and distribution in the central part of the Venice lagoon
No full textWe investigated the phytoplankton in the central area of the Venice lagoon in the 1990s. Water samples were collected at 46 sites in June 1993 and June 1998 and at four stations every month from November 1998 to October 1999. Collected data include community composition, cell abundance and cell bio-volumes, estimated with an inverted light microscope. Cell abundance distribution maps display mean (6.9 x106 and 2.5 x 106 cells dm3 in June 1993 and June 1998, respectively). The reasons for differences in blooming taxa were investigated by processing environmental variables of the most abundant taxa by Canonical Correspondence Analysis. The environmental parameters that most affected the species spatial distribution were salinity and temperature in June 1993 and salinity and water transparency in June 1998. Phytoplankton temporal trends confirmed the spatial distribution: the blooming period was in June and the highest cell abundances occurred close to the mainland
Phytoplankton in a transitional ecosystem of the Northern Adriatic Sea and its putative role as an indicator for water quality assessment.
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Spring phytoplankton community structure in the Thau lagoon, France (May, 2002).
No full textPhytoplankton communities were investigated in the French Mediterranean Thau lagoon (75
km2, mean depth ca. 4.5 m), where there is important shellfish farming activity. Samples were collected
during spring (May 2002), mainly at two sites (MPN and LP) characterised by different
eutrophication levels and macrophyte populations, Zostera marina L. and Gracilaria bursa-pastoris
(Gmelin) Silva. Taxonomy, cell abundance, carbon content and SHANNON-WEAVER diversity index
were determined.
The most common taxa were nanoflagellates (between 40 and 50% in terms of cells dm-3),
diatoms (between 20% and 35%) and Cryptophyceae (around 12%). The sites differed in that
Dinophyceae (<5%) were present at the oligotrophic station (MPN) and epiphytic and benthic
diatoms at the eutrophicated station (LP). Chlorophyll a and abundance were higher at LP than at
MPN. Carbon content was 8 times higher at MPN than at LP, because of the presence of big
dinoflagellates, representing 90% of the biomass, at MPN.
The authors discuss the species, which are indicators of the eutrophication level, and the important
role of taxonomic analysis and compare the results with those of the Venice lagoon
Studi sulla distribuzione spazio-temporale delle macroalghe planctoniche e bentoniche nella laguna di Venezia.
No full textPrime indicazioni sulla flora e lo stato ecologico delle lagune venete del delta del Po.
No full textQuali-quantitatrive Rapid-Macrophyte Index (R-MaQI) set up to meet the WFD(2000/60/EC) requirements
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