Seamounts are unique marine ecosystems, rich in biodiversity and highly vulnerable. One of the most recent definitions, often adopted in ecological studies, defines a seamount as “any topographically distinct elevation of the seafloor with a height of at least 100 meters, but which does not emerge above the sea surface.” In the Mediterranean Sea alone, there are more than 200 seamounts, 65 of which are in the Tyrrhenian Sea. Among them are the Vavilov and Vercelli seamounts (located in the central and northern Tyrrhenian Sea, respectively), which are the focus of this study. Between July and August 2024, an oceanographic survey was conducted to collect physical, chemical, and biological data on each seamount. Sampling stations were located on the summit of the seamount and progressively farther away along two perpendicular transects. The goal was to investigate the presence of a potential “seamount effect,” which involves the upwelling of deep, nutrient-rich waters into the photic zone, thereby stimulating phytoplankton production. Oceanographic parameters were measured using a multi-sensor CTD probe, and water samples were collected with Niskin bottles at various depths (surface, base of the mixed layer, base of the euphotic zone, depth of maximum fluorescence, and for nutrients also the deep scattering layer and the seafloor). Nutrient and chlorophyll analyses were performed using standardized protocols (autoanalyzer and spectrophotometer, respectively), while phytoplankton was identified and counted under an optical microscope using the Utermöhl method. Statistical analyses (Wilcoxon, Kruskal-Wallis, Dunn tests) were used to compare the two areas and the different transects. The behavior of the physical parameters showed marked variability in the surface water column, more pronounced at Vercelli, likely due to mesoscale dynamics and wind forcing, which is typically more intense in the northern Tyrrhenian compared to the central area. At Vavilov, a stable stratification was observed, with “stair-step” structures typical of the central Tyrrhenian, due to double diffusion processes. In contrast, such structures were less evident at Vercelli, except at some southeastern stations. At Vercelli, a potential upwelling of Levantine Intermediate Water (LIW) was detected, indicated by salinity maxima at shallower depths than expected, possibly caused by interactions between water flows and the seamount’s topography. However, the lack of current velocity data did not allow confirmation of structures consistent with a Taylor column. From a biochemical standpoint, both areas were oligotrophic, with very low concentrations of nutrients and chlorophyll-a in the mixed layer and a peak at the Deep Chlorophyll Maximum (DCM), consistent with observations from other parts of the Mediterranean. In the mixed layer, the phytoplankton community was dominated by small phytoflagellates, with similar abundances in both areas (on the order of 10⁵ cells/L) and homogeneous values across stations. At the DCM, not only were higher phytoplankton abundances and biomasses observed, but also greater diversity, particularly of diatoms and coccolithophores. No seamount effect was detected at either Vavilov or Vercelli, confirming the results of previous studies. Any differences between stations close to or farther from the seamount appear to be more attributable to hydrodynamic and seasonal factors rather than to effects induced by the underwater topography. As noted in the literature, the short temporal duration and limited spatial scale of the sampling likely do not provide high-resolution insights for such complex areas.
I seamounts sono ecosistemi marini unici, ricchi di biodiversità e molto vulnerabili. Una delle più recenti definizioni, spesso adottata in ambito ecologico, identifica come seamount “qualsiasi rilievo topograficamente distinto del fondo marino con un dislivello di almeno 100 metri, ma che non emerge dalla superficie del mare”. Solo nel Mar Mediterraneo ce ne sono oltre 200 di cui 65 nel Mar Tirreno, in cui si trovano i seamounts Vavilov e Vercelli (nel Tirreno centrale e settentrionale, rispettivamente), oggetto dello studio. Tra luglio e agosto 2024 è stata condotta una campagna oceanografica per la raccolta di dati fisici, chimici e biologici su ognuno dei seamount, in cui le stazioni erano localizzate sulla sommità del seamount e progressivamente più lontane, lungo due transetti perpendicolari tra loro. Lo scopo era quello di verificare la presenza di un possibile “effetto seamount” che consiste nella risalita di acque profonde ricche di nutrienti nella zona fotica, con conseguente stimolo della produzione fitoplanctonica.I parametri oceanografici sono stati misurati con una sonda CTD multi-sensore e campioni d’acqua prelevati con bottiglie Niskin a diverse profondità (superficie, base dello strato rimescolato, base dello strato eufotico e quota di massima fluorescenza e, solo per i nutrienti anche il deep scattering layer e il fondo). Le analisi dei nutrienti e della clorofilla sono state eseguite secondo protocolli standardizzati (autoanalyzer espettrofotometro rispettivamente), mentre il fitoplancton è stato identificato e quantificato al microscopio ottico con il metodo Utermöhl. Analisi statistiche (Wilcoxon, Kruskal-Wallis, Dunn test) sono state usate per confrontare le due aree e i diversi transetti. L’andamento dei parametri fisici ha mostrato una marcata variabilità nella colonna d'acqua superficiale, più evidente nel Vercelli, probabilmente dovuta a dinamiche mesoscalari e forzanti eoliche tipicamente più intense nel Tirreno settentrionale rispetto a quello centrale. Al Vavilov è stata osservata una stratificazione stabile con strutture “a gradini” tipiche del Tirreno centrale dovute alla doppia diffusione, mentre al Vercelli tale struttura è poco visibile, a eccezione di alcune stazioni sud-orientali. Nel Vercelli è stata rilevata una possibile risalita della Levantine Intermediate Water (LIW), indicata da massimi di salinità a profondità minori del previsto, probabilmente causata dall’interazione tra flussi e topografia del seamount. Tuttavia, l’assenza di dati di velocità di corrente non ha permesso di confermare la presenza di strutture coerenti con una colonna di Taylor. Dal punto di vista biochimico, entrambe le aree si sono rivelate oligotrofiche, con concentrazioni di nutrienti e clorofilla-a molto basse nello strato rimescolato e un picco al livello del Deep Chlorophyll Maximum (DCM), in linea con quanto osservato in altre zone del Mediterraneo. Nello strato rimescolato, la comunità fitoplanctonica era dominata da piccole fitoflagellate, con abbondanze simili tra le due aree (ordine di grandezza 105 cells/L) e valori omogenei tra le stazioni. Al DCM sono state riscontrate, non solo abbondanze e biomasse fitoplanctoniche maggiori, ma è anche una maggiore diversità, in particolare per le diatomee e le coccolitine. Un effetto seamount non è stato rilevato né al Vavilov né al Vercelli, confermando i risultati di studi precedenti. Le eventuali differenze tra le stazioni più vicine e più lontane dal seamount sembrano più attribuibili a fattori idrodinamici e stagionali piuttosto che a effetti indotti dal rilievo sottomarino. Così come riscontrato anche in letteratura, la breve durata temporale e la ridotta scala spaziale dei campionamenti probabilmente non è in grado di fornire informazioni ad alta risoluzione per queste aree così complesse.
Caratterizzazione fisica chimica e biologica di due seamounts nel Mar Tirreno
CHIAINO, ANTONIA
2024/2025
Abstract
Seamounts are unique marine ecosystems, rich in biodiversity and highly vulnerable. One of the most recent definitions, often adopted in ecological studies, defines a seamount as “any topographically distinct elevation of the seafloor with a height of at least 100 meters, but which does not emerge above the sea surface.” In the Mediterranean Sea alone, there are more than 200 seamounts, 65 of which are in the Tyrrhenian Sea. Among them are the Vavilov and Vercelli seamounts (located in the central and northern Tyrrhenian Sea, respectively), which are the focus of this study. Between July and August 2024, an oceanographic survey was conducted to collect physical, chemical, and biological data on each seamount. Sampling stations were located on the summit of the seamount and progressively farther away along two perpendicular transects. The goal was to investigate the presence of a potential “seamount effect,” which involves the upwelling of deep, nutrient-rich waters into the photic zone, thereby stimulating phytoplankton production. Oceanographic parameters were measured using a multi-sensor CTD probe, and water samples were collected with Niskin bottles at various depths (surface, base of the mixed layer, base of the euphotic zone, depth of maximum fluorescence, and for nutrients also the deep scattering layer and the seafloor). Nutrient and chlorophyll analyses were performed using standardized protocols (autoanalyzer and spectrophotometer, respectively), while phytoplankton was identified and counted under an optical microscope using the Utermöhl method. Statistical analyses (Wilcoxon, Kruskal-Wallis, Dunn tests) were used to compare the two areas and the different transects. The behavior of the physical parameters showed marked variability in the surface water column, more pronounced at Vercelli, likely due to mesoscale dynamics and wind forcing, which is typically more intense in the northern Tyrrhenian compared to the central area. At Vavilov, a stable stratification was observed, with “stair-step” structures typical of the central Tyrrhenian, due to double diffusion processes. In contrast, such structures were less evident at Vercelli, except at some southeastern stations. At Vercelli, a potential upwelling of Levantine Intermediate Water (LIW) was detected, indicated by salinity maxima at shallower depths than expected, possibly caused by interactions between water flows and the seamount’s topography. However, the lack of current velocity data did not allow confirmation of structures consistent with a Taylor column. From a biochemical standpoint, both areas were oligotrophic, with very low concentrations of nutrients and chlorophyll-a in the mixed layer and a peak at the Deep Chlorophyll Maximum (DCM), consistent with observations from other parts of the Mediterranean. In the mixed layer, the phytoplankton community was dominated by small phytoflagellates, with similar abundances in both areas (on the order of 10⁵ cells/L) and homogeneous values across stations. At the DCM, not only were higher phytoplankton abundances and biomasses observed, but also greater diversity, particularly of diatoms and coccolithophores. No seamount effect was detected at either Vavilov or Vercelli, confirming the results of previous studies. Any differences between stations close to or farther from the seamount appear to be more attributable to hydrodynamic and seasonal factors rather than to effects induced by the underwater topography. As noted in the literature, the short temporal duration and limited spatial scale of the sampling likely do not provide high-resolution insights for such complex areas.| File | Dimensione | Formato | |
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Thesis Chiaino PDFA.pdf
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https://hdl.handle.net/20.500.12075/22593