Alfredo Boldrin

Can plankton communities be considered as bio-indicators of water quality in the Lagoon of Venice?

This study examines whether plankton of the Lagoon of Venice could be considered as a bio-indicator of areas subjected to various anthropogenic influences. This study was a two year hydrochemical and biological survey in five areas of the Lagoon of Venice, each with different environmental conditions due to pollution from urban, industrial, thermal and agricultural wastes. Phytoplankton associations did not show any promising species. In the different lagoonal areas, this community was differentiated into its major groups. In contrast, the copepod Acartia tonsa Dana could be considered as a target species in highly eutrophic areas.

Particulate matter in the southern Adriatic and Ionian Sea: characterisation and downward fluxes

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. D 2002 Elsevier Science B.V. All rights reserved.

Nutrient, particulate matter and phytoplankton variability in the photic layer of the Otranto strait

The distribution of nutrients, suspended matter and phytoplankton in the photic layer of the Otranto Strait, as observed in four seasonal situations (February, May, August and November 1994), is analysed in relation to hydrography. In winter, two water masses were found: the Adriatic Surface Water (ASW) and the Ionian Surface Water (ISW), located at the western and eastern side of the Strait, respectively. In the fresher and cooler ASW, nutrients, suspended matter and phytoplankton contents were higher than in the warmer and saltier ISW (on average: N–NO3=2.3 μM, POC=3.6 μM, chl.a=0.4 μg dm−3 in the ASW, against N–NO3=1.3 μM, POC=2.5 μM, chl.a=0.3 μg dm−3 in the ISW). In the ASW, the mean N:P ratio (50) revealed an excess of nitrogen with respect to phosphorus; the nitrogen supply, as well as the significant presence of diatoms, made us suppose that new production processes were occurring here. In summer, the main features were: (i) high water column stability, (ii) small horizontal differences in hydrological and biological features, (iii) extremely low concentration of nutrients at the surface and (iv) a deep chlorophyll maximum (DCM) at the nutricline level. Phytoplankton summer communities, mainly consisting of nanoflagellates, coccolithophorids and small naked dinoflagellates, were present in low quantities. The low phytoplankton carbon:chlorophyll a ratio (<20), observed at DCM, indicated a high chlorophyll content per single cell, probably as a physiological response to low irradiance. As to the phytoplankton annual cycle, sediment traps and water column observations were highly correlated, evidencing two abundance peaks in spring and autumn, and a summer minimum. Hydro-chemical and biological data suggest that the winter spread of the ASW was the main factor favouring the enhancement of phytoplankton growth and controlling the species composition in the strait, while the DCM formation represents a more typical characteristic of ISW, with an oligotrophic feature similar to that of other Eastern Mediterranean water masses.

Variability and downward fluxes of particulate matter in the Otranto Strait area

Abstract The space-time distribution and vertical fluxes of particulate matter in the Otranto channel area have been studied. A three-layered structure was observed: (i) a surface layer with abundance of suspended matter, characterised by the presence of two water masses (the Adriatic Surface Water and the Ionian Surface Water); (ii) an intermediate layer (Levantine Intermediate Water) with very low particle content; and (iii) a bottom nepheloid layer. In winter, the particulate matter concentration at the surface revealed an east–west gradient that was not evident in summer, when the water exchange through the strait was reduced and local recirculation phenomena prevailed. Sediment traps and current meters were moored on the Italian shelf and on the shelf slope. Total vertical flux values of particulate matter, which were by one-tenth lower in the slope area with respect to the shelf, were in good agreement with the distribution of suspended matter in the water column observed during the seasonal cruises. Flux values measured at the shelf station appear to be related both to biological and advective processes, whereas the main source in the slope area appears to be the vertical transfer of biogenic material.

Massive shelf dense water flow influences plankton community structure and particle transport over long distance

Dense waters (DW) formation in shelf areas and their cascading off the shelf break play a major role in ventilating deep waters, thus potentially affecting ecosystem functioning and biogeochemical cycles. However, whether DW flow across shelves may affect the composition and structure of plankton communities down to the seafloor and the particles transport over long distances has not been fully investigated. Following the 2012 north Adriatic Sea cold outbreak, DW masses were intercepted at ca. 460 km south the area of origin and compared to resident ones in term of plankton biomass partitioning (pico to micro size) and phytoplankton species composition. Results indicated a relatively higher contribution of heterotrophs in DW than in deep resident water masses, probably as result of DW-mediated advection of fresh organic matter available to consumers. DWs showed unusual high abundances of Skeletonema sp., a diatom that bloomed in the north Adriatic during DW formation. The Lagrangian numerical model set up on this diatom confirmed that DW flow could be an important mechanism for plankton/particles export to deep waters. We conclude that the predicted climate-induced variability in DW formation events could have the potential to affect the ecosystem functioning of the deeper part of the Mediterranean basin, even at significant distance from generation sites.