S. Fonda Umani

Particulate matter and plankton dynamics in the Ross Sea Polynya of Terra Nova Bay during the Austral Summer 1997/98

Abstract The structure and variability of the plankton community and the distribution and composition of suspended particulate matter, were investigated in the polynya of Terra Nova Bay (western Ross Sea) during the austral summer 1997/1998, with the ultimate objective of understanding the trophic control of carbon export from the upper water column. Sampling was conducted along a transect parallel to the shore, near the retreating ice edge at the beginning of December, closer to the coast at the beginning of February, and more offshore in late February. Hydrological casts and water sampling were performed at several depths to measure total particulate matter (TPM), particulate organic carbon (POC), biogenic silica (BSi), chlorophyll a (Chl a ) and phaeopigment (Phaeo) concentrations. Subsamples were taken for counting autotrophic and heterotrophic pico- and nanoplankton and to assess the abundance and composition of microphyto- and microzooplankton. Statistical analysis identified two major groups of samples: the first included the most coastal surface samples of early December, characterized by the prevalence of autotrophic nanoplankton biomass; the second included all the remaining samples and was dominated by microphytoplankton. With regard to the relation of the plankton community composition to the biogenic suspended and sinking material, we identified the succession of three distinct periods. In early December Phaeocystis dominated the plankton assemblage in the well-mixed water column, while at the retreating ice-edge a bloom of small diatoms (ND) was developing in the lens of superficial diluted water. Concentrations of biogenic particulates were generally low and confined to the uppermost layer. The very low downward fluxes, the near absence of faecal pellets and the high Chl a /Phaeo ratios suggested that the herbivorous food web was not established yet or, at least, was not working efficiently. In early February the superficial pycnocline and the increased water column stability favoured the development of the most intense bloom of the season, essentially sustained by micro-sized diatoms (MD). The shift of the autotrophic community toward this size component produced major changes in the composition of particulate matter and determined its export to depth. The particulate organic carbon (POC)/chlorophyll a (Chl a ) and Chl a /Phaeo ratios more than halved, biogenic silica (BSi)/POC and BSi/Chl a strongly increased. Downward fluxes were greatly enhanced (reaching the yearly maximum) and essentially occurred via faecal pellets, underscoring the high efficiency of the herbivorous food web. In late February the deepening of the pycnocline, together with the decrease in light intensity, contributed to halting the diatom bloom. The biomass of small heterotrophs (HNF and MCZ) significantly increased relative to the previous period, favouring the shift toward a mistivorous food web (sensu [Ophelia 41 (1995) 153]) and resulting in the retention of biogenic matter in the superficial layer. Only in early February, with the increase in the size of primary producers (essentially represented by micro-sized diatoms), did the grazing food web become efficient [S. Afr. J. Mar. Sci. 12 (1992) 477], fuelling the long-lived carbon pool and enhancing export to depth (and hence carbon sequestration) via the sinking of large diatoms and high amounts of faecal pellets. The conditions predominating in the Terra Nova Bay polynya in mid-summer probably increased the efficiency of the CO 2 pump, possibly causing the Bay to act as a carbon sink.

Integrated approach to sediment pollution: a case study in the Gulf of Trieste.

The Gulf of Trieste is located at the northernmost part of the Adriatic Sea (Italy), and is a shallow embayment with an average depth of 17 m. The gulf is an interesting site for studies on anthropic impacts because it has been populated for at least the last 2000 years and activities such as marble quarrying and oyster culture have been practised for centuries; today, it houses several ports and an industrial zone which generate noticeable anthropogenic pressures. The gulf’s peculiar geomorphologic and hydrologic conditions make it prone to the accumulation of pollutants, since it is an elongated, sheltered bay with reduced hydrodynamism (SolisWeiss et al., 2004). The Servola sewage disposal plant is the most important sewage plant of the city, serving up to 270,000 inhabitants. Its primary treatment has been based on chemical precipitation since 1992. The treated water is discharged via two adjacent submarine ducts (6.5 and 7.5 km) ending at 23 m depth with several diffusers (Novelli, 1996). In many marine environments, benthic communities constitute the most effective tool for assessing environmental variations of any given habitat. This is because benthos, in contrast to nekton and plankton, remain in place and are thus subjected to different environmental stresses, and react to these synergetically without any escape possibility. The resulting composition of species, replacements, eliminations, diversity or abundance changes, can indicate the recent history of events affecting the area (Solis-Weiss et al., 2001). The aim of this study was to assess the sediment quality near the outlet of the main wastewater pipeline of the city, applying

Mucous aggregates under natural and laboratory conditions: a review

Abstract During the years 1991 and 1992 several experiments were carried out in order to study the phenomenon of mucous aggregates in the Northern Adriatic Sea. Samples of surface and bottom sea water were treated in different ways, according to the various experiments, and then placed in 10 or 20-1 tanks. The tanks were stored in thermostatic cells at a temperature of 20°C and a 16:8 h light:darkness period. After 8–10 days the formation of mucous filaments occurred. The genus Nitzschia was essential for the formation of mucous filaments, under the conditions of light and temperature applied during the experiments. Appropriate concentrations of phytoplankton and nutrients seemed to be very important also. It was possible to observe a sequence of species in the formation of mucilaginous filaments. There was a transition from species belonging to centric diatoms toward pennate diatoms. The centric diatoms (genus Chaetoceros ) formed the first small aggregates which later, with the colonisation of the genus Nitzschia , developed into real filaments.

Seasonal variations in carbon and nitrogen content of Acartia clausi (Copepoda, Calanoida) in the Gulf of Trieste (Northern Adriatic Sea)

A yearly study was made on total and cephalothorax lengths, dry weight, carbon and nitrogen contents and C:N ratio of the species Acartia clausi (Copepoda, Calanoida) which is always abundant, particularly in spring, in the net zooplankton community of the Gulf of Trieste (Northern Adriatic Sea). The samples of net zooplankton were carried out every month at the permanent station located 200 m offshore and preserved for about one year in 4% buffered formalin to insure weight loss stabilization. A. clausi was separated from other zooplankton specimens; washed and dried. For each monthly sample the average total and cephalothorax lengths and the average dry weight of adult organisms of A. clausi were measured. The mean carbon and nitrogen contents, as a percentage of dry weight, were determined by using a CHN analyser. Linear regression models were computed on the log-transformed data in order to check the relationships between dry weight, total and cephalothorax lengths, carbon and nitrogen contents and C:N ratio of A. clausi. The best of the various established correlations, was between dry weight and carbon content. Our study pinpointed a high seasonal variability of the C:N ratio, mainly due to seasonal fluctuations in the nitrogen content.

Mucilaginous aggregates in the Gulf of Trieste (Northern Adriatic Sea): analysis of the phytoplanktonic communities in the period June—August 1989

Intensive phytoplankton monitoring was carried out at a coastal station on the Gulf of Trieste in June—August 1989 during the massive presence of mucilaginous aggregates. The phytoplankton data were processed using multivariate analysis and compared with those of previous years. The classification clearly separated the samples before the appearance of gelatine (June) from those after this phenomenon (August). The samples taken in July during the presence of mucilaginous aggregates were of a less orderly distribution.

Temporal variability in bacterioplanktonic abundance in coastal waters of the Northern Adriatic Sea

The purpose of this study is to analyse the heterotrophic bacterial abundance (HBA) distribution in the water column at a coastal site of the Gulf of Trieste (Northern Adriatic Sea), from 1999 to 2003, by adopting three different sampling frequencies: monthly, twice monthly, and daily. The HBA ranged from 1.6 to 54.6×108 cell l−1, showing a high variability of more than two orders of magnitude over the study period, with a mean annual variation of one order of magnitude and a 60% annual mean coefficient of variation. Similar seasonal patterns were observed every year: these were characterized by low bacterial abundances during the winter which increased in the summer. Intensifying samplings from monthly to twice monthly did not provide more detailed information on the seasonal HBA distribution but allowed us to detect sporadic HBA hot spots perhaps as the consequence of micro-environmental changes. The results of a daily sampling carried out for 4 weeks, during winter 2002, did not show any significant changes in HBA, ranging from 0.8 to 2.4×108 cell l−1. The highest variations between two consecutive days was of about twofold, and the CV over the period was lower than 30%. A principal-component analysis was used to compare HBA, temperature, salinity, dissolved organic carbon, and chlorophyll a. Different gradients related to the surface water hydrological and biochemical characteristics resulted from the long- and short-term study periods. The study of different timescales allowed us to obtain a complete view of HBA temporal distribution confirming an annual pattern that could be affected by sporadic HBA hot spots as the consequence of changes of local environmental conditions.

The carbon budget in the northern Adriatic Sea, a winter case study

This paper presents a winter carbon budget for the northern Adriatic Sea, obtained through direct measurements during two multidisciplinary cruises and literature data. A box model approach was adopted to integrate estimates of stocks and fluxes of carbon species over the total area. The oligotrophy at the basin scale and the start of primary productivity well before the onset of spring stratification were observed. In winter, the system underwent a complete reset, as the mixing of water masses erased any signal of previous hypoxia or anoxia episodes. The northern Adriatic Sea was phosphorus depleted with respect to C and N availability. This fact confirms the importance of mixing with deep-sea water for P supply to biological processes on the whole. Despite the abundant prokaryotic biomass, the microbial food web was less efficient in organic C production than phytoplankton. In the upper layer, the carbon produced by primary production exceeded the fraction respired by planktonic community smaller than 200 µm. On the contrary, respiration processes prevailed in the water column below the pycnocline. The carbon budget also proved that the northern Adriatic Sea can be an effective sink for atmospheric CO2 throughout the entire winter season.

Tintinnid distributions in the Strait of Magellan (Chile)

In the Strait of Magellan and in the adjacent Pacific and Atlantic Ocean shelf tintinnids were studied during three oceanographic cruises (November 1989, March–April 1991, and April 1995). Total tintinnid abundances were higher in the first cruise and dramatically decreased in the other two late summer cruises. As a general rule, abundances were higher at the surface and in the Atlantic sector. A total of 47 tintinnid species were found. There was not a single species, which could be identified as indicator of a specific water mass. For each cruise, cluster analysis applied on a species/samples matrix identified station sets characterized by a specific tintinnid association. Angosturas entrance was characterized by agglutinated lorica tintinnids, typical of the Atlantic waters, while the Pacific sector by hyaline species. In Punta Arenas basin, a peculiar tintinnid community was clearly identified, although water masses derived from the mixing process between the Pacific and Atlantic waters. Overall, tintinnid communities maintained the structure typical of the water mass despite advection and/or mixing. The main result of this study was to demonstrate that the structure of the tintinnid species association could be used as indicator of the origin of water masses.

Mucilaginous agglomerations in the Gulf of Trieste (Northern Adriatic Sea): analysis of the microzooplankton populations in the period June-August 1989

Microzooplankton populations were analyzed at a hydrological station in the Gulf of Trieste from June to August 1989 during mucilaginous agglomerations presence. During the mucilaginous agglomerations phenomenon in the Gulf of Trieste in the summer of 1989, superfacial samples of water for qualitative and quantitative determination of the microzooplankton populations were collected every three days. Multivariate analysis on the quantitative data were carried out and diversity indexes calculated

Microbial Loop Structure along Trophic Gradients in the Adriatic Sea

Seasonal observations (May 1995–February 1996) carried out along the whole Adriatic basin have significantly contributed to a better understanding of the distribution of bacterial and heterotrophic nanoplankton communities in different trophic areas. The average density values along the water column ranged from 1 × 105 to 1.6 × 106 cell mL−1 and from 1.2 × 102 to 1.9 × 103 cell mL−1 for bacteria and nanoheterotrophs respectively.