P. Del Negro

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.

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.

Effects of the physical properties of water masses on microbial activity during an Ice Shelf Water overflow in the central Ross Sea

During the 1997–98 Italian Expedition to Antarctica a five-day mesoscale experiment was carried out on the continental shelf-break in the central Ross Sea. This area is oceanographically characterized by shelf/slope interactions, through intense mixing processes, between the Circumpolar Deep Water (CDW) and the Ice Shelf Water (ISW), coming from beneath the Ross Ice Shelf and spilling over the shelf edge. The export of dense shelf waters is of crucial importance not only for the mass balance of the basin, but also for carbon export from the upper layers into the abyssal ones. The study investigated how the ISW interactions with the CDW may influence bacterial metabolism during an ISW downslope event. In particular, what effect does this have on the bacterial activities related to the utilization and transformation of the organic carbon substrate (ectoenzymatic activities, carbon production, growth rate) within the ISW and the CDW cores? Our data show that in the CDW the metabolic response was to increase the biomass and enzymes were less active due to a higher nutritional value for the substrate. In the ISW the bacterial metabolic activity shifted towards degradative processes. These results suggest differences in the quality of the organic carbon pool with a greater concentration of labile organic matter in the CDW and of low-degradable compounds in the ISW. The use of microbial parameters seems to be very promising in the evaluation of the carbon export during mixing processes, when the refractory fraction of the organic carbon pool might play a key role.

Microbial mechanisms coupling carbon and phosphorus cycles in phosphorus-limited northern Adriatic Sea

The coastal northern Adriatic Sea receives pulsed inputs of riverine nutrients, causing phytoplankton blooms and seasonally sustained dissolved organic carbon (DOC) accumulation-hypothesized to cause episodes of massive mucilage. The underlying mechanisms regulating P and C cycles and their coupling are unclear. Extensive biogeochemical parameters, processes and community composition were measured in a 64-day mesocosms deployed off Piran, Slovenia. We followed the temporal trends of C and P fluxes in P-enriched (P+) and unenriched (P-) mesocosms. An intense diatom bloom developed then crashed; however, substantial primary production was maintained throughout, supported by tightly coupled P regeneration by bacteria and phytoplankton. Results provide novel insights on post-bloom C and P dynamics and mechanisms. 1) Post-bloom DOC accumulation to 186 μM remained elevated despite high bacterial carbon demand. Presumably, a large part of DOC accumulated due to the bacterial ectohydrolytic processing of primary productivity that adventitiously generated slow-to-degrade DOC; 2) bacteria heavily colonized post-bloom diatom aggregates, rendering them microscale hotspots of P regeneration due to locally intense bacterial ectohydrolase activities; 3) Pi turnover was rapid thus suggesting high P flux through the DOP pool (dissolved organic phosphorus) turnover; 4) Alpha- and Gamma-proteobacteria dominated the bacterial communities despite great differences of C and P pools and fluxes in both mesocosms. However, minor taxa showed dramatic changes in community compositions. Major OTUs were presumably generalists adapted to diverse productivity regimes.We suggest that variation in bacterial ectohydrolase activities on aggregates, regulating the rates of POM→DOM transition as well as dissolved polymer hydrolysis, could become a bottleneck in P regeneration. This could be another regulatory step, in addition to APase, in the microbial regulation of P cycle and the coupling between C and P cycles.

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.

Chrysaora hysoscella in the Gulf of Trieste: presence, evolution and cutaneous toxicity in man

In the past few years important changes in the occurrence of scyphomedusae have been observed in the North Adriatic Sea and in particular in the Gulf of Trieste, often reaching alarming proportions. Spring coastal blooms of C. hysoscella were observed in 1989. Until 1989, this jellyfish was considered to be innocuous but the data presented show its cutaneous toxicity in man.

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.

Effects of CO2 Induced pH Decrease on Shallow Benthic Microbial Communities

Two mesocosm experiments (18 and 10°C) have been carried out in order to investigate the effects of pH reduction due to pCO2 increase on benthic microbial communities collected from the Gulf of Trieste (Northern Adriatic Sea). During each experiment 3 mesocosms with different pH values were set up (6.5, 7 and an aerated control ~ 8). The analyses focussed on prokaryotic abundance, degradation processes and prokaryotic carbon production. Preliminary results highlight differences in metabolic response to CO2 in terms of organic matter degradation patterns whereas prokaryotic numbers and C production seemed to be less affected by pH decrease.

Field Experiment to Evaluate Impacts of pH Decrease on Marine Microbial Assemblage

We investigated the role of pH reduction, due to natural CO2 release, on the abundance, distribution and diversity of planktonic prokaryotes in seawater. A field experiment has been carried out during May 2008 on the seafloor east of Panarea Island in the Southern Thyrrenian Sea (Mediterranean Sea). Approaching the vent field pH values decreased from 8.1 to 6.3. Microbial assemblages are under analysis.