Roberta Azzoni

Nutrient and iron limitation to Ulva blooms in a eutrophic coastal lagoon (Sacca di Goro, Italy)

Growth patterns and bloom formation of the green seaweed Ulva rigida were analysed in the eutrophic Sacca di Goro lagoon (Po River Delta, Italy). Variations of standing biomasses and elemental composition of Ulva were analysed through an annual cycle with respect to nitrogen, phosphorus and iron. Growth rates, nutrient and iron uptake and nitrate storage by macroalgal thalli were also assessed with field experiments during the formation of a spring bloom. The control of Ulva growth and the bloom formation depended on multiple factors, especially on nitrogen availability and iron deficiency. In the nitrate rich waters of the Sacca di Goro lagoon, nitrate accumulation in Ulva thalli was inversely related with Fe uptake, indicating an influence of Fe limitation on N acquisition. Since length and magnitude of nitrate luxury uptake are inversely related to the size of the intracellular nitrate pools, in nitrate rich waters the fast growing Ulva may face risk of N-limitation not only when exposed to low N concentrations or at high biomass levels, but also when exposed to pulsed dissolved nitrate concentrations at low iron availability. The potential Fe limitation could be affected by processes controlled by geochemical reactions and by macroalgal growth and decomposition. Both Fe oxidation during the active macroalgal growth and the formation of insoluble FeS and FeS2 during bloom collapse can result in a drastic decrease of soluble iron. Thus, a potential limitation of Fe to macroalgae can occur, determining positive feedbacks and potentially controlling the extent of bloom development and persistence.

Seasonal variations of sulphate reduction rates sulphur pools and iron availability in the sediment of a dystrophic lagoon (Sacca di Goro, Italy)

The aim of this work was to analyse factors which regulate sulphide mobility in the sediments of a dystrophic lagoon (Sacca di Goro, the southern lagoon of the Po river delta, Italy). In 1995–96, sediment oxygen demand and variations in organic matter content, redox potential, iron availability, inorganic sulphur concentrations and sulphate reduction rates were measured in sediment profiles at three stations (G, 4 and 17) representative of the main areas of the lagoon. Stations differed mainly in their salinity range and primary producer communities. High concentrations of reactive iron (110-275 pmol ml-1) and low sulphate reduction rates (0.8–16.1 mmol m−2 d−1) were measured in the sediment. Moreover, high concentrations of reactive ferric iron were detected in winter and spring at the stations closest to the freshwater inputs. Nevertheless, in summer, high concentrations of free sulphides were detected in the porewater, although most of the reactive ferrous iron was not sulphide-bound, indicating that not all of the reactive iron pool was available to buffer against sulphide release and thus measures of reactive iron pools may not be a good measure of the true buffering capacity of the sediment. Furthermore, a considerable production of sulphide may occur in the decayingUlva biomass in the water column, where its concentration will be independent of the potential buffering capacity of iron in the sediment. Therefore when assessing the vulnerability of coastal lagoons to dystrophic events, both the size and availability of the reactive iron pool as well as the site of sulphide production must be taken into consideration.

Short-term effect of oxic to anoxic transition on benthic microbial activity and solute fluxes in organic-rich phytotreatment ponds

Manipulative experiments to test the short-term effect of oxygen depletion events on microbial activity and benthic fluxes in organic-rich sediments were carried out in March and June 2004. Oxic–anoxic transitions were induced by prolonged dark incubation of sealed sediment cores collected in phytotreatment ponds. Benthic fluxes of oxygen (O2), carbon dioxide (CO2), inorganic nutrients, and free sulfides were measured before (oxic) and after (anoxic) the transition occurred. A multifactorial design was employed for monitoring esoenzymatic activity, heterotrophic bacterial production, total prokaryotic abundance, actively respiring bacterial cells, and the biochemical composition of sedimentary organic matter. The oxic to anoxic transition resulted in a significant increase of esoenzymatic activity and bacterial production in March, due to the profound modification of the benthic community and the release of labile organic compounds which followed the onset of anoxia. In parallel, net efflux rates of dissolved inorganic carbon (DIC) and ammonium (NH4+) sharply decreased, soluble reactive phosphorus (SRP) influx reversed, and sulfide was buffered within the oxidized sediments. From March to June, ponds evolved toward oxygen deficit and reducing conditions in the upper sediment horizon, losing benthic fauna and biogeochemical buffering capacity. Thus, the oxic to anoxic transition had a much smaller effect on microbial activity and net flux exchange, while S2− was consistently delivered from the sediment to the water column. Overall data from this study suggest that the short-term response of benthic microbial activity and solute fluxes to anoxic events may have a significant impact on sediment biogeochemistry (e.g., at the oxic–anoxic interface), and that this impact may vary greatly depending on the sediment features, mainly its organic content and redox condition.

Persistence of meromixis and its effects on redox conditions and trophic status in Lake Idro (Southern Alps, Italy)

This paper reports a study of oxygen and redox conditions, trophic status, and phytoplankton community in the meromictic Lake Idro (Italy) from 2010 to 2014. The sequence of causes and effects of meromixis are also evaluated by comparing recent research with studies conducted from the late 1960s to the mid-1990s. In the last half century, Lake Idro was steadily meromictic due to solutes which accumulated in its deep waters, along with both dissolved nutrients and chemically reduced substances produced by the anaerobic microbial metabolism. These substances were retained in bottom waters and made unavailable to upper layers until stratification broke. Mixing episodes occurred in 2005–2006 altering stratification, and oxygen and nutrient distribution within the lake. The potential full overturn effects were also evaluated as potential oxygen consumption due to the oxidation of reduced substances to forecast possible oxygen exhaustion and collapse of biological communities. Finally, meromixis is discussed as a potential threat for deep perialpine lakes using Lake Idro as a reference to comparatively evaluate the present status and possible future trends.