Adriano Sfriso

Macroalgae, nutrient cycles, and pollutants in the lagoon of Venice

The Lagoon of Venice is a wide, shallow coastal basin that extends for about 50 km along the northwest coast of the Adriatic Sea. The lagoon has been substantially modified through the actions of man over the last century through the artificial control of the hydraulic dynamics of the laggon including the construction of channels to facilitate navigation. The lagoon is subjected to considerable pollutant loading through the drainage of land under cultivation, municipal sewage, and industrial effluents. In this paper are reported the results of observations designed to document recent changes in macroalgal species composition, seasonal cycles of primary producers and nutrient levels, and the effects of the macroalgal community on concentrations of organic and inorganic pollutants. The dominant macroalgae in the lagoon wasUlva rigida, and the levels of plant nutrients and pollutants were influenced by the seasonal cycles of the macroalgal community.

Relationships between macroalgal biomass and nutrient concentrations in a hypertrophic area of the Venice Lagoon

Abstract Macroalgae biomass and concentrations of nitrogen, phosphorus and chlorophyll a were determined weekly or biweekly in water and sediments, during the spring-summer of 1985 in a hypertrophic area of the lagoon of Venice. Remarkable biomass production (up to 286 g m −2 day −1 , wet weight), was interrupted during three periods of anoxia, when macroalgal decomposition (rate: up to 1000 g m −2 day −1 ) released extraordinary amounts of nutrients. Depending on the macroalgae distribution in the water column, the nutrients released in water varied from 3·3 to 19·1 μg-at litre −1 for total inorganic nitrogen and from 1·8 to 2·7 μg-at litre −1 for reactive phosphorus. Most nutrients, however, accumulated in the surficial sediment (up to 0·640 and to 3·06 mg g −1 for P and N respectively) redoubling the amounts already stored under aerobic conditions, Phytoplankton, systematically below 5 mg m −3 as Chl. a , sharply increased up to 100 mg m −3 only after the release of nutrients in water by anaerobic macroalgal decomposition. During the algal growth periods, the N:P atomic ratio in water decreased to 0·7, suggesting that nitrogen is a growth-limiting factor. This ratio for surficial sediment was between 6·6 and 13·1, similar to that of macroalgae (8·6–12·0).

Heavy metal contamination in the seaweeds of the Venice lagoon

The concentrations of heavy metals (Fe, Zn, Cu, Cd, Ni, Pb, Cr, As) were determined in seven seaweeds of environmental and commercial relevance (Ulva rigida C. Ag., Gracilaria gracilis (Stackhouse) Steentoft, L. Irvine and Farnham, Porphyra leucosticta Thuret, Grateloupia doryphora (Montagne) Howe., Undaria pinnatifida (Harv.) Suringar, Fucus virsoides J. Agardh, Cystoseira barbata (Good. et Wood.) Ag.) collected in four sampling sites in the lagoon of Venice, in spring and autumn 1999. Metals were extracted using hot concentrated acids in a Microwave Digestion Rotor and analysed by absorption spectrophotometry using a flame mode for Fe and Zn and a graphite furnace for Pb, Cr, Cd, Cu, Ni and As. High contamination levels, especially for Pb, were detected in Ulva and to a lesser extent in Gracilaria. Brown seaweeds, especially Cystoseira was highly contaminated by As. The least contaminated genera with all metals except As were Porphyra and Undaria. A concentration decrease for Zn and Cd was observed from the inner parts of the central lagoon, close to the industrial district, towards the lagoon openings to the sea.

Benthic macrofauna changes in areas of Venice lagoon populated by seagrasses or seaweeds

Two areas of the Venice lagoon populated by seagrasses (three stations covered by Cymodocea nodosa (Ucria) Asherson, Zostera marina Linnaeus, Zostera noltii Hornemann) or seaweeds (two stations: one covered by Ulva rigida C. Agardh and another at present without seaweed biomass) were monitored by means of six surveys over a year in order to study macrofaunal composition and seasonal changes. The seagrass stations showed a mean species richness (28-30 S m(-2)), individual abundance (1854-4018 N m(-2)) and biomass (22.3-37.7 g m(-2) ash-free-dry-weight, AFDW) ca. 3-8 times higher than those populated by seaweeds (10-15 S m(-2), 494-1395 N m(-2) and 5.6-13.7 g m(-2) AFDW). Differences among seagrass or seaweed stations were much lower. The Ulva-dominated station showed a macrofauna completely different both from the other stations and the communities recorded ca. 30 years ago, before the prolific growth of Ulva. In this station, frequent biomass decompositions and anoxic crises created critical conditions for life favouring organisms with reduced life cycles, younger individuals and the epifaunal species instead of the infaunal ones. In particular, Ulva grazers and scrapers such as Gammarus aequicauda Stock and Gibbula adriatica Philippi were found to be by far the most abundant species, whereas the taxa characteristic of the associations found in the past, in the presence of seagrasses or seaweeds and typical of low eutrophicated environments, appear strongly reduced. Marked differences in the macrophyte dominance and in the bio-physico-chemical variables which characterise the main environmental conditions of the Venice lagoon support the different distribution and composition of macrofaunal communities. Seaweed stations appear mainly governed by the seasonal cycles of these un-rooted macrophytes which, by alternating periods of production and decomposition, are responsible for the drastic reduction of macrofauna biodiversity and biomass. Conversely, seagrass stations exhibit a better oxidisation of the environment and show conditions more favourable for macrofauna colonisation, especially in the presence of macrophytes which are characterised by very well developed below-ground systems such as Cymodocea nodosa.

Persistent metabolites of alkylphenol polyethoxylates in the marine environment

The persistent metabolites of the nonionic surfactant nonylphenol polyethoxylates (NPnEO, n=1–18), namely nonylphenol (NP), nonylphenol monoethoxylate (NP1EO) and nonylphenol diethoxylate (NP2EO), were analysed in marine samples including sediment, artificially resuspended sediment and water. The UV-fluorescence high-performance liquid chromatography determination was carried out after Soxhlet extraction with hexane from the sedimentary matrices followed by clean-up on aminosilica minicolumns. Four sampling campaigns at five representative stations of the Venice lagoon were conducted to ascertain temporal and spatial variability of the examined chemicals. A portable resuspending device allowed us to analyse the first 0.01–0.15-mm sediment layer, where the sum of NP, NP1EO and NP2EO was in the range 0.15–13.7 μg g−1 (dry weight basis), at least five times higher than in the underlying 5 cm of sediment. Amounts of resuspended material, and concentrations of NP, NP1EO, NP2EO bound to it, showed a marked seasonal dependence: as much as twice the resuspended material, per unit of sediment surface, was measured in April and July, compared with that in February, but resuspended NP, NP1EO and NP2EO per unit of sediment surface were in February one order of magnitude higher. A major factor responsible for this trend was related to the proliferation of macroalgae which contained an average NP+NP1EO+NP2EO concentration of 0.25±0.15 μg g−1 (dry wt.). In water, NPEO oligomers with up to 13 ethoxy units were found at an overall concentration range of 0.6–4.5 μg l−1.

Annual variations of nutrients in the Lagoon of Venice

Abstract Nutrient concentrations (P, N compounds and C org ) and chemical-physical parameters (T, DO%, pH, Eh) were monitored in water and sediment in the Lagoon of Venice for one year. Large variations appeared to depend mainly on macroalgal biomass fluctuations which occurred during spring-summer. Sediment was the main sink of nutrients from decomposing macroalgae. Sedimentary phosphorus, nitrogen and carbon increased 75, 160, and 70%, respectively. When macroalgae were negligible, in August–March, nutrients accumulated in sediment were released to the overlying water. Due to the occurrence of anaerobic conditions, the highest concentrations of phosphorus in water were observed in spring-summer. The release of sedimentary nitrogen was higher in September–March. Macroalgal assimilation varied the N:P atomic ratio in water from 50–78 in winter to 0.7–5 in spring-summer. In addition, the N:P ratio in sediment increased from values around 7 in winter to more than 13 between May and July, approaching the N:P values found in macroalgae (8.5–12). Therefore, in spring-summer, the availability of nitrogen in water appeared to be the limiting factor for the macroalgal growth.

Temporal and spatial changes of macroalgae and phytoplankton in a Mediterranean coastal area: the Venice lagoon as a case study.

Since the late 1980s the lagoon of Venice, a shallow Mediterranean coastal area, has experienced strong environmental changes. Macroalgae, which were the predominant primary producers of the lagoon, reduced markedly, but neither phytoplankton nor seagrasses replaced them. Temporal and spatial changes in macroalgal standing crop (SC) and phytoplankton concentration were investigated between 1987 and 1998. Maps of macroalgal SC show a marked declining trend. Biomass in fresh weight decreased from: 558 ktonnes in 1987, to 85 ktonnes in 1993 and to 8.7 ktonnes in 1998. As a whole, the biomass in 1998 was only 1.6% of the biomass recorded in 1987. Similarly the macroalgal net (NPP) and gross (GPP) primary production decreased from ca. 1502 and 9721 ktonnes year(-1) to ca. 44 and 229 ktonnes year(-1), respectively. In the early 1990s the clam Tapes philippinarum Adams & Reeve and seagrasses, especially Zostera marina Linnaeus, colonised the bottoms free of macroalgae, but the development of intense clam-fishing activities prevented both phytoplankton blooms and seagrass spreading. Maps of chlorophyll a drawn according to data collected in parallel to macroalgal standing crop show unchanged concentrations. Macroalgae changes are enhanced by comparing annual trends in four areas of the central lagoon during 1989-1992 and 1998-1999. In those areas phytoplankton also decreased significantly. Marked changes of some environmental variables strongly associated with the primary production were recorded both during the lagoon mapping and in the areas studied on a yearly basis.

Macroalgae and phytoplankton standing crops in the central Venice lagoon: Primary production and nutrient balance

Biomass measurements carried out in the central part of the Venice lagoon during the season of the highest macroalgal production, at the beginning of June 1987, showed that 85 km2, corresponding to 65% of the examined area, were covered by remarkable amounts of nitrophile macroalgae. Total macroalgal biomass was 546 900 tons, accounting for 17 679, 1911 and 170 tons of carbon, nitrogen and phosphorus, respectively. The annual net primary production, calculated from the maximum production/biomass (P/B) ratio determined in a hypertrophic area of the central lagoon, was estimated to be 866 160–1154 880 tons (wet wt). Therefore, in Spring-Summer, macroalgae recycled 78–104% and 38–51% of the total annual nitrogen and phosphorus which entered the central lagoon. Phytoplankton standing crop appeared negligible in most of the studied area. High chlorophyll-a concentrations were only measured near the Porto Marghera industrial zone where macroalgae were almost absent; in the area between the islands of S. Angelo and Gidecca, where the water depth was > 1 m; and in the area facing the island of Lido used by the Municipality of Venice as a test area for algal removal experiments. Based on both space and time measurements, nanoplankton accounted for about 80% of the total phytoplankton.

Decline of Ulva growth in the lagoon of Venice

Abstract Causes and effects of the remarkable regression of the macroalga Ulva rigida C. Ag. in the central part of the Venice lagoon since 1990 are reported. Climatic changes triggered the progressive reduction of Ulva coverage and production (−80% in 1993; −95% in 1995) until its almost complete disappearance in 1996. Grazers, especially Gammaridae, controlled 70% of the current biomass production, on a yearly basis, and largely overcame the biomass production in summer. Concurrently, the sediment resuspension increased by a factor of 6–10 in the lagoon areas where the biomass disappeared, contributing significantly to control of the growth of the residual biomass by light limitation, especially by settling of a microlayer of sediment over the laminar thalli. Finally, biomass harvesting, now effective, and the heavy disturbance caused by fishing boats equipped with hydraulic dredges to catch Tapes philippinarum, a bivalve species recently introduced into the lagoon and spread in the areas previously covered by Ulva, currently play an important synergistic role in Ulva control. Conversely, the overall availability of nutrients did not appear to have any significant influence on the observed macroalgal decline.

Macrophyte production in a shallow coastal lagoon. Part I: Coupling with chemico-physical parameters and nutrient concentrations in waters

Abstract Standing crop and production of macroalgae and seagrasses were recorded, together with the main physical and chemical parameters, including nutrient concentrations of the water column and 5-cm top sediment porewater, the grazing pressure and the settled paniculate matter (SPM) in two areas of the central (Lido station) and southern (Petta di Bo station) parts of the Venice lagoon. Then the whole set of data was analysed by multivariate analysis. The highest standing crops of Ulva (Lido station) and Zostera (Petta di Bo station) monitored throughout the year (February 1994–February 1995) were ~ 6.5 and ~ 11.0 kg fwt m −2 , accounting for an annual net production of ~ 20.4 and ~ 20.9 kg fwt m −2 , respectively. The estimated gross production of Zostera was, however, ca 35–55% lower than that of Ulva because of the higher decomposition rate and grazing pressure suffered by the markedly stratified and light-limited free-floating fronds of the macroalga. At the Lido station, the overall grazing pressure accounted for ca 65% of the net Ulva production, but it was found to exceed the total production in the July–August period. Ammonium and orthophosphate concentrations in the water column and sediment porewater were ca 2–3 times higher at the Ulva than at the Zostera station. Considering the N:P atomic ratios, nitrogen, during the quick spring-summer biomass increase, could be temporarily critical for the macrophytic growth, especially at the Lido station. The rates of sediment resuspension and settlement were ca six times higher at the Zostera than at the Ulva station, mainly because of higher sediment coverage by the large free-floating fronds of Ulva . For the contribution of individual variables explained by the principal component analysis, it is shown that the Ulva decomposition at the Lido station and the Zostera production at Petta di Bo were the major factors affecting the total variance.