Danilo Degobbis

Long-term changes in the northern Adriatic ecosystem related to anthropogenic eutrophication

Knowledge of the mechanisms driving long-term changes in the nutrient and primary production cycles represents a basic step to verify real eutrophication trends in shallow marine ecosystems, such as the northern Adriatic open waters. In fact, this ecosystem appears to be especially sensitive to seasonal and long-term variations of the anthropogenic nutrient load (primarily the Po River discharges), which impact can be significantly modified by changes in the oceanographic conditions, due to climatic fluctuations. To verify this assumption, the data for several parameters related to eutrophication (salinity, temperature, chlorophyll a, primary production rate, nutrients, water transparency, dissolved oxygen), collected in the period 1966–1995 in open waters, were analysed and compared with changes of the Po River flow-rate. The data were grouped in typical seasons, periods, and subareas, characterised by different dominant relevant processes. The changes of mean values for salinity and temperature were well correlated with the Po flow-rates in all periods, except during the late 1980s. In these years, the surface salinity was generally lower and the temperature was higher than expected from the flow-rate values. This departure was explained by unusual hydrometeorological conditions, characterised by a calm sea and sunny weather, due to long periods of high barometric pressure. In these conditions, the freshwater mixing was limited to a thinner surface layer, in which heat accumulation was favoured. The long-term changes of the nutrient concentrations were even less correlated with the Po flow-rates. The mean orthophosphate concentration of the surface layer in the season February–April of the late 1980s appeared to be lower, and that of the total inorganic nitrogen (TIN) higher than expected from the pattern of the Po flow-rate. These deviations can be explained by long-term changes of the river nutrient composition. In relation to this, when compared at the same salinity (i.e. same dilution degree, independently on the freshwater discharge rate), the chlorophyll a concentrations and the photosynthetic activities (estimated by seawater incubation at constant light with 14C-bicarbonate) were higher in periods of higher river orthophosphate concentrations, but not of TIN concentrations. Furthermore, the data analysis suggested that more favourable hydrometeorological conditions for phytoplankton growth in spring and summer occurred during the late 1980s than other periods with similar phosphorus availabilities (i.e. the 1970s). Intense surface phytoplankton blooms, localised off the Po Delta, were induced in unusually long periods of calm sea and sunny weather. Long-term changes of the primary production parameters in the autumn and winter were not related to the nutrient composition of the Po waters. Probably, the primary production in these seasons is mainly controlled by nutrient recycling within the marine ecosystem. The bottom oxygen saturation during summer and autumn of the late 1980s was lower than expected from the Po flow-rate, especially if the decrease of the phosphorus load is considered. In fact, the near-anoxic events (and the autumn 1989 anoxia) in the bottom layer of large areas probably occurred as a consequence of a delayed reactivation of the autumn vertical and horizontal water circulation, due to unusual meteorological conditions. This is also confirmed by the fact that similar events were not observed in the next period (1990–95), which was characterised by different meteorological and hydrological cycles. The analysis of the long-term change of the Po flow-rate was expanded to the entire measurement period (since 1917). It was concluded that during the period 1969–1992 the flow-rate inspring was on average higher than in the previous period, and that the autumn peak shifted from November to October. These changes might have had some influence on the northern Adriatic ecosystem. Moreover, particularly frequent was the occurrence of a secondary peak flow in March during the 1970s. It was concluded that significant fluctuations of primary production in the open northern Adriatic have occurred since the 1970s, caused by variations of the nutrient load and oceanographic conditions.

Changes in the northern Adriatic ecosystem and the hypertrophic appearance of gelatinous aggregates

Biweekly to monthly measurements of a large number of physical, chemical, and biological parameters and visual observations (by scuba divers and underwater video cameras) were performed at 80 stations in the northern Adriatic in the framework of an ‘Alpe-Adria’ research of a phenomenon of gelatinous aggregate hypertrophy. This phenomenon was observed in the entire investigated region during the summers of 1988, 1989, and 1991 and only in the Kvarner areas in 1990. Results for some parameters were compared with available historical data series collected since 1966. Some qualitative changes in the phytoplankton communities (increased diatom contribution, decreased diversity, different dominant species) were evident during the eighties compared with the seventies. However, chlorophyl a and nutrient concentrations remained approximately at the same level. The dynamics of the Po river discharge (and nutrient inputs) during the spring, which was the critical period for aggregate formation, was different during the eighties from that in the preceding decade. An hypothesis is developed which relates the appearance of large quantities of gelatinous material to modifications of the environmental conditions (climatic, hydrology, and oceanographic).

Increased eutrophication of the Northern Adriatic sea

Abstract The Northern Adriatic is one of the most productive regions of the Mediterranean, primarily due to the eutrophicating influence of Italian rivers, in particular the Po. In June 1977 unusually high phytoplankton standing crops (13 μg Chl a l.−1) occurred, together with extremely low dissolved oxygen concentrations in bottom waters (to 13%) which persisted for 4–6 months. Such conditions were associated with unusual climatic conditions creating increased discharge from the Po. The data indicate long term meteorological changes which may alter the biological characteristics of the region to a degree equal to, or exceeding, those created by anthropomorphic eutrophication. The 1977 data are presented, discussed, and contrasted with a 10 year data series.

An annotated nitrogen budget calculation for the northern Adriatic Sea

Abstract The nitrogen budget of the northern Adriatic, one of the most productive subregions in the Mediterranean area, was estimated from data sets collected since 1966 and from results reported in the literature. River, wastewater and atmospheric contributions, water mass exchange, losses by sedimentation and in fish catches accounted for the major nitrogen inputs and outputs of the investigated area. The calculated nitrogen inputs (17320 × 10 6 moly −1 ) were significantly higher than the outputs (11870 × 10 6 moly −1 ). Nitrogen losses by denitrification in sediments can account for the major part of this difference. The results highlight the relative importance of the nitrogen contribution by the Po River (50% of the input) whose waters influence a large part of the northern Adriatic, and the loss by water mass transport, as the principal mechanisms balancing the nitrogen budget in the northern Adriatic. A quantity about twice the yearly input is biologically recycled annually in the northern Adriatic. Calculated assimilation and regeneration rates show a difference of about 40%, which can be ascribed to uncertainties in some of the data used, particularly the possible understimation of the primary production measured by the 14 C uptake method.

Historical development of the Venice lagoon contamination as recorded in radiodated sediment cores

Abstract Three cores (length 60 cm, i.d. 10 cm) collected in the most contaminated area of the lagoon of Venice (Italy) were sectioned (14, 12 and 15 sections each respectively) and analysed. Concentrations of heavy metals (Hg, Pb, Cd, Co, Cu, Zn, Cr, Ni, Fe), organic carbon, total nitrogen and total phosphorus were determined. Eh and pH profiles are discussed. Background (pre-industrial) levels for metals were estimated from the deepest sections. The principal component analysis performed on the metal concentrations, normalized on the backgrounds, revealed that two components can account for 99% of the variability. The first clearly represents the mercury pollution, the second weights the contribution of zinc and, to a lesser degree, of other metals (Cd, Cu, Co, Pb) to a contamination mainly caused by discharges of industrial tailings. Radiodating of two cores permitted us to reconstruct the historical evolution of the sediment pollution as related to the development of specific activities in the industrial zone of Porto Marghera. Total phosphorus and organic carbon accumulation was significant in the upper 10–20 cm of the cores.

A stoichiometric model of nutrient cycling in the northern adriatic sea and its relation to regeneration processes

From a linear regression analysis of an extended set of concentration data collected between 1972 and 1981 (n=20 000) a stoichiometric model of nutrient regeneration was developed for the shallow northern Adriatic, the most eutrophic region in the Mediterranean Sea. The model (AOU:N:Si:P=−610:15:47:1) differs significantly from a widely accepted ‘oceanic’ model (AOU:N:Si:P=−276:16:15:1), mainly because of denitrification in the sediment, different regeneration rates of phosphorus and nitrogen (regenerated mainly in the water column) vs. biogenic silicon (regenerated mainly in the sediments), and a phosphorus deficiency in decomposing organic matter from primary and secondary production. In water masses, mainly in surface layers, in which freshwater nutrient input and phytoplankton assimilation are dominant, the nutrient ratios (N/P=62, Si/N=1.1, and Si/P=65) are significantly different from ratios in subeuphotic regeneration layers (N/P=15, Si/N=3.3, and Si/P=48). These differences were ascribed to nutrient ratios in river waters which are ‘anomalous’, and to faster utilization of phosphorus by northern Adriatic phytoplankton compared with nitrogen and particularly silicon.

The role of alkaline phosphatase in the sediments of Venice Lagoon on nutrient regeneration

Abstract Potential alkaline phosphatase activity and nutrient concentration have been determined in the seawater, interstitial water and sediments of the most polluted area in the Venice Lagoon, at 6 stations, during three cruises performed in the October/November 1982 period, when a minimum phytoplankton standing crop existed. The relationships between parameters were established by linear regression analysis. The results clearly indicate the important role of alkaline phosphatase (APA) in phosphorus regeneration. Indeed, APA appears to be a good indicator of the degree of nutrient regeneration occurring in surface sediments on a global basis. In the Venice Lagoon a significant nutrient release from the sediments to the overlying lagoon water takes place. This process appears to be at least as important as external contributions in both establishing and controlling the concentration of the nutrients in lagoon waters.

Nutrient enrichment and phytoplankton response in an Adriatic karstic estuary

Abstract Nutrients, chlorophyll a , primary production ( 14 C), and standard oceanographic parameters were measured seasonally from 1983 to 1988 along the axis of a karstic estuary of the central Adriatic Sea (the Krka River estuary). Because of anthropogenic phosphorus discharges, the surface-layer orthophosphate concentrations (up to 1.7 mmol m −3 ), phytoplankton biomass (chlorophyll a up to 23 mg m −3 ) and primary production (up to 108 mg C m −3 h −1 ) were significantly higher in Sibenik Bay (lower estuary) than in the other estuarine subregions, and the coastal sea in particular. In contrast, nitrate and orthosilicate (up to 59 and 65 mmol m −3 , respectively) distributions during autumn and winter were ascribed to dilution of Krka River nutrients along the estuary. As a consequence, the surface-layer inorganic N/P ratio was extremely high in the upper estuary (averages up to 180), but this ratio was reduced up to three times in Sibenik Bay and the coastal sea. In spring and summer, nitrate and orthosilicate, but not orthophosphate, were almost exhausted from the water because of biological utilization. In the saline layer below the halocline (depth 2–5 m) oxygen saturation varied over a large range, particularly in the upper estuary (16–176%), and nutrient concentration ratios differed from those in the surface layer. A nutrient regeneration stoichiometric model was derived, based on a linear regression analysis: AOU:Si:N:P = 276:16:6:0.4. Anthropogenic nutrient inputs should be urgently reduced to re-establish a natural nutrient environment.

The relation of nutrient regeneration in the sediments of the northern adriatic to eutrophication, with special reference to the lagoon of Venice

Abstract An understanding of the dominant processes leading to eutrophication in the northern adriatic requires an integrated research program on the nutrient fluxes within the regional lagoons and between the lagoons and the adjacent sea. The first phase of the joint research was focused on nutrient exchange at the sediment-water interface and primary production dynamics in the Venice Lagoon, in which the habitat quality is seriously compromised. The first results obtained are briefly described and the future joint research plans presented.

SPACE AND TIME VARIABILITY OF NUTRIENTS IN THE VENICE LAGOON

We examined nutrient variability in the sediments, interstitial and lagoon waters at 15 stations in the central, most-polluted part of the Venice Lagoon, with 7 cruises between February 1984 and June 1985. Strong concentration gradients were observed, decreasing from the internal border of the lagoon towards the sea. Total phosphorus and total nitrogen in the sediments ranged from 261 to 599 μg g -1 and from 200 to 1800 μg g -1 , respectively. The total inorganic nitrogen (TIN), mainly ammonium, and reactive phosphorus (RP) in the interstitial waters varied from 4 to 1800 and from 0.1 to 38 μg-at l -1 , respectively. The most marked changes, both in time and space, were found in the overlying waters (TIN from 0.7 to 106 and RP from 0.03 to 15.7 μg-at l -1 ). The higher concentrations of TIN were observed in October and February, whereas the higher RP concentrations appeared in June and October. The maximum phytoplankton biomass occurred in April and June in correspondence with lower nitrogen concentrations. External nutrient contributions, biological recycling and tidal mixing determine high nutrient concentration variability in the Venice Lagoon waters. The results suggest that nutrient release from the sediments can additionally influence this variability, particularly during warm seasons, when maximum rates were measured (up to 23.5 and 1.58 μg-at m -2 d -1 for TIN and RP, respectively).