Papers by Mariachiara Naldi
Estuarine Coastal and Shelf Science, Jul 1, 2020
Abstract An increasing number of coastal lagoons are being exploited for intensive farming of biv... more Abstract An increasing number of coastal lagoons are being exploited for intensive farming of bivalve filter-feeders. One of the most important farming sites in Europe for the Manila clam (Ruditapes philippinarum) is the Sacca di Goro lagoon (Po River Delta, Italy). Here, while on the one hand river runoff enhances high clam productivity, on the other hand it stimulates extensive blooms of green seaweeds of the genus Ulva. The latter can affect clams when macroalgal biomass collapses and decomposes. We hypothesized that the filtration of suspended particulate matter by farmed clams could enhance inorganic nitrogen and phosphorus recycling and simultaneously reduce phytoplankton biomass, thus facilitating macroalgal growth. An experiment simulating the effects of clams on early Ulva development was conducted over 7 days in outdoor flow-through mesocosms containing lagoon sediments with and without clams. On day 1, 3 and 7, macroalgal biomass and thallus elemental composition, water-dissolved nutrients and chlorophyll-a concentrations were recorded. On day 7, sediment-water nutrient fluxes were also measured on sediment cores sampled from each mesocosm. The presence of clams clearly stimulated Ulva growth and enhanced nutrient recycling rates. Furthermore, Ulva grown with clams had higher chlorophyll-a, nitrogen and phosphorus content. While sediments showed a net release of both soluble reactive phosphorus and ammonium when clams were present, in mesocosms without clams sediments constituted a sink for both dissolved inorganic phosphorus and nitrogen. Based on this evidence we can conclude that intensive clam farming over wide lagoon areas could significantly affect nutrient cycling, thus facilitating macroalgal blooms, with serious consequences for farming sustainability and for the functioning of the whole lagoon ecosystem.
Biogeochemistry, May 1, 2015
Organic enrichment may deeply affect benthic N cycling in macrophyte meadows, either promoting N ... more Organic enrichment may deeply affect benthic N cycling in macrophyte meadows, either promoting N loss or its recycling. This depends upon the plasticity of plants and of the associated microbial communities, as those surrounding the rhizosphere. Rates of denitrification, dissolved inorganic N fluxes and N uptake were measured in sediments vegetated by the submerged macrophyte Vallisneria spiralis L. under increasing organic matter loads. The aim was to investigate how the combined N assimilation and denitrification, which subtract N via temporary retention and permanent removal, respectively, do vary along the gradient. Results showed that V. spiralis meadows act as regulators of benthic N cycling even in organic enriched sediments, with negative feedbacks for eutrophication. A moderate organic load stimulates N uptake and denitrification coupled to nitrification in the rhizosphere. This is due to a combination of weakened competition between macrophytes and N cycling bacteria and enhanced radial oxygen loss by roots. An elevated organic enrichment affects N uptake due to hostile conditions in pore water and plant stress and impairs N mineralisation and its removal via denitrification coupled to nitrification. However, the loss of plant performance is almost completely compensated by increased denitrification of water column nitrate, resulting in a shift between the relative relevance of temporary and permanent N removal processes.
Verhandlungen, Dec 1, 1993
Verhandlungen, Mar 1, 1998
Ecological Engineering, Oct 1, 2012
ABSTRACT The effects of organic enrichment on pore water chemistry of bare and Vallisneria spiral... more ABSTRACT The effects of organic enrichment on pore water chemistry of bare and Vallisneria spiralis L. colonized sediments were investigated. Substrates of three organic levels were created by adding different amounts of powdered fish feed (0, 5 and 10g/l of sediment, respectively) to homogenized sediment and incubated with and without plants. Redox potential (Eh), reduced compounds (CH 4, Fe 2+, Mn 2+) and nutrients (PO 4 3-, NH 4 +) were analyzed at time zero and after 6, 10, 13 and 17 days. In control microcosms V. spiralis sediments displayed significantly higher Eh and lower CH 4, Fe 2+, Mn 2+, PO 4 3- and NH 4 + concentrations than bare ones. In organic enriched microcosms methanogenesis became the main degradation pathway when other electron acceptor pools were depleted. However, lower levels of interstitial Fe 2+, Mn 2+ and PO 4 3- were found in vegetated sediments compared to bare ones and this difference was maintained during the whole experimental time. Root oxygen release in the rizosphere seemed to be the main responsible of this outcome, as also suggested by the nitrification potential assay, indicating the maintenance of oxic microniches. V. spiralis can act as an engineer species in urban, organic impacted sediments due to its high tolerance against reduced conditions, which makes this macrophyte an interesting option in aquatic ecosystems restoration programs. © 2012 Elsevier B.V.
Journal of Phycology, Feb 1, 2002
Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlor... more Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) were determined by 15N accumulation in algal tissue and by disappearance of nutrient from the medium in long‐term (4–13 days) incubations. Nitrogen‐rich algae (total nitrogen> 4% dry weight [dw]) were used to detect isotope dilution by release of inorganic unlabeled N from algal thalli. Uptake of NH4+ was similar for the two macroalgae, and the highest rates were observed on the first day of incubation (45 μmol N·g dw−1·h−1 in U. fenestrata and 32 μmol N·g dw−1·h−1 in G. pacifica). A significant isotope dilution (from 10 to 7.9 atom % enrichment) occurred in U. fenestrata cultures during the first day, corresponding to a NH4+ release rate of 11 μmol N·g dw−1·h−1. Little isotope dilution occurred in the other algal cultures. Concurrently to net NH4+ uptake, we observed a transient free amino acid (FAA) release on the first day in both macroalgal cultures. The uptake rates estimated by NH4+ disappearance and 15N incorporation in algal tissue compare well (82% agreement, defined as the percentage ratio of the lower to the higher rate) at high NH4+ concentrations, provided that isotope dilution is taken into account. On average, 96% of added 15NH4+ was recovered from the medium and algal tissue at the end of the incubation. Negligible uptake of NO3− was observed during the first 2–3 days in both macroalgae. The lag of uptake may have resulted from the need for either some N deprivation (use of NO3− pools) or physiological/metabolic changes required before the uptake of NO3−. During the subsequent days, NO3− uptake rates were similar for the two macroalgae but much lower than NH4+ uptake rates (1.97–3.19 μmol N·g dw−1·h−1). Very little isotope dilution and FAA release were observed. The agreement between rates calculated with the two different methods averaged 91% in U. fenestrata and 95% in G. pacifica. Recovery of added 15NO3− was virtually complete (99%). These tracer incubations show that isotope dilution can be significant in NH4+ uptake experiments conducted with N‐rich macroalgae and that determination of 15N atom % enrichment of the dissolved NH4+ is recommended to avoid poor isotope recovery and underestimation of uptake rates.
Journal of Phycology, Feb 1, 1999
The accumulation of nitrogen in different cellular pools by the macroalgae Ulva fenestrata (Poste... more The accumulation of nitrogen in different cellular pools by the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) was studied in a laboratory experiment. After 8 or 9 days of nitrogen enrichment, nitrate, ammonium, free amino acid (FAA), protein, chlorophyll (chl), phycoerythrin (PE), and insoluble nitrogen pools were extracted and analyzed, and their relative contribution to total nitrogen (TN) was assessed. In U. fenestrata, the nitrate and ammonium enrichments resulted in a significant increase of TN from 2.41% dry weight (dw) to 4.19% and 4.71% dw, respectively. All the extracted N pools increased significantly. In G. pacifica, TN increased more under ammonium enrichment than under nitrate enrichment. In both macroalgae, proteins and FAA were the most important N storage pools. Protein‐N ranged from 700 to 2300 μmol N·g dw−1 (43%–66% of TN) and contributed the most to TN increase (41%–89%). The FAA pool was always larger in G. pacifica than in U. fenestrata. In both species, the FAA pool accounted for 4%–17% of TN (70–600 μmol N·g dw−1). In U. fenestrata, nitrate can represent a temporary storage pool: it accumulated up to 200 μmol N·g dw−1 (7% of TN) and contributed more than FAA to overall increase in cellular nitrogen. In contrast, G. pacifica had a small nitrate pool. The PE pool in G. pacifica increased with TN but was never more than 9% of total protein‐N or 6% of TN, and it was less important than FAA as a storage pool. All TN was recovered in the extracted and insoluble N pools at the end of the experiment in U. fenestrata. In G. pacifica, the extracted and insoluble N pools accounted on average for 83%–90% of TN.
Hydrobiologia, Jun 26, 2010
This study aims to investigate the effects of benthic vegetation colonization on pore water chemi... more This study aims to investigate the effects of benthic vegetation colonization on pore water chemistry in river sediments with different organic matter and nutrient loadings. Shoots of Vallisneria spiralis L., surface sediments and water were collected in March 2009 from two fluvial reaches, upstream (U) and downstream (D) an urban wastewater treatment plant. Laboratory microcosms were created with homogenised sediments and macrophytes from each sampling site, half with bare sediments (U B and D B) and half with transplanted shoots of V. spiralis (U V and D V). Microcosms were then incubated over 25 days in two tanks with water from U and D. Approximately every 4 days, three microcosms from each treatment (U B , D B , U V and D V) were terminated, and pore water was analysed for Eh, pH, O 2 , CO 2 , CH 4 , Fe 2? , PO 4 3-, NH 4 ? , NO 3 and DRSi. Significant effects of both vegetation and wastewater loadings were found. Vegetated sediments of both sites, especially U V , displayed significantly higher Eh and O 2 and significantly lower dissolved CH 4 , NH 4 ? and DRSi in pore water compared to bare sediments. At site D, despite an elevated nitrate availability, pore water NH 4 ? was the preferred N-source for V. spiralis uptake. Unvegetated sediments downstream the sewage plant (D B) exhibited the lowest redox potential and CH 4 , NH 4 ? , PO 4 3and Fe 2? accumulation in the pore water. Overall, results from this study suggest that early colonisation by rooted macrophytes affects pore water chemistry towards more oxidized conditions, along with nutrient retention within sediments, which are related to the uptake capacity and oxygen release by roots. They also suggest an elevated physiological plasticity for V. spiralis, as higher organic matter content and lower redox potential in downstream sediments did not affect its functions of ecosystem engineer as benthic metabolism regulator.
Hydrobiologia, Aug 1, 1996
Growth of the seaweed Ulva rigida C. Agardh was investigated in relation to biomass densities, in... more Growth of the seaweed Ulva rigida C. Agardh was investigated in relation to biomass densities, internal nutrient pools and external nutrient supply. Research was carried out from 23 March to 5 July 1994 in the Sacca di Goro (Po Delta, Northern Italy), whose southeastern part was covered by extensive mats of Ulva rigida. Two types of field experiments were conducted by incubating Ulva thalli inside large cages. In the first experiment, beginning on 23 March, 100 g of wet thalli were placed into the cages, allowed to grow for two weeks, then collected and replaced. This procedure was repeated 8 times over the study period. In the second experiment, Ulva thalli were left inside the cages and collected at selected time intervals (14, 27, 41, 64 and 76 days) in order to simulate the effects of increased density on growth and nutrient storage. We recorded specific growth rates (NGR) ranging from 0 .025 to 0 .081 d-' for a period up to two months in the repeated short-term experiments performed at relatively low initial algal densities (300-500 g AFDW m-3). These NGR resulted significantly related to dissolved inorganic nitrogen (DIN) in the water column. Tissue concentrations of total Kjeldahl nitrogen (TN) were almost constant, while extractable nitrate decreased in a similar manner to DIN in the water column. Total phosphorus showed considerable variation, probably linked to pulsed freshwater inflow. In the long-term incubation experiment, NGR of Ulva was inversely related to density. Internal concentrations of both total P and TN reached maximum values after one month ; thereafter P concentration remained almost constant, while TN decreased below 2% w/w (by dry weight). The TN decrease was also accompanied by an abrupt decrease in nitrate tissue concentration. The biomass incubated over the two month period suffered a progressive N limitation as shown by a decreasing N :P ratio (49 .4 to 14 .6). The reciprocal control of Ulva against biogeochemical environment and viceversa is a key factor in explaining both resource competition and successional stages in primary producer communities dominated by Ulva. However, when the biomass exceeds a critical threshold level, approximately 1 kg AFDW m-', the macroalgal community switches from active production to rapid decomposition, probably as a result of selfshading, biomass density and development of anaerobic conditions within the macroalgal beds .
Verhandlungen, Mar 1, 1998
Chemistry and Ecology, Apr 1, 2003
In this work we propose an integrated model to simulate the oxygen balance of a eutrophic lagoon ... more In this work we propose an integrated model to simulate the oxygen balance of a eutrophic lagoon exploited for mollusks farming. The balance is determined by macroalgal primary production and respiration rates plus the oxygen demand by clams and sediment. The aim is to evaluate the impact of intensive clam rearing on the vulnerability of the lagoon ecosystem to anoxic crises. The model is based on field data collected in the Sacca di Goro lagoon (Po River Delta) and has a stochastic formulation accounting for environmental unpredictability. The results show that clams have a considerable impact on the ecosystem, i.e. densities of 500 clams m À2 can cause hypoxic events (DO < 2 mgO 2 L À1) in June and September, whilst densities over 1000 clams m À2 (one half the maximum observed seeding densities) can determine a state of chronic hypoxia during the whole summer period, with minimum DO values lower than 1 mgO 2 L À1. The model provides a valuable tool for assessing the sustainability of different rearing policies.
Hydrobiologia, Nov 1, 2005
Net daily budgets of dissolved oxygen (O 2), dissolved inorganic carbon (DIC), dissolved inorgani... more Net daily budgets of dissolved oxygen (O 2), dissolved inorganic carbon (DIC), dissolved inorganic nitrogen (DIN = NH 4 + +NO 2) +NO 3)) and soluble reactive phosphorus (SRP) were determined in a pond colonised by Ulva spp. This pond received wastewater from a land-based fish farm and was used as a phytotreatment plant. Three consecutive 24-h cycles of measurements were performed with 8-14 samplings per day. Water samples were collected at the inlet and outlet of the pond and budgets were estimated from differences between inlet and outlet loadings. The first cycle was started when Ulva biomass was 8 kg m)2 , as wet weight. The second cycle was performed after the harvest of $20% of the macroalgal biomass and the third after the harvest of another $20% of the remaining biomass. Ulva removal was very fast (<1 h) and samplings for cycles 2 and 3 were started two hours after harvesting, so that the whole experiment lasted $80 h. When Ulva biomass was at its maximum, the aquatic system was heterotrophic with an O 2 demand of 519 mol d)1 and a net regeneration of DIC (2686 mol d)1), NH 4 + (49 mol d)1) and SRP (2.5 mol d)1). The DIC to O 2 ratio was an indicator of persistent anaerobic metabolism. Following the first harvest intervention, this system displayed a prompt response and shifted toward a lower O 2 demand (from)519 to)13 mol d)1), with a lesser regeneration degree of NH 4 + (11.4 mol d)1) and DIC (1066 mol d)1). After the second Ulva removal the net budget of SRP became negative ()1.0 mol d)1). By integrating these results over the three days cycle we estimated that in order to operate an efficient nutrient control and maintain macroalgal mats in a healthy status the optimal Ulva biomass should be well below $4 kg m)2 as wet weight. Above this threshold, self-limitation would render most of the algal mat unable to exploit light and nutrients. An efficient removal of nitrogen and phosphorus could be attained through the management of macroalgal biomass only with an optimisation of recipient surface to nutrient loading ratio.
Springer eBooks, 1998
Microbial processes often dominate biogeochemical cycling within ecosystems. Therefore, microbial... more Microbial processes often dominate biogeochemical cycling within ecosystems. Therefore, microbial ecology interfaces with ecosystem ecology as appropriately as plant or animal ecology does. However, it is difficult for many microbiologists to interpret ecosystem-level phenomena. Often field measurements are made separately on individual components of an ecosystem. Thus one measures a single process, such as phytoplankton primary productivity, or a single assemblage of organisms, such as benthic (i.e., referring to sediments) microalgae. The interactions of components are inferred from observations of patterns of several variables over time or space. Or one conducts experiments using a limited number of variables where inferences involve only direct interactions of a few components. It is more difficult to examine indirect influences of one component on another or to simultaneously evaluate numerous interactions. We have found that constructing and analyzing simple mathematical models can be useful in greater synthesis of microbiological and ecological information (Christian et al. 1986; Christian and Wetzel 1991). Modeling provides a mechanism by which one can extend inferences to include numerous components and both direct and indirect interactions simultaneously.
Advances in Oceanography and Limnology, Dec 8, 2010
Coastal lagoons are threatened by natural and anthropogenic factors that have been accelerated in... more Coastal lagoons are threatened by natural and anthropogenic factors that have been accelerated in the last decades, with detrimental effects on ecosystem and water quality, especially in sheltered systems. Eutrophication, macroalgal blooms, hypoxia and anoxia are among the main threats, which also impact on human activities, e.g. aquaculture and fishery. Study were performed from late 1980s to date in several Mediterranean lagoons, mainly in the Po river delta, in order to elucidate the main causes of such degenerative processes. Here we review (1) relationships between benthic vegetation and sediment biogeochemistry, with focus on factors determining eutrophication; (2) development of macroalgal blooms, with focus on causes and biogeochemical triggers of bloom onset; (3) perturbation of nitrogen cycling due to changes in benthic vegetation assemblages; (4) relationships between primary producers and benthic fauna, especially farmed species; (5) possible feedback loops among shellfish and primary producers.
Chemistry and Ecology, Apr 1, 2003
ABSTRACT In this work we tested whether macroalgal growth can be stimulated by clams filtration a... more ABSTRACT In this work we tested whether macroalgal growth can be stimulated by clams filtration activity and nutrient excretion. Thalli of Ulva rigida were grown suspended in semi-opened microcosms containing sand, in presence and absence of the clam Tapes philippinarum. Macroalgal growth and nitrogen pools were measured on subsamples collected during the experiment whilst oxygen and inorganic nitrogen fluxes were measured via short light and dark microcosm incubations. The presence of the clams stimulated significantly primary production of both macroalgae and microphytobenthos; in the light. gross O2 fluxes up to 52.5 ± 6.9 and 99.2±4.6 mmol m−2 h−1 were measured respectively in the chambers without and with clams. Daily inorganic nitrogen fluxes were mostly negative with a peak (−49.4 mmol N m−2 d−1) measured in the clam chambers. U. rigida cultured with clams had a higher growth rate and maintained a higher nitrogen content than U. rigida in bare sediment chambers.
Ecological Modelling, Jun 1, 1996
We have used ecological network analysis to compare nitrogen cycles from five well-researched coa... more We have used ecological network analysis to compare nitrogen cycles from five well-researched coastal ecosystems. These included a representative ricefield and two lagoons (Tancada and Encanysada lagoons) in the Ebro River delta, Spain; a region of the Sacca di Goro, a lagoon at the mouth of the Po River, Italy; and a drowned river estuary in North Carolina, USA, the Neuse River estuary. We constructed networks for the various systems and ranked them by trophic status (i.e., degree of eutrophication) using four indices. We then considered the importance of (1) trophic status, (2) growth form of dominant primary producer and (3) water residence time to the intensity and pattern of recycling and to the manner in which the systems can "filter" N. Three indices of flux (rate of import, primary producivity and total systems throughput) gave similar rankings of trophic status among ecosystems with the Italian and U.S. systems being most eutrophic, ricefields next, and then the two Spanish lagoons. Patterns of N export and of cycling within the systems were most closely related to the growth form of dominant primary producers. Phytoplankton, with their rapid turnover rate, foster rapid recycling within the water column and continuous transfer to sediments and export. Submersed and emergent aquatic vegetation and macroalgae create lags and pulses within systems by sequestering N during growth and releasing it during senescence, death and decomposition. Trends in cycling among systems relative to trophic status or water residence appear largely secondary to primary producer growth form.
Journal of Experimental Marine Biology and Ecology, Mar 1, 2002
The seasonal cycle of biomass and tissue composition of Ulva rigida C. Agardh, in relation to nit... more The seasonal cycle of biomass and tissue composition of Ulva rigida C. Agardh, in relation to nitrogen availability in the water column, was studied in 1991-1992 in the Sacca di Goro, a highly eutrophic lagoon in the Po River Delta (Italy). Nitrate uptake rates and storage capacity were also determined in laboratory experiments. The seasonal growth of U. rigida was related to the seasonal trend of nitrogen concentration in the water column. U. rigida biomass increased exponentially during spring and attained peaks of about 300-400 g dry mass (DM) m À 2 in June. As biomass increased, U. rigida depleted nitrate in the water column. Thallus nitrate reserves also declined from 100 mmol N (g DM) À 1 to almost undetectable levels, and total thallus nitrogen declined from 4% to 2.5% DM and 1.25% DM in 1991 and 1992, respectively. During summer, U. rigida decomposition increased, and organic nitrogen concentrations in the water column increased. The uptake experiments demonstrated an inverse relationship between thallus nitrate content and nitrate uptake rates. A modified Michaelis-Menten equation that accounts for thallus nitrate fit the uptake data well. U. rigida can accumulate up to about 400-500 mmol nitrate (g DM) À 1 in cellular reserves. U. rigida in the Sacca di Goro has higher K m and lower V max /K m ratios for nitrate uptake than other chlorophycean species, indicating a low efficiency of uptake at low nitrate concentrations. This low uptake efficiency, and the ability to exploit N availability by storing cellular nitrate pools in excess of immediate growth needs, may represent a physiological response to an eutrophic environment where nitrate is in large supply for most of the year.
The Po River watershed accounts for about 25% of the total surface and about 40% of the gross dom... more The Po River watershed accounts for about 25% of the total surface and about 40% of the gross domestic product of Italy. Agricultural, industrial and urban development, along with hydromorphological modification of rivers and canals, are responsible for water quality deterioration. Frequent and persistent summer droughts and extreme floods have occurred concurrently in the last two decades, likely as early signals of exacerbation of global change effects. In this contribution we review the evolution of hydrological regime and nutrient loadings in the last three decades, short-term studies (2003-2007) on the effects of persistent drought conditions on river discharge, nutrient loadings and stoichiometry, and salt wedge intrusion. To date, diffuse nitrate contamination has been one of the major threats. We identified and assessed possible nitrogen sources in the watersheds of four tributaries of the Po River (Parma, Mincio, Oglio and Po di Volano) with different livestock pressure, crop production and population densities.
Science of The Total Environment, Oct 1, 2018
The aim of the present study is to analyze relationships between land uses and anthropogenic pres... more The aim of the present study is to analyze relationships between land uses and anthropogenic pressures, and nutrient loadings in the Po river basin, the largest hydrographic system in Italy, together with the changes they have undergone in the last half century. Four main points are addressed: 1) spatial distribution and time evolution of land uses and associated N and P budgets; 2) long-term trajectories of the reactive N and P loadings exported from the Po river; 3) relationships between budgets and loadings; 4) brief review of relationships between N and P loadings and eutrophication in the Northern Adriatic Sea. Net Anthropogenic N (NANI) and P (NAPI) inputs, and N and P surpluses in the cropland between 1960 and 2010 were calculated. The annual loadings of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) exported by the river were calculated for the whole 1968-2016 period. N and P loadings increased from the 1960s to the 1980s, as NAPI and NANI and N and P surpluses increased. Thereafter SRP declined, while DIN remained steadily high, resulting in a notable increase of the N:P molar ratio from 47 to 100. In the same period, the Po river watershed underwent a trajectory from net autotrophy to net heterotrophy, which reflected its specialization toward livestock farming. This study also demonstrates that in a relatively short time, i.e. almost one decade, N and P sources were relocated within the watershed, due to discordant environmental policies and mismanagement on the local scale, with frequent episodes of heavy pollution. This poses key questions about the spatial scale on which problems have to be dealt with in order to harmonize policies, set sustainable management goals, restore river basins and, ultimately, protect the adjacent coastal seas from eutrophication.
Aquatic Conservation-marine and Freshwater Ecosystems, Aug 19, 2008
The succession of primary producer communities in coastal lagoons is analysed in the light of the... more The succession of primary producer communities in coastal lagoons is analysed in the light of the regime shift theory. Pristine coastal lagoons are considered to be dominated by extensive meadows of seagrass species, which are assumed to take advantage of nutrient supply from sediments. An increasing nutrient input is thought to favour phytoplankton and/or epiphytic micro-, macroalgae as well as opportunistic ephemeral macroalgae that coexist with seagrasses. In the latest stages of this succession, the imbalance of phosphorus to nitrogen ratio can favour macroalgal, cyanobacteria and/or picoplankton blooms, often causing dystrophy. (2) The primary causes of shifts and succession in the macrophyte community are nutrient loadings, mainly nitrogen, as well as changes in coastal hydrology or interactions between them. To some extent, in very shallow choked lagoons, benthic vegetation is mainly controlled by loading rates, while in open deep estuaries hydromorphological factors predominate. (3) External stressors/perturbations cause an amplification in benthic biogeochemical processes, e.g. wide variations in primary productivity and dark respiration, with large oscillations in oxygen and sulphide concentrations. Altered biogeochemical processes can determine positive feedbacks inducing a shift from pristine to altered macrophyte communities, which in turn amplify the perturbation until the shift becomes irreversible. (4) Macrophyte typology, organic matter composition and sedimentary geochemistry are primary factors in controlling feedbacks and shifts. For example, the sedimentary buffering capacity of iron controls sulphide and phosphates, while nitrogen cycling is mainly controlled by primary producers-microbial process interactions. (5) The alternative states which occur through the transition from pristine to modified primary producer communities can also be viewed as a sequence of stable states with different degrees of embedded information and with different ecological functions.
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Papers by Mariachiara Naldi