Daniele Nizzoli

Impact of Tapes philippinarum farming on nutrient dynamics and benthic respiration in the Sacca di Goro

AbstractThe introduction of the short-necked clam Tapes philippinarum into the Sacca di Goro has over a short period made this coastal environment one of the top European clam production sites. In recent years, this activity has been seriously impacted due to the appearance in the lagoon of large macroalgal beds and the occurrence of dystrophic events causing anoxia and massive deaths of molluscs in the cultivated areas. Tapes cultivation sites now cover more than one third of the lagoon surface at densities sometimes attaining 2000–2500 adult individuals m−2; such densities and the harvesting methods, based on sediment dredging, probably have a strong impact on the benthic system. Whilst a number of studies have reported water–sediment interface induced modifications due to oyster or mussel farming there have been few attempts to quantify how clam farming affects biogeochemical cycles of oxygen and nutrients, in particular in the Sacca di Goro. Two areas, a farmed and a control one, were compared for benthic fluxes and results were correlated with clam biomass. Oxygen, carbon dioxide, ammonium, reactive silica and phosphorus fluxes were stimulated several fold by the presence of Tapes due to the clams, respiration and excretion activities, but also to the reducing conditions in the surface sediments. On average, the whole lagoon dark sediment O2 demand and CO2 production were stimulated by a factor of, respectively, 1.8 and 3.3, whilst nutrient release was 6.5 times higher for NH4+ and 4.6 times higher for PO43-. Our results indicate that clam farmers should carefully consider sustainable densities of Tapes in order to prevent the risk of sediment and water anoxia. Rapid nutrient recycling (up to 4000 μmol NH4+ m−2 h−1 and 150 μmol PO43- m−2 h−1) stimulated by the high biodegradability of clam faeces and pseudofaeces could in turn favour macroalgal growth.

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.

Microphytobenthos activity and fluxes at the sediment-water interface: interactions and spatial variability

In this study oxygen and nutrient fluxes and denitrification rates across the sediment-water interface were measured via intact core incubations with a twofold aim: show whether microphytobenthos activity affects these processes and analyse the dispersion of replicate measurements. Eighteen intact sediment cores (i.d. 8 cm) were randomly sampled from a shallow microtidal brackish pond at Tjarno, on the west coast of Sweden, and were incubated in light and in darkness simulating in situ conditions. During incubation O2, inorganic N and SiO2 fluxes and denitrification rates (isotope pairing) were measured. Assuming mean values of 18 cores as best estimate of true average (BEA), the accuracy of O2, NH4+, NO3- and SiO2 fluxes calculated for an increasing number of subsamples was tested. At the investigated site, microalgae strongly influenced benthic O2, inorganic N and SiO2 fluxes and coupled (Dn) and uncoupled (Dw) denitrification through their photosynthetic activity. In the shift between dark and light conditions NH4+ and SiO2 effluxes (60 and 110 µmol m-2h-1) and Dn (5 µmol m-2 h-1) dropped to zero, NO3- uptake (70 µmol m-2 h-1) showed a 30% increase, while Dw (20 µmol m-2 h-1) showed an 80% decrease. For O2 and NO3- dark fluxes, 4 core replicates were sufficient to obtain averages within 5-10% of the best estimated mean, while 10-20% accuracy was obtained with 4-12 replicates for SiO2 and >10 replicates for NH4+ dark fluxes. Mean accuracy was considerably lower for all light incubations, probably due to the patchy distribution of the benthic microalgal community.

Short-term influence of recolonisation by the polycheate worm [2pt] Nereis succinea on oxygen and nitrogen fluxes and [2pt] denitrification: a microcosm simulation

The short-term effects of sediment recolonisation by Nereis succinea on sediment-water column fluxes of oxygen and dissolved inorganic nitrogen, and rates of denitrification, were studied in microcosms of homogenised, sieved sediments. The added worms enhanced oxygen uptake by the sediments, due to the increased surface area provided by the burrow walls and the degree of stimulation was stable with time. Similarly, ammonium fluxes to the water column were stimulated by N. succinea, but declined over the 3 day incubation in all microcosms including the controls. Nitrate fluxes were generally greater in the faunated microcosms, but highly variable with time. Denitrification rates were positively stimulated by N. succinea populations, denitrification of water column nitrate was stimulated 10-fold in comparison to denitrification coupled to nitrification in the sediments. Rates of denitrification of water column nitrate were not significantly different from rates in undisturbed sediment cores with similar densities of N. succinea, whereas rates of coupled nitrification–denitrification were 3-fold lower in the experimental set-up. These results may reflect the relative growth rates of nitrifying and denitrifying bacteria, which allow more rapid colonisation of new burrow surfaces by denitrifier compared to nitrifier populations. The data indicate that recolonisation by burrowing macrofauna of the highly reduced sediments of the Sacca di Goro, Lagoon, Italy, following the annual dystrophic crisis, may play a significant role in the reoxidation and detoxification of the sediments. The increased rates of denitrification associated with the worm burrows, may promote nitrogen losses, but due to the low capacity of nitrifying bacteria to colonise the new burrow structures, these losses would be highly dependent upon water column nitrate concentrations.

Impact of a trout farm on the water quality of an Apennine creek from daily budgets of nutrients

A detailed 24-h investigation in August 2005 evaluated net dissolved and particulate nutrient budgets in a small trout farm located in the Parma Apennines. During the monitoring period, due to water shortage, the Cedra Creek was almost entirely diverted into the farm; the water flow was 190 l s−1, and the fish standing stock about 20 t. Inflow and outflow waters were characterized for dissolved gases (O2 and CO2) and dissolved and particulate inorganic nutrients ( , , , , PN, and PP). Solute concentrations in outflowing waters were found to fluctuate considerably during the day, due to fish metabolic activity and farm-management practices. Despite the small amount of feed supplied to the fish (75 kg d−1) due to high water temperatures (∼20 °C) and the high feed conversion factor (∼1.2), the farm released net amounts of 2.20 and 0.76 kg d−1 of nitrogen and phosphorus, respectively, to the Cedra Creek. Of the nutrients produced, 68% of the nitrogen was as , while 67% of the phosphorus was particulate. Significantly different , , and PP concentrations were measured 500 m downstream of the fish farm compared with inflowing water. This study supports the hypothesis that the ecological quality of creeks or streams receiving fish farm effluents can be seriously affected due to fine particle sedimentation, interstice clogging, simplification of benthic macrofauna communities, and stimulation of microfitobenthos growth.

Oxygen and ammonium dynamics during a farming cycle of the bivalve Tapes philippinarum

Fluxes of dissolved oxygen and ammonium across the water sediment interface were measured in a control and in an experimental area farmed with the clam Tapes philippinarum. Young clams were seeded in March 2003 at mean (~500 ind m−2) and high (~1500 ind m−2) densities in a sandy area (2100 m2) of the Sacca di Goro Lagoon, Italy. Approximately every two months, until October 2003, intact sediment cores were collected and incubated in the light and in the dark and surface sediments (0–2 cm) were analysed for organic matter and nitrogen content. Clams farming induced pronounced changes in sediment characteristics and metabolism. Oxygen consumption and ammonium production at the high density area were, on average, 3 to 4 and 1.9 to 4.9 folds higher than those measured in the control field respectively; rates were positively correlated with clams biomass. Experimental fields resulted “Net and Total Heterotrophyc” in 3 out of 4 sampling dates and clams were the major factor shifting the benthic system towards this status. In only one occasion the appearance of the macroalgae Ulva spp. pushed the system rapidly towards hyperautotrophic conditions. Our results indicated that clams have the potential to drive benthic metabolism in farmed areas and to sustain macroalgal growth through regeneration of a limiting nutrient for seawater as inorganic N.

Assessing the Potential Impact of Clam Rearing in Dystrophic Lagoons: An Integrated Oxygen Balance

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 maeroalgal 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−2 can cause hypoxic events (DO < 2mgO2 L−1) in June and September, whilst densities over 1000 clamsm-2 (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 mgO2 L−1. The model provides a valuable tool for assessing the sustainability of different rearing policies.

Influence of Clam Farming on Macroalgal Growth: A Microcosm Experiment

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.

Short term changes of benthic fluxes during clam harvesting in a coastal lagoon (Sacca di Goro, Po River Delta)

In this paper the relationships between the Manila clam (Ruditapes philippinarum) fanning and benthic fluxes in the eutrophic Sacca di Goro lagoon (Po River Delta, Italy) are studied considering (1) the analysis of spatial patterns of sedimentary organic matter, acid volatile sulphide (AVS), chromium reducible sulphur (CRS) and labile amorphous iron in a grid of nine stations; (2) an assessment of the short-term oxygen consumption caused by sediment resuspension in oxygen-rich water; (3) a mesocosm experiment for evaluating the potential effects of sediment dredging on benthic fluxes of oxygen, soluble reactive phosphorus (SRP), ammonium, nitrate and dissolved reactive silica (DRSi). Our data suggest that in the farming area benthic fluxes of oxygen, ammonium and silica are driven mainly by clam respiration, whilst harvesting causes only transient modification of benthic fluxes. Therefore, we expect that clam metabolism is responsible for a persistent impact at the lagoon scale, whilst harvesting activities induce local and acute effects.

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.