Diagenesis of carbon and nutrients and benthic exchange in sediments of the Northern Adriatic Sea

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Abstract

The characteristics of diagenesis and benthic biogeochemical cycling were studied at six stations in the Northern Adriatic during September, 1988. The objectives of this work were to quantify the mechanisms responsible for mass transport and to establish the stoichiometry of reactions involving carbon and nutrients. Stations were chosen to include sites near the Po delta that have rapid sediment accumulation, sites south of the Po delta that lie beneath its nutrient-rich plume but beyond the region of rapid accumulation, and one site further offshore in a zone with little or no modern sediment accumulation. Benthic flux measurements of oxygen, TCO2, ammonia, alkalinity, phosphate, silicate and radon were made at five of the study sites. Synthesis of this data shows that fluxes of oxygen and nutrients are similar at both high and low accumulation rate sites, but that the offshore site has fluxes that are at least two times smaller for all parameters except silica and radon. Cores were collected, and analyses of solid phases (organic carbon, nitrogen, sulfur, total and organic phosphorus, and 210Pb) and pore waters (alkalinity, pH, ammonia, silica, calcium, iron, and phosphate) were carried out. Calculations indicated that pore waters were near equilibrium or slightly supersaturated with calcite, supersaturated with apatite, and undersaturated with vivianite. Pore water profiles of TCO2 and silica were used to calculate diffusive fluxes across the sediment–water interface. Comparison of these calculated fluxes to in situ flux measurements indicated solute transport at the rapid accumulation sites is dominated by diffusive fluxes, while at the other sites about half is accomplished by irrigation; the contrast reflects the abundance of macrofauna. Rate constants for organic matter degradation were estimated and reflect the presence of fractions with mean lifetimes from a few months to several years. Diagenetic stoichiometry is dominated by degradation of organic carbon, which accounts for about 95% of the TCO2 flux. Ultimately, oxygen is the principal terminal electron acceptor, although ferric iron, nitrate and sulfate must be important intermediates. Carbonate dissolution accounts for the remaining 5% of the TCO2 flux. The average C/N ratio of degrading organic material derived from pore water profiles of TCO2 and ammonia is 5±2; this is 30–50% of the ratio measured in solid phases, demonstrating preferential degradation of compounds rich in N. However, the C/N ratio observed in flux measurements averages 11.0±1.7, suggesting that about half of the fixed nitrogen remineralized is lost as N2 during diagenesis. The TCO2/alkalinity diagenetic ratio in the anoxic pore waters was uniform at all sites and averaged 1.11±0.02 mol/equivalent, a result 30% greater than predicted for the formation of FeS2 via sulfate reduction with ferric hydroxide as the iron source. Several explanations for this difference are possible. The observed ratio is more consistent with precipitation of FeS using ferric hydroxide as an iron source; additional factors may be preferential de-carboxylation of organic matter or proton uptake by DOC during alkalinity titrations.

Introduction

The Northern Adriatic Sea (Fig. 1) has long been recognized for its value as a rich commercial fishery and as a recreation site. Recently, dystrophic episodes have had undesirable side effects along substantial portions of the western coast (Curzi and Tombolini, 1989; Marchetti, 1992). These episodes have included massive algal blooms, development of bottom water anoxia, fish kills, and appearance of gelatinous material. While the precise causes of these phenomena are not well understood, they are probably related to the large input of nutrients from the Po and other smaller rivers into the Northern Adriatic. At present, their occurrence cannot be predicted, demonstrating that our knowledge of the trophic balance of this marine ecosystem is still rather poor. In order to develop any predictive model of the ecosystem, it is necessary to understand the biogeochemical cycling of nutrients in the Adriatic, identifying reaction pathways and developing budgets for the system.

The research described here was undertaken in order to understand nutrient cycling at and beneath the sediment–water interface and to assess the contribution of sediments to the nutrient mass balance in NW Adriatic coastal waters, because sedimentary processes have a major impact on biogeochemical cycles in this rather shallow system. Earlier measurements (Hammond et al., 1984; Giordani and Hammond, 1985) had indicated that the benthic flux of recycled nutrients to the Northern Adriatic may be comparable to the input of nutrients from the Po. In situ flux data collected during 1988 confirmed this finding and a discussion of this data and sediment accumulation rates derived from 210Pb and seismic profiling has been given by Giordani et al. (1992). They developed budgets for carbon, nitrogen, phosphorus and silica reaching the sea floor. Principal conclusions were that approximately half of the primary productivity of this shallow system reaches the sea floor as particulate matter, and that about 85% of the organic carbon, 60% of the fixed nitrogen, and 85% of the silica in these particulates is recycled into the water column. The remaining organic carbon and silica are buried in sediments, while much of the remaining nitrogen could be lost through denitrification. Benthic fluxes are sufficient to account for approximately half of the oxygen depletion and nutrient enrichment seen in bottom waters during summer stratification. These budgets confirmed suggestions made by Degobbis et al. (1986)and Degobbis (1990)that extensive denitrification in sediments might account for low ratios of nitrate production to oxygen consumption observed in bottom waters. Subsequent observations in 1992–1993 (Tahey et al., 1996; Epping and Helder, 1997) have defined the seasonality of fluxes and improved the spatial resolution of flux measurements and observations of benthic fauna.

The primary purpose of this contribution is to define the stoichiometry of diagenetic reactions and the processes that are of importance in the recycling and burial of nutrients in Adriatic sediments with particular emphasis on carbon, oxygen, nitrogen, and phosphorus. Mineral stabilities and the importance of diffusion and irrigation on solute transport are evaluated. The co-variance of solutes in pore waters is utilized to deduce net reaction stoichiometries and these results are compared to those derived from solid phase composition and in situ benthic flux measurements. The approaches used here should be useful in analyzing data from systems like this one that are strongly impacted by macrofaunal activity and the observations described here are probably typical of many coastal and estuarine settings.

Section snippets

Study area

The Northern Adriatic (Fig. 1), north of a line joining Ancona and Pag, is a shallow basin (40 m average depth) receiving about 2550 m3/s of fresh water through river discharge. The majority (60%) of this input comes from the Po, which crosses an area intensively exploited by industrial and agricultural activities. The Adige, Brenta, Isonzo, and Reno rivers also contribute significant flow. The Italian rivers provide nearly all of the fresh water input and have a combined drainage area of

Methods

Replicate gravity cores (6.5 cm i.d.) were collected at each of six stations. The water overlying the cores was generally clear, suggesting minimal disturbance, although the sediment surface was often uneven. Observations of the bottom by divers confirm this is the actual appearance. Cores were kept cool until their return to the laboratory on shore. One core was used for analyses of pore water and bulk solid phase chemistry and a second for measurement of radioisotopes. The core for pore water

In situ flux measurements

Chamber deployments successfully determined radon, silica, oxygen, ammonia, nitrate, phosphate and alkalinity fluxes at five sites (Stations 1, 2, 4, 6, and 7 in Fig. 1). The fluxes measured during this study (Table 1) are similar at all stations except Station 4. Fluxes of bio-active compounds at this station are significantly lower than for the other sites, although the radon flux is similar. Station 4 is deeper than the other stations and lies beyond the primary plume of southward-flowing,

O2:C:N stoichiometry derived from chambers

Benthic chambers reflect the net stoichiometry of all reactions throughout the sediment column, although it is important to remember that depth-dependent zonation may exist for any single reaction. The relative fluxes of alkalinity, oxygen, TCO2, nitrate and ammonia measured using chambers in 1988 (Table 1) and the solid phase sulfur burial fluxes (Table 3) were used to calculate net reaction stoichiometries (Table 4) using logic described below. Additional details are in the table notes.

Conclusions

These coordinated studies of solid phases, pore waters, and in situ benthic flux measurements permit several conclusions to be reached about the solute transport processes and diagenetic reactions occurring in Adriatic sediments, and their role in recycling nutrients.

(1) Study sites may be divided into three categories: (a) northern sites near the Po Delta, characterized by high accumulation rates (about 1 g/cm2 year), low macrofaunal abundances, and relatively high (about 50%—see Giordani et

Acknowledgements

We wish to acknowledge invaluable assistance in carrying out this experiment from Dr. Giuseppe Montanari, Dr. Anna Milandri, and Dr. Attilio Rinaldi of the Marine Center in Cesenatico; and from the Captain and crew of the R.V. Daphne II, Vittorio and Dino Pagan. Prof. Ernesto Rabbi of the Universita di Bologna and Gabriella Rovatti of IGM-CNR assisted with fieldwork, analytical work, and gastronomical inspiration. Giovanni Bortoluzzi of IGM-CNR provided invaluable assistance in computer

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      Also, suspended particles in the two rivers have similar POC/TN mass ratios (ca. 14.2) while surface and sediment cores presented higher values for POC/TN ratio (up to 44.9). Even when considering a potential contribution of marine plankton (POC/TN ratio of 6.6, (Redfield et al., 1963)), the observed increase in POC/TN ratio in sediments compared to sources (terrestrial particles and/or marine planktons) could be due to the diagenetic preferential degradation of N-rich compounds (e.g., amino acids, proteins (Burdige, 2007; Hammond et al., 1999)). In fact, a previous study has shown that the DOC/TN mass ratio in porewaters at station 12 was 8.6 (r2 = 0.9, n = 275) (Dang et al., 2014a) when the POC/TN ratio observed in station-12 sediments was 18.6 (Fig. 3A).

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