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