RMZ - Materials and Geoenvironment, Vol. 52, No. 1, 17-20, 2005
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Diel metabolism and nutrient cycling in the northern Adriatic lagoonary sediments
Stefano Covelli1, Jadran Faganeli2, Cinzia De Vittor3, Raffaela Piani1, Nives Ogrinc4
'Dept. of Geol., Environ. & Mar. Sciences, University of Trieste, Trieste, Italy; covelli@univ.trieste.it, piani@univ.trieste.it PMarine Biological Station, National Institute of Biology, Piran, Slovenia; jadran.faganeli@uni-lj.si laboratory of Marine Biology, Trieste, Italy; devittor@univ.trieste.it RDept. Environ. Sci., "Jozef Stefan" Institute, Ljubljana, Slovenia; nives.ogrinc@ijs.si
Abstract: Metabolism and nutrient fluxes were studied in the Lagoon of Grado (Gulf of Trieste, N Adriatic) using in situ deployed clear benthic chambers. It was found that there were several processes influencing the biogeochemistry of this environment including various microbial processes, degradation of organic matter, carbonate dissolution and precipitation and exchange between pore water and overlaying water.
Key words: lagoon, incubation experiment, nutrient fluxes, metabolism
Introduction
Lagoonary systems, areas of great economic importance and subjected to pollution, are characterized by intense benthic primary production. The aim of this study is to present the metabolism and nutrient fluxes found in the Lagoon of Grado (The Gulf of Trieste, N Adriatic) using in situ deployed light benthic chambers. Field experiments were performed in June and September/October 2003, and March 2004 at stations Artalina (ART) at depth of 1.5-1.8 m and Barbana (BAR) at depth of 0.5-1.5 m. The surficial sediments are composed of sand (10%) and pellites (90%) and are bioturbated mainly by poly-chaetes. The carbonate content averages about 50%. The sediment is inhabited by extensive beds of marine diatoms and at station BAR also by some patches of macroalgae Gracilaria. The chamber consists of a 30x50x30 cm Plexiglass box, which is open
at the bottom, with valves on the lid for sampling and a flexible polyethylene bag to compensate for the volume removed during sampling. Attached to the inside of the chamber lid is a stirring mechanism coupled to a motor driven at a speed of 50 rpm. After 2, 3 and 12 hours samples were collected by divers. In addition, virtually undisturbed sediment cores were collected by divers inserting a Plexiglass tube (12.5 cm i.d.) directly into the sediment. The sediment was extruded and sectioned into 1 and 2 cm intervals in a N2-filled glove bag, and pore water extracted by centrifugation. The water and pore water samples, except for oxygen, were filtered through 0.45 mm Millipore HA membrane filters. Diel fluxes of solutes across the sediment-water interface were evaluated from the significant differences between extremes and by integrating the area under rate of chang-
Short scientific paper
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Covelli, S. et al.
ing diel curves. Nitrate, ammonium, phosphate, dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) were determined using standard procedures (Grasshoff et al., 1983). Oxygen was determined by the Winkler method (Grasshoff et al., 1983). Alkalinity was measured by titration with HCl, and dissolved inorganic carbon (DIC) using a Shimadzu TOC 5000A analyzer. The HCl acidified samples were analyzed for dissolved organic carbon (DOC) using a high temperature catalytic oxidation (Shimadzu TOC 5000A) analyzer. The isoto-pic composition of DIC (513Cdic) was analyzed in COP liberated from water samples after acidification, using a Europa 20-20 mass spectrometer.
Results and discussion
Pore waters and solid phase
A general increase in concentrations with sediment depth were observed for DIC, NH4, and P043", while for DOC, DON and DOP an irregular depth pattern was observed. The inorganic N/P ratios were mostly >20 (atomic). Steep near-surface gradients were observed for all solutes. Higher concentra-
tions of DIC, NH;
po43-:
DON and DOP,
and DOC in autumn were observed at station ART compared to station BAR. The d13CDIC values show a decreasing trend with depth and lower values were always found at station ART influenced by more pronounced degradation of organic matter. Sedimentary Cor levels were also higher at station ART (2.7-1.8 %) compared to BAR (approx. 1.2 %).
Benthic processes
Substantial diel patterns of dissolved 0P, DIC, NH4c, N03", P043", DOC, DON and DOP were observed during the series of measurements at both stations except in September at station ART where a temporal decrease of OP and an increase of DIC and NH4+ concentrations appeared. Normally, inverse correlations appeared between diel OP and DIC changes. An inverse correlation was also observed between diel fluctuations of NH4+ concentrations and OP fluctuations. The lowest NH4c concentrations appeared during the photoperiod despite the fact that benthic micro- and macroalgae may show a reduced but still substantial uptake of ammonia at night (Nixon et al., 1976). Ammonia is suggested to be an active inorganic nutrient in N as-
Table I. Apparent daytime benthic community production during incubation experiments at stations Artalina (ART) and Barbana (BAR).
Date	Station	Temperature	Production	Production
		(°C)	(mM 02 m"2d"')	(mM C m"2d_1)
June	ART	26-30	8	13
2003	BAR	26-29	40	35
Sept	ART	20-23	12	55*
2003	BAR	18-19	13	75**
March	ART	2-4	17	28
2004	BAR	4-9	29	16
C0Ig mineralization; ** carbonate dissolution
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Diel metabolism and nutrient cycling in the northern Adriatic lagoonary sediments
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similation processes and supported high biomass in the lagoon indicating a highly efficient recycling of N within the lagoon community. Generally, the high inorganic N/P ratios observed suggest that an excess of N was present and that the whole lagoon eco-
system is most probably P-limited. Nutrient regeneration at the sediment-water interface and in surficial sediment is the primary source of nutrients available for assimilation processes, especially in the warmer period of the year (June, September) when the natu-
5 10 15 20 25 Time [hours]
10 15 20 25 Time [hours]
6
13.0, 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0-
0
10 15 20 Time [hours]
25 30
8.0, 7.57.0 6.5 6.0 5.55.0 4.5 4.a

0
10 15 20 Time [hours]
25 30
Figure I. Example of the diel cycle of Op, DIC, DOC, SI3Cdic, Ca, NHR+ concentrations at station BAR in June 2003.
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Covelli, S. et al.
ral nutrient input from fresh water inflows is limited. The N03", P043", DOC, DON and DOP variations were apparently not related to changes of 02 and DIC. The decoupling between 02-DIC concentrations and the nutrient results indicate that the benthic primary producers are capable of storing nutrients or that other un-measured nutrient pools are supporting primary production.
The apparent community production measured from DIC concentration differences were higher than the 02-based production estimations (Table I) in September probably due to intense mineralization, decoded by lower 813Cdic values averaging -6.7 ± 0.3 %%, at station ART and carbonate dissolution, 513Cdic values approaching 0 %, at station BAR. This trend was also confirmed by pore water 513Cdic values. In June at station BAR, the DIC mostly originated from Corg mineralization while in other periods both the carbonate dissolution and C mineralization
org
were important. However, no attempt was
made to differentiate between micro- and macroalgae production at BAR and we consider the benthic microalgae as the most important primary producer.
In conclusion, the Grado lagoonary system is characterized by the coupling of metabolism and mineralization. This lagoon environment accumulates rather high nutrient concentrations through benthic micro- and also macroalgae assimilation and decomposition. These benthic biogeochemical processes are probably also important for trace metal cycling, especially Hg mobilization and sequestration in the studied area.
References
Grasshoff, P.N., Ehrhardt, M., Kremling, K. (1983): Methods of Seawater Analysis, Verlag Chemie, Weiheim.
Nixon, S.W., Oviatt, C.A., Garber, J., Lee, V. (1976): Diel metabolism and nutrient dynamics in a salt marsh embayment, Ecology 57, 740-750.
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