CHAPTER 58
Dissolved Organic Carbon Seasonality in the Northern Adriatic and its
Possible Involvement in the Aggregate Formation
A. Puddu, R. Pagnotta, L. Patrolecco, M. Pettine, and A. Zoppini
ABSTRACT
Dissolved organic carbon (DOC) concentrations in northern Adriatic coastal waters show seasonal variations with increasing values from winter (75-95 pM in February) to spring-summer
periods (120-190 JlM in June 1996 and 85-123 JlM in June 1997). Riverine inputs of refractory
organic matter and abiotic transformations of in situ produced biogenic matter contribute to the
accumulation of DOC. Phosphorus limitation of bacteria, whose productivity varied widely with
time and space (0.01 to 7]J.g C 1-' h-'), probably concurs to this seasonal DOC behaviour making
microbial community unable to contrast the enrichment in DOC concentrations. Possible
involvement of DOC accumulation in the gel production in the Adriatic sea are discussed.
Introduction
Dissolved organic carbon is by far the major carbon pool in oceans, outweighing any other
marine source of organic carbon by at least a factor of 10 (Kepkay 1994). Furthermore, despite the
fact that algal biomass in oceans is only 0.2% of
that of terrestrial plants, annual phytoplankton
photosynthesis is roughly equivalent to that of
plants since algae have duplication rates on the
order of days compared to months to years for
plants (Chisholm 1992). Biogenic carbon which
is produced in the photic layer of oceans is transferred to deep and bottom waters by the biological pump consisting of sinking particles
(Thingstad and Rassoulzadegan 1995), and by
diffusion and downwelling of DOC which may
seasonally accumulate in surface waters (Carlson
et al. 1994). While this organic matter descends
to deep and bottom waters it is slowly degraded
to CO 2 , making deep oceans a massive oversaturated reservoir of CO 2 , The average C age of DOC
in deep waters is between 4,000 and 6,000 years
(Druffel et ala 1992), thus suggesting a strong
refractive character of this pool. However, even
in surface waters only a fraction of the freshly
produced biogenic carbon may be biodegradable. The magnitude of this fraction ranges from
10 to 50% of the DOC with 19% found as a mean
in a recent review (Semdergaard and Middelboe
1995). For this reason algal blooms are generally
followed by partial accumulation of the DOC
produced. and the quantity of accumulated carbon may increase significantly in the case that
phytoplankton blooms deplete nutrients necessary for bacterial activity. Thingstad et al. (1997)
have summarized surface and deep water concentrations of DOC in different areas, highlighting that concentrations differed in many cases
between surface and deep waters due to a fraction of slowly degradable DOC. The fate of carbon in oceans has a large influence on the trophic food chain and affects the exchange of carbon
at seawater-atmosphere interface. Seasonal variations in the productiOn/respiration ratio produce an annual cycle in the concentrations of
CO. and 02 in the atmosphere (Sherr and Sherr
1996). Seawater/atmosphere exchange processes
have strongly stimulated scientists interest in
studying limiting factors in the so-called high
nutrient, low chlorophyll (HNLC) central equatorial Pacific. Research efforts have been addressed
to a possible limitation by iron (Martin 1992;
Landry et al. 1997) and silicon (Dugdale and
Wilkerson 1998).
The North Adriatic sea is a small basin and
its influence on a planetary scale is limited.
However, this basin is one of the most productive
Istituto di Ricerca sulle Acque, CNR, Via Reno 1,00198 Roms, Italy
F.M. Faranda, L. Guglielmo, G. Spezie (elis)
Mediterranean Ecosystems: Structures and Processes
@ Springer-Verlag Italia 2001
Dissolved Organic Carbon Seasonality in the Northern Adriatic and its
Possible Involvement in the Aggregate Formation
A. Puddu, R. Pagnotta, L. Patrolecco, M. Pettine, and A. Zoppini
ABSTRACT
Dissolved organic carbon (DOC) concentrations in northern Adriatic coastal waters show seasonal variations with increasing values from winter (75-95 pM in February) to spring-summer
periods (120-190 JlM in June 1996 and 85-123 JlM in June 1997). Riverine inputs of refractory
organic matter and abiotic transformations of in situ produced biogenic matter contribute to the
accumulation of DOC. Phosphorus limitation of bacteria, whose productivity varied widely with
time and space (0.01 to 7]J.g C 1-' h-'), probably concurs to this seasonal DOC behaviour making
microbial community unable to contrast the enrichment in DOC concentrations. Possible
involvement of DOC accumulation in the gel production in the Adriatic sea are discussed.
Introduction
Dissolved organic carbon is by far the major carbon pool in oceans, outweighing any other
marine source of organic carbon by at least a factor of 10 (Kepkay 1994). Furthermore, despite the
fact that algal biomass in oceans is only 0.2% of
that of terrestrial plants, annual phytoplankton
photosynthesis is roughly equivalent to that of
plants since algae have duplication rates on the
order of days compared to months to years for
plants (Chisholm 1992). Biogenic carbon which
is produced in the photic layer of oceans is transferred to deep and bottom waters by the biological pump consisting of sinking particles
(Thingstad and Rassoulzadegan 1995), and by
diffusion and downwelling of DOC which may
seasonally accumulate in surface waters (Carlson
et al. 1994). While this organic matter descends
to deep and bottom waters it is slowly degraded
to CO 2 , making deep oceans a massive oversaturated reservoir of CO 2 , The average C age of DOC
in deep waters is between 4,000 and 6,000 years
(Druffel et ala 1992), thus suggesting a strong
refractive character of this pool. However, even
in surface waters only a fraction of the freshly
produced biogenic carbon may be biodegradable. The magnitude of this fraction ranges from
10 to 50% of the DOC with 19% found as a mean
in a recent review (Semdergaard and Middelboe
1995). For this reason algal blooms are generally
followed by partial accumulation of the DOC
produced. and the quantity of accumulated carbon may increase significantly in the case that
phytoplankton blooms deplete nutrients necessary for bacterial activity. Thingstad et al. (1997)
have summarized surface and deep water concentrations of DOC in different areas, highlighting that concentrations differed in many cases
between surface and deep waters due to a fraction of slowly degradable DOC. The fate of carbon in oceans has a large influence on the trophic food chain and affects the exchange of carbon
at seawater-atmosphere interface. Seasonal variations in the productiOn/respiration ratio produce an annual cycle in the concentrations of
CO. and 02 in the atmosphere (Sherr and Sherr
1996). Seawater/atmosphere exchange processes
have strongly stimulated scientists interest in
studying limiting factors in the so-called high
nutrient, low chlorophyll (HNLC) central equatorial Pacific. Research efforts have been addressed
to a possible limitation by iron (Martin 1992;
Landry et al. 1997) and silicon (Dugdale and
Wilkerson 1998).
The North Adriatic sea is a small basin and
its influence on a planetary scale is limited.
However, this basin is one of the most productive
Istituto di Ricerca sulle Acque, CNR, Via Reno 1,00198 Roms, Italy
F.M. Faranda, L. Guglielmo, G. Spezie (elis)
Mediterranean Ecosystems: Structures and Processes
@ Springer-Verlag Italia 2001