CHAPTER 59
Evolution of the Trophic Conditions and Dystrophic Outbreaks
in the Sacca di Goro Lagoon (Northern Adriatic Sea)
P. Viaroli, R. Azzoni, M. Bartoli, G. Giordani, and L. Taje
ABSTRACT
Dystrophy and vulnerability have been analysed in the Sacca di Goro lagoon (Po river Delta,
Italy), where long term research was started in 1989 with respect to the abnormal growth and
decomposition of the seaweed Ulva rigida. The research addresses: (1) trends in hydrochemical
variables; (2) biomass, growth rates and the elemental composition of the dominant macroalgae; (3) nutrient -uptake and retention within the macroalgal biomass; (4) oxygen metabolism;
(5) decomposition processes and their effects on benthic fluxes of nitrogen, phosphorus and
sulphide; (6) iron buffering of the phosphorus and sulphur cycles.
In the last decade, annual patterns have been observed with the abnormal spring growth of
the seaweed Ulva rigida,which is usually followed by a sudden collapse and a prolonged oxygen
deficiency in early summer. Nitrates are the most important nitrogen source for the macroalgae.
Soluble reactive phosphorus concentrations attain significant peaks only during summer anoxia. In the worst years (1989-92 and 1997), Ulva growth commences in early Spring at temperatures above 10
0
e. The biomass increases at high rates (0.10-0.25 d- 1 ) and reaches the highest
standing crop 00 3 g dw m- 2 } and the maximum spreading (10 km 2 ) at the end of May. From mid
June, the macroalgal biomass starts to decompose causing anoxia and sulphide production.The
biomass accumulation results in the temporary nitrogen retention within the organic pool leading to a prolonged nitrogen deficiency in the water column which allows only Ulva to grow. The
impact of the dystrophy is related to the sedimentary cycles of sulphur and iron.
Microbiologically reducible iron seems to buffer against free sulphide, precipitating it as insoluble iron monosulphide and pyrite. However, a considerable production of sulphide may occur in
the water column associated with the decomposition of the thick floating macroalgal matsl
where its concentration is independent of the potential iron buffering capacity of the sediment.
In conclusion, the vulnerability (or buffer capacity) of such coastal systems seems to depend on
the extent of the macroalgal blooms, amplitude of dissolved oxygen fluctuations at different
time scales (from days to weeks), and sedimentary iron availability.
Introduction
In coastal areas, the development of large and fast
growing primary producers plays a key role in
oxygen production and consumption, and regulation of the entire benthic metabolism (Borum
1996; Castel et al. 1996). Usually. fast growing
macroalgae exhibit nutrient uptake and storage
capacities greatly in excess of their physiological
needs. There is also experimental evidence of a
strict relationship between growth and decomposition rates: fast growing plants tend to have high
nutrient concentrations and low fibre content and
therefore also decompose more rapidly because
their high N/C and PIC ratios stimulate microbial
growth. Fast macro algal growth followed by
decomposition of the accumulated biomass is also
considered to be one of the most important factors in the occurrence of dystrophic crises. Hence,
growth patterns and distribution of macroalgae
may explain the changes which occur in other
processes within coastal ecosystems (for detailed
reviews see Sand-Jensen and Borum 1991; Borum
1996; Flechter 1996; Valiela et aL 1997).
Dipartirnento di Sdenze Ambientali, UniversitA degli Studi di Parma, Parco Area delle Scienze 33/A, 43100 Parma, Italy
F.M. Faranda, L. Guglielmo, G. Spezie (eds)
Mediterranean Ecos}'lItems: Structures and Processes
@ Springer-Verlag Italia 2001
Evolution of the Trophic Conditions and Dystrophic Outbreaks
in the Sacca di Goro Lagoon (Northern Adriatic Sea)
P. Viaroli, R. Azzoni, M. Bartoli, G. Giordani, and L. Taje
ABSTRACT
Dystrophy and vulnerability have been analysed in the Sacca di Goro lagoon (Po river Delta,
Italy), where long term research was started in 1989 with respect to the abnormal growth and
decomposition of the seaweed Ulva rigida. The research addresses: (1) trends in hydrochemical
variables; (2) biomass, growth rates and the elemental composition of the dominant macroalgae; (3) nutrient -uptake and retention within the macroalgal biomass; (4) oxygen metabolism;
(5) decomposition processes and their effects on benthic fluxes of nitrogen, phosphorus and
sulphide; (6) iron buffering of the phosphorus and sulphur cycles.
In the last decade, annual patterns have been observed with the abnormal spring growth of
the seaweed Ulva rigida,which is usually followed by a sudden collapse and a prolonged oxygen
deficiency in early summer. Nitrates are the most important nitrogen source for the macroalgae.
Soluble reactive phosphorus concentrations attain significant peaks only during summer anoxia. In the worst years (1989-92 and 1997), Ulva growth commences in early Spring at temperatures above 10
0
e. The biomass increases at high rates (0.10-0.25 d- 1 ) and reaches the highest
standing crop 00 3 g dw m- 2 } and the maximum spreading (10 km 2 ) at the end of May. From mid
June, the macroalgal biomass starts to decompose causing anoxia and sulphide production.The
biomass accumulation results in the temporary nitrogen retention within the organic pool leading to a prolonged nitrogen deficiency in the water column which allows only Ulva to grow. The
impact of the dystrophy is related to the sedimentary cycles of sulphur and iron.
Microbiologically reducible iron seems to buffer against free sulphide, precipitating it as insoluble iron monosulphide and pyrite. However, a considerable production of sulphide may occur in
the water column associated with the decomposition of the thick floating macroalgal matsl
where its concentration is independent of the potential iron buffering capacity of the sediment.
In conclusion, the vulnerability (or buffer capacity) of such coastal systems seems to depend on
the extent of the macroalgal blooms, amplitude of dissolved oxygen fluctuations at different
time scales (from days to weeks), and sedimentary iron availability.
Introduction
In coastal areas, the development of large and fast
growing primary producers plays a key role in
oxygen production and consumption, and regulation of the entire benthic metabolism (Borum
1996; Castel et al. 1996). Usually. fast growing
macroalgae exhibit nutrient uptake and storage
capacities greatly in excess of their physiological
needs. There is also experimental evidence of a
strict relationship between growth and decomposition rates: fast growing plants tend to have high
nutrient concentrations and low fibre content and
therefore also decompose more rapidly because
their high N/C and PIC ratios stimulate microbial
growth. Fast macro algal growth followed by
decomposition of the accumulated biomass is also
considered to be one of the most important factors in the occurrence of dystrophic crises. Hence,
growth patterns and distribution of macroalgae
may explain the changes which occur in other
processes within coastal ecosystems (for detailed
reviews see Sand-Jensen and Borum 1991; Borum
1996; Flechter 1996; Valiela et aL 1997).
Dipartirnento di Sdenze Ambientali, UniversitA degli Studi di Parma, Parco Area delle Scienze 33/A, 43100 Parma, Italy
F.M. Faranda, L. Guglielmo, G. Spezie (eds)
Mediterranean Ecos}'lItems: Structures and Processes
@ Springer-Verlag Italia 2001