Evolution of the Trophic Conditions and Dystrophic Outbreaks in the Sacca di Goro Lagoon (Northern Adriatic Sea)
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°C. The biomass increases at high rates (0.10–0.25 d−1) and reaches the highest standing crop (103 g dw m−2) and the maximum spreading (10 km2) 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 mats, 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.
KeywordsAcid Volatile Sulphide Sulphate Reduction Rate Soluble Reactive Phosphorus Concentration Benthic Flux Macroalgal Biomass
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