Growth tests of Gongolaria barbata (Ochrophyta Sargassaceae), a native species producing pleustophitic blooms in a hypertrophic Mediterranean lagoon

Abstract

In the Orbetello lagoon (Italy), the Ochrophyta Gongolaria barbata has shown in recent years a tendency to form vegetative blooms in the winter-spring period, with decay in midsummer. Growth tests were conducted in the laboratory and the field to understand the reasons for these unexpectedly opportunistic characteristics of this species. In the laboratory, different salinities and concentrations of P and N and N/P molar ratios were tested using nitrate and ammonium nitrogen. In the field, tests were carried out in three lagoon areas with different macroalgal development scenarios dominated by the Chlorophyta Chaetomorpha linum, analysing the water and using the physico-chemical data provided continuously by two stations equipped with multi-parameter probes. Laboratory tests showed: Ochrophyta's ability to grow in oligo-, meso- and hypertrophic conditions, both for N and P, even for very unbalanced N/P molar ratios, although intolerance to ammonium nitrogen-dominant conditions was observed; wide tolerance to salinity; increased growth for temperatures between 26°C and 30°C. In the lagoon, tests showed increased growth for strongly nitrate-nitrogen-dominated hypertrophic conditions, in areas with dense outcropping mats of C. linum, where G. barbata overgrew. SGRs were not elevated either in the laboratory or the lagoon, compared to those of C. linum. SRG values ​​were significantly higher in the West basin than in the East basin. The hypothesis we draw from the results is that the species, previously confined with modest biomass development in oligo-mesotrophic areas close to the sea mouth, was able to develop in bloom when the lagoon reached hypertrophic conditions, i.e. when nutrient availability became unlimited. In the past, it probably could not compete for the resources with opportunistic and nitrophilous species with high nutrient uptake capacities. G. barbata develops, in fact, above all in the over-layer of dense Chlorophyta mats, which by creating anoxic conditions in the substratum allow substantial nutrient releases that not only support the mat itself but also the development of the Ochrophyta. In winter-spring, ammonium releases are oxidised to nitrate in the pathway from the bottom to the surface of the water column, but the nitrification process fails in summer and its mats decay as a result of the increased summer decay of Chlorophyta mats, which results in the dominance of ammonia nitrogen and releases of hydrogen sulphide.