In 2007, the Rhodophyceae Alsidium corallinum C. Ag., a marine taxon, bloomed in the eutrophic lagoon of
Orbetello (Tuscany, Italy) for the first time, becoming the dominant species in spring and summer. In
November, its biomass collapsed. The hypothesis examined in this study is that the bloom expressed a relatively
low eutrophic level of the ecosystem after intense disposal of accumulated sedimentary organic
matter (OM) by dystrophic processes in the two years preceding the bloom. To verify the hypothesis, we
compared water physical–chemical variables, sediment redox (Eh) and OM, and standing crops of macroalgae
and seagrass from the database of routine monitoring between 2005 and 2008.Wealso used dissolved
nutrient data obtained in 2007 and 2008, as well as data on chlorophyll and total suspended matter in the
water column during the microalgal bloom of 2007, and C, N and P content in thalli of the Chlorophycea
Chaetomorpha linum and the Rhodophyceae Gracilariopsis longissima and A. corallinum obtained in 2007.
In 2007, unusually low values of dissolved inorganic nitrogen (DIN) were recorded. Combined with stable
values of soluble reactive phosphorus (SRPs), low DIN led to a reduction of about one order of magnitude in
the DIN:SRP atomic ratio with respect to the past and to 2008. G. longissima accumulated C, N and P more
than the other species and A. corallinum proved to be less demanding. Sediment OM was lower in the
autumn of years characterized by dystrophy, confirming that summer dystrophic events coincided with
maximum energy dissipation in this ecosystem. However, as soon as OM and DIN values increased
(2008), the vegetation shifted towards blooms of G. longissima and C. linum, while A. corallinum almost
disappeared. The results sustain the hypothesis that the bloom of A. corallinum was due to a decline in
DIN that limited G. longissima, and to intense turbidity of the water caused by microphytes that developed
after the dystrophic event of summer 2006. The latter probably limited the development of C. linum, which
could only develop at the edges of the lagoon.
