Climatic Changes Shift Macroalgal Assemblages from Cold- to Warm-Adapted Species: The Venice Lagoon as a Study Case

Temperature increase is one of the main effects of climate change occurring worldwide, with drastic impacts on both terrestrial and aquatic biota. Changes in the dominant macroalgal taxa in the Venice Lagoon have been analyzed in relation to the rise in air temperature recorded since 1973, highlighting the significant decline in cold-adapted species, which have been replaced by taxa more tolerant of higher temperatures. Cold-adapted species such as the native Fucus virsoides, Punctaria latifolia, Scytosiphon lomentaria, and many other Phaeophyceae are in decline, whereas thermophilic species such as the non-indigenous species (NIS) Gracilaria vermiculophylla, Agardhiella subulata, Solieria filiformis, Hypnea cervicornis, Caulacanthus okamurae, and many others have replaced the species that once dominated the lagoon. These changes have been associated with an average air temperature increase of approximately 2.5 °C. The highest increase has mostly been recorded for average minimum temperatures (+2.8 °C), compared to average maximum temperatures (+2.0 °C). As a result, Phaeophyceae have declined, while Rhodophyceae, especially recent NIS introductions, have colonized the lagoon bottoms. Changes in Chlorophyceae, on the other hand, appear to be more linked to the reduction of the lagoon’s trophic conditions, although the currently dominant species is Ulva australis, a NIS that has replaced the native Ulva rigida almost everywhere.