Living coralligenous as geo-historical structure built by coralline algae

Abstract

The most important reef of the Mediterranean is the Coralligène (Coralligenous = C), including several types of calcareous algal-invertebrate build-ups growing in normal open marine conditions. We analyzed and compared two C samples from the Ligurian Sea developed in different environmental settings: 1) off Portofino on a rocky cliff, at a depth of about 40 m and 2) in front of Bogliasco, on a sub-horizontal substrate at a depth of 10 m. The maximum AMS radiocarbon dating provided an older age for Bogliasco (about 5 ka BP) than for Portofino (about 3.6 ka BP), and the mean accumulation rate of the Portofino build-up (about 80 µm y−1) was found to be higher than the one in Bogliasco (about 65 µm y−1). The different sides of each build-up showed a remarkable heterogeneity in the dominant cover by living organisms, and the comparison between the two build-ups highlighted an evident diversity in their taxonomic composition and structure, although crustose coralline algae (CCA) are the dominant framework builder and major autogenic ecosystem engineers at both localities, in the present as in the past millennia. Other major components of the structure are bryozoans and serpulids, and an important role is played by sediment filling. In Bogliasco, extreme climate events and major peaks of fine matrix and terrigenous grains are observed, lithologically related to the drainage basin of the Poggio creek and associated with charophyte occurrence and reduced CCA abundance. The occurrence of the rare Sporolithon ptychoides was observed both in Portofino at about 750 BCE and in Bogliasco. These Sporolithon phases are likely related to warm and humid spells punctuating the Holocene climate fluctuations in the Ligurian Sea. Because coralline algae are confirmed to be the most important habitat engineer of the Mediterranean reefs, they deserve more attention in the framework of any monitoring initiative aimed at C management and conservation.

Habit and Behaviour of the Nestling Bivalve Gregariella semigranata (Reeve, 1858) from the Mediterranean Coralligenous

“…Carbonate concretions formed by in situ reef-building, crustose coralline algae, known as Coralligenous, constitute important bioconstructions that grow in dim light conditions in the Mediterranean midshelf. Coralligenous is the result of a process of framework building by superimposition of several generations of calcareous algae and other sessile organisms (Basso et al, 2022, and references therein). It shows high biodiversity and represents the habitat for several organisms that compete for the colonization of this substratum (Pérès & Picard, 1964; Hong, 1982; Pérès, 1982; Bellan-Santini et al, 1994; Ballesteros, 2006).…”

Halimeda tuna (Bryopsidales, Ulvophyceae) calcification on the depth transect in the northern Adriatic Sea; carbonate production on the microscale of individual segments

“…Assemblages of H. tuna thrive in relatively shallow waters. Halimeda tuna may also occur in the Mediterranean Coralligenous formations, a complex of predominantly calcareous-invertebrate biocenoses with important concretion potential, so Halimeda fragments may enter the sedimentary record ( Basso et al, 2022 ). In the northern Adriatic, it is also among the ten dominant seaweeds in terms of abundance in spring and autumn samples collected at reference sites for macroalgae in the upper-infralittoral belt ( Orlando-Bonaca & Rotter, 2018 ).…”