Mediterranean shallow-water soft bottoms are characterized by extensive meadows of the endemic seagrass Posidonia oceanica (L.) Delile that support abundant benthic biota including numerous epiphytic foraminiferal taxa. The biomass of the epiphytic communities varies with the P. oceanica cycle, especially influencing those taxa with higher abundances in summer, when the foliar surface is maximum. During the past decades exotic macrophyte species have invaded habitats formerly dominated by P. oceanica. Two of these taxa are the green algae Caulerpa taxifolia (Vahl) Agardh, 1817 and C. racemosa (Forsskål) Agardh, 1873, that, along with the non-invasive C. prolifera (Forsskål) Lamouroux, 1809, produce defensive, secondary metabolites such as caulerpenyne that affects turnover rates of P. oceanica leaves. As a consequence of different architectural features of the algal substrate, replacement of P. oceanica by Caulerpa spp. results in the change from a complex three-dimensional, long duration substrate into a simpler, two-dimensional one with a shorter life span. Epiphytic foraminifers can be clustered into functional groups according to their shape, structure, behavior and life span. The foraminiferal dead assemblage includes a total of 110 species, that included 43 species in sediments colonized by P. oceanica, 82 species in sediments with C. prolifera, 78 in sediments invaded by C. taxifolia, and 55 in sediments invaded by C. racemosa. Taxonomic composition of all assemblages is similar, though differences occur in the relative abundance of each taxon. Sediments in P. oceanica meadows are characterized by flat, encrusting, long life-span species (e.g., Planorbulina mediterranensis d’Orbigny, 1826), whereas in Caulerpa spp. habitat, temporarily motile, shorter life-span taxa (e.g., Lobatula lobatula (Walker and Jacob, 1798) and Rosalina bradyi Cushman, 1915) tend to dominate. Multivariate analysis shows that only the thanathocoenosis of P. oceanica sediments is representative of the P. oceanica epiphytic foraminiferal assemblage (Planorbulinatum medi-terranensaeColom, 1942). Hence, differences among the foraminiferal assemblages in sediments colonized by different phytal substrates occur prior to taphonomic and dissolution processes and may be applicable to paleoecological interpretations.

Mediterranean infralittoral soft bottoms are dominated by extensive seagrass meadows of the endemic phanerogam Posidonia oceanica (L.) Delile. Both leaves and rhizomes are substrates suitable for colonization by a vast array of epiphytic organisms, including foraminifera. Epiphytic foraminifera are very useful bioindicators, because of their relatively short life cycles and their quick reaction to environmental changes at global and local scales. To evaluate the environmental conditions reflected by the foraminiferal assemblages in P. oceanica-dominated environments, the FORAM Index (FI) has been slightly modified to include changes to the ecological categories defined in the original Langer A–D morphotype scheme. Sensitive, long-lived species are now represented by the SB and A* groups; the stress-tolerant taxa correspond to D*; and the small heterotrophic forms are still represented by B and C. Consequently, the FI was modified as follows: FI′ = 10 3 (P_A*+P_SB) + P_D* + 2 × (P_B+P_C). The Posidonia oceanica foraminiferal assemblage contains very abundant B and C forms that remain more or less constant regardless of environmental conditions as they do not have as strict requirements for light and substrate as the A* and SB groups. Moreover, B and C taxa are not as well-adapted to stressful conditions as is D*. Therefore, to magnify the differences between, a priori, minimally altered and stressed areas, a new index is calculated. The “long vs. short life span” index (I_LS), which is expressed as: I_LS = (3.5 × (P_A* + P_SB) + 0.01)/(P_D* + 0.01). Index I_LS has a high correlation with the modified FORAM Index, which was developed to characterize water quality. Index I_LS, can be used as a reference for the quality assessment and preservation of P. oceanica meadows.