Thibaud Mascart

Meiofauna and harpacticoid copepods in different habitats of a Mediterranean seagrass meadow

This study investigated whether associated meiobenthic communities, especially harpacticoid copepods, differed amongst habitats. Five pre-defined habitats within and next to the Posidonia oceanica seagrass meadow were sampled: living seagrass canopy leaves (LL), small (SMF) and large (LMF) macrophytodetritus fragment accumulations and sand, bare (BS) and covered (CS). The highest meiofauna abundances were recorded in the BS for the core sampled habitats (BS, CS, SMF and LMF) and in the LMF for seagrass material habitats (SMF, LMF and LL). Harpacticoid copepods were the most abundant taxon in all habitats. The assemblage composition at copepod family level showed two distinct habitats clusters: a leaf (LMF and LL) and a sediment cluster (BS, CS and SMF). Subsequently, stable isotope analyses were conducted to analyse the relationship between copepods and their potential food sources in seagrass material habitats. Based on delta C-13 isotopic analyses and SIAR mixing model, harpacticoid copepods relied for 70% on epiphytes and for 30% on P. oceanica leaf material in the LMF and LL habitats.

Response of Posidonia oceanica seagrass and its epibiont communities to ocean acidification

The unprecedented rate of CO2 increase in our atmosphere and subsequent ocean acidification (OA) threatens coastal ecosystems. To forecast the functioning of coastal seagrass ecosystems in acidified oceans, more knowledge on the long-term adaptive capacities of seagrass species and their epibionts is needed. Therefore we studied morphological characteristics of Posidonia oceanica and the structure of its epibiont communities at a Mediterranean volcanic CO2 vent off Panarea Island (Italy) and performed a laboratory experiment to test the effect of OA on P. oceanica photosynthesis and its potential buffering capacity. At the study site east of Basiluzzo Islet, venting of CO2 gas was controlled by tides, resulting in an average pH difference of 0.1 between the vent and reference site. P. oceanica shoot and leaf density was unaffected by these levels of OA, although shorter leaves at the vent site suggest increased susceptibility to erosion, potentially by herbivores. The community of sessile epibionts differed in composition and was characterized by a higher species richness at the vent site, though net epiphytic calcium carbonate concentration was similar. These findings suggest a higher ecosystem complexity at the vent site, which may have facilitated the higher diversity of copepods in the otherwise unaffected motile epibiont community. In the laboratory experiment, P. oceanica photosynthesis increased with decreasing pHT (7.6, 6.6, 5.5), which induced an elevated pH at the leaf surfaces of up to 0.5 units compared to the ambient seawater pHT of 6.6. This suggests a temporary pH buffering in the diffusive boundary layer of leaves, which could be favorable for epibiont organisms. The results of this multispecies study contribute to understanding community-level responses and underlying processes in long-term acidified conditions. Increased replication and monitoring of physico-chemical parameters on an annual scale are, however, recommended to assure that the biological responses observed during a short period reflect long-term dynamics of these parameters.