Seagrass macrophytodetritus: a copepod hub - Species diversity, dynamics and trophic ecology of the meiofauna community in Posidonia oceanica leaf litter accumulations

Macrophytodetritus is a heterogeneous mixture of detrital material that accumulates on submerged unvegetated sand patches amid vast Mediterranean Posidonia oceanica seagrass meadows. Several vagile invertebrates are present in substantial biomass and biodiversity. Among these invertebrates, meiofauna (fauna between 38µm and 1mm) is ubiquitous and seems to play a key-feature in this dynamic and patchy system. Coastal ecosystems are under the direct effect of anthropogenic disturbance and degradation. Extra research is crucially needed to understand better the dynamics of coastal vegetation, in order to have a more successful restauration of these regressing ecosystems. In this context, the main goal of this PhD research was triple: (1) characterising in situ the physico-chemistry and the composition of the macrophytodetritus accumulations in the Calvi Bay, Corsica, (2) identifying the diversity of the associated meiofauna communities, especially harpacticoid copepods together with unravelling the origin of the present copepods and (3) characterizing the trophic ecology of the copepod communities in the macrophytodetritus at the specific and eco-morphological level.
This research showed that macrophytodetritus biomass is composed on average for 75% of dead P. oceanica seagrass leaves shed after senescence. Attached to the surface of the seagrass leaves numerous micro- and macroepiphytes are present, representing on average 10% of the total biomass. The remaining part is mainly constituted of drift material, like detached P. oceanica shoots and epilithic macroalgae. A seasonal pattern is observed regarding the amount of accumulated material and the physico-chemical composition inside the accumulation. Wind-induced hydrodynamics is the responsible driver behind the variability of the macrophytodetritus and consequently it has a major impact on the faunal communities already present in a macrophytodetritus accumulation.
Previous studies showed that the presence of macrofaunal invertebrates (> 1 mm) in high amounts contributes to the degradation of the detritus. Similarly, this study proves the ubiquitous presence of meiofauna in macrophytodetritus. Depending on the season, densities from 20.10³ to 160.10³ meiofaunal organisms per square metre of accumulation were recorded. Copepods were the most abundant taxon (> 50%) of which 87% belonged to the order Harpacticoida. Nematodes were the second most abundant taxon, representing on average 18% of the total meiofauna densities. A total of 61 copepod species were found in Calvi Bay macrophytodetritus accumulations and adjacent habitats (bare sand, seagrass and water column), wherefrom 85% were shared amongst these habitats, underlining the high colonization capabilities of copepods. Active colonization occurred within 24h through species-specific dispersal pathways. Certain species were more avid to colonize, resulting in a colonizer-competitor trade-off among the copepod community. Eco-morphological characteristics seemed to be responsible for the dispersal potential. However, the variety of the composition of the copepod community suggested that other factors also contributed to the attractiveness of the structurally complex macrophytodetritus habitat.
The isotopic niches of four abundant copepod species, representing four different eco-morphological groups were identified: Ectinosoma dentatum (mesopsammic-type), Diosaccus tenuiremis (phytal-type), Tisbe furcata (epibenthic-type) and Clausocalanus arcuicornis (water-column-type). Based on stable isotope analysis, fatty acid profiling and Bayesian mixing model, results suggested an interspecific diversity which would indicate a species-specific resource partitioning. C. arcuicornis mainly fed on suspended organic matter, while D. tenuiremis thrived mainly on epiphytes (mostly diatoms). E. dentatum was dependent on the seasonal availability of food sources, while T. furcata fed on a heterogeneous mixture of sources. Presumably none of the species directly assimilated dead seagrass leaf litter.
Overall, by combining in situ sampling, novel mesocosm experiment, biomarkers and mixing models, this study displayed the carrying capacity of macrophytodetritus to support a large amount of meiofauna and a wide diversity of copepod species. The morphological differences among copepod species seem to allow specialization towards habitat preferences, (physical habitat preferences and colonization potential) and towards resource preferences (food partitioning). Macrophytodetritus seems thus to be a suitable home, or a temporary hub for a diverse copepod community. Finally, this dynamic and patchy habitat, prone to swift changes and situated at the crossing of different ecosystems, plays a major role in coastal ecology.