University of Lisbon

About: University of Lisbon is a education organization based out in Lisbon, Lisboa, Portugal. It is known for research contribution in the topics: Population & Context (language use). The organization has 19122 authors who have published 48503 publications receiving 1102623 citations. The organization is also known as: Universidade de Lisboa & Lisbon University.

Lagrangian Observations of Meddy Formation during A Mediterranean Undercurrent Seeding Experiment

Abstract: Mediterranean eddies (meddies) play an important role in maintaining the temperature and salinity distributions in the North Atlantic, but relatively little is known about their early life histories, including where, how often, and by what mechanism they form. A major field program, called A Mediterranean Undercurrent Seeding Experiment, has been carried out to directly observe meddy formation and the spreading pathways of Mediterranean Water into the North Atlantic. Between May 1993 and March 1994, 49 RAFOS floats were deployed sequentially in the Mediterranean Undercurrent south of Portugal and tracked acoustically for up to 11 months. The float deployments were accompanied by high-resolution XBT sections across the undercurrent. Nine meddy formation events were observed in the float trajectories, six near Cape St. Vincent, at the southwestern corner of the Iberian Peninsula, and three near the Estremadura Promontory, along the western Portuguese continental slope. Meddy formation thus occurs where the continental slope turns sharply to the right (when facing in the downstream direction of the undercurrent). After conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent, the authors have estimated a meddy formation rate of 15‐20 meddies per year. The timescale for meddy formation at Cape St. Vincent was found to be 3‐7 days, shorter than previous estimates based on the volume of larger meddies. Meddies were observed to form most frequently when the speed of the Mediterranean Undercurrent was relatively fast. The meddy formation process at Cape St. Vincent resembles the conceptual model of E. A. D’Asaro, whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer (with negative relative vorticity) at a sharp corner. Comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent. This confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices, by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean, may not be the principle mechanism at work in the formation of meddies at Cape St. Vincent.

Host identity is a dominant driver of mycorrhizal fungal community composition during ecosystem development.

Abstract: Summary Little is known about the response of arbuscular mycorrhizal fungal communities to ecosystem development. We use a long-term soil chronosequence that includes ecosystem progression and retrogression to quantify the importance of host plant identity as a factor driving fungal community composition during ecosystem development. We identified arbuscular mycorrhizal fungi and plant species from 50 individual roots from each of 10 sites spanning 5–120 000 yr of ecosystem age using terminal restriction fragment length polymorphism (T-RFLP), Sanger sequencing and pyrosequencing. Arbuscular mycorrhizal fungal communities were highly structured by ecosystem age. There was strong niche differentiation, with different groups of operational taxonomic units (OTUs) being characteristic of early succession, ecosystem progression and ecosystem retrogression. Fungal alpha diversity decreased with ecosystem age, whereas beta diversity was high at early stages and lower in subsequent stages. A total of 39% of the variance in fungal communities was explained by host plant and site age, 29% of which was attributed to host and the interaction between host and site (24% and 5%, respectively). The strong response of arbuscular mycorrhizal fungi to ecosystem development appears to be largely driven by plant host identity, supporting the concept that plant and fungal communities are tightly coupled rather than independently responding to habitat.