Community cascades in a marine pelagic food web controlled by the non-visual apex predator Mnemiopsis leidyi

Trophic cascades are a ubiquitous feature of many terrestrial and fresh-water food webs, but have been difficult to demonstrate in marine systems with multispecies trophic levels. Here we describe significant trophic cascades in an open coastal planktonic ecosystem exposed to an introduced top predator. The ctenophore Mnemiopsis leidyi was monitored for an 8-year period concurrent with measures of the food web structure of the plankton and strong trophic cascades were evident. In the 5 years when M. leidyi were found, their target prey (grazing copepods) were reduced 5-fold and the primary producers doubled their biomass when released from the grazing pressure. The increased phytoplankton biomass could unequivocally be assigned to grazing release since concurrent measurements of primary production did not differ between years with or without M. leidyi. Copepod biomass prior to the mass occurrence of the ctenophore was important. The years without M. leidyi had significantly higher biomass of copepods in July, the month preceding the outburst of the ctenophore. The profound changes of the pelagic ecosystem faced with a non-selective apex predator shows that marine communities are not exceptions from trophic cascade mechanisms

Conversion efficiency and bandwidth of inter-modal four wave mixing in two-mode optical fibres

FWM is a non-linear process where two or more wave propagating throughout a fibre result in a production of frequencies different to that of the input waves. Space division multiplexing and in particular mode division multiplexing (MDM) have shown promises in overcoming the capacity limit of single-mode fibres for optical telecommunications. Over long distances MDM systems would result in processes like inter modal FWM (IM-FWM). If such systems are to be used commercially, they will require methods of switching data signals between wavelengths and spatial modes. An attractive solution is provided by four-wave mixing (FWM) where two strong pump fields convert the signal into an idler field at another wavelength. However, applications of intermodal FWM (IM-FWM) for telecommunications in multimode fibres are relatively new. Recently, two potentially interesting IM-FWM processes have been identified: phase conjugation (PC) and Bragg scattering (BS) [Essiambre et al., IEEE Photon. Technol. Lett. 25, 539 (2013)].Here we investigate conversion efficiencies and spectral bandwidths of PC and BS in a two-mode optical fibre as potential mode/wavelength conversion systems using two numerical models and compare with experimental results.The first model uses the coupled amplitude equations for FWM [Agrawal, Nonlinear fiber optics, Academic Press (2013)] which take into account self and cross phase modulation and FWM between the four wavelength channels involved. This model was found to agree with experimental results of the PC idler. However discrepancies are found for BS, which we attribute to simultaneous and/or cascaded FWM processes within the same fibre. Thus we use a second, more sophisticated model, the multimode nonlinear Schrödinger equation [Poletti & Horak, JOSAB 25, 1645 (2008)] which, contrary to the first model, includes all third order nonlinear processes simultaneously and is additionally able to predict cascaded FWM processes that occur throughout the spectrum. We compare the differences between the two models, analyse the contributions to the BS and PC idlers from multiple FWM processes within the spectrum, and finally present comparisons to experimental results