Ecoacoustics and multispecies semiosis: naming, semantics, semiotic characteristics, and competencies

Biosemiotics to date has focused on the exchange of signals between organisms, in line with bioacoustics; consideration of the wider acoustic environment as a semiotic medium is under-developed. The nascent discipline of ecoacoustics, that investigates the role of environmental sound in ecological processes and dynamics, fills this gap. In this paper we introduce key ecoacoustic terminology and concepts in order to highlight the value of ecoacoustics as a discipline in which to conceptualise and study intra- and interspecies semiosis. We stress the inherently subjective nature of all sensory scapes (vivo-, land-, vibro- and soundscapes) and propose that they should always bear an organismic attribution. Key terms to describe the sources (geophony, biophony, anthropophony, technophony) and scales (sonotopes, soundtopes, sonotones) of soundscapes are described. We introduce epithets for soundscapes to point to the degree to which the global environment is implicated in semiosis (latent, sensed and interpreted soundscapes); terms for describing key ecological structures and processes (acoustic community, acoustic habitat, ecoacoustic events) and examples of ecoacoustic events (choruses and noise) are described. The acoustic eco-field is recognized as the semiotic model that enables soniferous species to intercept core resources like food, safety and roosting places. We note that whilst ecoacoustics to date has focused on the critical task of the development of metrics for application in conservation and biodiversity assessment, these can be enriched by advancing conceptual and theoretical foundations. Finally, the mutual value of integrating ecoacoustic and biosemiotics perspectives is considered

On the influence of bottom topography and the Deep Western Boundary Current on Gulf Stream separation

The Gulf Stream separates abruptly from the North American coastline at Cape Hatteras. The absence of significant seasonal and interannual variability in the separation point, compared with that of other separating boundary currents, suggests that Gulf Stream separation is locally controlled. In this paper we consider the possible influence of bottom topography and the Deep Western Boundary Current (DWBC), which descends underneath the Gulf Stream at Cape Hatteras. The path of the DWBC is strongly constrained by bottom topography. At Cape Hatteras, the continental shelf widens and the DWBC is forced to swing offshore and pass beneath the Gulf Stream. Three possible mechanisms by which bottom topography and the DWBC can affect the separation of the Gulf Stream are proposed and investigated: (i) topography modifies the background potential vorticity contours; (ii) the DWBC 'advects' the Gulf Stream separation point southward; (iii) intense downwelling as the DWBC passes beneath the Gulf Stream induces an adverse pressure gradient in the Gulf Stream, leading to its separation. Results from a series of idealized numerical experiments with a 'geostrophic vorticity' model are presented to investigate these mechanisms. Topography alone does have an impact on the separation point, broadly consistent with modification of the background potential vorticity. We also show that the presence of a DWBC does, indeed, push the time average separation of the Gulf Stream farther southward, consistent with both the advection and adverse pressure gradient mechanisms. However, the time-dependent boundary current separation is more nonlinear than suggested by each of the above mechanisms, undergoing a series of abrupt transitions between northern and southern separation states. As the DWBC transport is increased, the southern separation state is occupied more and more frequently

Minimal change in Antarctic Circumpolar Current flow speed between the last glacial and Holocene

The Antarctic Circumpolar Current is key to the mixing and ventilation of the world’s oceans1, 2, 3, 4, 5. This current flows from west to east between about 45° and 70° S (refs 1, 2, 3) connecting the Atlantic, Pacific and Indian oceans, and is driven by westerly winds and buoyancy forcing. High levels of productivity in the current regulate atmospheric CO2 concentrations6. Reconstructions of the current during the last glacial period suggest that flow speeds were faster7 or similar8 to present, and it is uncertain whether the strength and position of the westerly winds changed9, 10, 11. Here we reconstruct Antarctic Circumpolar Current bottom speeds through the constricting Drake Passage and Scotia Sea during the Last Glacial Maximum and Holocene based on the mean grain size of sortable silt from a suite of sediment cores. We find essentially no change in bottom flow speeds through the region, and, given that the momentum imparted by winds, and modulated by sea-ice cover, is balanced by the interaction of these flows with the seabed, this argues against substantial changes in wind stress. However, glacial flow speeds in the sea-ice zone12 south of 56° S were significantly slower than present, whereas flow in the north was faster, but not significantly so. We suggest that slower flow over the rough topography south of 56° S may have reduced diapycnal mixing in this region during the last glacial period, possibly reducing the diapycnal contribution to the Southern Ocean overturning circulation