Characterisation of pelagic seascapes through micronektonic and zooplanktonic scattering layers

Landscape ecology is a key discipline for studying the relationships between spatial patterns and ecological processes, as well as for monitoring macro-scale changes in ecosystems. Unlike terrestrial landscapes, which have been extensively studied, the open-ocean pelagic environment presents unique complexities that require innovative approaches for its understanding. Aggregations of micronektonic and macrozooplanktonic organisms in layers are prominent features of the open-ocean pelagic zone. Such visually cryptic features can be revealed by echosounders as pelagic Sound Scattering Layers (SSLs). We characterise various pelagic seascapes and their relationships with environmental parameters across three oceanographically contrasting tropical regions, characterised by diverse ecological patterns, using an integrated methodological framework that combines dual-frequency acoustic analysis (18 and 38 kHz). Diel vertical migration is a common feature that involves epipelagic and mesopelagic SSLs. Nevertheless, there are significant regional contrasts in SSL spatial distribution as oceanographic features influence SSL patterns and micronektonic acoustic backscatter. Acoustically defined pelagic seascapes reveal biological-physical coupling and SSL responses to oceanographic variability at meso- and macro-scales. SSL distribution was significantly driven by oceanographic variables such as temperature, chlorophyll a, salinity, oxygen, and PAR, as well as by mesoscale eddies that structured their spatial patterns, with anticyclonic eddies concentrating SSLs’ acoustic backscatter at their centres and cyclonic eddies exhibiting scattered acoustic backscatter at their peripheries. This framework enhances our ability to assess how climate variability and changing ocean conditions influence open-ocean pelagic ecosystems. Developed and demonstrated at a broad regional scale, this validated approach establishes a transferable framework for characterising pelagic habitats through integrated SSL structures, enabling its application across wider spatial and temporal domains to advance understanding of global biophysical and ecosystem dynamics.