Diversity and distribution patterns of ichthyoplankton assemblages in the subtropical northeastern Atlantic Ocean

Ichthyoplankton comprises the early developmental stages of most oceanic bony fishes and plays a key role in marine ecosystems, as it influences recruitment success and sustainability of commercial fisheries. Despite their ecological importance, diversity, vertical distribution and ontogenetic migrations of mesopelagic fish larvae remain poorly understood. Thus, this study analyzes ichthyoplankton communities sampled in oceanic waters during winter along a transect of 11 stations in the subtropical northeastern Atlantic Ocean during day- and nighttime. Biological sampling was performed using a MOCNESS-1 system, allowing collection of ichthyoplankton at 8 depth layers from the surface to 1800 m. We identified four water masses across all stations (NACW, MW, AAIW, and EASIW), which generated vertical gradients throughout the water column. Temperature and salinity were highest in the upper 200 m at the central stations influenced by the NACW, whereas dissolved oxygen decreased between 500 and 1200 m under the influence of the AAIW. Chlorophyll-a and particulate carbon concentration values were highest near the Northwest African upwelling region, indicating enhanced surface productivity. A total of 3244 larval specimens were collected, comprising at least 137 distinct taxa. Diversity index revealed the lowest values in proximity to the Northwest African region despite the highest overall larval abundance, reflecting the predominance of a few highly abundant taxa. Larvae of the most abundant mesopelagic fish families displayed different vertical distribution patterns throughout development, indicating differing ontogenetic migration habits. A distance-based redundancy analysis (dbRDA), based on a Bray-Curtis similarity matrix and temperature, chlorophyll-a, and oxygen concentration values across stations, identified three distinct larval assemblages which were found to be significant according to a SIMPROF analysis. Overall, these findings provide new insights into the processes shaping ichthyoplankton diversity across the subtropical northeastern Atlantic Ocean.