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Water depth and mesoscale oceanography drive neustonic polychaete assemblages in the SE Pacific Ocean

Joao B. Gusmao, Oscar Diaz, Camila Gallardo, Valeria Hidalgo-Ruz, Tim Kiessling, Ariadna Mecho, Erika Meerhoff, Nicolas Rozbaczylo, Martin Thiel*

*Corresponding author:

ABSTRACT: This study aimed to assess how oceanographic variables (depth, water temperature, and phytoplankton biomass) and the proximity to islands and seamounts affect the structure of neustonic polychaete assemblages across the Southeastern Pacific Ocean. We used data from two oceanographic cruises and online data bases to analyze how different environmental variables affect polychaete assemblages sampled at the sea surface. We hypothesized that (1) the proximity to islands and seamounts would have a positive effect on polychaete diversity since islands and seamounts cause changes in local currents that can promote primary productivity, and that (2) changes in species composition would be related to large spatial scales since major gradients in oceanographic variables are observed along a longitudinal gradient of the SE Pacific. Thirty polychaete taxa belonging to 14 different families were identified. All polychaetes were collected no further than 350 km from shallow geological features, such as seamounts or oceanic islands, indicating that mesoscale oceanographical processes drive neustonic polychaete assemblages. Polychaete abundance increased towards shallow waters and decreased with increasing distances from shallow geological features. Changes in species composition were related to gradients in latitude, longitude, and increasing distances to coastal systems. The most notable changes in assemblage structure were observed along the longitudinal gradient. These results indicate that the assemblage structure of neustonic polychaetes is the result of the interaction between mesoscale oceanographic processes related to shallow geological features and the major longitudinal gradients in productivity and temperature observed across the SE Pacific Ocean.