Fine-scale habitat associations of medusae and ctenophores along a gradient of river influence and dissolved oxygen

Abstract

Environmental drivers of abundance and distribution are poorly described for many gelatinous zooplankton (GZ), yet this knowledge is essential to understanding the role of GZ in ecosystems and potential ecological impacts. In situ imaging systems are a contemporary approach to describe the relationship between GZ and their environment, providing detailed information on distribution and habitat preference. Here we quantify patterns in the abundance and distribution of medusae and ctenophores (>1 cm) related to oceanographic data collected with in situ imaging on the stratified northern Gulf of Mexico shelf. Fourteen taxa were identified, typically to genus level, including ctenophores, scyphomedusae, and hydromedusae. The site with the most freshwater influence and widespread bottom water hypoxia contained the highest taxonomic diversity of GZ, with Aequorea, Chrysaora, and Pelagia occurring only at this site. Although most taxa aggregated in particular oceanographic micro-habitats (e.g., surface, low salinity, high temperature, or deeper, high salinity, low temperature), some were habitat generalists, found evenly dispersed throughout the water column (e.g., Eutima) or bimodally distributed near the surface and bottom (e.g., Liriope). Paired day/night sampling revealed that most taxa did not vertically migrate, suggesting they remain in their preferred micro-habitats for extended periods of time and potentially serve as biological tracers of water masses. Imaging offers the ability to monitor these under-described size classes of GZ with higher taxonomic resolution and describe their fine-scale habitat preferences and vertical migration behaviors, which ultimately improves our understanding of how GZ disperse and interact with other zooplankton in stratified shelf waters.