Zooplankton release complex dissolved organic matter to aquatic environments

Zooplankton communities are important components of aquatic food webs in part because they recycle nutrients and carbon. Lacking a complete understanding of the composition of organic material cycled by zooplankton, the breadth of their biogeochemical and ecological contributions to aquatic ecosystems is uncertain. Here, we show that large-bodied zooplankton communities from three diverse biomes across North America release dissolved organic matter (DOM) that is chemically complex. We applied optical techniques and ultrahigh-resolution mass spectrometry to samples of DOM released by zooplankton in leachate incubations and compared these to the composition of aquatic DOM from ambient environments. The leachates captured the DOM released from zooplankton before exposure to environmental conditions that alter its chemical composition. Surprisingly, leachates from all sites contained substantial quantities of all five major fluorescence peaks representing distinct DOM compositions. When averaged, leachates consistently had low optical absorbance and elevated protein-like fluorescence, relative to lake water. Ultrahigh-resolution characterization showed that leachate DOM contained over 7000 detected molecular formulae (MF), many of which were N- and S- rich, with > 10% MF unique to the leachate and not detected in lake DOM. Together, these results help to define the composition of animal DOM inputs, a widely-overlooked end-member in aquatic DOM studies. Given the chemical breadth of zooplankton-derived DOM, this source may partially explain how N- and S- rich, reduced MF persist in these environments. These findings further underscore the rich chemical diversity of linkages between animals and lower trophic levels.