Mark Gessner is a limnologist with a keen interest in streams, lakes and wetlands. Much of his research has focused on understanding the consequences of global environmental change on the functioning and biodiversity of inland waters. The fundamental underlying questions are how organisms affect ecosystem processes and how these relationships are shaped by environmental context. For many years, Mark’s work has focused on plant litter decomposition, but he has also worked extensively on microbial productivity, whole-ecosystem metabolism, greenhouse gas dynamics, and primary productivity.