Special issue  

The worldwide increase in coastal hypoxia and anoxia over the past decades is an alarming signal that underlines profound changes in and collapses of (“dead zone”) ecosystem processes in shallow shelf seas. This special issue is largely based on experimentally induced small-scale anoxia in a benthic soft-bottom community at 24m depth (northern Adriatic, Mediterranean). The approach combines Plexiglas chambers with time-lapse photography, microsensors, core sampling and the DET technique (diffusive equilibrium in thin films). This yielded detailed data on

• macrofauna behavioral responses and tolerances,
• meiofauna (harpacticoid copepods and foraminiferans) responses,
• sediment geochemistry processes, and
• post-anoxia scavenging and recovery.
Macrofauna behaviour links physiological functions and ecological processes and therefore depicts multi-level effects of oxygen deficiency. A detailed series of behavioral responses reveals more tolerant and more sensitive species and emphasizes the broad range of possible responses in a community setting. Among the meiofauna, the focus is on harpacticoid copepod and foraminiferan abundance and diversity. A fluorescent staining technique (cell tracker green: CTG) is applied for the first time to copepods, improving the distinction between living and dead components. An accompanying pulse-chase lab experiment with pre-labeled diatoms reveals functional responses of primary consumers (copepods) to anoxia. The in situ sediment geochemistry processes (key pore water and solid phase parameters in response to migration of redox layers, mobility of trace elements) are documented (millimeter-scale resolution). Finally, post-anoxia time-lapse films examine the sequence of scavenger/predator arrivals and (the lack of) macroepifauna recolonization even after 2 years.

Combined, these perspectives yield a more comprehensive picture of events during and after anoxia, improving on the information available from laboratory experiments and fragmentary in situ documentations. This is a step forward in interpreting post-hypoxia/anoxia community composition and recovery potential in the past and present.