Biodiversity and diel variation of the benthohyponeuston: A case study of the Northeast Black Sea

The neustal is a specific habitat of oceans, which significantly differs in abiotic parameters from the waters below. One of the most significant components of the coastal neustonic fauna is the benthohyponeuston migrating diurnally between benthic and neustonic realms. Data on this fauna are fragmentary and contradictory, partly due to lack of the criteria to distinguish benthohyponeuston from other benthopelagic animals diurnally migrating to the bulk water from the seafloor. We propose a criterion to quantify the degree of aggregation/avoidance of the neustal zone, reveal four distinct ecological groups and describe patterns of their overnight dynamics. Benthohyponeuston appears in open water at sunset, its biomass most rapidly increases one hour after sunset. Cumaceans, mysids and polychaetes make significant contribution during first three hours after sunset. Decapods are important around midnight and 3 h later. Amphipods are significant overnight. By analogy with the benthopelagic species, we define the benthohyponeuston as benthic animals, which are associated with the neustal zone at least at one stage of their life cycle. This association is necessary for reproduction, dispersal or feeding – that represent three basic pathways connecting neustonic and benthic/benthopelagic coastal communities below. The data on benthohyponeuston and patterns of its overnight dynamics will help in a better understanding of vertical migrations in the coastal zone and in estimating diurnal fluxes of organic matter.