Is Oithona the most important copepod in the world's oceans?

Oithona has been described as the most ubiquitous and abundant copepod in the world's oceans. Most of our knowledge of zooplankton abundance and distribution is derived from net samples whose mesh size is often 200 mum or greater, and researchers have commented on losses of smaller organisms such as Oithona and Oncaea, as well as juvenile forms of larger copepods, from these nets. We review the literature on this subject over the last 50 years, and note that such nets remain in common use for estimating the abundance, biomass and productivity of mesozooplankton. We show that an important fraction of mesozooplankton between 200 and 800 mum in length is significantly under-represented in many current and historical data sets. A 5 year study of the abundance and size distribution of zooplankton biomass on the Atlantic Meridional Transect has produced a very large data set covering a wide range of ecosystem types across the Atlantic Ocean, from subtropical oligotrophic to areas of upwelling and vernal blooming. We use these data to derive estimates of mesh selection effects for commonly used nets on measures of zooplankton abundance, biomass and secondary production, and compare these estimates to those derived from the literature. We estimate that the conventionalWP-2 net with a 200 mum mesh may capture < 10% of conventional mesozooplankton numbers, whilst underestimating biomass by one-third and leading to an underestimate of secondary production by two-thirds. This has serious implications for estimates of zooplankton-mediated fluxes and for the modelling of ecosystem dynamics.