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Is Oithona the most important copepod in the world’s oceans? C. P. GALLIENNE AND D. B. ROBINS PLYMOUTH MARINE LABORATORY, WEST HOE, PLYMOUTH PL DH, UK CORRESPONDING AUTHOR: C. P. GALLIENNE, E-MAIL:
[email protected]
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 µm 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 µm 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 conventional WP-2 net with a 200 µm mesh may capture 200 µm was underestimated by a factor of 4.4 using a 200 µm mesh Juday–Bogorov net (Calbet et al., 2001). Copepodite and adult stages of important groups such as Acartia, Calocalanus, Clausocalanus, Corycaeus, Microcalanus, Paracalanus, Pseudocalanus, Oithona, Oncaea, Temora and most harpacticoids, as well as polychaete larvae, invertebrate eggs and naupliar stages, are therefore undersampled or missed altogether in many studies. The consensus is that a mesh size of 75% of the width of the smallest animal to be sampled is required for quantitative sampling (Bernhard et al., 1973; Nichols and Thompson, 1991). Towed filtration methods improve on the spatial resolution of traditional net sampling methods, but may suffer even more from these losses. A recent study in the NE Atlantic (Gallienne et al., 2001) using an optical plankton counter [OPC; (Herman, 1992)] and WP-2 net samples showed that smaller taxa often dominate numbers, and sometimes biomass. A towed filtration sampler (fixed depth ~10 m, 200 µm mesh) used concurrently with the OPC in this study gave abundances 2–3 orders of magnitude lower than those found by Gallienne et al. (Gallienne et al., 2001), and by other researchers in the same area. Numerical dominance by copepods
