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 <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.

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

From the Hensen net toward four-dimensional biological oceanography

Plankton tows from the northern California Current constrain biological and physical influences on living planktonic foraminifera. In this region, the dominant factors controlling the size and distribution of symbiotic and asymbiotic species are light and food. Food decreases off- shore. Light, needed for symbiont photosynthesis, increases offshore as water turbidity lessens. Asymbiotic foraminifera (e.g., right-coiling Neogloboquadrina pachyderma, Globigerina quinqueloba, and Globigerina bulloides), which survive by grazing, dominate the coastal fauna. The most abundant of these species, right-coiling Neogloboquadrina pachyderma, did not change in size in response to increasing food. Species that benefit from symbiont photosynthesis (OrbMina universa, Neogloboquadrina dutertrei, Globigerinoides ruber, and Globigerinita glutinata) dominate the offshore fauna. Individuals of these species are rare and have smaller shells in turbid waters where light is limited. G. ruber, which is near its thermal tolerance limit of --14oC, is the only species to demonstrate a clear temperature response. Although temperature may control a foraminiferal species' distribution near the limits of its thermal tolerance, food and light appear to provide the primary control under more favorable thermal conditions. We infer that gradients in food and light can result in quantifiable sedimentary patterns related to oceanic productivity through changes in plankton biomass and turbidity.

https://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/15603/Ortiz%20et%20al%20Paleo%201995.pdf;sequence=1

Environmental control of living symbiotic and asymbiotic foraminifera of the California Current

Summary 1. Megaplanktivores such as filter-feeding sharks and baleen whales are at the apex of a short food chain (phytoplankton‐zooplankton‐vertebrate) and are sensitive indicators of sea-surface plankton availability. Even though they spend the majority of their time below the surface it is still not known how most of these species utilize vertical habitat and adapt to short-term changes in food availability. 2. A key factor likely to control vertical habitat selection by planktivorous sharks is the diel vertical migration (DVM) of zooplankton; however, no study has determined whether specific ocean-habitat type influences their behavioural strategy. Based on the first high-resolution dive data collected for a plankton-feeding fish species we show that DVM patterns of the basking shark Cetorhinus maximus reflect habitat type and zooplankton behaviour. 3. In deep, well-stratified waters sharks exhibited normal DVM (dusk ascent‐dawn descent) by tracking migrating sound-scattering layers characterized by Calanus and euphausiids. Sharks occupying shallow, inner-shelf areas near thermal fronts conducted reverse DVM (dusk descent‐dawn ascent) possibly due to zooplankton predator‐prey interactions that resulted in reverse DVM of Calanus . 4. These opposite DVM patterns resulted in the probability of daytime-surface sighting differing between these habitats by as much as two orders of magnitude. Ship-borne surveys undertaken at the same time as trackings reflected these behavioural differences. 5. The tendency of basking sharks to feed or rest for long periods at the surface has made them vulnerable to harpoon fisheries. Ship-borne and aerial surveys also use surface occurrence to assess distribution and abundance for conservation purposes. Our study indicates that without bias reduction for habitat-specific DVM patterns, current surveys could under- or overestimate shark abundance by at least 10-fold.

Habitat-specific normal and reverse diel vertical migration in the plankton-feeding basking shark

Mesopelagic mesozooplankton communities of an oligotrophic (Hawaii Ocean Time series-HOT station ALOHA) and a mesotrophic (Japanese time-series station K2) environment in the North Pacific Ocean are compared as part of a research program investigating the factors that control the efficiency of particle export to the deep sea (VERtical Transport In the Global Ocean—VERTIGO). We analyzed zooplankton (>350 μm) collected from net tows taken between 0 and 1000 m at each site to investigate the biomass size structure and the abundance of the major taxonomic groups in discrete depth intervals throughout the water column. Biomass of zooplankton at K2 over all depths was approximately an order of a magnitude higher than at ALOHA, with a significantly higher proportion of the biomass at K2 in the larger (>2 mm) size classes. This difference was mostly due to the abundance at K2 of the large calanoid copepods Neocalanus spp. and Eucalanus bungii, which undergo ontogenetic (seasonal) vertical migration. The overall strength of diel vertical migration was higher at K2, with a mean night:day biomass ratio in the upper 150 m of 2.5, vs. a ratio of 1.7 at ALOHA. However, the amplitude of the diel migration (change in weighted mean depth between day and night) was higher at ALOHA for all biomass size classes, perhaps due to deeper light penetration causing deeper migration to avoid visual predators. A number of taxa known to feed on suspended or sinking detritus showed distinct peaks in the mesopelagic zone, which affects particle transport efficiency at both sites. These taxa include calanoid and poecilostomatoid (e.g., Oncaea spp.) copepods, salps, polychaetes, and phaeodarian radiolaria at K2, harpacticoid copepods at ALOHA, and ostracods at both sites. We found distinct layers of carnivores (mainly gelatinous zooplankton) in the mesopelagic at K2 including chaetognaths, hydrozoan medusae, polychaetes, and gymnosome pteropods, and, in the upper mesopelagic zone, of ctenophores and siphonophores; at both sites a mesopelagic layer of hyperiid amphipods was found. The large population of ontogenetically migrating calanoid copepods is likely supporting large carnivorous populations at depth at K2. The contrasting zooplankton taxonomic structure at the two sites helps explain the higher efficiency of the biological pump at K2. Factors responsible for increased transport efficiency at K2 include rapid transport of POC via larger fecal pellets produced by zooplankton at K2, and enhanced active carbon export at K2 vs. ALOHA, due to the greater strength of diel vertical migration and to additional ontogenetic migration at K2.

A comparison of mesopelagic mesozooplankton community structure in the subtropical and subarctic North Pacific Ocean

Four variants of the Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) have been constructed to sample a broad size spectrum of oceanic animals from microzooplankton to micronekton. The systems differ in mouth opening dimensions (ranging from 1/4 to 20 m2), the number of nets carried (from 5 to 20), and the mesh size of the netting (from 64 μm to 3.0 mm). A new electronics package enables an operator to send commands down a single conductor, armored cable to open/close the nets and provides 12-bit resolution for the environmental (temperature, depth, conductivity) and net operation data (flow, net-frame angle, net-bar release), which are transmitted up the cable to the deck unit at 2-s intervals. A microcomputer system, interfaced to the deck unit, calculates salinity, volume filtered by a net, net trajectory velocity, and vertical velocity. The data are printed out and stored on disc, and profiles of temperature and salinity versus depth are plotted during the course of the tow. Analysis of the relationship between the geometry of the MOCNESS under tow and the past and present methods used to estimate the water filtered by a net revealed that significant bias is introduced when the ascent or descent angle of a net is disregarded. The bias is a function of the ratio of vertical velocity to net trajectory velocity and results in an underestimate of volume filtered while shooting a net and an overestimate while hauling.

V. Barber

Papers

New development in the MOCNESS, an apparatus for sampling zooplankton and micronekton