The abundance and composition of zooplankton down to 3000 m depth was studied in the subtropical and tropical latitudes across the Atlantic, Pacific and Indian Oceans (35 °N–40 °S). Samples were collected from December 2010 to June 2011 during the Malaspina Circumnavigation Expedition. Usually, low abundances were observed with the highest values found in the North Pacific Ocean, Benguela, and off Mauritania, and the lowest in the South Pacific Ocean. No significant differences in abundance and zooplankton composition were found among oceans, with depth being consistently the most important factor affecting their distribution. Each depth strata were inhabited by distinct copepod assemblages, which significantly differed among the strata. The contribution of copepods to the zooplankton community increased with the depth although, as expected, their abundance strongly decreased. Among the copepods, 265 species were identified but 85% were rare and contributed less than 1% in abundance. Clausocalanus furcatus and Nannocalanus minor dominated the epipelagic strata. Pleuromamma abdominalis and Lucicutia clausi were of importance in the mesopelagic layer, and Pareucalanus, Triconia, Conaea and Metridia brevicauda in the bathypelagic layer. Our results provide a global-scale assessment of copepod biodiversity and distribution, providing a contemporary benchmark to follow future ocean changes at low latitudes.

Decadal trends in the zooplankton community were seasonally investigated in multiple monitoring transects of the western Mediterranean Sea during a period of increasing and record-high water temperatures. The transition area around the Baleares archipelago can be separated into two regions: the Alborán Sea and the Balearic Sea. Differences were found in the abundance and structure of the dominant zooplankton groups: the cladocerans and copepods. The highest zooplankton abundances were found during the stratified summer season, when cladoceran numbers peaked. However, copepods were more abundant during the winter. A marked seasonality was found in all groups and the dominant taxa: 114 species of copepods and 5 species of cladocerans were identified, but less than 8 species of copepods and 2 species of cladocerans dominated the populations. During the 2007–2017 study, warm and salty waters were observed in the Balearic Sea. In the Alborán Sea, cool and fresh waters were observed, along with a decline in chlorophyll. Irregular group and species trends were observed in both regions, exhibiting both increases and decreases depending on region and season. Trends differed, often oppositely, for the Alborán versus the Balearic Sea, with the transition region between them mirroring one or the other, switching with the seasons. The most dominant species, including Penilia avirostris and Clausocalanus arcuicornis, but also Evadne spinifera, C. lividus, C. furcatus, Paracalanus parvus, Acartia clausi, Centropages typicus, Subeucalanus monachus, and Calanus helgolandicus, are proposed as biological tracers to be used in further studies on climate and zooplankton community changes in the western Mediterranean Sea.

The mesozooplankton community was analyzed over a 6-year period (2013-2018) during the post-eruptive stage of the submarine volcano Tagoro, located south of the island of El Hierro (Canary Archipelago, Spain). Nine cruises from March 2013 to March 2018 were carried out in two different seasons, spring (March-April) and autumn (October). A high-resolution study was carried out across the main cones of Tagoro volcano, as well as a large number of reference stations surrounding El Hierro (unaffected by the volcano). The zooplankton community at the reference stations showed a high similarity with more than 85% of the variation in abundance and composition attributable to seasonal differences. Moreover, our data showed an increase in zooplankton abundance in waters affected by the volcano with a higher presence of non-calanoid copepods and a decline in the diversity of the copepod community, indicating that volcanic inputs have a significant effect on these organisms. Fourteen different zooplankton groups were found but copepods were dominant (79%) with 59 genera and 170 species identified. Despite the high species number, less than 30 presented a larger abundance than 1%. Oncaea and Clausocalanus were the most abundant genera followed by Oithona and Paracalanus (60%). Nine species dominated (>2%):O. media, O. plumifera, andO. setigeraamong the non-calanoids andM. clausi, P. nanus,P. parvus, C. furcatus, C. arcuicornis, andN. minoramong the calanoids. After the initial low abundance of the copepods as a consequence of the eruption, an increase was observed in the last years of the study, where besides the small Paracalanus and Clausocalanus, the Cyclopoids seem to have a good adaptive strategy to the new water conditions. The increase in zooplankton abundance and the decline in the copepod diversity in the area affected by the volcano indicate that important changes in the composition of the zooplankton community have occurred. The effect of the volcanic emissions on the different copepods was more evident in spring when the water was cooler and the mixing layer was deeper. Further and longer research is recommended to monitor the zooplankton community in the natural laboratory of the Tagoro submarine volcano.

Vertical migration is a rather complex behavior by which multiple species of different sizes move from the meso- and bathy-pelagic zones, where they reside during day to avoid predators, to the epipelagic zone for feeding. Understanding this behavior of zooplankton organisms is key to assess their role in the active transport of carbon in the oceans. The present study disentangles the diel vertical distribution of zooplankton community (mainly copepods) during spring in the Northeast Atlantic Ocean. To address that, 10 stations down to 1900 m depth were sampled using an opening-closing MOCNESS during day- and nighttime. Additionally, we also sampled the epipelagic strata (0–200 m) for microzooplankton (50-200 µm) as well as the mesozooplankton community (>200 µm). A total of 15 zooplankton groups were found (>85% were copepods), and more than 250 species of copepods were identified. The results showed a latitudinal decreasing gradient northward in the number of genera and species, and an increasing gradient in their abundances. A frontal system was observed in the epipelagic layer between 45° and 47°N promoting sharp differences among the northern and southern communities. In this frontal zone, small copepod nauplii and copepodites were rather abundant, being Paracalanus parvus the dominant copepod. In the most northern region, we found Calanus finmarchicus and a high abundance of Oithona spp, while in the southern region C . helgolandicus, Nannocalanus minor and Mecynocera clausi were dominant. A decreasing trend of copepod abundance was observed with depth, segregating the upper 700 m from below. Using the 50 dominant copepod species, five different groups were distinguished in their vertical distributions: (1) the non-migrant epipelagic copepods, (2) copepod migrating from mid-waters to the epipelagic zone at night, (3) copepods abundant in the epipelagic layer but performing reverse migration, (4) the strong migrants moving from the meso- and bathypelagic zones to the epipelagic zone at night, and (5) those just moving into the twilight zone. Our findings highlight the complexity of the diel vertical migration pattern of copepods during spring and their relevant role in the deep-sea of the North Atlantic Ocean.

Hernández-León, Santiago (2020): Carbon export and sequestration in the ocean. PANGAEA, https://doi.org/10.1594/PANGAEA.923832