Deep Sea Research Part II: Topical Studies in Oceanography
Deep-sea sampling on CMarZ cruises in the Atlantic Ocean – an Introduction
Introduction
The ocean ecosystem is home to at least 7000 holozooplankton species (Table 1). They are distributed throughout the world’s oceans in biogeographic provinces that were first described nearly a century ago (e.g. Steuer, 1933, McGowan, 1971, McGowan, 1974, Longhurst, 1998). These patterns are based almost exclusively on species that inhabit the upper 500 to 1000 m of the water column, i.e., the epipelagic and mesopelagic zones. Below these depths – in the bathypelagic and abyssopelagic zones – far less is known about zooplankton species diversity, distribution, and abundance. This lack of knowledge is even more notable since the bathypelagic zone accounts for 60% of the ocean’s volume, making it the largest marine habitat on earth (Table 2).
Comprehensive understanding of zooplankton biodiversity has eluded oceanographers because of the fragility, rarity, small size, and/or systematic complexity of many taxa. For many zooplankton groups, there are long-standing and unresolved questions of species identification, systematic relationships, genetic diversity and structure, and biogeography. The global geographic scale of the investigations required to address these issues, as well as the three-dimensional complexity of the world ocean, make complete knowledge of marine zooplankton diversity challenging. With increasing depths, the need for fundamental knowledge of pelagic biodiversity becomes even more marked.
Coarse-resolution sampling in the deep ocean from the late 1800s (the early days of ocean exploration) to the mid-1900s used mechanically activated closing or opening/closing nets (Wiebe and Benfield, 2003). During this era, deep-sea zooplankton species were described and their depth distributions were characterized (Banse, 1964). With the advent of multiple opening-closing net systems in the mid-1900s, high-resolution sampling of the bathypelagic and abyssopelagic zones became possible. More recently, observations and collections made from submersibles, ROVs (Robison, 2004, Osborn et al., 2009), and free-vehicles have shaped our understanding of the deep sea (Smith et al., 1979).
A number of studies have reported that copepods dominate the deep sea in terms of numbers of species, and that a number of these are many new or undescribed species. Of 67 copepod species found deeper than 2000 m in the Guaymas Basin, about half were new to science (Wiebe et al., 1988). Wishner (1980) identified over 100 copepod species in deep-sea near-bottom tows, including many that were undescribed. Despite these evident prospects for species discovery, taxonomic work on deep-sea zooplankton moves at a snail’s pace. Until recently, few studies have sought to integrate morphological and molecular systematic approaches to the study of deep-sea zooplankton (Bucklin et al., 2007, Bucklin et al., 2010, Jennings et al., 2010, Ortman et al., 2010).
The focus of the Census of Marine Zooplankton (CMarZ) program, a component of the Census of Marine Life, is on the development of a taxonomically comprehensive assessment of biodiversity of animal plankton throughout the world’s oceans. The project goal is to answer the question “what lives in the ocean?” The taxonomic focus is the animals that drift with ocean currents throughout their lives (i.e., the holozooplankton). Traditional morphologically-based analysis of zooplankton samples is being enhanced by the parallel use of molecular genetic characters (a barcode) in order to form a basis for accurate assessment of zooplankton species diversity (Bucklin et al., 2010).
As part of the CMarZ program, two cruises were undertaken to collect and identify the zooplankton distributed throughout the entire water column, with a particular focus on the under-sampled mesopelagic, bathypelagic, abyssopelagic zones, and then to sequence them genetically at sea. One was to the tropical/subtropical waters of the Atlantic Ocean west of the mid-Atlantic ridge in 2006 and the other to the Atlantic tropical/subtropical waters east of the mid-Atlantic ridge in 2007.
Section snippets
Methods
Sampling on the first cruise aboard the R/V Ronald H. Brown 06–03 was conducted along a transect extending from the northern Sargasso Sea to the equatorial waters east of the eastern Caribbean sea island of Martinique (Fig. 1a). At five primary stations, environmental data and zooplankton samples were collected using three Multiple Opening/Closing Nets and Environmental Sensing Systems (MOCNESS – Table 3). Other samples were collected with ring nets and water bottles, and by blue water SCUBA
Water column structure at the stations
The station locations on the Ron Brown cruise (Fig. 1) provided contrasting physical settings for the zooplankton collections. At station 1 in the Northern Sargasso Sea, the “eighteen degree water” was present from near the surface (∼40 m) to more than 400 m with only a shallow mixed layer of slightly warmer water at the surface (Fig. 2). The main thermocline and halocline occurred between 500 and 1000 m below which temperatures gradually decreased from around 5 °C to below 3 °C at 5000 m. At station
Summary
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Two CMarZ expeditions brought together classical taxonomists and gene sequencing experts to collaborate at sea and produce impressive results in just three weeks on each cruise in 2006 and 2007.
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Sampling was successfully completed from 5000 m to the surface at five stations in the Western North Atlantic Ocean and four stations in the eastern Atlantic Ocean.
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For several taxonomic groups, a significant fraction of the known species were collected and identified (e.g., Ostracods, Shelled Pteropods).
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A
Acknowledgments
The success of these two CMarZ cruises was due to the collective efforts of Captain, Officers, Crew, and all members of the Scientific Party on the R/V Ron Brown and the FS Polarstern. The UConn Team DNA (Rob Jennings, Paola Batta Lona, Brian Ortman, Lisa Nigro, Leo Blanco-Bercial and Christopher Sweetman) carried out the at-sea sequencing. Nancy Copley and Dicky Allison were principally responsible for arranging logistical elements for the cruises. Support for the R/V Ron Brown and R/V
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Our good friend and colleague Francesc died on 5 May 2007 while undergoing medical treatment. It is with great sorrow that we mourn his passing.