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Ecosystem Structure and Dynamics in the North Pacific Subtropical Gyre

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Ecosystem Structure and Dynamics in the North Pacific Subtropical Gyre Ecosystems DOI: 10.1007/s10021-017-0117-0 Ó 2017 The Author(s). This article is published with open access at Springerlink.com 20th Anniversary Paper Ecosystem Structure and Dynamics in the North Pacific Subtropical Gyre: New Views of an Old Ocean David M. Karl1,2* and Matthew J. Church1,2,3 1Center for Microbial Oceanography: Research and Education, C-MORE Hale, University of Hawaii at Manoa, 1950 East-West Rd., Honolulu, Hawaii 96822, USA; 2School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA; 3Present address: Flathead Lake Biological Station, University of Montana, Polson, MT 59860, USA ABSTRACT The North Pacific Subtropical Gyre (NPSG) is one of community with a relatively stable plankton com- the largest biomes on Earth. It has a semi-enclosed munity structure and relatively low variability in key surface area of about 2 9 107 km2 and mean depth of microbiological rates and processes. Now, after nearly nearly 5 km and includes a broad range of habitats three decades of observations and experimentation from warm, light-saturated, nutrient-starved surface we present a new view of this old ocean, one that waters to the cold, nutrient-rich abyss. Microorgan- highlights temporal variability in ecosystem pro- isms are found throughout the water column and are cesses across a broad range of scales from diel to vertically stratified by their genetically determined decadal and beyond. Our revised paradigm is built on metabolic capabilities that establish physiological the strength of high-quality time-series observations, tolerances to temperature, light, pressure, as well as on insights from the application of state-of-the-art organic and inorganic growth substrates. Despite the –omics techniques (genomics, transcriptomics, pro- global significance of the NPSG for energy and matter teomics and metabolomics) and, more recently, the transformations and its role in the oceanic carbon discoveries of novel microorganisms and metabolic cycle, it is grossly undersampled and not well char- processes. Collectively, these efforts have led to a new acterized with respect to ecosystem structure and understanding of trophic dynamics and population dynamics. Since October 1988, interdisciplinary interactions in the NPSG. A comprehensive under- teams of scientists from the University of Hawaii and standing of the environmental controls on microbial around the world have been investigating the NPSG rates and processes, from genomes to biomes, will be ecosystem at Station ALOHA (A Long-term Olig- required to inform the scientific community and the otrophic Habitat Assessment), a site chosen to be public at large about the potential impacts of human- representative of this expansive oligotrophic habitat, induced climate change. The pace of new discovery, with a focus on microbial processes and biogeo- and the importance of integrating this new knowl- chemistry. At the start of this comprehensive field edge into conceptual paradigms and predictive study, the NPSG was thought to be a ‘‘Climax’’ models, is an enormous contemporary challenge with great scientific and societal relevance. Received 27 July 2016; accepted 6 January 2017; Key words: microbial oceanography; North Paci- Data related to this manuscript can be found at hanaha.soest.hawaii.edu fic Subtropical Gyre; biogeochemistry; primary (HOT), scope.soest.hawaii.edu (SCOPE), cmore.soest.hawaii.edu (CMORE). production; oceanic carbon cycle; climate change; Authors’ contributions DMK conceived the study. DMK and MJC performed research, analyzed data and contributed new methods. DMK time series. and MJC wrote the paper. *Corresponding author; e-mail: [email protected] D. M. Karl and M. J. Church INTRODUCTION programs served to consolidate the importance of Station ALOHA as a site to advance the discipline of In 1999, one of us (Karl 1999) published a ‘‘mini microbial oceanography. Between 2006 and 2016, review’’ in volume #2 of Ecosystems that summa- C-MORE scientists made numerous discoveries of rized the state of knowledge on microbial and novel microorganisms, metabolic pathways and biogeochemical variability in the North Pacific population interactions. They also facilitated an Subtropical Gyre (NPSG). At that time, the Hawaii understanding of the time–space continuum by Ocean Time-series (HOT) program had already conducting higher-frequency temporal sampling amassed one of the most comprehensive data sets (hourly to weekly) than was possible to achieve in for any open ocean ecosystem (10 years of the HOT program structure and expanded regional approximately monthly observations) and was analyses using ships and remotely operated vehi- beginning to erode the ‘‘chronic undersampling’’ cles to better understand mesoscale variability (10– barrier that existed at that time. Because previous 100 km) in microbial dynamics in the NPSG. In zonal and meridional transects across the NPSG had 2014, a second complementary field-based research documented weak horizontal gradients in many program was established at Station ALOHA. The physical, chemical and biological characteristics, Simons Collaboration on Ocean Processes and relative to other regions of the ocean (Venrick Ecology (SCOPE) is an international research pro- 1971; Hayward and McGowan 1985), serial obser- gram with a mission to characterize the flows of vations at a strategically located sampling site might energy and matter through NPSG microbial be scaled up to the larger region of interest (Karl assemblages and into the mesopelagic zone. The 1999). This basic premise underlies the establish- primary stated goal for this planned, decade-long ment of Station ALOHA (A Long-term Oligotrophic program is to quantify the inputs to, and outputs Habitat Assessment) at 22°N45¢N, 158°W (Fig- from, the upper ocean (0–200 m) at Station ure 1A) as a NPSG benchmark (Karl and Winn ALOHA and to incorporate this new knowledge 1991; Karl and Lukas 1996). The HOT program was into ocean models. In November 2015, the Amer- designed to extend the knowledge base that was ican Society for Microbiology designated Ocean gained during the extensive study of the climax Station ALOHA as a ‘‘milestones in microbiology’’ region at 28°N, 155°W (Hayward and others 1983), site in recognition of historic and visionary during which time the NPSG exhibited a doubling accomplishments. in the concentrations of chlorophyll a and rates of This review will focus on four complementary primary production (Venrick and others 1987; Karl themes: (1) the fundamental role of sunlight in the and others 2001a; Figure 1B, C). The establishment structure and function of the NPSG, (2) phyto- of Station ALOHA would also provide an oppor- plankton diversity, (3) controls on primary pro- tunity to test novel methods and instrumentation duction and carbon export and (4) habitat and to serve as a natural laboratory for hypothesis variability, including the time–space continuum testing, experimentation and ocean science educa- and potential impacts of human-induced climate tion and training. These data sets would eventually change. The review concludes with some thoughts be used to develop a predictive capability for global on future challenges and opportunities. environmental change, by elucidating the path- ways and controls on physical and biological pro- cesses that govern the fluxes of carbon (C) into and STATION ALOHA: A PROVING GROUND from the ocean, especially the key roles of FOR ICROBIAL CEANOGRAPHY microorganisms (Figure 2). M O The immediate successes of the HOT program Research at Station ALOHA has had a major impact also brought attention to the poor state of knowl- on how we study, interpret and model microbial edge about the sea around us and catalyzed the processes in oligotrophic ecosystems. Many of the establishment of similar programs at strategic general characteristics of the NPSG habitat were coastal and open ocean locations worldwide. In known prior to the establishment of Station ALO- 2006, a US National Science Foundation Center of HA (Karl and Lukas 1996), but other key features Excellence in Microbial Oceanography was estab- were either poorly understood or not yet discov- lished at the University of Hawaii in collaboration ered. For example, the three major groups of with six partner institutions. The Center for microorganisms that are numerically dominant Microbial Oceanography: Research and Education members of the NPSG ecosystem (that is, (C-MORE) was literally built on a ‘‘HOT founda- Prochlorococcus spp., the SAR11 clade of Alphapro- tion.’’ The colocation of these two major field teobacteria and planktonic archaea) were all dis- Ecosystem Structure and Dynamics in the North Pacific Subtropical Gyre Figure 1. (A) Map of the North Pacific Ocean showing the location of Station ALOHA (22°45¢N, 158°W) within the NPSG. The background colors depict summer chlorophyll a distributions as determined by the SeaWiFS satellite mission (oceancolor.gsfc.nasa.gov/SeaWiFS/) with blue regions corresponding to the lowest concentrations (<0.1 mg m-3) and green-yellow-red corresponding to higher concentrations (>1mgm-3). The white contours represent mean annual surface ocean phosphate concentrations (lmol l-1) and emphasize the correspondence between chlorophyll a and nutrient concentrations. (B) Euphotic zone depth-integrated chlorophyll a as determined by the standard fluorometric method. The solid and dashed lines for the Station ALOHA-era data set represent the mean ± 1 SD (mean = 23.76, SD = 4.05, n = 131). (C) Euphotic zone depth-integrated rates of primary production as determined by the standard 14C method. Most of the data prior to the Station ALOHA-era were collected at or near the Climax study region and, where appropriate, represent the mean ± 1 SD for multiple observations for a given cruise, season or study. Horizontal bars indicate the duration of multicruise data sets (‘‘C’’ indicates Climax program; see Karl and others 2001a). The solid and dashed lines for the Station ALOHA-era data set represent the mean ± 1 SD (mean = 577.4, SD = 129.1, n = 127). covered after the establishment of Station ALOHA that quantitative estimates were available (Bergh in 1988 (Karl and Church 2014).
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