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Pakhomov E.A.,University of British Columbia | Hunt B.P.V.,University of British Columbia | Hunt B.P.V.,Hakai Institute
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2017

The distribution, density, demography and feeding dynamics of the pelagic tunicate Salpa thompsoni were investigated during the trans-Atlantic ANTARKTIS XXVIII/3 expedition to the Southern Ocean, on board RV Polarstern, between January and March 2012. Net samples were collected using an RMT-8 at four major sampling regions in the proximity of the Antarctic Polar Front (APF): along a 10°E transect, Salpastan, 12°W eulerian study, and the South Georgia Basin. Salps, mostly S. thompsoni, were found at all but one station and with few exceptions contributed <<40% to total zooplankton in terms of both abundance and biomass. Salp abundance and biomass was lowest (6.2 ind.1000m-3 and 0.8mgDWm-3) north of the APF along the 10°E transect and highest (3150 ind.1000m-3 and 20.5mgDWm-3) at the Salpastan station south of the APF. Size distributions of S. thompsoni aggregates and solitaries, and their developmental dynamics, were similar in all locations except the Salpastan region. In aggregates, the overall salp size distributions were strongly right-tailed with the mass of the distribution always concentrated in the left where small aggregates (6-10mm) dominated. During the eulerian study at 12°W, several cohorts of aggregates and solitaries were traceable allowing assessment of the daily growth rates of S. thompsoni. Estimated in situ growth rates of aggregates and solitaries were 0.53±0.18 and 2.83±0.42mmday-1, respectively, and the complete S. thompsoni life cycle duration (sexual+asexual) during austral summer at the APF region may have been as short as 2-3 months. The rapid growth rates of S. thompsoni found in this study urgently require further research to re-evaluate salp life cycle in the Southern Ocean. © 2017 Elsevier Ltd.

Donoso K.,Aix - Marseille University | Carlotti F.,Aix - Marseille University | Pagano M.,Aix - Marseille University | Hunt B.P.V.,Aix - Marseille University | And 4 more authors.
Journal of Geophysical Research: Oceans | Year: 2017

The Gulf of Lion is an important area of deep convection, where intense winter vertical mixing brings nutrients up from deeper layers, promoting the largest bloom in the Mediterranean at the end of winter/early spring. The DEWEX program conducted cruises in February and April 2013 to investigate the ecosystem level impacts of deep water convection. Zooplankton data were collected through net sampling and imaging with an Underwater Vision Profiler. In winter, low zooplankton abundance and biomass were observed in the Deep Convection Zone (DCZ) and higher values on its periphery. In spring, this pattern reversed with high biomass in the DCZ and lower values on the periphery. On average for the whole area, the potential grazing impact was estimated to increase by one order of magnitude from winter to spring. In April, all areas except the DCZ incurred top-down control by zooplankton on the phytoplankton stock. In the DCZ, the chlorophyll-a values remained high despite the high zooplankton biomass and carbon demand, indicating a sustained bottom-up control. The zooplankton community composition was comparable for both periods, typified by high copepod dominance, but with some differences between the DCZ and peripheral regions. In spring the DCZ was characterized by a strong increase in herbivorous species such as Centropages typicus and Calanus helgolandicus, and an increase in the number of large zooplankton individuals. Our study indicates that the DCZ is likely an area of both enhanced energy transfer to higher trophic levels and organic matter export in the North Western Mediterranean Sea. © 2017. American Geophysical Union. All Rights Reserved.

Darimont C.T.,University of Victoria | Darimont C.T.,Raincoast Conservation Foundation | Darimont C.T.,Hakai Institute | Fox C.H.,University of Victoria | And 5 more authors.
Science | Year: 2015

Paradigms of sustainable exploitation focus on population dynamics of prey and yields to humanity but ignore the behavior of humans as predators. We compared patterns of predation by contemporary hunters and fishers with those of other predators that compete over shared prey (terrestrial mammals and marine fishes). Our global survey (2125 estimates of annual finite exploitation rate) revealed that humans kill adult prey, the reproductive capital of populations, at much higher median rates than other predators (up to 14 times higher), with particularly intense exploitation of terrestrial carnivores and fishes. Given this competitive dominance, impacts on predators, and other unique predatory behavior, we suggest that humans function as an unsustainable "super predator," which - unless additionally constrained by managers - will continue to alter ecological and evolutionary processes globally. © 2015, American Association for the Advancement of Science. All rights reserved.

News Article | August 31, 2016
Site: www.rdmag.com

Human occupation is usually associated with deteriorated landscapes, but new research shows that 13,000 years of repeated occupation by British Columbia's coastal First Nations has had the opposite effect, enhancing temperate rainforest productivity. Andrew Trant, a professor in the Faculty of Environment at the University of Waterloo, led the study in partnership with the University of Victoria and the Hakai Institute. The research combined remote-sensed, ecological and archaeological data from coastal sites where First Nations' have lived for millennia. It shows trees growing at former habitation sites are taller, wider and healthier than those in the surrounding forest. This finding is, in large part, due to shell middens and fire. "It's incredible that in a time when so much research is showing us the negative legacies people leave behind, here is the opposite story," said Trant, a professor in Waterloo's School of Environment, Resources and Sustainability. "These forests are thriving from the relationship with coastal First Nations. For more than 13,000 years --500 generations -- people have been transforming this landscape. So this area that at first glance seems pristine and wild is actually highly modified and enhanced as a result of human behaviour." Fishing of intertidal shellfish intensified in the area over the past 6,000 years, resulting in the accumulation of deep shell middens, in some cases more than five metres deep and covering thousands of square metres of forest area. The long-term practice of harvesting shellfish and depositing remnants inland has contributed significant marine-derived nutrients to the soil as shells break down slowly, releasing calcium over time. The study examined 15 former habitation sites in the Hakai Lúxvbálís Conservancy on Calvert and Hecate Islands using remote-sensed, ecological and archaeological methods to compare forest productivity with a focus on western red cedar. The work found that this disposal and stockpiling of shells, as well as the people's use of fire, altered the forest through increased soil pH and important nutrients, and also improved soil drainage. This research is the first to find long-term use of intertidal resources enhancing forest productivity. Trant says it is likely similar findings will occur at archaeological sites along many global coastlines. "These results alter the way we think about time and environmental impact," he said. "Future research will involve studying more of these human-modified landscapes to understand the extent of these unexpected changes."

News Article | November 3, 2016
Site: www.eurekalert.org

EUGENE, Ore. -- Nov. 3, 2016 -- Fire-scarred trees on isolated Hecate Island off British Columbia have helped researchers from two Pacific Northwest universities isolate patterns of wildfires over the past 700 years of climate data. Indigenous populations, they found, likely had a profound effect on fire activity. The human connection became clear after determining that the last fire occurred in 1893, around the time the island was abandoned as indigenous populations were moved to permanent settlements on the mainland, said Daniel G. Gavin of the University of Oregon and Kira M. Hoffman of the University of Victoria. "Fire is historically important in the West," said Gavin, a professor of geography. "There is a lot of evidence from fire scars throughout the Pacific Northwest, not just in the dry pine forests, that point to more frequent fires having occurred before 1900." The study, published Oct. 26 in the open-access journal Royal Society Open Science, was the first to use tree-ring records obtained through fire-scarred trees spanning such long time scales in a temperate coastal rainforest. The island has a short dry season, experiences more than 4,000 millimeters (157 inches) of rain annually and is located in a northern region where both human- and lightning-caused fires have been considered to be rare. The study helps to begin filling a void in the historical record about the frequency of fire activity in coastal temperate rainforests in the Pacific Northwest. The island also is in a place little affected by fire suppression or 20th century land uses, which makes attributing pre-20th century fire to human causes simpler than is possible in other places in the Pacific Northwest. Hoffman, a former forest firefighter, chose the island for the study after finding a fire-damaged tree while she was there examining soils during a project for a master's of science degree. "Finding this fire evidence was an accident and I was really surprised because there had been no evidence of fire on the central coast of British Columbia," she said. "The more I looked, the more I found. We know that the fires were smoldering ground fires because they didn't kill the trees that they scarred, and they didn't fully burn ground fuels." To help pursue her study, Hoffman spent four months in 2015 as a courtesy research associate with Gavin's UO research group to learn a statistical analysis approach that Gavin had created for correlating wildfire occurrences with respect to other fires and past climate events. The technique was based on one developed by neuroscientists studying connections between firing neurons and movements elsewhere in the body. Gavin also had identified frequent fire patterns on Vancouver Island prior to the 1870s during his doctoral research at the University of Washington in Seattle. The new study by Hoffman, Gavin and co-author Brian M. Starzomski, a professor in UVic's School of Environmental Studies, drew data from some 3,000 trees and other vegetation at 30 plots within 300 hectares (741 acres) -- an area that has three archaeological sites where indigenous populations had lived. In a 517-year period, from 1376 to 1893, 13 fire events occurring roughly every 39 years were documented. The fires tended to follow El Nino events by six to nine and 13 to 17 years. The final fire, in 1893, was the largest and occurred during one of the wettest years on record and possibly coincided with undetermined events outside the regular cycle of fires. The farther away from the living areas, evidence for fires became scarcer. The researchers reported that fire occurrences during their study period were 25 times more likely than previous estimates of greater than 1,000-year intervals, in which tree-ring records had not been used. Previous estimates were drawn mostly from air photos and charcoal buried in soils and preserved in lake sediments. They found little direct relationship between drought and fire years. "It seems that fire events don't have a pattern if you look only at weather in any given year," Hoffman said. "When we pulled back and look at longer climate patterns, we began to see that fires were linked to prolonged periods of warm and dry conditions that build up over time." The team's findings were in agreement with earlier research on the scarcity of fires in the region, but varied in that the researchers were able to tie fire events to human activity. The absence of fire after 1893, the researchers concluded, suggests that indigenous peoples had likely used fire as a tool for resource management. "It's possible they would have relied heavily on burning forests and single trees to create dry firewood for cooking and heating, clothing, canoes, food and many other resources," Hoffman said. The Natural Sciences and Engineering Research Council of Canada, Hakai Institute and Canada Foundation for Innovation primarily funded the research. Hoffman also was supported by a Dr. Ian and Joyce McTaggart Cowan Scholarship at UVic. Sources: Kira Hoffman, doctoral student, University of Victoria School of Environmental Studies, 805-256-4465, khoff@uvic.ca; and Dan Gavin, associate professor of geography, University of Oregon, 541-346-9380, dgavin@uoregon.edu

News Article | March 2, 2017
Site: www.eurekalert.org

The Ecological Society of America (ESA) will present the 2017 awards recognizing outstanding contributions to ecology in new discoveries, teaching, sustainability, diversity, and lifelong commitment to the profession during the Society's Annual Meeting in Portland, Ore. The awards ceremony will take place during the Scientific Plenary on Monday, August 7, at 8 AM in the Oregon Ballroom, Oregon Convention Center. Learn more about ESA awards on our home website. The Eminent Ecologist Award honors a senior ecologist for an outstanding body of ecological work or sustained ecological contributions of extraordinary merit. Soil ecologist Diana Wall, the founding director of the Colorado State University's School of Global Environmental Sustainability, is world-renowned for uncovering the importance of below-ground processes. Best known for her outstanding quarter century of research in the McMurdo Dry Valleys in Antarctica, one of the more challenging environments of the planet. Her research has revealed fundamental soil processes from deserts and forests to grasslands and agricultural ecosystems to New York City's Central Park. Dr. Wall's extensive collaborative work seeks to understand how the living component of soil contributes to ecosystem processes and human wellbeing--and to in turn uncover how humans impact soils, from local to global scales. In landmark studies, she revealed the key role of nematodes and other tiny animals as drivers of decomposition rates and carbon cycling. The biodiversity in soils, she found, influences ecosystem functioning and resilience to human disturbance, including climate change. She demonstrated that the biodiversity belowground can at times be decoupled from biodiversity aboveground. Her focus on nematodes in soils in very harsh environments, from the cold, dry Antarctic to hot, dry deserts, opened up a perspective on how life copes with extreme environments. She has a laudable record of publishing excellent papers in top-ranked scientific journals. Dr. Wall has played a vital role as an ecological leader, chairing numerous national and international committees and working groups and serving as president of the Ecological Society of America in 1999. She is a Fellow of ESA, the American Association for the Advancement of Science, and the Society of Nematologists. In 2013, she received the Tyler Prize for Environmental Achievement for her outspoken efforts as an ambassador for the environmental and economic importance of soils and ecology. Currently, she is scientific chair of the Global Soil Biodiversity Initiative, which works to advance soil biodiversity for use in policy and management of terrestrial ecosystems. Dr. Wall is well-respected in her role as mentor of young scientists, over several generations, and as a communicator of science outside the usual academic arenas. Odum Award recipients demonstrate their ability to relate basic ecological principles to human affairs through teaching, outreach, and mentoring activities.? Kathleen Weathers is a senior scientist and the G.Evelyn Hutchinson chair of ecology at the Cary Institute of Ecosystem Studies, where she focuses on freshwater ecosystems. For more than a decade, she has been dedicated to advancing bottom-up network science, creating training opportunities for graduate students and tools for citizen science engagement. Her efforts strive to equip the next generation of ecologists and managers with the skills needed to protect freshwater resources. Dr Weathers played a guiding role in the formation of the Global Lake Ecological Observatory Network (GLEON), and currently acts as co-chair. A part of this international grassroots collaboration she helped develop Lake Observer, a crowd-sourcing App that streamlines the way that researchers and citizen scientists record water quality observations in lakes, rivers, and streams. Dr. Weathers has made it a priority to mentor students and early-career scientists participating in GLEON, with an eye toward diversity, inclusion, and instruction. She helped empower GLEON's student association, which contributes meaningfully to governance and training within the broader network. She also spearheaded the development of the GLEON Fellows Program, a two-year graduate immersion in data analysis, international collaboration, effective communication, and team science. The GLEON Fellows Program has emerged as a model for training initiatives in macrosystem ecology, and will affect the ecological community positively for decades to come, as participants carry their training forward to other institutions and endeavors. The Distinguished Service Citation recognizes long and distinguished volunteer service to ESA, the scientific community, and the larger purpose of ecology in the public welfare. Debra Peters is the founding editor-in-chief of ESA's newest journal, Ecosphere, created in 2010 to offer a rapid path to publication for research reports from across the spectrum of ecological science, including interdisciplinary studies that may have had difficulty finding a home within the scope of the existing ESA family of journals. In her hands the online-only, open-access journal has claimed a successful niche in the ecological publications landscape, expanding to publish over 400 manuscripts in 2016. Dr. Peters, an ecologist for the United States Department of Agriculture Agricultural Research service's (USDA-ARS) Jornada Experimental Range and lead principal investigator for the Jornada Basin Long Term Ecological Research program in Las Cruces, New Mexico, has served on the editorial boards of ESA's journals Ecological Applications, Ecology and Ecological Monographs. She chaired the society's Rangeland Section, was a founding member and chair of the Southwest Chapter, and has served as member-at-large on the Governing Board. As program chair for the 98th Annual Meeting of the society, she inaugurated the wildly popular Ignite talks, which give speakers the opportunity to present conceptual talks that do not fit into the standard research presentation format. Dr. Peters has greatly contributed to the broader research enterprise as senior advisor to the chief scientist at the USDA, and as a member of the National Ecological Observatory Network's (NEON) Board of Directors. She has provided this quite amazing array of services in support of the society and her profession while maintaining an outstanding level of research productivity and scientific leadership in landscape-level, cross-scale ecosystem ecology. Many of her more than 100 research publication have been cited more than 100 times. Her fine record of research led to her election as a Fellow of ESA and the American Association for the Advancement of Science. In all respects, Debra Peters exemplifies distinguished service to the ESA, and to science. ESA's Commitment to Human Diversity in Ecology award recognizes long-standing contributions of an individual towards increasing the diversity of future ecologists through mentoring, teaching, or outreach. Gillian Bowser, research scientist in Colorado State University's Natural Resource Ecology Laboratory, is honored for her joyful and successful recruitment and retention of under-represented students to the study of ecology, to public service in support of the natural world, and to empowerment of women and minorities worldwide. The Cooper Award honors the authors of an outstanding publication in the field of geobotany, physiographic ecology, plant succession or the distribution of plants along environmental gradients. William S. Cooper was a pioneer of physiographic ecology and geobotany, with a particular interest in the influence of historical factors, such as glaciations and climate history, on the pattern of contemporary plant communities across landforms. University of Waterloo, Ontario professor Andrew Trant and colleagues at the University of Victoria and the Hakai Institute in British Columbia revealed a previously unappreciated historical influence on forest productivity: long-term residence of First Nations people. Counter to a more familiar story of damage to ecosystems inflicted by people and their intensive use of resources, the activities of native people on the Central Coast of British Columbia enhanced the fertility of the soil around habitation sites, leading to greater productivity of the dominant tree species, the economically and culturally valuable western redcedar (Thuja plicata Donn ex D. Don). Through a combination of airborne remote sensing and on-the-ground field work, the authors showed that forest height, width, canopy cover, and greenness increased on and near shell middens. They presented the first documentation of influence on forest productivity by the daily life activities of traditional human communities. The Mercer Award recognizes an outstanding and recently-published ecological research paper by young scientists. Biological invasions, and migrations of native species in response to climate change, are pressing areas of interest in this time of global change. Fragmentation of the landscape by natural and human-made barriers slows the velocity of spread, but it is not known how patchy habitat quality might influence the potential for evolution to accelerate invasions. Jennifer Williams, an assistant professor at the University of British Columbia, and colleagues implemented a creative experimental design using the model plant species Arabidopsis thaliana that allowed them to disentangle ecological and evolutionary dynamics during population expansion. Some plant populations were allowed to evolve, while others were continually reset to their original genetic composition. The authors convincingly demonstrate that rapid evolution can influence the speed at which populations spread, especially in fragmented landscapes. The Sustainability Science Award recognizes the authors of the scholarly work that makes the greatest contribution to the emerging science of ecosystem and regional sustainability through the integration of ecological and social sciences. Sustainability challenges like air pollution, biodiversity loss, climate change, energy and food security, disease spread, species invasion, and water shortages and pollution are often studied, and managed, separately, although they the problems they present are interconnected. Jianguo Liu and colleagues provide a framework for addressing global sustainability challenges from a coupled human and natural systems approach that incorporates both socioeconomic and environmental factors. They review several recent papers that have quantified at times conflicting efforts to provide ecosystem services, when these efforts are examined in a global perspective. The authors argue for the need to quantify spillover systems and feedbacks and to integrate analyses over multiple spatial and temporal scales. This will likely require the development of new analytical frameworks both to understand the social ecological mechanisms involved and to inform management and policy decisions for global sustainability. The Innovation in Sustainability Science Award recognizes the authors of a peer-reviewed paper published in the past five years exemplifying leading-edge work on solution pathways to sustainability challenges. One of the biggest challenges facing development of effective policy to address sustainability issues is that the concepts and vocabulary used by scientists to define and promote sustainability rarely translate into effective policy, because they do not include measures of success. This challenge is particularly apparent in the concept of stability and resilience, terms which are frequently used in policy statements and have long been the subject of empirical and theoretical research in ecology, but for which there are no easily defined and quantified metrics. Ian Donohue and colleagues argue that much of the fault for this disconnect lies with the academic community. They summarize and analyze a number of examples to support their claim that ecologists have taken a one-dimensional approach to quantifying stability and disturbance when these are actually multi-dimensional processes. They argue that this has led to confused communication of the nature of stability, which contributes to the lack of adoption of clear policies. They propose three areas where future research is needed and make clear recommendations for better integrating the multidimensional nature of stability into research, policy and actions that should become a priority for all involved in sustainability science. The Whittaker Award recognizes an ecologist with an earned doctorate and an outstanding record of contributions in ecology who is not a U.S. citizen and who resides outside the United States. Petr Pyšek, the chair of the Department of Invasion Ecology at the Academy of Sciences of the Czech Republic, is honored for his pioneering and insightful work in invasion ecology. Dr. Pyšek is editor-in-chief of Preslia (Journal of the Czech Botanical Society) and serves on the editorial boards of Biological Invasions, Diversity and Distributions, Folia Geobotanica, and Perspectives on Plant Ecology, Evolution and Systematics. The Shreve award supplies $1,000-2,000 to support ecological research by graduate or undergraduate student members of ESA in the hot deserts of North America (Sonora, Mohave, Chihuahua, and Vizcaino). Daniel Winkler, a PhD student with Travis Huxman at University of California Irvine, studies the invasion of Sahara mustard (Brassica tournefortii) in the Mojave, Sonoran, and Chihuahuan deserts. His dissertation focuses on determining the source populations of Sahara mustard and whether plasticity in functional traits is allowing the species to spread. Funds from the Forrest Shreve Student Research Fund will be used to process samples for leaf stable isotopes and elemental stoichiometry, allowing for a comparison of functional traits indicative of local adaptation and the species' plasticity. Daniel was a National Park Service Young Leaders in Climate Change Fellow and a NSF EAPSI Research Fellow. Learn more about the August 7-12, 2017 ESA Annual Meeting on the meeting website: http://esa. ESA welcomes attendance from members of the press and waives registration fees for reporters and public information officers. To apply, please contact ESA Communications Officer Liza Lester directly at llester@esa.org. The Ecological Society of America (ESA), founded in 1915, is the world's largest community of professional ecologists and a trusted source of ecological knowledge, committed to advancing the understanding of life on Earth. The 10,000 member Society publishes five journals and a membership bulletin and broadly shares ecological information through policy, media outreach, and education initiatives. The Society's Annual Meeting attracts 4,000 attendees and features the most recent advances in ecological science. Visit the ESA website at http://www. .

Finke J.F.,University of British Columbia | Hunt B.P.V.,University of British Columbia | Hunt B.P.V.,Hakai Institute | Winter C.,University of British Columbia | And 2 more authors.
Viruses | Year: 2017

Virus particles are highly abundant in seawater and, on average, outnumber microbial cells approximately 10‐fold at the surface and 16‐fold in deeper waters; yet, this relationship varies across environments. Here, we examine the influence of a suite of environmental variables, including nutrient concentrations, salinity and temperature, on the relationship between the abundances of viruses and prokaryotes over a broad range of spatial and temporal scales, including along a track from the Northwest Atlantic to the Northeast Pacific via the Arctic Ocean, and in the coastal waters of British Columbia, Canada. Models of varying complexity were tested and compared for best fit with the Akaike Information Criterion, and revealed that nitrogen and phosphorus concentrations, as well as prokaryote abundances, either individually or combined, had significant effects on viral abundances in all but hypoxic environments, which were only explained by a combination of physical and chemical factors. Nonetheless, multivariate models of environmental variables showed high explanatory power, matching or surpassing that of prokaryote abundance alone. Incorporating both environmental variables and prokaryote abundances into multivariate models significantly improved the explanatory power of the models, except in hypoxic environments. These findings demonstrate that environmental factors could be as important as, or even more important than, prokaryote abundance in describing viral abundance across wide‐ranging marine environments. © 2017 by the authors. Licensee MDPI, Basel, Switzerland.

PubMed | Hakai Institute and University of Waterloo
Type: | Journal: Nature communications | Year: 2016

Human occupation is usually associated with degraded landscapes but 13,000 years of repeated occupation by British Columbias coastal First Nations has had the opposite effect, enhancing temperate rainforest productivity. This is particularly the case over the last 6,000 years when intensified intertidal shellfish usage resulted in the accumulation of substantial shell middens. We show that soils at habitation sites are higher in calcium and phosphorous. Both of these are limiting factors in coastal temperate rainforests. Western redcedar (Thuja plicata) trees growing on the middens were found to be taller, have higher wood calcium, greater radial growth and exhibit less top die-back. Coastal British Columbia is the first known example of long-term intertidal resource use enhancing forest productivity and we expect this pattern to occur at archaeological sites along coastlines globally.

Hunt B.P.V.,University of British Columbia | Hunt B.P.V.,Hakai Institute | Bonnet S.,Aix - Marseille University | Berthelot H.,Aix - Marseille University | And 3 more authors.
Biogeosciences | Year: 2016

In oligotrophic tropical and subtropical oceans, where strong stratification can limit the replenishment of surface nitrate, dinitrogen (N2) fixation by diazotrophs can represent a significant source of nitrogen (N) for primary production. The VAHINE (VAriability of vertical and tropHIc transfer of fixed N2 in the south-wEst Pacific) experiment was designed to examine the fate of diazotroph-derived nitrogen (DDN) in such ecosystems. In austral summer 2013, three large (∼50m3) in situ mesocosms were deployed for 23 days in the New Caledonia lagoon, an ecosystem that typifies the low-nutrient, low-chlorophyll environment, to stimulate diazotroph production. The zooplankton component of the study aimed to measure the incorporation of DDN into zooplankton biomass, and assess the role of direct diazotroph grazing by zooplankton as a DDN uptake pathway. Inside the mesocosms, the diatom-diazotroph association (DDA) het-1 predominated during days 5-15 while the unicellular diazotrophic cyanobacteria UCYN-C predominated during days 15-23. A Trichodesmium bloom was observed in the lagoon (outside the mesocosms) towards the end of the experiment. The zooplankton community was dominated by copepods (63% of total abundance) for the duration of the experiment. Using two-source N isotope mixing models we estimated a mean ∼28% contribution of DDN to zooplankton nitrogen biomass at the start of the experiment, indicating that the natural summer peak of N2 fixation in the lagoon was already contributing significantly to the zooplankton. Stimulation of N2 fixation in the mesocosms corresponded with a generally low-level enhancement of DDN contribution to zooplankton nitrogen biomass, but with a peak of ∼73% in mesocosm 1 following the UCYN-C bloom. qPCR analysis targeting four of the common diazotroph groups present in the mesocosms (Trichodesmium, het-1, het-2, UCYN-C) demonstrated that all four were ingested by copepod grazers, and that their abundance in copepod stomachs generally corresponded with their in situ abundance. 15N2 labelled grazing experiments therefore provided evidence for direct ingestion and assimilation of UCYN-C-derived N by the zooplankton, but not for het-1 and Trichodesmium, supporting an important role of secondary pathways of DDN to the zooplankton for the latter groups, i.e. DDN contributions to the dissolved N pool and uptake by nondiazotrophs. This study appears to provide the first evidence of direct UCYN-C grazing by zooplankton, and indicates that UCYN-C-derived N contributes significantly to the zooplankton food web in the New Caledonia lagoon through a combination of direct grazing and secondary pathways. © 2016 Author(s).

Harding J.M.S.,Simon Fraser University | Segal M.R.,Simon Fraser University | Reynolds J.D.,Simon Fraser University | Reynolds J.D.,Hakai Institute
PLoS ONE | Year: 2015

Estuaries are amongst the world's most productive ecosystems, lying at the intersection between terrestrial and marine environments. They receive substantial inputs from adjacent landscapes but the importance of resource subsidies is not well understood. Here, we test hypotheses for the effects of both terrestrial- and salmon-derived resource subsidies on the diet (inferred from stable isotopes of muscle tissue), size and percent nitrogen of the softshell clam (Mya arenaria), a sedentary estuarine consumer. We examine how these relationships shift across natural gradients among 14 estuaries that vary in upstream watershed size and salmon density on the central coast of British Columbia, Canada. We also test how assimilation and response to subsidies vary at smaller spatial scales within estuaries. The depletion and enrichment of stable isotope ratios in soft-shell clam muscle tissue correlated with increasing upstream watershed size and salmon density, respectively. The effects of terrestrial- and salmon-derived subsidies were also strongest at locations near stream outlets. When we controlled for age of individual clams, there were larger individuals with higher percent nitrogen content in estuaries below larger watersheds, though this effect was limited to the depositional zones below river mouths. Pink salmon exhibited a stronger effect on isotope ratios of clams than chum salmon, which could reflect increased habitat overlap as spawning pink salmon concentrate in lower stream reaches, closer to intertidal clam beds. However, there were smaller clams in estuaries that had higher upstream pink salmon densities, possibly due to differences in habitat requirements. Our study highlights the importance of upstream resource subsidies to this bivalve species, but that individual responses to subsidies can vary at smaller scales within estuaries. © 2015 Harding et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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