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    Experimental results of tethered amphipod and isopod survival in global eelgrass habitats, summer 2015 (Zostera Experimental Network 2; ZEN2)

Experimental results of tethered amphipod and isopod survival in global eelgrass habitats, summer 2015 (Zostera Experimental Network 2; ZEN2)

Website: https://www.bco-dmo.org/dataset/724015
Data Type: experimental, Other Field Results
Version: 2
Version Date: 2018-01-24

» Global biodiversity and functioning of eelgrass ecosystems (Zostera Experimental Network 2) (ZEN 2)
Hovel, KevinSan Diego State University (SDSU)Principal Investigator
Duffy, J EmmettSmithsonian Institution (TMON)Co-Principal Investigator
Stachowicz, John J.University of California-Davis (UC Davis)Co-Principal Investigator
Copley, NancyWoods Hole Oceanographic Institution (WHOI BCO-DMO)BCO-DMO Data Manager


Spatial Extent: N:60.1084 E:0.001 S:31.753 W:135
Temporal Extent: 2013-05-01

Dataset Description

This dataset includes survival of tethered amphipods and isopods in eelgrass plots, and the biomass of other organisms in the plots. The eelgrass beds were located globally.

Data version 2018-01-24 replaced version 2018-01-17: longitudes were corrected for sites VA and GA.

Acquisition Description

In the summer of 2015 we quantified predation risk for mesograzers using identical experiments conducted at 17 sites spread across North America, Europe, and Asia (see Table 1 at end of this document). In each experiment, we used tethering to determine relative predation risk for locally collected organisms along patch edges and in patch interiors under three levels of simulated eelgrass degradation (0, 50, and 80% shoot loss) in a crossed design. Epifauna selected for tethering were small (approximately 2 – 20 mm in length) mesograzers (gammarid amphipods and isopods) that are commonly found in the guts of small fishes (Table 1). Tethering measures the relative mortality rate of prey among different treatments, and because tethered prey cannot flee from predators, represents the relative mortality rate for prey that are readily available to predators (Aronson & Heck 1995). Though organisms used in tethering experiments differed in species and sizes among sites, we only used taxa of sizes that are commonly found in the guts of predators at each site.  

To set up experiments, at each site we first selected a large eelgrass bed (typically > 5,000 m2) in shallow water (0.5 – 1.5 m water depth at low tide) with a distinct edge formed by an abrupt transition from eelgrass to unvegetated sand or mud. Edge habitat was defined as being within eelgrass but within 1 m of the transition from eelgrass to unvegetated sediment, and interior habitat was > 5 m from this transition. We chose these distances because in seagrass habitat edge effects on mortality and abundance of small epifauna typically occur within 1 m from patch edges (Tanner 2005, MacReadie et al. 2010). Patch vegetation consisted exclusively of eelgrass, except for epibionts or sparse drift algae. At each site, we created 21 experimental blocks along the edge and 21 experimental blocks within the interior of the eelgrass bed. Each block consisted of two 1 m x 1 m eelgrass plots separated by a distance of 30 cm. One randomly selected plot in each block was designated for tethering mesograzers, and the other plot was used to tether larger organisms in a companion experiment (data are not listed for this companion experiment). We randomly selected seven of the 21 blocks at the edge and in the interior, and after obtaining shoot counts within these plots, haphazardly pulled shoots by hand to thin each plot to 50% of its ambient shoot density, creating 50% shoot loss plots. Another randomly selected seven blocks were thinned to 20% ambient shoot density (80% shoot loss plots), and the remaining seven remained at ambient shoot density.

To conduct experimental trials, we affixed locally collected mesograzers to 10 cm pieces of monofilament (Fireline™; dia. 0.13 mm) tied near the top of 40 cm clear acrylic rods. After being tethered in the lab, each mesograzer was held in seawater overnight before being deployed to the center of a randomly chosen plot, 15 cm above the sediment surface, between 0800 – 1100 h the next morning. Trials lasted 24 h, at which time we retrieved acrylic rods and scored each individual as alive, eaten (fragments of the carapace remaining on the tether), missing, or molted (entire carapace remaining on the tether). We considered organisms that went missing to have been consumed by predators because no organisms tethered in predator-free controls at three sites (n = 20 mesograzers at Bodega Bay, Finland, and San Diego) fell off tethers after 48 h. Few animals molted on tethers, and any that did were removed from the analysis. Four trials of the experiment were conducted over a 7 – 10 day period at each site (N = 7 individuals per treatment per trial * 6 treatments * 4 trials = 168 organisms tethered per site).

Immediately after trials concluded at a site we sampled plots in which mesograzers were tethered for epibiont biomass and epifaunal biomass. Epibiont biomass represented the degree to which eelgrass shoots were colonized by epiphytic algae and sessile epifauna such as bryozoans; these organisms contribute to variability in structural complexity at very small scales. We used the biomass of mobile crustacean epifauna as a proxy for prey density. To quantify epibiont biomass, three shoots near the center of each plot were haphazardly selected and removed from the plot, and returned to the laboratory where all epibionts were scraped from shoots, dried, and weighed. Scraped shoots also were dried and weighed to calculate epibiont biomass per unit eelgrass biomass. Epifauna were sampled by placing a 500 m mesh bag with a 20 cm diameter opening over eelgrass in a haphazardly selected area of each plot. This method targets small mobile mesograzers, but not larger mesopredators. Captured organisms were removed from eelgrass blades in the laboratory, separated into crustaceans vs. others taxa (primarily gastropods), and weighed. Eelgrass collected in the bag was dried and weighed to standardize epifaunal biomass per unit eelgrass biomass.

Instruments: Experiments were conducted individually at each site, so instrumentation varied among sites. At each site, a balance was used to measure biomass of organisms collected in plots. In the field, sampling of organisms was performed by collecting organisms in mesh bags or by clipping shoots. Shoot counts were made using PVC or wire rings laid over plots.

Table 1. (A) Sites used in the tethering experiment, their locations, and principle investigators involved in the study. (B) Taxa used for the tethering experiment at each site.



Principle Investigator




Bodega Bay, California, USA

J. Stachowicz




Posejarje, Adriatic Sea, Croatia

C. Kruschel




Angso Island, Baltic Sea, Finland

C. Boström




Bouzigues, Mediterranean Sea, France

F. Rossi




Shinryu, Hokkaido, Japan

M. Nakaoka




Akiwan Bay, Hiroshima, Japan

M. Hori




Dong-dae Bay, Korea

K-S Lee




Koje Bay, Korea

K-S Lee




Punt Banda Estuary, Baja, Mexico

C. Hereu, P. Jorgensen




Back Sound, North Carolina, USA

J. Fodrie




Greyabbey, Irish Sea, Northern Ireland

N. O'Connor




Sally's Bend, Oregon, USA

F. Nash




Point-Lebel, Quebec, Canada

M. Cusson




San Diego Bay, California, USA

K. Hovel




San Francisco Bay, California, USA

K. Boyer




Chesapeake Bay, Virginia, USA

E. Duffy




Willapa Bay, Washington, USA

J. Ruesink



Processing Description

Data were submitted to the PI from each site in the form of Microsoft Excel 2013 spreadsheets. The final collated data set is available as a Microsoft Excel spreadsheet.

BCO-DMO Processing Notes:
- added conventional header with dataset name, PI name, version date
- modified parameter names to conform with BCO-DMO naming conventions

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Taxontype of organism tethered: 0 = alive; 1 = dead unitless
Siteone of 17 unique sites unitless
BasinAtlantic or Pacific unitless
Latitudelatitude of the site; north is positive decimal degrees
Longitudelongitude of the site; east is positive decimal degrees
Patch_locationwhether organism was tethered at patch edge or in patch interior: edge or interior unitless
Shoot_reductionone of 3 levels of shoot reduction: ambient = no reduction; fifty = 50% shoot reduction; eighty = 80% shoot reduction unitless
Tether_nounique number for each tethered organism unitless
Size_mmsize of the tethered organism in mm millimeters (mm)
Crust_biomassbiomass of all crustaceans captured in samples taken in the plot in which this organism was tethered. grams per shoot
Other_biomassbiomass of all non-crustaceans captured in samples taken in the plot in which this organism was tethered. grams per shoot
Total_biomassbiomass of all organisms captured in samples taken in the plot in which this organism was tethered. grams per shoot
Epiphyte_biomassbiomass of all organisms living on eelgrass leaves in samples taken in the plot in which this organism was tethered. grams per shoot
Starting_shoot_densshoot density of the plot in which this organism was tethered; before any shoot reduction. shoots per square meter
Calc_shoot_densshoot density of the plot after shoot reduction. shoots per square meter
Statusstatus of the tethered organism after a 24 hour deployment: 0 = alive and 1 = eaten unitless

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Dataset-specific Instrument Name
Generic Instrument Name
Dataset-specific Description
Used to measure biomass
Generic Instrument Description
An instrument used to measure weight or mass.

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Start Date
End Date
eelgrass community studies

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Project Information

Global biodiversity and functioning of eelgrass ecosystems (Zostera Experimental Network 2) (ZEN 2)

Coverage: 20+ sites located throughout the northern hemisphere

Description from NSF award abstract:
This research will produce the second generation of a global collaborative research project, the Zostera Experimental Network (ZEN), to quantify the interacting influences of environmental forcing, biodiversity, and food-web perturbations on structure and functioning of eelgrass (Zostera marina) beds, the foundation of important but threatened coastal ecosystems worldwide. Partners at 40 sites in 14 countries will conduct parallel, standardized field sampling of producer and consumer biomass and diversity, and measure grazing and predation rates, to produce a global map of biodiversity, biomass distribution among trophic levels, and ecosystem processes in eelgrass habitats. Partners at a subset of core sites will conduct factorial experiments to characterize the interaction of nutrient loading, predator loss, and biogenic habitat structure (eelgrass density) in mediating producer growth and trophic processes in eelgrass. Finally, guided by the results from mechanistic experiments, the global field data will be used to test specific hypotheses about impacts of climate warming, nutrient loading, and declining biodiversity on eelgrass ecosystems via structural equation modeling, a uniquely powerful approach to dissecting complex interacting networks of causality. The proposed research will characterize in unprecedented detail how environmental forcing, biodiversity, and food-web processes interact to mediate functioning of a coastal ecosystem on a global scale. There are four general objectives:

1. Quantify linkages between eelgrass genetic diversity, growth, and provision of animal habitat;

2. Quantify the influence of eelgrass habitat structure on consumer-prey interactions, secondary production, and trophic transfer;

3. Identify mechanisms for the influence of grazer diversity on algal control;

4. Develop a global map of grazing and predation intensity to assess the relative importance of bottom-up and top-down forcing in eelgrass beds.

This program's integrated characterization of biodiversity, ecosystem state variables, and process rates across the globe is arguably unique in any marine system. It builds on promising results from the first generation of ZEN to allow for the first time a rigorous analysis of links between biodiversity and ecosystem functioning in a natural system on a global scale. As part of this analysis, the proposed research will provide the most comprehensive analysis yet of the controversial question of the relative importance of bottom-up and top-down forcing in seagrass ecosystems, an issue of fundamental importance to management and conservation.

Seagrasses and the many ecosystem services they provide are declining worldwide. This project's data on higher trophic levels and food-web interactions will provide a valuable and overdue complement to the many monitoring programs around the world that focus primarily on seagrasses and water quality, and will ultimately be made available to parameterize and test models of threatened seagrass ecosystems at a higher level of resolution ecological reality than previously possible. The success of the Zostera Experimental Network (ZEN) is evidenced by the continuation of all but one partner in the second generation (ZEN 2), and recruitment of nearly the same number of new partners to this global collaboration. This research will solidify and expand this network by more than doubling the number of participating sites, collaborating with parallel European Union and Japanese efforts, and integrating the world's largest and most successful seagrass restoration project at the Virginia Coast LTER site.

Note: This is an NSF-funded Collaborative Research project.

Also see: ZEN project BCO-DMO page.

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Funding SourceAward
NSF Division of Ocean Sciences (NSF OCE)

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This document is created by info v 4.1f 5 Oct 2018 from the content of the BCO-DMO metadata database.    2021-02-28  17:23:46