Basal Food Web Dynamics in a Natural Eelgrass (Zostera marina) Community: Cage-Free Field Experimentation

The relative strength of bottom-up and top-down processes operating within food webs is a fundamental determinant of community structure and function. In marine systems, inconspicuous but often highly abundant invertebrate herbivores (mesograzers) are implicated as strong consumers of primary production and important prey for higher-order consumers. Because of their small size, however, mesograzer abundance is not easily manipulated in the field, which limits our ability to adequately assess their grazing impacts. Seagrass systems present a pressing need for the study of food web dynamics because anthropogenic nutrient and sediment inputs decrease the amount of light reaching seagrass leaves, which limits the depth distribution of seagrasses via reduced photosynthesis to respiration ratios. Mesograzers benefit seagrass through their consumption of epiphytic algae and thus may mitigate the loss of seagrass beds due to nutrient enrichment. I test the relative impacts of nutrient enrichment and crustacean mesograzer abundance on epiphytes in a natural seagrass bed without using cages. My work presents the first cage-free tests of crustacean mesograzing impacts in natural seagrass communities. I successfully decreased crustacean abundance for extended periods of time in multiple experiments using a degradable chemical deterrent. Crustacean mesograzer reduction led to concomitant increases in epiphytic algal biomass, while nutrients increased epiphytes only in the absence of mesograzers. My results validate early work from mesocosm and field cage studies designed to test grazing impacts of mesograzers and support the hypothesis that mesograzers indirectly benefit seagrass through a positive indirect interaction

A bacterial quorum-sensing precursor induces mortality in the marine coccolithophore, Emiliania huxleyi

© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Frontiers in Microbiology 7 (2016): 59, doi:10.3389/fmicb.2016.00059.Interactions between phytoplankton and bacteria play a central role in mediating biogeochemical cycling and food web structure in the ocean. However, deciphering the chemical drivers of these interspecies interactions remains challenging. Here, we report the isolation of 2-heptyl-4-quinolone (HHQ), released by Pseudoalteromonas piscicida, a marine gamma-proteobacteria previously reported to induce phytoplankton mortality through a hitherto unknown algicidal mechanism. HHQ functions as both an antibiotic and a bacterial signaling molecule in cell–cell communication in clinical infection models. Co-culture of the bloom-forming coccolithophore, Emiliania huxleyi with both live P. piscicida and cell-free filtrates caused a significant decrease in algal growth. Investigations of the P. piscicida exometabolome revealed HHQ, at nanomolar concentrations, induced mortality in three strains of E. huxleyi. Mortality of E. huxleyi in response to HHQ occurred slowly, implying static growth rather than a singular loss event (e.g., rapid cell lysis). In contrast, the marine chlorophyte, Dunaliella tertiolecta and diatom, Phaeodactylum tricornutum were unaffected by HHQ exposures. These results suggest that HHQ mediates the type of inter-domain interactions that cause shifts in phytoplankton population dynamics. These chemically mediated interactions, and other like it, ultimately influence large-scale oceanographic processes.This research was support through funding from the Gordon and Betty Moore Foundation through Grant GBMF3301 to MJ and TM; NIH grant from the National Institute of Allergy and Infectious Disease (NIAID – 1R21Al119311-01) to TM and KW; the National Science Foundation (OCE – 1313747) and US National Institute of Environmental Health Science (P01-ES021921) through the Oceans and Human Health Program to BM. Additional financial support was provided to TM from the Flatley Discovery Lab

Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate

The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10?4) m2 s?1 and above 1000-m depth is O(10?5) m2 s?1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins

Spitzer Space Telescope observations of the Carina Nebula: The steady march of feedback-driven star formation

We report the first results of imaging the Carina Nebula with Spitzer/IRAC, providing a catalog of point sources and YSOs based on SED fits. We discuss several aspects of the extended emission, including dust pillars that result when a clumpy molecular cloud is shredded by massive star feedback. There are few "extended green objects" (EGOs) normally taken as signposts of outflow activity, and none of the HH jets detected optically are seen as EGOs. A population of "extended red objects" tends to be found around OB stars, some with clear bow-shocks. These are dusty shocks where stellar winds collide with flows off nearby clouds. Finally, the relative distributions of O stars and subclusters of YSOs as compared to dust pillars shows that while some YSOs are located within pillars, many more stars and YSOs reside just outside pillar heads. We suggest that pillars are transient phenomena, part of a continuous outwardly propagating wave of star formation driven by massive star feedback. As pillars are destroyed, they leave newly formed stars in their wake, which are then subsumed into the young OB association. Altogether, the current generation of YSOs shows no strong deviation from a normal IMF. The number of YSOs suggests a roughly constant star-formation rate over the past 3Myr, implying that star formation in pillars constitutes an important mechanism to construct unbound OB associations. Accelerated pillars may give birth to O-type stars that, after several Myr, could appear to have formed in isolation.Comment: 25 pages, 15 figures, MNRAS accepte

Latitude, temperature, and habitat complexity predict predation pressure in eelgrass beds across the Northern Hemisphere

Latitudinal gradients in species interactions are widely cited as potential causes or consequences of global patterns of biodiversity. However, mechanistic studies documenting changes in interactions across broad geographic ranges are limited. We surveyed predation intensity on common prey (live amphipods and gastropods) in communities of eelgrass (Zostera marina) at 48 sites across its Northern Hemisphere range, encompassing over 370 of latitude and four continental coastlines. Predation on amphipods declined with latitude on all coasts but declined more strongly along western ocean margins where temperature gradients are steeper. Whereas in situ water temperature at the time of the experiments was uncorrelated with predation, mean annual temperature strongly positively predicted predation, suggesting a more complex mechanism than simple increased metabolic activity at the time of predation. This large-scale biogeographic pattern was modified by local habitat characteristics; predation declined with higher shoot density both among and within sites. Predation rates on gastropods, by contrast, were uniformly low and varied little among sites. The high replication and geographic extent of our study not only provides additional evidence to support biogeographic variation in intensity, but also insight into the mechanisms that relate temperature and biogeographic gradients in species interactions

A Pleistocene legacy structures variation in modern seagrass ecosystems

Distribution of Earth's biomes is structured by the match between climate and plant traits, which in turn shape associated communities and ecosystem processes and services. However, that climate-trait match can be disrupted by historical events, with lasting ecosystem impacts. As Earth's environment changes faster than at any time in human history, critical questions are whether and how organismal traits and ecosystems can adjust to altered conditions. We quantified the relative importance of current environmental forcing versus evolutionary history in shaping the growth form (stature and biomass) and associated community of eelgrass (Zostera marina), a widespread foundation plant of marine ecosystems along Northern Hemisphere coastlines, which experienced major shifts in distribution and genetic composition during the Pleistocene. We found that eelgrass stature and biomass retain a legacy of the Pleistocene colonization of the Atlantic from the ancestral Pacific range and of more recent within-basin bottlenecks and genetic differentiation. This evolutionary legacy in turn influences the biomass of associated algae and invertebrates that fuel coastal food webs, with effects comparable to or stronger than effects of current environmental forcing. Such historical lags in phenotypic acclimatization may constrain ecosystem adjustments to rapid anthropogenic climate change, thus altering predictions about the future functioning of ecosystems.This work was supported by the US NSF (OCE-1031061, OCE-1336206, OCE0-1336741, OCE-1336905) and the Smithsonian Institution. F.T. was supported by José Castillejo Award CAS14/00177. A.H.E. was supported by the FCT (Foundation for Science and Technology) through Project UIDB/04326/2020 and Contract CEECINST/00114/2018. This is Contribution 106 from the Smithsonian’s MarineGEO and Tennenbaum Marine Observatories Network and Contribution 4105 of the Virginia Institute of Marine Science, College of William & Mary

Climate Process Team on internal wave–driven ocean mixing

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 98 (2017): 2429-2454, doi:10.1175/BAMS-D-16-0030.1.Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatiotemporal patterns of mixing are largely driven by the geography of generation, propagation, and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last 5 years and under the auspices of U.S. Climate Variability and Predictability Program (CLIVAR), a National Science Foundation (NSF)- and National Oceanic and Atmospheric Administration (NOAA)-supported Climate Process Team has been engaged in developing, implementing, and testing dynamics-based parameterizations for internal wave–driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here, we review recent progress, describe the tools developed, and discuss future directions.We are grateful to U.S. CLIVAR for their leadership in instigating and facilitating the Climate Process Team program. We are indebted to NSF and NOAA for sponsoring the CPT series.2018-06-0

A Pleistocene legacy structures variation in modern seagrass ecosystems

Distribution of Earth’s biomes is structured by the match between climate and plant traits, which in turn shape associated communities and ecosystem processes and services. However, that climate–trait match can be disrupted by historical events, with lasting ecosystem impacts. As Earth’s environment changes faster than at any time in human history, critical questions are whether and how organismal traits and ecosystems can adjust to altered conditions. We quantified the relative importance of current environmental forcing versus evolutionary history in shaping the growth form (stature and biomass) and associated community of eelgrass ( Zostera marina ), a widespread foundation plant of marine ecosystems along Northern Hemisphere coastlines, which experienced major shifts in distribution and genetic composition during the Pleistocene. We found that eelgrass stature and biomass retain a legacy of the Pleistocene colonization of the Atlantic from the ancestral Pacific range and of more recent within-basin bottlenecks and genetic differentiation. This evolutionary legacy in turn influences the biomass of associated algae and invertebrates that fuel coastal food webs, with effects comparable to or stronger than effects of current environmental forcing. Such historical lags in phenotypic acclimatization may constrain ecosystem adjustments to rapid anthropogenic climate change, thus altering predictions about the future functioning of ecosystems

Climate Process Team on Internal-Wave Driven Ocean Mixing

Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean, and consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Climate models have been shown to be very sensitive not only to the overall level but to the detailed distribution of mixing; sub-grid-scale parameterizations based on accurate physical processes will allow model forecasts to evolve with a changing climate. Spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and destruction of internal waves, which are thought to supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF and NOAA supported Climate Process Team has been engaged in developing, implementing and testing dynamics-base parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions

Climate Process Team On Internal Wave-Driven Ocean Mixing

The study summarizes recent advances in our understanding of internal wave–driven turbulent mixing in the ocean interior and introduces new parameterizations for global climate ocean models and their climate impacts