Publications

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2014
Smith, KL, Sherman AD, Huffard CL, McGill PR, Henthorn R, Von Thun S, Ruhl HA, Kahru M, Ohman MD.  2014.  Large salp bloom export from the upper ocean and benthic community response in the abyssal northeast Pacific: Day to week resolution. Limnology and Oceanography. 59:745-757.   10.4319/lo.2014.59.3.0745   AbstractWebsite

A large bloom of Salpa spp. in the northeastern Pacific during the spring of 2012 resulted in a major deposition of tunics and fecal pellets on the seafloor at similar to 4000 m depth (Sta. M) over a period of 6 months. Continuous monitoring of this food pulse was recorded using autonomous instruments: sequencing sediment traps, a time-lapse camera on the seafloor, and a bottom-transiting vehicle measuring sediment community oxygen consumption (SCOC). These deep-sea measurements were complemented by sampling of salps in the epipelagic zone by California Cooperative Ocean Fisheries Investigations. The particulate organic carbon (POC) flux increased sharply beginning in early March, reaching a peak of 38 mg C m(-2) d(-1) in mid-April at 3400 m depth. Salp detritus started appearing in images of the seafloor taken in March and covered a daily maximum of 98% of the seafloor from late June to early July. Concurrently, the SCOC rose with increased salp deposition, reaching a high of 31 mg C m(-2) d(-1) in late June. A dominant megafauna species, Peniagone sp. A, increased 7-fold in density beginning 7 weeks after the peak in salp deposition. Estimated food supply from salp detritus was 97-327% of the SCOC demand integrated over the 6-month period starting in March 2012. Such large episodic pulses of food sustain abyssal communities over extended periods of time.

Martz, T, Send U, Ohman MD, Takeshita Y, Bresnahan P, Kim HJ, Nam S.  2014.  Dynamic variability of biogeochemical ratios in the Southern California Current System. Geophysical Research Letters. 41:2496-2501.   10.1002/2014gl059332   AbstractWebsite

We use autonomous nitrate (NO3-), oxygen (O-2), and dissolved inorganic carbon (DIC) observations to examine the relationship between ratios of C:N:O at an upwelling site in the Southern California Current System. Mean ratios and 95% confidence intervals observed by sensors over 8 months were NO3-:O-2=-0.110.002, NO3-:DIC=0.140.001, and DIC:O-2=-0.830.01, in good agreement with Redfield ratios. Variability in the ratios on the weekly time scale is attributable to shifts in biological demand and nutrient availability and shown to exhibit a spectrum of values ranging from near 100% New Production to 100% Regenerated Production.

2013
Di Lorenzo, E, Ohman MD.  2013.  A double-integration hypothesis to explain ocean ecosystem response to climate forcing. Proceedings of the National Academy of Sciences. 110:2496-2499.   10.1073/pnas.1218022110   AbstractWebsite

Long-term time series of marine ecological indicators often are characterized by large-amplitude state transitions that can persist for decades. Understanding the significance of these variations depends critically on the underlying hypotheses characterizing expected natural variability. Using a linear autoregressive model in combination with long-term zooplankton observations off the California coast, we show that cumulative integrations of white-noise atmospheric forcing can generate marine population responses that are characterized by strong transitions and prolonged apparent state changes. This model provides a baseline hypothesis for explaining ecosystem variability and for interpreting the significance of abrupt responses and climate change signatures in marine ecosystems.

Combes, V, Chenillat F, Di Lorenzo E, Riviere P, Ohman MD, Bograd SJ.  2013.  Cross-shore transport variability in the California Current: Ekman upwelling vs. eddy dynamics. Progress in Oceanography. 109:78-89.   10.1016/j.pocean.2012.10.001   AbstractWebsite

The low-frequency dynamics of coastal upwelling and cross-shelf transport in the Central and Southern California Current System (CCS) are investigated using the Regional Ocean Modeling System (ROMS) over the period 1965-2008. An ensemble of passive tracers released in the numerical model is used to characterize the effects of linear (Ekman upwelling) and non-linear (mesoscale eddies) circulation dynamics on the statistics of advection of coastal waters. The statistics of passive tracers released in the subsurface show that the low-frequency variability of coastal upwelling and cross-shelf transport of the upwelled water mass are strongly correlated with the alongshore wind stress, and are coherent between the central and southern CCS. However, the offshore transport of tracers released at the surface is not coherent between the two regions, and is modulated by intrinsic mesoscale eddy activity, in particular cyclonic eddies. The transport of cyclonic eddies extends with depth and entrains water masses of southern origin, advected by the poleward California Undercurrent (CUC). The CUC water masses are not only entrained by eddies but also constitute a source for the central California upwelling system. The interplay between intrinsic (eddy activity) and deterministic (Ekman upwelling) dynamics in controlling the cross-shelf exchanges in the CCS may provide an improved framework to understand and interpret nutrients and ecosystem variability. Published by Elsevier Ltd.

Ohman, MD, Rudnick DL, Chekalyuk A, Davis RE, Feely RA, Kahru M, Kim HJ, Landry MR, Martz TR, Sabine CL, Send U.  2013.  Autonomous ocean measurements in the California Current ecosystem. Oceanography. 26:18-25. AbstractWebsite

Event-scale phenomena, of limited temporal duration or restricted spatial extent, often play a disproportionately large role in ecological processes occurring in the ocean water column. Nutrient and gas fluxes, upwelling and downwelling, transport of biogeochemically important elements, predator-prey interactions, and other processes may be markedly influenced by such events, which are inadequately resolved from infrequent ship surveys. The advent of autonomous instrumentation, including underwater gliders, profiling floats, surface drifters, enhanced moorings, coastal high-frequency radars, and satellite remote sensing, now provides the capability to resolve such phenomena and assess their role in structuring pelagic ecosystems. These methods are especially valuable when integrated together, and with shipboard calibration measurements and experimental programs.

Ohman, MD, Barbeau K, Franks PJS, Goericke R, Landry MR, Miller AJ.  2013.  Ecological transitions in a coastal upwelling ecosystem. Oceanography. 26:210-219. AbstractWebsite

The southern California Current Ecosystem (CCE) is a dynamic eastern boundary current ecosystem that is forced by ocean-atmosphere variability on interannual, multidecadal, and long-term secular time scales. Recent evidence suggests that apparent abrupt transitions in ecosystem conditions reflect linear tracking of the physical environment rather than oscillations between alternative preferred states. A space-for-time exchange is one approach that permits use of natural spatial variability in the CCE to develop a mechanistic understanding needed to project future temporal changes. The role of (sub)mesoscale frontal systems in altering rates of nutrient transport, primary and secondary production, export fluxes, and the rates of encounters between predators and prey is an issue central to this pelagic ecosystem and its future trajectory because the occurrence of such frontal features is increasing.

Litchman, E, Ohman MD, Kiorboe T.  2013.  Trait-based approaches to zooplankton communities. Journal of Plankton Research. 35:473-484.   10.1093/plankt/fbt019   AbstractWebsite

Zooplankton are major primary consumers and predators in most aquatic ecosystems. They exhibit tremendous diversity of traits, ecological strategies and, consequently, impacts on other trophic levels and the cycling of materials and energy. An adequate representation of this diversity in community and ecosystem models is necessary to generate realistic predictions on the functioning of aquatic ecosystems but remains extremely challenging. We propose that the use of trait-based approaches is a promising way to reduce complexity while retaining realism in developing novel descriptions of zooplankton in ecosystem models. Characterizing zooplankton traits and trade-offs will also be helpful in understanding the selection pressures and diversity patterns that emerge in different ecosystems along major environmental gradients. Zooplankton traits can be characterized according to their function and type. Some traits, such as body size and motility, transcend several functions and are major determinants of zooplankton ecological strategies. Future developments of trait-based approaches to zooplankton should assemble a comprehensive matrix of key traits for diverse groups and explore it for general patterns; develop novel predictive models that explicitly incorporate traits and associated trade-offs; and utilize these traits to explain and predict zooplankton community structure and dynamics under different environmental conditions, including global change scenarios.

Brinton, E, Townsend AW, Knight MD, Ohman MD.  2013.  Development of thysanopoda egregia (euphausiacea) furciliae and early juvenile. Journal of Crustacean Biology. 33:244-252.   10.1163/1937240x-00002139   AbstractWebsite

Nine furcilia and the early juvenile phase of the deep-sea euphausiid Thysanopoda egregia Hansen, 1905 are described and illustrated for the first time. This identification of the bathypelagic species is made on the basis of the very large, well-developed and dark-brown eyes, short sixth pleomere, large body size relative to stage of development, ripple-like sculpturing of the posterior and lateral parts of the carapace, and the number of terminal telson spines in the first five stages, F1-F5. The ripple marks on the carapace are a particularly distinctive characteristic of T. egregia. Because of the rarity of the nauplius, metanauplius, and calyptopis phases for this species and the unknown identification of the other three deep-living species of Thysanopoda, we are not yet able to positively identify these phases with confidence and that information is not presented here. A distribution map of records of furciliae of T. egregia from our samples and published sources corresponds with the previously described distribution of the adults, showing a cosmopolitan distribution in waters mainly equator-ward of the subpolar ocean provinces.

Batchelder, HP, Daly KL, Davis CS, Ji RB, Ohman MD, Peterson WT, Runge JA.  2013.  Climate impacts on zooplankton population dynamics in coastal marine ecosystems. Oceanography. 26:34-51. AbstractWebsite

The 20-year US GLOBEC (Global Ocean Ecosystem Dynamics) program examined zooplankton populations and their predators in four coastal marine ecosystems. Program scientists learned that environmental controls on zooplankton vital rates, especially the timing and magnitude of reproduction, growth, life-cycle progression, and mortality, determine species population dynamics, seasonal and spatial distributions, and abundances. Improved knowledge of spatial-temporal abundance and distribution of individual zooplankton taxa coupled with new information linking higher trophic level predators (salmon, cod, haddock, penguins, seals) to their prey yielded mechanistic descriptions of how climate variation impacts regionally important marine resources. Coupled ecological models driven by improved regional-scale climate scenario models developed during GLOBEC enable forecasts of plausible future conditions in coastal ecosystems, and will aid and inform decision makers and communities as they assess, respond, and adapt to the effects of environmental change. Multi-region synthesis revealed that conditions in winter, before upwelling, or seasonal stratification, or ice melt (depending on region) had significant and important effects that primed the systems for greater zooplankton population abundance and productivity the following spring-summer, with effects that propagated to higher trophic levels.

Di Lorenzo, E, Combes V, Keister JE, Strub PT, Thomas AC, Franks PJS, Ohman MD, Furtado JC, Bracco A, Bograd SJ, Peterson WT, Schwing FB, Chiba S, Taguchi B, Hormazabal S, Parada C.  2013.  Synthesis of Pacific Ocean climate and ecosystem dynamics. Oceanography. 26:68-81. AbstractWebsite

The goal of the Pacific Ocean Boundary Ecosystem and Climate Study (POBEX) was to diagnose the large-scale climate controls on regional transport dynamics and lower trophic marine ecosystem variability in Pacific Ocean boundary systems. An international team of collaborators shared observational and eddy-resolving modeling data sets collected in the Northeast Pacific, including the Gulf of Alaska (GOA) and the California Current System (CCS), the Humboldt or Peru-Chile Current System (PCCS), and the Kuroshio-Oyashio Extension (KOE) region. POBEX investigators found that a dominant fraction of decadal variability in basin- and regional-scale salinity, nutrients, chlorophyll, and zooplankton taxa is explained by a newly discovered pattern of ocean-climate variability dubbed the North Pacific Gyre Oscillation (NPGO) and the Pacific Decadal Oscillation (PDO). NPGO dynamics are driven by atmospheric variability in the North Pacific and capture the decadal expression of Central Pacific El Ninos in the extratropics, much as the PDO captures the low-frequency expression of eastern Pacific El Ninos. By combining hindcasts of eddy-resolving ocean models over the period 1950-2008 with model passive tracers and long-term observations (e.g., CalCOFI, Line-P, Newport Hydrographic Line, Odate Collection), POBEX showed that the PDO and the NPGO combine to control low-frequency upwelling and alongshore transport dynamics in the North Pacific sector, while the eastern Pacific El Nino dominates in the South Pacific. Although different climate modes have different regional expressions, changes in vertical transport (e.g., upwelling) were found to explain the dominant nutrient and phytoplankton variability in the CCS, GOA, and PCCS, while changes in alongshore transport forced much of the observed long-term change in zooplankton species composition in the KOE as well as in the northern and southern CCS. In contrast, cross-shelf transport dynamics were linked to mesoscale eddy activity, driven by regional-scale dynamics that are largely decoupled from variations associated with the large-scale climate modes. Preliminary findings suggest that mesoscale eddies play a key role in offshore transport of zooplankton and impact the life cycles of higher trophic levels (e.g., fish) in the CCS, PCCS, and GOA. Looking forward, POBEX results may guide the development of new modeling and observational strategies to establish mechanistic links among climate forcing, mesoscale circulation, and marine population dynamics.

Stukel, MR, Ohman MD, Benitez-Nelson CR, Landry MR.  2013.  Contributions of mesozooplankton to vertical carbon export in a coastal upwelling system. Marine Ecology Progress Series. 491:47-+.   10.3354/meps10453   AbstractWebsite
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2012
Li, QP, Franks PJS, Ohman MD, Landry MR.  2012.  Enhanced nitrate fluxes and biological processes at a frontal zone in the southern California current system. Journal of Plankton Research. 34:790-801.   10.1093/plankt/fbs006   AbstractWebsite

Processes that occur at mesoscale and submesoscale features such as eddies and fronts are important for marine ecosystem dynamics and biogeochemical fluxes. However, their impacts on the fate of biogenic organic carbon in coastal oceans are not well quantified because physical and biological interactions at such features are very complex with short time-and small spatial scales variability. As part of the California Current Ecosystem Long-Term Ecological Research (CCE-LTER) Process studies in the southern California Current in October 2008, we sampled across a strong temperature and chlorophyll front ('A-Front') separating water masses with distinct hydrographic and biogeochemical characteristics and a modified biological assemblage at the frontal interface. Thorpe-scale analyses of the hydrographic data from a free-fall moving vessel profiler suggested an increased diapycnal diffusive nitrate flux at the front zone. Based on these field data, we use data-driven diagnostic biogeochemical models to quantify how the front-induced physical mixing influenced the production, grazing and transport of phytoplankton carbon in the southern California Current. Our results suggest that enhanced diffusive diapycnal fluxes of nutrients stimulated phytoplankton primary production at the front; this effect, together with reduced microzooplankton grazing, increased net growth of the phytoplankton community leading to locally enhanced biomass of large phytoplankton, such as diatoms, in the frontal zone.

Ohman, MD, Powell JR, Picheral M, Jensen DW.  2012.  Mesozooplankton and particulate matter responses to a deep-water frontal system in the southern California Current System. Journal of Plankton Research. 34:815-827.   10.1093/plankt/fbs028   AbstractWebsite

We analyzed the abundance of mesozooplankton and suspended particulate matter across the deep-water A-Front in the southern sector of the California Current System. We characterized the A-Front with two novel devices, a free-fall Moving Vessel Profiler (MVP) and an Underwater Vision Profiler 5 (UVP5), together with quantitative bongo samples analyzed by ZooScan. The MVP permitted real-time visualization of vertical density structure, chlorophyll a fluorescence and particle size structure (from a laser optical particle counter) across the front to a depth of 200 m with the research vessel moving at 6 m s(1). The UVP5 quantified in situ vertical distributions from digital images of planktonic organisms and particles in profiles to 300 m. Both the MVP and UVP5 indicated that organic aggregates increased several-fold at the A-Front. The A-Front was a region of elevated abundance of mainly particle-grazing mesozooplankton, including calanoid copepods, Oithona spp., appendicularians and euphausiids, as well as a site of elevated ratio of nauplii copepod(1). In contrast, poecilostomatoid copepods, ostracods, chaetognaths and radiolaria, most of which are more carnivorous or omnivorous, were all elevated in abundance to the south of the front. We provide evidence that submesoscale fronts can be regions of locally elevated plankton abundance and production, as well sites of faunal transitions.

Landry, MR, Ohman MD, Goericke R, Stukel MR, Barbeau KA, Bundy R, Kahru M.  2012.  Pelagic community responses to a deep-water front in the California Current Ecosystem: overview of the A-Front Study. Journal of Plankton Research. 34:739-748.   10.1093/plankt/fbs025   AbstractWebsite

In October 2008, we investigated pelagic community composition and biomass, from bacteria to fish, across a sharp frontal gradient overlying deep waters south of Point Conception, California. This northsouth gradient, which we called A-Front, was formed by the eastward flow of the California Current and separated cooler mesotrophic waters of coastal upwelling origin to the north, from warm oligotrophic waters of likely mixed subarcticsubtropical origin to the south. Plankton biomass and phytoplankton growth rates were two to three times greater on the northern side, and primary production rates were elevated 5-fold to the north. Compared with either of the adjacent waters, the frontal interface was strongly enriched and uniquely defined by a subsurface bloom of large diatoms, elevated concentrations of suspension-feeding zooplankton, high bioacoustical estimates of pelagic fish and enhanced bacterial production and phytoplankton biomass and photosynthetic potential. Such habitats, though small in areal extent, may contribute disproportionately and importantly to regional productivity, nutrient cycling, carbon fluxes and trophic ecology. As a general introduction to the A-Front study, we provide an overview of its design and implementation, a brief summary of major findings and a discussion of potential mechanisms of plankton enrichment at the front.

Ohman, MD.  2012.  Estimation of mortality for stage-structured zooplankton populations: What is to be done? Journal of Marine Systems. 93:4-10.   10.1016/j.jmarsys.2011.05.008   AbstractWebsite

Estimation of zooplankton mortality rates in field populations is a challenging task that some contend is inherently intractable. This paper examines several of the objections that are commonly raised to efforts to estimate mortality. We find that there are circumstances in the field where it is possible to sequentially sample the same population and to resolve biologically caused mortality, albeit with error. Precision can be improved with sampling directed by knowledge of the physical structure of the water column, combined with adequate sample replication. Intercalibration of sampling methods can make it possible to sample across the life history in a quantitative manner. Rates of development can be constrained by laboratory-based estimates of stage durations from temperature- and food-dependent functions, mesocosm studies of molting rates, or approximation of development rates from growth rates, combined with the vertical distributions of organisms in relation to food and temperature gradients. Careful design of field studies guided by the assumptions of specific estimation models can lead to satisfactory mortality estimates, but model uncertainty also needs to be quantified. We highlight additional issues requiring attention to further advance the field, including the need for linked cooperative studies of the rates and causes of mortality of co-occurring holozooplankton and ichthyoplankton. (C) 2011 Elsevier B.V. All rights reserved.

Alin, SR, Feely RA, Dickson AG, Hernandez-Ayon JM, Juranek LW, Ohman MD, Goericke R.  2012.  Robust empirical relationships for estimating the carbonate system in the southern California Current System and application to CalCOFI hydrographic cruise data (2005-2011). Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007511   AbstractWebsite

The California Current System (CCS) is expected to experience the ecological impacts of ocean acidification (OA) earlier than most other ocean regions because coastal upwelling brings old, CO2-rich water relatively close to the surface ocean. Historical inorganic carbon measurements are scarce, so the progression of OA in the CCS is unknown. We used a multiple linear regression approach to generate empirical models using oxygen (O-2), temperature (T), salinity (S), and sigma theta (sigma(theta)) as proxy variables to reconstruct pH, carbonate saturation states, carbonate ion concentration ([CO32-]), dissolved inorganic carbon (DIC) concentration, and total alkalinity (TA) in the southern CCS. The calibration data included high-quality measurements of carbon, oxygen, and other hydrographic variables, collected during a cruise from British Columbia to Baja California in May-June 2007. All resulting empirical relationships were robust, with r(2) values >0.92 and low root mean square errors. Estimated and measured carbon chemistry matched very well for independent data sets from the CalCOFI and IMECOCAL programs. Reconstructed CCS pH and saturation states for 2005-2011 reveal a pronounced seasonal cycle and inter-annual variability in the upper water column. Deeper in the water column, conditions are stable throughout the annual cycle, with perennially low pH and saturation states. Over sub-decadal time scales, these empirical models provide a valuable tool for reconstructing carbonate chemistry related to ocean acidification where direct observations are limited. However, progressive increases in anthropogenic CO2 content of southern CCS water masses must be carefully addressed to apply the models over longer time scales.

Stukel, MR, Landry MR, Ohman MD, Goericke R, Samo T, Benitez-Nelson CR.  2012.  Do inverse ecosystem models accurately reconstruct plankton trophic flows? Comparing two solution methods using field data from the California Current Journal of Marine Systems. 91:20-33.   10.1016/j.jmarsys.2011.09.004   AbstractWebsite

Despite the increasing use of linear inverse modeling techniques to elucidate fluxes in undersampled marine ecosystems, the accuracy with which they estimate food web flows has not been resolved. New Markov Chain Monte Carlo (MCMC) solution methods have also called into question the biases of the commonly used L(2) minimum norm (L(2)MN) solution technique. Here, we test the abilities of MCMC and L(2)MN methods to recover field-measured ecosystem rates that are sequentially excluded from the model input. For data, we use experimental measurements from process cruises of the California Current Ecosystem (CCE-LTER) Program that include rate estimates of phytoplankton and bacterial production, micro- and mesozooplankton grazing, and carbon export from eight study sites varying from rich coastal upwelling to offshore oligotrophic conditions. Both the MCMC and L(2)MN methods predicted well-constrained rates of protozoan and mesozooplankton grazing with reasonable accuracy, but the MCMC method overestimated primary production. The MCMC method more accurately predicted the poorly constrained rate of vertical carbon export than the L(2)MN method, which consistently overestimated export. Results involving DOC and bacterial production were equivocal. Overall, when primary production is provided as model input, the MCMC method gives a robust depiction of ecosystem processes. Uncertainty in inverse ecosystem models is large and arises primarily from solution under-determinacy. We thus suggest that experimental programs focusing on food web fluxes expand the range of experimental measurements to include the nature and fate of detrital pools, which play large roles in the model. (C) 2011 Elsevier B.V. All rights reserved.

Macias, D, Franks PJS, Ohman MD, Landry MR.  2012.  Modeling the effects of coastal wind- and wind-stress curl-driven upwellings on plankton dynamics in the Southern California current system. Journal of Marine Systems. 94:107-119.   10.1016/j.jmarsys.2011.11.011   AbstractWebsite

We use a Nitrogen-Phytoplankton-Zooplankton-Detritus (NPZD) biogeochemical model implemented in a time-dependent box model scheme to simulate the temporal dynamics of the pelagic ecosystem in the Southern California Current System (SCCS). The model was forced by winds, sea surface temperature and light. Nutrient inputs to the modeled box were driven by coastal upwelling or upwelling due to wind-stress curl in order to assess the importance of each process in the temporal dynamics of the SCCS ecosystem. Model results were compared to the CalCOR dataset, both in terms of climatological annual cycles and actual values. This comparison led to modifications of the basic model structure to better represent the coastal ecosystem, particularly phytoplankton growth and zooplankton mortality terms. Wind-stress curl-induced upwelling was found to be significant only in the offshore regions while coastal upwelling better represented the dynamics of the inshore areas. The two upwelling mechanisms work in synchrony, however, to bring nutrients to surface waters during the same time periods. Finally, the effect of low-frequency perturbations, such as those associated with the ENSO and NPGO, were assessed by comparing model results and data. Since the NPGO cycle largely impacts the SCCS through modifications of upwelling-favorable winds, its effects were well represented in the model results. In contrast, ENSO responses were poorly captured in the simulations because such perturbations alter the system by changing surface water mass distributions via mechanisms that were not included in the model forcing. (C) 2011 Elsevier B.V. All rights reserved.

Powell, JR, Ohman MD.  2012.  Use of glider-class acoustic Doppler profilers for estimating zooplankton biomass. Journal of Plankton Research. 34:563-568.   10.1093/plankt/fbs023   AbstractWebsite

We evaluate the feasibility of estimating zooplankton biomass using acoustic Doppler profilers (ADP) as deployed on autonomous ocean gliders. A Sontek Spray ADP mounted on a Mocness net was used for simultaneous collection of zooplankton and measurement of acoustic backscatter (ABS). Zooplankton biomass was estimated from optically scanned plankton samples. We found a direct proportionality between ABS and the summed cross-sectional area of the zooplankton as well as with the estimated zooplankton carbon biomass. ABS at 750kHz was most closely related to the summed cross-sectional area of zooplankton and micronekton greater than 1.6 mm equivalent circular diameter.

Ohman, MD, Rau GH, Hull PM.  2012.  Multi-decadal variations in stable N isotopes of California Current zooplankton. Deep-Sea Research Part I-Oceanographic Research Papers. 60:46-55.   10.1016/j.dsr.2011.11.003   AbstractWebsite

We analyzed variations in naturally occurring delta N-15 in four species of zooplankton as an index of climate influences on pelagic food web structure in a major eastern boundary current ecosystem. Our analyses focused on two species of particle-grazing copepods (Calanus pacificus and Eucalanus californicus) and two species of carnivorous chaetognaths (Sagitta bierii and Sagitta euneritica), drawing on the CalCOFI zooplankton time series from both the southern and central sectors of the California Current System. We detected a significant difference between regions in average stable N isotope content of the two species of copepods, with delta N-15 elevated by 0.5-1.1 per mil in the southern region, but no regional differences in the isotopic content of the chaetognaths. We address climate influences over a 54-year time period, on three different time scales: interannual (dominated by ENSO), decadal, and multi-decadal. Three of four species showed evidence of an ENSO-related isotopic shift toward increased N-15 during El Nino conditions. In addition, in Southern California waters, C. pacificus and S. euneritica showed elevated delta N-15 in the warm phase of the NE Pacific between 1978 and 1998 relative to the preceding and following time periods. When considered over the entire 5 1/2 decades treated here, for most species there was remarkable long-term stability in stable isotope content in both southern and central California waters, despite interannual and decadal perturbations. Only E. californicus in the southern sector showed a significant downward secular trend in delta N-15. Variability of delta N-15 in 3 species covaried with the average nitrate concentration in the mixed layer, suggesting altered nitrate utilization at the base of the food web as a primary mechanism underlying interannual changes in zooplankton isotopic content. (C) 2011 Elsevier Ltd. All rights reserved.

Bargu, S, Silver MW, Ohman MD, Benitez-Nelson CR, Garrison DL.  2012.  Mystery behind Hitchcock's birds. Nature Geoscience. 5:2-3.   10.1038/ngeo1360   AbstractWebsite
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Peters, DPC, Laney CM, Lugo AE, Collins SL, Driscoll CT, Groffman PM, Grove JM, Knapp AK, Kratz TK, Ohman MD, Waide RB, Yao JW.  2012.  Long-Term Trends in Ecological Systems: A Basis for Understanding Responses to Global Change. U.S. Department of Agriculture, Technical Bulletin Number. :647., Washington, D. C. Abstract
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2011
Todd, RE, Rudnick DL, Davis RE, Ohman MD.  2011.  Underwater gliders reveal rapid arrival of El Nino effects off California's coast. Geophysical Research Letters. 38   10.1029/2010gl046376   AbstractWebsite

The 2009-2010 El Nino marked the first occurrence of this climate phenomenon since the initiation of sustained autonomous glider surveillance in the California Current System (CCS). Spray glider observations reveal the subsurface effects of El Nino in the CCS with spatial and temporal resolutions that could not have been obtained practically with any other observational method. Glider observations show that upper ocean waters in the CCS were unusually warm and isopycnals were abnormally deep during the El Nino event, but indicate no anomalous water masses in the region. Observed oceanic anomalies in the CCS are nearly in phase with an equatorial El Nino index and local anomalies of atmospheric forcing. These observations point toward an atmospheric teleconnection as an important mechanism for the 2009-2010 El Nino's remote effect on the mid-latitude CCS. Citation: Todd, R. E., D. L. Rudnick, R. E. Davis, and M. D. Ohman (2011), Underwater gliders reveal rapid arrival of El Nino effects off California's coast, Geophys. Res. Lett., 38, L03609, doi:10.1029/2010GL046376.

Mazzillo, FFM, Field JC, Staaf DJ, Carter ML, Ohman MD.  2011.  A note on the detection of the neurotoxin domoic acid in beach-stranded Dosidicus gigas in the Southern California Bight. California Cooperative Oceanic Fisheries Investigations Reports. 52:109-115. AbstractWebsite

The first occurrence of the neurotoxin domoic acid (DA) in Humboldt squid (Dosidicus gigas) during a toxic Pseudo-nitzschia bloom in the Southern California Bight is reported. Bloom levels of cells within the Pseudo-nitzschia delicatissima group were detected on 6 July 2009 at 4 nearshore collection sites in the Southern California Bight (Scripps Pier, Newport Pier, Goleta Pier and Sterns Wharf). Particulate DA was detected in all of these locations, except for Newport Pier. Stranded Humboldt squid were found south of the Scripps pier 5 days after the toxic bloom was detected. DA was measured using ELISA and low DA concentrations were detected in the stomach or mantle tissue of the stranded specimens. Stomach content analysis indicated that possible DA vectors to Humboldt squid included both pelagic (Pacific hake, Merluccius productus, and Pacific sardine, Sardinops sagax) and nearshore (pile surfperch, Damalichthys vacca, and shiner surfperch, Cymatogaster aggregata) fish species. Although low DA levels were detected in stranded squid specimens, neurological symptoms of DA toxicity were not observed and low DA concentrations alone may not have been the cause of the strandings. Further studies should focus on DA toxic effects in D. gigas to verify whether this pelagic predator can be affected by a toxin frequently detected in pelagic ecosystems influenced by the California Current System.

Bestelmeyer, BT, Ellison AM, Fraser WR, Gorman KB, Holbrook SJ, Laney CM, Ohman MD, Peters DPC, Pillsbury FC, Rassweiler A, Schmitt RJ, Sharma S.  2011.  Analysis of abrupt transitions in ecological systems. Ecosphere. 2:art129.: Ecological Society of America   10.1890/ES11-00216.1   AbstractWebsite

The occurrence and causes of abrupt transitions, thresholds, or regime shifts between ecosystem states are of great concern and the likelihood of such transitions is increasing for many ecological systems. General understanding of abrupt transitions has been advanced by theory, but hindered by the lack of a common, accessible, and data-driven approach to characterizing them. We apply such an approach to 30–60 years of data on environmental drivers, biological responses, and associated evidence from pelagic ocean, coastal benthic, polar marine, and semi-arid grassland ecosystems. Our analyses revealed one case in which the response (krill abundance) linearly tracked abrupt changes in the driver (Pacific Decadal Oscillation), but abrupt transitions detected in the three other cases (sea cucumber abundance, penguin abundance, and black grama grass production) exhibited hysteretic relationships with drivers (wave intensity, sea-ice duration, and amounts of monsoonal rainfall, respectively) through a variety of response mechanisms. The use of a common approach across these case studies illustrates that: the utility of leading indicators is often limited and can depend on the abruptness of a transition relative to the lifespan of responsive organisms and observation intervals; information on spatiotemporal context is useful for comparing transitions; and ancillary information from associated experiments and observations aids interpretation of response-driver relationships. The understanding of abrupt transitions offered by this approach provides information that can be used to manage state changes and underscores the utility of long-term observations in multiple sentinel sites across a variety of ecosystems.