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Kahru, M, Lee ZP, Mitchell BG.  2017.  Contemporaneous disequilibrium of bio-optical properties in the Southern Ocean. Geophysical Research Letters. 44:2835-2842.   10.1002/2016gl072453   AbstractWebsite

Significant changes in satellite-detected net primary production (NPP, mgCm(-2)d(-1)) were observed in the Southern Ocean during 2011-2016: an increase in the Pacific sector and a decrease in the Atlantic sector. While no clear physical forcing was identified, we hypothesize that the changes in NPP were associated with changes in the phytoplankton community and reflected in the concomitant bio-optical properties. Satellite algorithms for chlorophyll a concentration (Chl a, mgm(-3)) use a combination of estimates of the remote sensing reflectance Rrs() that are statistically fitted to a global reference data set. In any particular region or point in space/time the estimate produced by the global mean algorithm can deviate from the true value. Reflectance anomaly (RA) is supposed to remove the first-order variability in Rrs() associated with Chl a and reveal bio-optical properties that are due to the composition of phytoplankton and associated materials. Time series of RA showed variability at multiple scales, including the life span of the sensor, multiyear and annual. Models of plankton functional types using estimated Chl a as input cannot be expected to correctly resolve regional and seasonal anomalies due to biases in the Chl a estimate that they are based on. While a statistical model using RA() time series can predict the times series of NPP with high accuracy (R-2=0.82) in both Pacific and Atlantic regions, the underlying mechanisms in terms of phytoplankton groups and the associated materials remain elusive.

Kahru, M, Lee ZP, Mitchell BG, Nevison CD.  2016.  Effects of sea ice cover on satellite-detected primary production in the Arctic Ocean. Biology Letters. 12   10.1098/rsbl.2016.0223   AbstractWebsite

The influence of decreasing Arctic sea ice on net primary production (NPP) in the Arctic Ocean has been considered in multiple publications but is not well constrained owing to the potentially large errors in satellite algorithms. In particular, the Arctic Ocean is rich in coloured dissolved organic matter (CDOM) that interferes in the detection of chlorophyll a concentration of the standard algorithm, which is the primary input to NPP models. We used the quasianalytic algorithm (Lee et al. 2002 Appl. Opti. 41, 575525772. (doi: 10.1364/AO.41.005755)) that separates absorption by phytoplankton from absorption by CDOMand detrital matter. We merged satellite data from multiple satellite sensors and created a 19 year time series (1997-2015) of NPP. During this period, both the estimated annual total and the summer monthly maximum pan-Arctic NPP increased by about 47%. Positive monthly anomalies in NPP are highly correlated with positive anomalies in open water area during the summer months. Following the earlier ice retreat, the start of the high-productivity season has become earlier, e.g. at a mean rate of -3.0 d yr(-1) in the northern Barents Sea, and the length of the high-productivity period has increased from15 days in 1998 to 62 days in 2015. While in some areas, the termination of the productive season has been extended, owing to delayed ice formation, the termination has also become earlier in other areas, likely owing to limited nutrients.

Anderson, CR, Kudela RM, Kahru M, Chao Y, Rosenfeld LK, Bahr FL, Anderson DM, Norris TA.  2016.  Initial skill assessment of the California Harmful Algae Risk Mapping (C-HARM) system. Harmful Algae. 59:1-18.   10.1016/j.hal.2016.08.006   AbstractWebsite

Toxic algal events are an annual burden on aquaculture and coastal ecosystems of California. The threat of domoic acid (DA) toxicity to human and wildlife health is the dominant harmful algal bloom (HAB) concern for the region, leading to a strong focus on prediction and mitigation of these blooms and their toxic effects. This paper describes the initial development of the California Harmful Algae Risk Mapping (C-HARM) system that predicts the spatial likelihood of blooms and dangerous levels of DA using a unique blend of numerical models, ecological forecast models of the target group, Pseudo-nitzschia, and satellite ocean color imagery. Data interpolating empirical orthogonal functions (DINEOF) are applied to ocean color imagery to fill in missing data and then used in a multivariate mode with other modeled variables to forecast biogeochemical parameters. Daily predictions (nowcast and forecast maps) are run routinely at the Central and Northern California Ocean Observing System (CeNCOOS) and posted on its public website. Skill assessment of model output for the nowcast data is restricted to nearshore pixels that overlap with routine pier monitoring of HABs in California from 2014 to 2015. Model lead times are best correlated with DA measured with solid phase adsorption toxin tracking (SPATI') and marine mammal strandings from DA toxicosis, suggesting long-term benefits of the HAB predictions to decision making. Over the next three years, the C-HARM application system will be incorporated into the NOAA operational HAB forecasting system and HAB Bulletin. (C) 2016 Elsevier B.V. All rights reserved.

Valente, A, Sathyendranath S, Brotas V, Groom S, Grant M, Taberner M, Antoine D, Arnone R, Balch WM, Barker K, Barlow R, Belanger S, Berthon JF, Besiktepe S, Brando V, Canuti E, Chavez F, Claustre H, Crout R, Frouin R, Garcia-Soto C, Gibb S, Gould R, Hooker S, Kahru M, Klein H, Kratzer S, Loisel H, McKee D, Mitchell BG, Moisan T, Muller-Karger F, O'Dowd L, Ondrusek M, Poulton AJ, Repecaud M, Smyth T, Sosik HM, Twardowski M, Voss K, Werdell J, Wernand M, Zibordi G.  2016.  A compilation of global bio-optical in situ data for ocean-colour satellite applications. Earth System Science Data. 8:235-252.   10.5194/essd-8-235-2016   AbstractWebsite

A compiled set of in situ data is important to evaluate the quality of ocean-colour satellite-data records. Here we describe the data compiled for the validation of the ocean-colour products from the ESA Ocean Colour Climate Change Initiative (OC-CCI). The data were acquired from several sources (MOBY, BOUSSOLE, AERONET-OC, SeaBASS, NOMAD, MERMAID, AMT, ICES, HOT, GeP&CO), span between 1997 and 2012, and have a global distribution. Observations of the following variables were compiled: spectral remote-sensing reflectances, concentrations of chlorophyll a, spectral inherent optical properties and spectral diffuse attenuation coefficients. The data were from multi-project archives acquired via the open internet services or from individual projects, acquired directly from data providers. Methodologies were implemented for homogenisation, quality control and merging of all data. No changes were made to the original data, other than averaging of observations that were close in time and space, elimination of some points after quality control and conversion to a standard format. The final result is a merged table designed for validation of satellite-derived ocean-colour products and available in text format. Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) were preserved throughout the work and made available in the final table. Using all the data in a validation exercise increases the number of matchups and enhances the representativeness of different marine regimes. By making available the metadata, it is also possible to analyse each set of data separately. The compiled data are available at doi: 10.1594/PANGAEA.854832 (Valente et al., 2015).

Martinez-Fuentes, LM, Gaxiola-Castro G, Gomez-Ocampo E, Kahru M.  2016.  Effects of interannual events (1997-2012) on the hydrography and phytoplankton biomass of Sebastian Vizcaino Bay. Ciencias Marinas. 42:81-97.   10.7773/cm.v42i2.2626   AbstractWebsite

Sebastian Vizcaino Bay (Baja California Peninsula, Mexico) presents hydrographic conditions and phytoplankton biomass corresponding to a temperate/subtropical transition zone affected by large-scale tropical and subtropical events and those events originating in the subpolar Pacific region. Conditions in the first 50 m depth of the bay are mostly temperate (average temperature: 15.5 degrees C; average salinity: 33.6) and mesotrophic (phytoplankton biomass: >1 mg m(-3)). During spring and summer the bay is heavily influenced by the water transported by the California Current and the coastal upwelling generated off Punta Canoas. During the rest of the year the hydrography and phytoplankton biomass are mostly associated with subtropical conditions. The ENSO events arising in the period 1997-2012 affected the bay's water column. The extreme 1997-1998 El Nino generated increases of similar to 8 degrees C in temperature and similar to 0.8 in salinity. Local dynamic processes decreased the effects of moderate and weak El Nino events on phytoplankton biomass, with possible changes in the plankton functional groups. Due to the mostly temperate environment of the bay, the moderate 1998-2000 and 2010-2011 La Nina events did not generate a substantial change in the hydrography and phytoplankton biomass. However, the abundant subarctic water inflow in the period 2002-2006 abruptly decreased salinity and led to increased stratification of the water column and a reduction in phytoplankton chlorophyll.

Melville, WK, Lenain L, Cayan DR, Kahru M, Kleissl JP, Linden PF, Statom NM.  2016.  The Modular Aerial Sensing System. Journal of Atmospheric and Oceanic Technology. 33:1169-1184.   10.1175/jtech-d-15-0067.1   AbstractWebsite

Satellite remote sensing has enabled remarkable progress in the ocean, earth, atmospheric, and environmental sciences through its ability to provide global coverage with ever-increasing spatial resolution. While exceptions exist for geostationary ocean color satellites, the temporal coverage of low-Earth-orbiting satellites is not optimal for oceanographic processes that evolve over time scales of hours to days. In hydrology, time scales can range from hours for flash floods, to days for snowfall, to months for the snowmelt into river systems. On even smaller scales, remote sensing of the built environment requires a building-resolving resolution of a few meters or better. For this broad range of phenomena, satellite data need to be supplemented with higher-resolution airborne data that are not tied to the strict schedule of a satellite orbit. To address some of these needs, a novel, portable, high-resolution airborne topographic lidar with video, infrared, and hyperspectral imaging systems was integrated. The system is coupled to a highly accurate GPS-aided inertial measurement unit (GPS IMU), permitting airborne measurements of the sea surface displacement, temperature, and kinematics with swath widths of up to 800 m under the aircraft, and horizontal spatial resolution as low as 0.2 m. These data are used to measure ocean waves, currents, Stokes drift, sea surface height (SSH), ocean transport and dispersion, and biological activity. Hydrological and terrestrial applications include measurements of snow cover and the built environment. This paper describes the system, its performance, and present results from recent oceanographic, hydrological, and terrestrial measurements.

Lin, JF, Lee Z, Ondrusek M, Kahru M.  2016.  Attenuation coefficient of usable solar radiation of the global oceans. Journal of Geophysical Research-Oceans. 121:3228-3236.   10.1002/2015jc011528   AbstractWebsite

Usable solar radiation (USR) represents spectrally integrated solar energy in the spectral range of 400-560 nm, a domain where photons penetrate the most in oceanic waters and thus contribute to photosynthesis and heating at deeper depths. Through purely numerical simulations, it was found that the diffuse attenuation coefficient of downwelling USR (K-d(USR), m(-1)) is nearly a constant vertically in the upper water column for clear waters and most turbid waters. Subsequently an empirical model was developed to estimate Kd(USR) based on the diffuse attenuation coefficient at 490 nm (Kd(490), m(-1)). We here evaluate this relationship using data collected from a wide range of oceanic and coastal environments and found that the relationship between Kd(490) and Kd(USR) developed via the numerical simulation is quite robust. We further refined this relationship to extend the applicability to "clearest" natural waters. This refined relationship was then used to produce sample distribution of Kd(USR) of global oceans. As expected, extremely low Kd(USR) (similar to 0.02 m(-1)) was observed in ocean gyres, while significantly higher Kd(USR) (similar to 5.2 m(-1)) was found in very turbid coastal regions. A useful application of Kd(USR) is to easily and accurately propagate surface USR to deeper depths, potentially to significantly improve the estimation of basin scale primary production and heat fluxes in the upper water column.

Kahru, M, Elmgren R, Savchuk OP.  2016.  Changing seasonality of the Baltic Sea. Biogeosciences. 13:1009-1018.   10.5194/bg-13-1009-2016   AbstractWebsite

Changes in the phenology of physical and ecological variables associated with climate change are likely to have significant effect on many aspects of the Baltic ecosystem. We apply a set of phenological indicators to multiple environmental variables measured by satellite sensors for 17-36 years to detect possible changes in the seasonality in the Baltic Sea environment. We detect significant temporal changes, such as earlier start of the summer season and prolongation of the productive season, in several variables ranging from basic physical drivers to ecological status indicators. While increasing trends in the absolute values of variables like sea-surface temperature (SST), diffuse attenuation of light (Ked490) and satellite-detected chlorophyll concentration (CHL) are detectable, the corresponding changes in their seasonal cycles are more dramatic. For example, the cumulative sum of 30 000 W m(-2) of surface incoming short-wave irradiance (SIS) was reached 23 days earlier in 2014 compared to the beginning of the time series in 1983. The period of the year with SST of at least 17 degrees C has almost doubled (from 29 days in 1982 to 56 days in 2014), and the period with Ked490 over 0.4 m(1) has increased from about 60 days in 1998 to 240 days in 2013 -i.e., quadrupled. The period with satellite-estimated CHL of at least 3 mg m(-3) has doubled from approximately 110 days in 1998 to 220 days in 2013. While the timing of both the phytoplankton spring and summer blooms have advanced, the annual CHL maximum that in the 1980s corresponded to the spring diatom bloom in May has now shifted to the summer cyanobacteria bloom in July.

Cloern, JE, Abreu PC, Carstensen J, Chauvaud L, Elmgren R, Grall J, Greening H, Johansson JOR, Kahru M, Sherwood ET, Xu J, Yin KD.  2016.  Human activities and climate variability drive fast-paced change across the world's estuarine-coastal ecosystems. Global Change Biology. 22:513-529.   10.1111/gcb.13059   AbstractWebsite

Time series of environmental measurements are essential for detecting, measuring and understanding changes in the Earth system and its biological communities. Observational series have accumulated over the past 2-5 decades from measurements across the world's estuaries, bays, lagoons, inland seas and shelf waters influenced by runoff. We synthesize information contained in these time series to develop a global view of changes occurring in marine systems influenced by connectivity to land. Our review is organized around four themes: (i) human activities as drivers of change; (ii) variability of the climate system as a driver of change; (iii) successes, disappointments and challenges of managing change at the sea-land interface; and (iv) discoveries made from observations over time. Multidecadal time series reveal that many of the world's estuarine-coastal ecosystems are in a continuing state of change, and the pace of change is faster than we could have imagined a decade ago. Some have been transformed into novel ecosystems with habitats, biogeochemistry and biological communities outside the natural range of variability. Change takes many forms including linear and nonlinear trends, abrupt state changes and oscillations. The challenge of managing change is daunting in the coastal zone where diverse human pressures are concentrated and intersect with different responses to climate variability over land and over ocean basins. The pace of change in estuarine-coastal ecosystems will likely accelerate as the human population and economies continue to grow and as global climate change accelerates. Wise stewardship of the resources upon which we depend is critically dependent upon a continuing flow of information from observations to measure, understand and anticipate future changes along the world's coastlines.

Kahru, M, Kudela RM, Anderson CR, Mitchell BG.  2015.  Optimized merger of ocean chlorophyll algorithms of MODIS-Aqua and VIIRS. Ieee Geoscience and Remote Sensing Letters. 12:2282-2285.   10.1109/lgrs.2015.2470250   AbstractWebsite

Standard ocean chlorophyll-a (Chla) products from currently operational satellite sensors Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua and Visible Infrared Imager Radiometer Suite (VIIRS) underestimate medium and high in situ Chla concentrations and have approximately 9% bias between each other in the California Current. By using the regional optimization approach of Kahru et al., we minimized the differences between satellite estimates and in situ match-ups as well as between estimates of the two satellite sensors and created improved empirical algorithms for both sensors. The regionally optimized Chla estimates from MODIS-Aqua and VIIRS have no bias between each other, have improved retrievals at medium to high in situ Chla, and can be merged to improve temporal frequency and spatial coverage and to extend the merged time series.

Stukel, MR, Kahru M, Benitez-Nelson CR, Decima M, Goericke R, Landry MR, Ohman MD.  2015.  Using Lagrangian-based process studies to test satellite algorithms of vertical carbon flux in the eastern North Pacific Ocean. Journal of Geophysical Research-Oceans. 120:7208-7222.   10.1002/2015jc011264   AbstractWebsite

The biological carbon pump is responsible for the transport of similar to 5-20 Pg C yr(-1) from the surface into the deep ocean but its variability is poorly understood due to an incomplete mechanistic understanding of the complex underlying planktonic processes. In fact, algorithms designed to estimate carbon export from satellite products incorporate fundamentally different assumptions about the relationships between plankton biomass, productivity, and export efficiency. To test the alternate formulations of export efficiency in remote-sensing algorithms formulated by Dunne et al. (2005), Laws et al. (2011), Henson et al. (2011), and Siegel et al. (2014), we have compiled in situ measurements (temperature, chlorophyll, primary production, phytoplankton biomass and size structure, grazing rates, net chlorophyll change, and carbon export) made during Lagrangian process studies on seven cruises in the California Current Ecosystem and Costa Rica Dome. A food-web based approach formulated by Siegel et al. (2014) performs as well or better than other empirical formulations, while simultaneously providing reasonable estimates of protozoan and mesozooplankton grazing rates. By tuning the Siegel et al. (2014) algorithm to match in situ grazing rates more accurately, we also obtain better in situ carbon export measurements. Adequate representations of food-web relationships and grazing dynamics are therefore crucial to improving the accuracy of export predictions made from satellite-derived products. Nevertheless, considerable unexplained variance in export remains and must be explored before we can reliably use remote sensing products to assess the impact of climate change on biologically mediated carbon sequestration.

Lee, ZP, Marra J, Perry MJ, Kahru M.  2015.  Estimating oceanic primary productivity from ocean color remote sensing: A strategic assessment. Journal of Marine Systems. 149:50-59.   10.1016/j.jmarsys.2014.11.015   AbstractWebsite

It has long been realized that approaches using satellite ocean-color remote sensing are the only feasible means to quantify primary productivity (PP) adequately for the global ocean. Through decades of dedicated efforts and with the help of various satellite ocean-color missions, great progresses have been achieved in obtaining global PP as well as its spatial and temporal variations. However, there still exist wide differences between satellite estimations and in situ measurements, as well as large discrepancies among results from different models. The reasons for these large differences are many, which include uncertainties in measurements, errors in satellite-derived products, and limitations in the modeling approaches. Unlike previous round-robin reports on PP modeling where the performance of specific models was evaluated and compared, here we try to provide a candid overview of three primary modeling strategies and the nature of present satellite ocean-color products. We further highlight aspects where efforts should be focused in the coming years, with the overarching goal of reducing the gaps between satellite modeling and in situ measurements. (C) 2014 Elsevier B.V. All rights reserved.

Kahru, M, Jacox MG, Lee Z, Kudela RM, Manzano-Sarabia M, Mitchell BG.  2015.  Optimized multi-satellite merger of primary production estimates in the California Current using inherent optical properties. Journal of Marine Systems. 147:94-102.   10.1016/j.jmarsys.2014.06.003   AbstractWebsite

Building a multi-decadal time series of large-scale estimates of net primary production (NPP) requires merging data from multiple ocean color satellites. The primary product of ocean color sensors is spectral remote sensing reflectance (Rrs). We found significant differences (13-18% median absolute percent error) between Rrs estimates at 443 nm of different satellite sensors. These differences in Rrs are transferred to inherent optical properties and further on to estimates of NPP. We estimated NPP for the California Current region from three ocean color sensors (SeaWiFS, MODIS-Aqua and MERIS) using a regionally optimized absorption based primary production model (Aph-PP) of Lee et al. (2011). Optimization of the Aph-PP model was required for each individual satellite sensor in order to make NPP estimates from different sensors compatible with each other. While the concept of Aph-PP has advantages over traditional chlorophyll-based NPP models, in practical application even the optimized Aph-PP model explained less than 60% of the total variance in NPP which is similar to other NPP algorithms. Uncertainties in satellite Rrs estimates as well as uncertainties in parameters representing phytoplankton depth distribution and physiology are likely to be limiting our current capability to accurately estimate NPP from space. Introducing a generic vertical profile for phytoplankton improved slightly the skill of the Aph-PP model. (C) 2014 Elsevier B.V. All rights reserved.

Kahru, M, Lee Z, Kudela RM, Manzano-Sarabia M, Mitchell GB.  2015.  Multi-satellite time series of inherent optical properties in the California Current. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 112:91-106.   10.1016/j.dsr2.2013.07.023   AbstractWebsite

Satellite ocean color radiometry is a powerful method to study ocean biology but the relationships between satellite measurements and the in situ ocean properties are not well understood. Moreover, the measurements made with one satellite sensor may not be directly compatible with similar measurements from another sensor. We estimate inherent optical properties (IOPs) in the California Current by applying empirically optimized versions of the Quasi-Analytical Algorithm (QAA) of Lee et al. (2002) to satellite remote sensing reflectance (Rrs) from four ocean color sensors (OCTS, SeaWiFS, MODISA and MERIS). The set of estimated IOPs includes the total absorption coefficient at 490 nm (a490), phytoplankton absorption coefficient at 440 nm (aph440), absorption by dissolved and detrital organic matter at 440 nm (adg440) and particle backscattering coefficient at 490 nm (bbp490). The empirical inversion models are created by minimizing the deviations between satellite match-ups with in situ measurements and between the estimates of individual overlapping satellite sensors. The derived empirical algorithms were then applied to satellite Level-3 daily Rrs to create merged multi-sensor time series of the near-surface optical characteristics in the California Current region for a time period of over 16 years (November 1996-December 2012). Due to the limited number of in situ match-ups and their uneven distribution as well as the large errors in the satellite-derived Rrs, the uncertainty in the retrieved lOPs is still significant and difficult to quantify. The merged time series show the dominant annual cycle but also significant variability at interannual time scales. The ratio of adg440 to aph440 is around 1 in the transition zone, is > 1 in the coastal zone and generally

Saba, VS, Hyde KJW, Rebuck ND, Friedland KD, Hare JA, Kahru M, Fogarty MJ.  2015.  Physical associations to spring phytoplankton biomass interannual variability in the US Northeast Continental Shelf. Journal of Geophysical Research-Biogeosciences. 120:205-220.   10.1002/2014jg002770   AbstractWebsite

The continental shelf of the Northeast United States and Nova Scotia is a productive marine ecosystem that supports a robust biomass of living marine resources. Understanding marine ecosystem sensitivity to changes in the physical environment can start with the first-order response of phytoplankton (i.e., chlorophyll a), the base of the marine food web. However, the primary physical associations to the interannual variability of chlorophyll a in these waters are unclear. Here we used ocean color satellite measurements and identified the local and remote physical associations to interannual variability of spring surface chlorophyll a from 1998 to 2013. The highest interannual variability of chlorophyll a occurred in March and April on the northern flank of Georges Bank, the western Gulf of Maine, and Nantucket Shoals. Complex interactions between winter wind speed over the Shelf, local winter water levels, and the relative proportions of Atlantic versus Labrador Sea source waters entering the Gulf of Maine from the previous summer/fall were associated with the variability of March/April chlorophyll a in Georges Bank and the Gulf of Maine. Sea surface temperature and sea surface salinity were not robust correlates to spring chlorophyll a. Surface nitrate in the winter was not a robust correlate to chlorophyll a or the physical variables in every case suggesting that nitrate limitation may not be the primary constraint on the interannual variability of the spring bloom throughout all regions. Generalized linear models suggest that we can resolve 88% of March chlorophyll a interannual variability in Georges Bank using lagged physical data.

Jacox, MG, Edwards CA, Kahru M, Rudnick DL, Kudela RM.  2015.  The potential for improving remote primary productivity estimates through subsurface chlorophyll and irradiance measurement. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 112:107-116.   10.1016/j.dsr2.2013.12.008   AbstractWebsite

A 26-year record of depth integrated primary productivity (PP) in the Southern California Current System (SCCS) is analyzed with the goal of improving satellite net primary productivity (PP) estimates. Modest improvements in PP model performance are achieved by tuning existing algorithms for the SCCS, particularly by parameterizing carbon fixation rate in the vertically generalized production model as a function of surface chlorophyll concentration and distance from shore. Much larger improvements are enabled by improving the accuracy of subsurface chlorophyll and light profiles. In a simple vertically resolved production model for the SCCS (VRPM-SC), substitution of in situ surface data for remote sensing estimates offers only marginal improvements in model r(2) (from 0.54 to 0.56) and total log(10) root mean squared difference (from 0.22 to 0.21), while inclusion of in situ chlorophyll and light profiles improves these metrics to 0.77 and 0.15, respectively. Autonomous underwater gliders, capable of measuring subsurface properties on long-term, long-range deployments, significantly improve PP model fidelity in the SCCS. We suggest their use (and that of other autonomous profilers such as Argo floats) in conjunction with satellites as a way forward for large-scale improvements in PP estimation. (C) 2013 Elsevier Ltd. All rights reserved.

Leising, AW, Schroeder ID, Bograd SJ, Bjorkstedt EP, Field J, Sakuma K, Abell J, Robertson RR, Tyburczy J, Peterson WT, Brodeur R, Barcelo C, Auth TD, Daly EA, Campbell GS, Hildebrand JA, Suryan RM, Gladics AJ, Horton CA, Kahru M, Manzano-Sarabia M, McClatchie S, Weber ED, Watson W, Santora JA, Sydeman WJ, Melin SR, DeLong RL, Largier J, Kim SY, Chavez FP, Golightly RT, Schneider SR, Warzybok P, Bradley R, Jahncke J, Fisher J, Peterson J.  2014.  State of the California Current 2013-14: El Nino looming. California Cooperative Oceanic Fisheries Investigations Reports. 55:51-87. AbstractWebsite

In 2013, the California current was dominated by strong coastal upwelling and high productivity. Indices of total cumulative upwelling for particular coastal locations reached some of the highest values on record. Chlorophyll a levels were high throughout spring and summer. Catches of upwelling-related fish species were also high. After a moderate drop in upwelling during fall 2013, the California current system underwent a major change in phase. Three major basin-scale indicators, the PDO, the NPGO, and the ENSO-MEI, all changed phase at some point during the winter of 2013/14. The PDO changed to positive values, indicative of warmer waters in the North Pacific; the NPGO to negative values, indicative of lower productivity along the coast; and the MEI to positive values, indicative of an oncoming El Nino. Whereas the majority of the California Current system appears to have transitioned to an El Nino state by August 2014, based on decreases in upwelling and chlorophyll a concentration, and increases in SST, there still remained pockets of moderate upwelling, cold water, and high chlorophyll a biomass at various central coast locations, unlike patterns seen during the more major El Ninos (e.g., the 97-98 event). Catches of rockfish, market squid, euphausiids, and juvenile sanddab remained high along the central coast, whereas catches of sardine and anchovy were low throughout the CCS. 2014 appears to be heading towards a moderate El Nino state, with some remaining patchy regions of upwelling-driven productivity along the coast. Superimposed on this pattern, three major regions have experienced possibly non-El Nino-related warming since winter: the Bering Sea, the Gulf of Alaska, and offshore of southern California. It is unclear how this warming may interact with the predicted El Nino, but the result will likely be reduced growth or reproduction for many key fisheries species.

Kahru, M, Kudela RM, Anderson CR, Manzano-Sarabia M, Mitchell BG.  2014.  Evaluation of satellite retrievals of ocean chlorophyll-a in the California Current. Remote Sensing. 6:8524-8540.   10.3390/rs6098524   AbstractWebsite

Retrievals of ocean surface chlorophyll-a concentration (Chla) by multiple ocean color satellite sensors (SeaWiFS, MODIS-Terra, MODIS-Aqua, MERIS, VIIRS) using standard algorithms were evaluated in the California Current using a large archive of in situ measurements. Over the full range of in situ Chla, all sensors produced a coefficient of determination (R-2) between 0.79 and 0.88 and a median absolute percent error (MdAPE) between 21% and 27%. However, at in situ Chla > 1 mg m(-3), only products from MERIS (both the ESA produced algal_1 and NASA produced chlor_a) maintained reasonable accuracy (R-2 from 0.74 to 0.52 and MdAPE from 23% to 31%, respectively), while the other sensors had R-2 below 0.5 and MdAPE higher than 36%. We show that the low accuracy at medium and high Chla is caused by the poor retrieval of remote sensing reflectance.

Kahru, M, Elmgren R.  2014.  Multidecadal time series of satellite-detected accumulations of cyanobacteria in the Baltic Sea. Biogeosciences. 11:3619-3633.   10.5194/bg-11-3619-2014   AbstractWebsite

Cyanobacteria, primarily of the species Nodularia spumigena, form extensive surface accumulations in the Baltic Sea in July and August, ranging from diffuse flakes to dense surface scums. The area of these accumulations can reach similar to 200 000 km(2). We describe the compilation of a 35-year-long time series (1979-2013) of cyanobacteria surface accumulations in the Baltic Sea using multiple satellite sensors. This appears to be one of the longest satellite-based time series in biological oceanography. The satellite algorithm is based on remote sensing reflectance of the water in the red band, a measure of turbidity. Validation of the satellite algorithm using horizontal transects from a ship of opportunity showed the strongest relationship with phycocyanin fluorescence (an indicator of cyanobacteria), followed by turbidity and then by chlorophyll a fluorescence. The areal fraction with cyanobacteria accumulations (FCA) and the total accumulated area affected (TA) were used to characterize the intensity and extent of the accumulations. The fraction with cyanobacteria accumulations was calculated as the ratio of the number of detected accumulations to the number of cloud-free sea-surface views per pixel during the season (July-August). The total accumulated area affected was calculated by adding the area of pixels where accumulations were detected at least once during the season. The fraction with cyanobacteria accumulations and TA were correlated (R-2 = 0.55) and both showed large interannual and decadal-scale variations. The average FCA was significantly higher for the second half of the time series (13.8 %, 1997-2013) than for the first half (8.6 %, 1979-1996). However, that does not seem to represent a long-term trend but decadal-scale oscillations. Cyanobacteria accumulations were common in the 1970s and early 1980s (FCA between 11-17 %), but rare (FCA below 4 %) during 1985-1990; they increased again starting in 1991 and particularly in 1999, reaching maxima in FCA (similar to 25 %) and TA (similar to 210 000 km(2)) in 2005 and 2008. After 2008, FCA declined to more moderate levels (6-17 %). The timing of the accumulations has become earlier in the season, at a mean rate of 0.6 days per year, resulting in approximately 20 days advancement during the study period. The interannual variations in FCA are positively correlated with the concentration of chlorophyll a during July-August sampled at the depth of similar to 5 m by a ship of opportunity, but interannual variations in FCA are more pronounced as the coefficient of variation is over 5 times higher.

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.

Neal, BP, Condit C, Liu G, dos Santos S, Kahru M, Mitchell BG, Kline DI.  2014.  When depth is no refuge: cumulative thermal stress increases with depth in Bocas del Toro, Panama. Coral Reefs. 33:193-205.   10.1007/s00338-013-1081-6   AbstractWebsite

Coral reefs are increasingly affected by high-temperature stress events and associated bleaching. Monitoring and predicting these events have largely utilized sea surface temperature data, due to the convenience of using large-scale remotely sensed satellite measurements. However, coral bleaching has been observed to vary in severity throughout the water column, and variations in coral thermal stress across depths have not yet been well investigated. In this study, in situ water temperature data from 1999 to 2011 from three depths were used to calculate thermal stress on a coral reef in Bahia Almirante, Bocas del Toro, Panama, which was compared to satellite surface temperature data and thermal stress calculations for the same area and time period from the National Oceanic and Atmospheric Administration Coral Reef Watch Satellite Bleaching Alert system. The results show similar total cumulative annual thermal stress for both the surface and depth-stratified data, but with a striking difference in the distribution of that stress among the depth strata during different high-temperature events, with the greatest thermal stress unusually recorded at the deepest measured depth during the most severe bleaching event in 2005. Temperature records indicate that a strong density-driven temperature inversion may have formed in this location in that year, contributing to the persistence and intensity of bleaching disturbance at depth. These results indicate that depth may not provide a stress refuge from high water temperature events in some situations, and in this case, the water properties at depth appear to have contributed to greater coral bleaching at depth compared to near-surface locations. This case study demonstrates the importance of incorporating depth-stratified temperature monitoring and small-scale oceanographic and hydrologic data for understanding and predicting local reef responses to elevated water temperature events.

Sydeman, WJ, Thompson SA, Garcia-Reyes M, Kahru M, Peterson WT, Largier JL.  2014.  Multivariate ocean-climate indicators (MOCI) for the central California Current: Environmental change, 1990-2010. Progress in Oceanography. 120:352-369.   10.1016/j.pocean.2013.10.017   AbstractWebsite

Temporal environmental variability may confound interpretations of management actions, such as reduced fisheries mortality when Marine Protected Areas are implemented. To aid in the evaluation of recent ecosystem protection decisions in central-northern California, we designed and implemented multivariate ocean-climate indicators (MOCI) of environmental variability. To assess the validity of the MOCI, we evaluated interannual and longer-term variability in relation to previously recognized environmental variability in the region, and correlated MOCI to a suite of biological indicators including proxies for lower- (phytoplankton, copepods, krill), and upper-level (seabirds) taxa. To develop the MOCI, we selected, compiled, and synthesized 14 well-known atmospheric and oceanographic indicators of large-scale and regional processes (transport and upwelling), as well as local atmospheric and oceanic response variables such as wind stress, sea surface temperature, and salinity. We derived seasonally-stratified MOCI using principal component analysis. Over the 21-year study period (1990-2010), the ENSO cycle weakened while extra-tropical influences increased with a strengthening of the North Pacific Gyre Oscillation (NPGO) and cooling of the Pacific Decadal Oscillation (PDO). Correspondingly, the Northern Oscillation Index (NOI) strengthened, leading to enhanced upwelling-favorable wind stress and cooling of air and ocean surface temperatures. The seasonal MOCI related well to subarctic copepod biomass and seabird productivity, but poorly to chlorophyll-a concentration and krill abundance. Our results support a hypothesis of enhanced sub-arctic influence (transport from the north) and upwelling intensification in north-central California over the past two decades. Such environmental conditions may favor population growth for species with sub-arctic zoogeographic affinities within the central-northern California Current coastal ecosystem. (C) 2013 Elsevier Ltd. All rights reserved.

Cape, MR, Vernet M, Kahru M, Spreen G.  2014.  Polynya dynamics drive primary production in the Larsen A and B embayments following ice shelf collapse. Journal of Geophysical Research-Oceans. 119:572-594.   10.1002/2013jc009441   AbstractWebsite

The climate-driven collapses of the Larsen A and B ice shelves have opened up new regions of the coastal Antarctic to the influence of sea ice resulting in increases in seasonal primary production. In this study, passive microwave remote sensing of sea ice concentration and satellite imagery of ocean color are employed to quantify the magnitude of and variability in open water area and net primary productivity (NPP) in the Larsen embayments between 1997 and 2011. Numerical model output provides context to analyze atmospheric forcing on the coastal ocean. Following ice shelf disintegration the embayments function as coastal, sensible heat polynyas. The Larsen A and B are as productive as other Antarctic shelf regions, with seasonally averaged daily NPP rates reaching 1232 and 1127 mg C m(-2) d(-1) and annual rates reaching 200 and 184 g C m(-2) yr(-1), respectively. A persistent cross-shelf gradient in NPP is present with higher productivity rates offshore, contrasting with patterns observed along the West Antarctic Peninsula. Embayment productivity is intimately tied to sea ice dynamics, with large interannual variability in NPP rates driven by open water area and the timing of embayment opening. Opening of the embayment is linked to periods of positive Southern Annular Mode and stronger westerlies, which lead to the vertical deflection of warm, maritime air over the peninsula and down the leeward side causing increases in surface air temperature and wind velocity. High productivity in these new polynyas is likely to have ramifications for organic matter export and marine ecosystem evolution. Key Points Primary production and sea ice dynamics after ice shelf disintegration Larsen embayments function as productive coastal sensible heat polynyas High sea ice interannual variability affects total production

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.

Frants, M, Gille ST, Hatta M, Hiscock WT, Kahru M, Measures CI, Mitchell BG, Zhou M.  2013.  Analysis of horizontal and vertical processes contributing to natural iron supply in the mixed layer in southern Drake Passage. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:68-76.   10.1016/j.dsr2.2012.06.001   AbstractWebsite

Horizontal advection, vertical mixing, and mixed-layer entrainment all affect iron concentrations and biological productivity in the Ona Basin, near the Shackleton Transverse Ridge (STR) in southern Drake Passage. Trace metal sampling in the region indicates that dissolved iron concentrations are significantly higher on the continental shelf near the Antarctic Peninsula and the South Shetland Islands than they are in the deep waters away from the shelf. Comparisons between satellite-derived sea surface height (SSH) and Chlorophyll-a (Chl-a) levels in the Ona Basin show > 95% correlation between Chl-a concentrations and horizontal advection of these iron-rich shelf waters during the months of November and December for the years 1997-2010. However, no significant correlations are found for January-April, while high Chl-a concentrations in the Ona Basin persist through March. Enhanced vertical (diapycnal) mixing and mixed-layer entrainment are considered as alternative mechanisms for delivering iron into the Ona Basin mixed layer and sustaining the high Chl-a concentrations. Estimates of iron flux based on in situ measurements of dissolved iron concentrations suggest that diapycnal mixing alone can supply iron to the base of the mixed layer at a rate of 64 +/- 2 nmol m(-2) day(-1) during the summer. In addition, the summer mixed layer in the Ona Basin deepens from January to April, allowing for iron-rich water to be steadily entrained from below. Estimates based on monthly mixed-layer climatologies produce average daily entrainment rates ranging from 5 to 25 nmol m(-2) day(-1). While neither diapycnal mixing nor entrainment alone is always sufficient to meet the estimated iron demand for the Ona Basin bloom, numerical simulation suggests that the combined effect of the two processes can consistently supply sufficient iron to sustain the bloom. (c) 2012 Elsevier Ltd. All rights reserved.