Publications

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2018
Smith, KL, Ruhl HA, Huffard CL, Messie M, Kahru M.  2018.  Episodic organic carbon fluxes from surface ocean to abyssal depths during long-term monitoring in NE Pacific. Proceedings of the National Academy of Sciences of the United States of America. 115:12235-12240.   10.1073/pnas.1814559115   AbstractWebsite

Growing evidence suggests substantial quantities of particulate organic carbon (POC) produced in surface waters reach abyssal depths within days during episodic flux events. A 29-year record of in situ observations was used to examine episodic peaks in POC fluxes and sediment community oxygen consumption (SCOC) at Station M (NE Pacific, 4,000-m depth). From 1989 to 2017, 19% of POC flux at 3,400 m arrived during high-magnitude episodic events (>= mean + 2 sigma), and 43% from 2011 to 2017. From 2011 to 2017, when high-resolution SCOC data were available, time lags between changes in satellite-estimated export flux (EF), POC flux, and SCOC on the sea floor varied between six flux events from 0 to 70 days, suggesting variable remineralization rates and/or particle sinking speeds. Half of POC flux pulse events correlated with prior increases in EF and/or subsequent SCOC increases. Peaks in EF overlying Station M frequently translated to changes in POC flux at abyssal depths. A power-law model (Martin curve) was used to estimate abyssal fluxes from EF and midwater temperature variation. While the background POC flux at 3,400-m depth was described well by the model, the episodic events were significantly underestimated by similar to 80% and total flux by almost 50%. Quantifying episodic pulses of organic carbon into the deep sea is critical in modeling the depth and intensity of POC sequestration and understanding the global carbon cycle.

2016
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.

2014
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, 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.

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.

2012
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.

Kahru, M, Di Lorenzo E, Manzano-Sarabia M, Mitchell BG.  2012.  Spatial and temporal statistics of sea surface temperature and chlorophyll fronts in the California Current. Journal of Plankton Research. 34:749-760.   10.1093/plankt/fbs010   AbstractWebsite

The statistics of sea-surface fronts detected with the automated histogram method were studied in the California Current using sea-surface temperature (SST) and chlorophyll-a concentration (Chl) images from various satellite sensors. Daily maps of fronts were averaged into monthly composites of front frequency (FF) spanning 29 years (19812009) for SST and 14 years (19972010) for Chl. The large-scale distributions of frontal frequency of both SST (FFsst) and of Chl (FFchl) had a 500700 km wide band of elevated values (47) along the coast that roughly coincided with the area of increased mesoscale eddy activity. FFsst and FFchl were positively correlated at monthly and seasonal frequencies, but the year-to-year variations were not significantly correlated. The long-period (1 year and longer) variability in FFsst is influenced by the large-scale SST gradient, while at shorter timescales the influence of the Coastal Upwelling Index is evident. In contrast with FFsst, FFchl variability is less related to the coherent large-scale forcing and has stronger sensitivity to local forcings in individual areas. Decadal-scale increasing trends in the frequency of both SST and Chl fronts were detected in the Ensenada Front area (general area of the A-Front study) and corresponded to, respectively, trends towards colder SST and increasing chlorophyll-a concentration.

Kahru, M, Kudela RM, Manzano-Sarabia M, Mitchell BG.  2012.  Trends in the surface chlorophyll of the California Current: Merging data from multiple ocean color satellites. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 77-80:89-98. Abstract

Standard remote sensing reflectance products from four ocean color sensors (OCTS, SeaWiFS, MODISA, MERIS) and over 10,000 in situ measurements of surface chlorophyll-a (Chl-a) concentration in the California Current were used to create empirical algorithms that are consistent with in situ data as well as between individual sensors. Using these algorithms, a merged multi-sensor time series of the surface Chl-a concentration in California Current region was created. The merged Oil-a time series (November 1996-December 2011) show a significant (P < 0.01) increasing trend off central California and significant (P < 0.01) decreasing trends in the central North Pacific gyre and off southern Baja California. Although this 15-year time series is too short to separate interannual and multidecadal cycles from climate trends, both of these trends are consistent with the predicted effects of global warming. The expected increase in vertical stratification of the water column and the resulting decreased vertical flux of nutrients would lead to lower Chl-a in the gyre but the increased upwelling-favorable winds leading to stronger upwelling off central California or the increased nitrate content of the upwelled water would lead to higher Chl-a in the upwelling region. The decreased Chl-a off southern Baja California resembles the effect of a decreased influence of strong El Nino events. (c) 2012 Elsevier Ltd. All rights reserved.

2011
McQuatters-Gollop, A, Reid PC, Edwards M, Burkill PH, Castellani C, Batten S, Gieskes W, Beare D, Bidigare RR, Head E, Johnson R, Kahru M, Koslow JA, Pena A.  2011.  Is there a decline in marine phytoplankton? Nature. 472:E6-E7.   10.1038/nature09950   AbstractWebsite

Phytoplankton account for approximately 50% of global primary production, form the trophic base of nearly all marine ecosystems, are fundamental in trophic energy transfer and have key roles in climate regulation, carbon sequestration and oxygen production. Boyce et al. compiled a chlorophyll index by combining in situ chlorophyll and Secchi disk depth measurements that spanned a more than 100-year time period and showed a decrease in marine phytoplankton biomass of approximately 1% of the global median per year over the past century. Eight decades of data on phytoplankton biomass collected in the North Atlantic by the Continuous Plankton Recorder (CPR) survey, however, show an increase in an index of chlorophyll (Phytoplankton Colour Index) in both the Northeast and Northwest Atlantic basinsFig. 1), and other long-term time series, including the Hawaii Ocean Time-series (HOT)8, the Bermuda Atlantic Time Series (BATS)8 and the California Cooperative Oceanic Fisheries Investigations (CalCOFI)9 also indicate increased phytoplankton biomass over the last 20–50 years. These findings, which were not discussed by Boyce et al.1, are not in accordance with their conclusions and illustrate the importance of using consistent observations when estimating long-term trends.

2006
Murakami, H, Sasaoka K, Hosoda K, Fukushima H, Toratani M, Frouin R, Mitchell BG, Kahru M, Deschamps PY, Clark D, Flora S, Kishino M, Saitoh S, Asanuma I, Tanaka A, Sasaki H, Yokouchi K, Kiyomoto Y, Saito H, Dupouy C, Siripong A, Matsumura S, Ishizaka J.  2006.  Validation of ADEOS-II GLI ocean color products using in-situ observations. Journal of Oceanography. 62:373-393.   10.1007/s10872-006-0062-6   AbstractWebsite

The Global Imager (GLI) aboard the Advanced Earth Observing Satellite-II (ADEOS-II) made global observations from 2 April 2003 to 24 October 2003. In cooperation with several institutes and scientists, we obtained quality controlled match-ups between GLI products and in-situ data, 116 for chlorophyll-a concentration (CHLA), 249 for normalized water-leaving radiance (nLw) at 443 nm, and 201 for aerosol optical thickness at 865 nm (Tau_865) and Angstrom exponent between 520 and 865 nm (Angstrom). We evaluated the GLI ocean color products and investigated the causes of errors using the match-ups. The median absolute percentage differences (MedPD) between GLI and in-situ data were 14.1-35.7% for nLws at 380-565 nm 52.5-74.8% nLws at 625-680 nm, 47.6% for Tau_865, 46.2% for Angstrom, and 46.6% for CHLA, values that are comparable to the ocean-color products of other sensors. We found that some errors in GLI products are correlated with observational conditions; nLw values were underestimated when nLw at 680 nm was high, CHLA was underestimated in absorptive aerosol conditions, and Tau_865 was overestimated in sunglint regions. The error correlations indicate that we need to improve the retrievals of the optical properties of absorptive aerosols and seawater and sea surface reflection for further applications, including coastal monitoring and the combined use of products from multiple sensors.

2004
Aguirre-Hernandez, E, Gaxiola-Castro G, Najera-Martinez S, Baumgartner T, Kahru M, Mitchell BG.  2004.  Phytoplankton absorption, photosynthetic parameters, and primary production off Baja California: summer and autumn 1998. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 51:799-816.   10.1016/j.dsr2.2004.05.015   AbstractWebsite

To estimate ocean primary production at large space and time scales, it is necessary to use models combined with ocean-color satellite data. Detailed estimates of primary production are typically done at only a few representative stations. To get survey-scale estimates of primary production, one must introduce routinely measured Chlorophyll-a (Chl-a) into models. For best precision, models should be based on accurate parameterizations developed from optical and photosynthesis data collected in the region of interest. To develop regional model parameterizations C-14- bicarbonate was used to estimate in situ primary production and photosynthetic parameters (alpha*, P-m*, and E-k) derived from photosynthesis-irradiance (P-E) experiments from IMECOCAL cruises to the southern California Current during July and October 1998. The P-E experiments were done for samples collected from the 50% surface light depth for which we also determined particle and phytoplankton absorption coefficients (a(p), a(phi), and a(phi)*). Physical data collected during both surveys indicated that the 1997-1998 El Ni (n) over tildeo was abating during the summer of 1998, with a subsequent transition to the typical California Current circulation and coastal upwelling conditions. Phytoplankton chl-a and in situ primary production were elevated at coastal stations for both surveys, with the highest values during summer. Phytoplankton specific absorption coefficients in the blue peak (a(phi)*(440)) ranged from 0.02 to 0.11 m(2) (mg Chl-a)(-1) with largest values in offshore surface waters. In general a(phi)*5 was lower at depth compared to the surface. P-E samples were collected at the 50% light level that was usually in the surface mixed layer. Using alpha* and spectral absorption, we estimated maximum photosynthetic quantum yields (phi(max); mol C/mol quanta). phi(max) values were lowest in offshore surface waters, with a total range of 0.01-0.07. Mean values of phi(max) for July and October were 0.011 and 0.022, respectively. In July P-m* was approximately double and alpha* was about 1.4 times the values for October. Since the P-E samples were generally within the upper mixed layer, these tendencies in the photosynthetic parameters are attributed to deeper mixing of this layer during October when the mean mixed layer for the photosynthesis stations was 35m compared to a mean of 10m in July. Application of a semi-analytical model using mean values of P-E parameters determined at the 50% light depth provided good agreement with C-14 in situ estimates at the discrete 50% light depth and for the water-column integrated primary production. (C) 2004 Elsevier Ltd. All rights reserved.

2000
Kahru, M, Mitchell BG.  2000.  Influence of the 1997-98 El Nino on the surface chlorophyll in the California Current. Geophysical Research Letters. 27:2937-2940.   10.1029/2000gl011486   AbstractWebsite

Satellite-derived time series for the California Current System (CCS) showed marked changes in the surface chlorophyll a concentration (Chl, mg m(-3)) associated with the 1997-98 El Nino. In addition to the previously known de crease in Chi off Southern California (Fiedler, 1984), we also observed a significant increase of Chi off Baja California. Whereas the extent of eutrophic (Chl > 1.0) areas decreased throughout the CCS, the extent of mesotrophic areas (0.2 < Chi < 1.0) off Baja California approximately doubled. The reduced area of eutrophic waters is attributed to weakened upwelling but the increase in the offshore mesotrophic area off Baja may be caused by blooms of nitrogen-fixing cyanobacteria. Using revised Coastal Zone Color Scanner data we detected similar changes during the 1982-83 El Nino.

1998
Kahru, M, Mitchell BG.  1998.  Spectral reflectance and absorption of a massive red tide off southern California. Journal of Geophysical Research-Oceans. 103:21601-21609.   10.1029/98jc01945   AbstractWebsite

Spectral reflectance and absorption of a massive Lingulodinium (Gonyaulax) polyedra red tide in March 1995 off southern California are compared to a "baseline" of biooptical measurements from the California Cooperative Oceanic Fisheries Investigations. The red tide was characterized by increased absorption and therefore reduced remote sensing reflectance (R(rs)) in the 340-400 nm spectral range. The increased ultraviolet absorption was probably caused by mycosporine-like amino acids in the particulate fraction as well as increased absorption by dissolved organic matter. The chlorophyll a (chl a) specific particulate absorption of the L. polyedra bloom in the visible spectral range remained relatively constant for the chi a range 1-150 mg m(-3) indicating accumulation of cells with similar optical characteristics. The difference in the R(rs) versus chl a relationship of the red tide and "normal" California Current phytoplankton diminished with increasing wavelength from 340 nm and disappeared at 412 nm. Ratios of R(rs) at 340 nm (or 380 nm) and 412 nm (or 443 nm) provided differentiation of the red tide starting at chi a concentration of 1-2 mg m(-3). The forthcoming Japanese Global Imager (GLI) satellite sensor has, among others, the 380 nm band. If the signal to noise ratio and atmospheric correction for the 380 nm band are sufficient to retrieve the dynamic range of the water leaving radiance, then it might be possible to differentiate red tides from other phytoplankton bloods with the algorithm described here.

Mitchell, BG, Kahru M.  1998.  Algorithms for SeaWiFS standard products developed with the CalCOFI big-optical data set. California Cooperative Oceanic Fisheries Investigations Reports. 39:133-147. AbstractWebsite

Funding from NASA's Ocean Biogeochemistry Program and the Goddard Space Flight Center SeaWiFS Project was used to implement an ocean optics program as part of the routine cruises of the California Cooperative Oceanic Fisheries Investigations (CalCOFI). Since August 1993, data from more than 300 bio-optical stations have been acquired, merged with complementary data, and made available for developing remote sensing algorithms. The profiling instrument consisted of a Biospherical Instruments, Inc. MER-2040/2041 radiometer integrated with CTD probes, a transmissometer, and a fluorometer. A detailed calibration time series of the radiance and irradiance sensors has been maintained to ensure maximum accuracy. The data set has been used to develop empirical algorithms for Sea WiFS standard products including chlorophyll a (chl a), "CZCS pigments," and diffuse attenuation coefficient K-d(490). Algorithms using cubic regressions of remote sensing reflectance (R-rs) ratios provided the best estimation of chi a and pigments over the full range of chl a (0.05-22.3 mg m(-3)). Multiple linear regressions of multiple-band ratios proved to be less robust. Relationships between spectral K and chi a suggest that previous K algorithms may have errors due to estimates of pure-water absorption.