Publications

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2018
Kahru, M, Jacox MG, Ohman MD.  2018.  CCE1: Decrease in the frequency of oceanic fronts and surface chlorophyll concentration in the California Current System during the 2014-2016 northeast Pacific warm anomalies. Deep-Sea Research Part I-Oceanographic Research Papers. 140:4-13.   10.1016/j.dsr.2018.04.007   AbstractWebsite

Oceanic fronts are sites of increased vertical exchange that are often associated with increased primary productivity, downward flux of organic carbon, and aggregation of plankton and higher trophic levels. Given the influence of fronts on the functioning of marine ecosystems, an improved understanding of the spatial and temporal variability of frontal activity is desirable. Here, we document changes in the frequency of sea-surface fronts and the surface concentration of chlorophyll-a (Chla) in the California Current System that occurred during the Northeast Pacific anomalous warming of 2014-2015 and El Nino of 2015-2016, and place those anomalies in the context of two decades of variability. Frontal frequency was detected with the automated histogram method using datasets of sea-surface temperature (SST) and Chla from multiple satellite sensors. During the anomalous 2014-2016 period, a drop in the frequency of fronts coincided with the largest negative Chla anomalies and positive SST anomalies in the whole period of satellite observations (1997-2017 for Chla and 1982-2017 for SST). These recent reductions in frontal frequency ran counter to a previously reported increasing trend, though it remains to be seen if they represent brief interruptions in that trend or a reversal that will persist going forward.

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
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.

2009
Kahru, M, Kudela R, Manzano-Sarabia M, Mitchell BG.  2009.  Trends in primary production in the California Current detected with satellite data. Journal of Geophysical Research-Oceans. 114   10.1029/2008jc004979   AbstractWebsite

Several ocean primary production algorithms using satellite data were evaluated on a large archive of net primary production (NPP) and chlorophyll-a (Chl-a) measurements collected by the California Cooperative Fisheries Investigations program in the California Current. The best algorithm matching in situ data was found by empirically adjusting the Behrenfeld-Falkowski Vertically Generalized Production Model. Satellite-derived time series of NPP were calculated for the California Current area. Significant increase in NPP and Chl-a annual peak levels, i.e., the "bloom magnitude,'' were found along the coasts of the California Current as well as other major eastern boundary currents for the period of modern ocean color data (1997-2007). The reasons for this increase are not clear but are associated with various environmental conditions.

2008
Manzano-Sarabia, M, Salinas-Zavala CA, Kahru M, Lluch-Cota SE, Gonzalez-Becerril A.  2008.  The impact of the 1997-1999 warm-SST and low-productivity episode on fisheries in the southwestern Gulf of Mexico. Hydrobiologia. 610:257-267.   10.1007/s10750-008-9440-y   AbstractWebsite

Satellite-derived time-series of sea surface temperature (SST), chlorophyll a, and net primary productivity showed a period of warm SST and low productivity during 1997 and 1999 in the southwestern Gulf of Mexico followed by a period of colder than average SST (2000-2001). This shift between the warm and cold oceanic conditions might have caused significant changes in the structure of the ecosystem that is shown by changes in primary productivity and fishery landings between those periods.

2006
Smith, KL, Baldwin RJ, Ruhl HA, Kahru M, Mitchell BG, Kaufmann RS.  2006.  Climate effect on food supply to depths greater than 4,000 meters in the northeast Pacific. Limnology and Oceanography. 51:166-176. AbstractWebsite

A long time-series Study was conducted over 15 yr (1989-2004) to measure particulate organic carbon (POC) flux as an estimate of food supply reaching > 4,000-m depth in the northeast Pacific. Sequencing sediment traps were moored at 3,500-and 4,050-m depth, 600 and 50 in above the seafloor, respectively, to collect sinking particulate matter with 10-d resolution. POC fluxes were compared with three climate indices in the Pacific: the basin-scale multivariate El Nino Southern Oscillation index (MEI) and northern Oscillation index (NOI) and the regional-scale Bakun upwelling index (BUI). The NOI and MEI correlated significantly with POC flux, lagged earlier by 6-10 months, respectively. The BUI also correlated with POC flux. lagged by 2-3 months, suggesting a direct relationship between upwelling intensity and rates Of POC Supply to abyssal depths. Satellite ocean color data for the surface above the study site were used to estimate chlorophyll a concentrations and, combined with sea surface temperature and photosynthetically available radiation, to estimate net primary production and export flux (EF) from the euphotic zone. EF was significantly correlated with POC flux. lagged earlier by 0-3 months. An empirical model to estimate POC flux, with the use of NOI, BUI, and EF yielded Significant agreement with measured fluxes. Modeling of deep-sea processes on broad spatial and temporal scales with climate indices and satellite sensing now appears feasible.

2004
Kahru, M, Marinone SG, Lluch-Cota SE, Pares-Sierra A, Mitchell BG.  2004.  Ocean-color variability in the Gulf of California: scales from days to ENSO. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 51:139-146.   10.1016/j.dsr2.2003.04.001   AbstractWebsite

Time series of surface chlorophyll a concentration (C-sat) and phytoplankton net primary production (NPP) in the Gulf of California were derived using satellite data from OCTS, SeaWiFS, MODIS, AVHRR and the VGPM primary productivity model. The 6-year (1997-2003) time series showed variability at a multitude of scales. The annual cycle was the dominant mode in Gat variability in the entire gulf, except just south of the midriff islands where the semiannual cycle dominated. The semiannual cycle has C-sat maxima during the spring and fall transition periods when the general circulation is switching between cyclonic in the summer and anticyclonic in the winter and is less developed, therefore allowing a more efficient tidal mixing. The spring and fall maxima often consisted of multiple peaks of about 10 days. A significant peak at about 1 month was often present in the short-term C-sat variability, especially in areas near the midriff islands, suggesting the influence of tidal mixing. The interannual variability was dominated by the 1997-98 El Nino and the following La Nina. During the El Nino period NPP decreased by 30-40% in the southern part of the gulf (by approximately 1 Tg C month(-1)), but the changes in the central and northern parts were less evident. (C) 2004 Elsevier Ltd. All rights reserved.

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.