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A
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.

Aitsam, A, Hansen HP, Elken J, Kahru M, Laanemets J, Pajuste M, Pavelson J, Talpsepp L.  1984.  Physical and Chemical Variability of the Baltic Sea - a Joint Experiment in the Gotland Basin. Continental Shelf Research. 3:291-310.   10.1016/0278-4343(84)90013-x   AbstractWebsite

An area of 25 × 30 nmi with a subarea of 20 × 20 nmi in the Baltic Sea Gotland Basin was surveyed for about one month by vertical CTD and fluorometric chlorophyll a casts, and by towed profiles with a chemical profiler (nutrients, CTD, O2, pH) and an undulating CTD. Autonomous current meter data were available for the same period.Five CTD surveys with a 5-mile spacing showed the existence of synoptic eddies and several other perturbations. A first mode eddy (with all the isopycnals lifted within the halocline) had reduced salinities in the upper layer and chlorophyll concentrations were reduced by 2 to 3-fold. A second mode eddy (with the isopycnals lifted in the top of the halocline and lowered in the bottom layers) with a diameter of 20 km was under prolonged observation. Anticyclonic currents at 96-m depth were up to 25 cm s−1. Along the section across the eddy at 80-m depth, the mean nitrate concentration dropped from about 7 to 8 to 5 μmol dm−3 while other chemical parameters lacked intense variations. An increased activity of the intrusive fine-structure was detected in the eddy center. The Baltic eddies are essentially non-linear, and transport water in their ‘nuclei’. An internal front, separating waters with different T,S composition, was discovered in the upper layers.The observations are discussed with respect to discovering long-term trends while monitoring the Baltic environment.

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.

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Barlow, J, Kahru M, Mitchell BG.  2008.  Cetacean biomass, prey consumption, and primary production requirements in the California Current ecosystem. Marine Ecology-Progress Series. 371:285-295.   10.3354/meps07695   AbstractWebsite

To better understand the role played by cetaceans as top-level predators in the California Current ecosystem, we estimate the fraction of annual net primary production (NPP) required to support the prey consumed by cetaceans, using a simple trophic transfer model. The biomass of cetacean species in the California Current is calculated as the product of their mean summer and fall abundance during 1991, to 2005 and estimates of mean mass ind.(-1). Total prey consumption by cetaceans is estimated from a mass-specific consumption model. NPP is estimated from remote satellite measurements using the Behrenfeld-Falkowski vertically-generalized production model for each of 4 geographic regions. The total biomass of baleen whales exceeds the biomass of toothed whales by a factor of similar to 2.5; however, the estimated prey consumption by these taxa is nearly equal. Assuming 10% trophic transfer efficiency, cetaceans are estimated to require 32.2 g C m(-2) yr(-1) of primary production, or similar to 12 % of the NPP in the study area, to sustain the prey that they directly consume. Because they feed at a lower trophic level, the primary production requirement (PPR) of baleen whales is similar to 13 % of that of toothed whales, despite their 2.5-fold greater biomass. Uncertainty in trophic transfer efficiency results in the greatest uncertainty in estimating PPR for these upper trophic predators.

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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

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.

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Frants, M, Gille ST, Hewes CD, Holm-Hansen O, Kahru M, Lombrozo A, Measures CI, Mitchell BG, Wang HL, Zhou M.  2013.  Optimal multiparameter analysis of source water distributions in the Southern Drake Passage. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:31-42.   10.1016/j.dsr2.2012.06.002   AbstractWebsite

In order to evaluate the effects of horizontal advection on iron supply in the vicinity of the Shackleton Transverse Ridge (SIR) in the southern Drake Passage, the water composition in the region is estimated along the isopycnal containing the subsurface iron peak. Optimal Multiparameter (OMP) analysis of temperature, salinity, oxygen and nutrient data is used to estimate the water composition at CID stations sampled in summer 2004 and winter 2006. The highest iron concentrations in the Ona Basin are found below the mixed layer, both in summer and in winter. The water composition derived from the OMP analysis is consistent with a scenario in which iron-rich shelf waters from the South Shetland Islands and the Antarctic Peninsula are advected northward on the eastern side of the SIR, where they interact with the low-iron waters of the Antarctic Circumpolar Current (ACC) in the Ona Basin. The shelf waters and the ACC waters appear to interact through a stirring process without fully mixing, resulting in a filamented distribution that has also been inferred from the satellite data. To the west of the STR, the shelf waters are primarily confined to the continental shelf, and do not extend northwards. This source of water distribution is consistent with the idea that iron enters the Ona Basin from the continental shelf through advection along an isopycnal, resulting in an iron concentration peak occurring below the winter mixed layer in the Ona Basin. (c) 2012 Elsevier Ltd. All rights reserved.

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.

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Hewes, CD, Reiss CS, Kahru M, Mitchell BG, Holm-Hansen O.  2008.  Control of phytoplankton biomass by dilution and mixed layer depth in the western Weddell-Scotia Confluence. Marine Ecology-Progress Series. 366:15-29.   10.3354/meps07515   AbstractWebsite

Hydrographic, nutrient and trace metal (iron, manganese, and aluminum) concentration data, collected as part of a 2-ship survey during austral summer 2004, were used to examine the influence of upwelling and horizontal mixing on phytoplankton biomass in the region of Elephant Island and South Shetland Islands, Antarctica. Temperature/salinity property analysis and changes in trace metal and nutrient concentrations show that horizontal mixing of shelf waters, not upwelling from depth, is correlated with phytoplankton biomass in the upper mixed layer (UML). The interaction between changing UML depth and nutrient and trace metal concentrations in the UML results in a unimodal distribution of phytoplankton biomass centered at intermediate surface salinities of similar to 34. Principal component (PC) analysis of hydrographic and chemical observations resolved 3 components that accounted for 99% of the variability in nutrient and trace metal concentrations. The first PC accounted for a conservative loss of nutrients through dilution across a latitudinal salinity gradient. The second and third PCs separated mixed layer depth and nutrient consumption. Although these 2 PCs accounted for just 20% of the variability in the data matrix, they accounted for 65% of the variability in mean phytoplankton biomass, and recreated the unimodal distribution of chlorophyll concentration when modeled across a salinity gradient. We propose that the distribution of phytoplankton biomass is structured by the horizontal mixing of nutrient rich waters, derived from Weddell Sea Shelf Waters, with Antarctic Surface Water that enhances stratification and shoaling of the UML.

Holm-Hansen, O, Naganobu M, Kawaguchi S, Kameda T, Krasovski I, Tchernyshkov P, Priddle J, Korb R, Brandon M, Demer D, Hewitt RP, Kahru M, Hewes CD.  2004.  Factors influencing the distribution, biomass, and productivity of phytoplankton in the Scotia Sea and adjoining waters. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 51:1333-1350.   10.1016/j.dsr2.2004.06.015   AbstractWebsite

During January and February 2000 four research vessels, from Russia, the UK, Japan, and the United States, conducted an oceanographic survey with 137 hydrographic stations within the Scotia Sea and adjoining waters as part of a survey sponsored by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to estimate the biomass and distribution of Antarctic krill in the Scotia Sea. Chlorophyll-a (Chl-a) measurements showed great variability in phytoplankton biomass within the Scotia Sea, with some areas having among the lowest Chl-a concentrations found in Antarctic waters (<0.1 mg m(-3) in surface waters) while other areas were among the richest with > 10 mg m(-3). This paper describes the distribution and concentration of Chl-a in the upper 100 m of the water column and relates the Chl-a profiles at individual stations to profiles of upper water-column stability, to the depth of the upper mixed layer, and to the mixing of different water masses. The 58 stations with the lowest Chl-a values in surface waters also had low values for integrated Chl-a (33.9 +/- 19.5 mg m(-2)) and a Chl-a maximum at depths of between 70 and 90 m, in contrast to all other stations where deep Chl-a maxima did not occur. The T/S diagrams at many of these stations were indicative of Antarctic Circumpolar Current (ACC) waters. The central Scotia Sea and areas to the west and north of South Georgia had significantly higher integrated Chl-a values (98.1+/-46.0 mg m(-2), n = 57), in addition to five stations with very high Chl-a values (mean of 359+/-270 mg m(-2)). The mean rate of integrated primary production, which was estimated using the Chl-a data and the mean incident solar radiation measured from previous cruises as well as from satellite data, was estimated to be 994 mg carbon m(-2) day(-1). The temperature profiles at these stations suggested that considerable interleaving and mixing of water types had occurred, which was also evident in the T/S diagrams, which indicated mixing of ACC waters with coastal waters originating from Bransfield Strait or the Weddell Sea. There was no significant correlation between integrated Chl-a values and the profiles of upper water column stability or the depth of the upper mixed layer. The spatial variability in phytoplankton biomass within the Scotia Sea is discussed in relation to the hypothesis that low iron concentrations are the major factor controlling phytoplankton biomass in these pelagic Antarctic waters and that concentrations of iron available for phytoplankton uptake are strongly influenced by fronts and the mixing of different water masses. Published by Elsevier Ltd.

Holm-Hansen, O, Kahru M, Hewes CD.  2005.  Deep chlorophyll a maxima (DCMs) in pelagic Antarctic waters. II. Relation to bathymetric features and dissolved iron concentrations. Marine Ecology-Progress Series. 297:71-81.   10.3354/meps297071   AbstractWebsite

A deep chlorophyll a maximum (DCM) at depths between 60 and 90 m in waters south of the Antarctic Polar Front (APF) occurs only in pelagic waters where the chlorophyll a concentrations in the upper mixed layer (UML) are very low (generally < 0.2 mg m(-3)). Dissolved Fe concentrations in these waters with DCMs are also very low (generally < 0.2 nM) and are probably a limiting factor for phytoplankton growth and biomass. DCMs occur in the upper portion of the temperature minimum layer (TML), which is the winter residue of the Antarctic Surface Water (AASW). The higher phytoplankton biomass at these depths is thought to result from higher Fe concentrations in the winter remnant of the AASW as compared to that found in the overlying UML. A survey of the literature indicates that DCMs are located predominately over the deep ocean basins where enrichment of surface waters with Fe from either coastal sediments or from upwelling processes would be minimal. DCMs are not found in coastal waters or in pelagic regions where complex bottom topography causes upwelling of deep water with sufficiently high Fe concentrations to enhance surface chlorophyll a concentrations. Such enrichment of surface waters overlying or downstream of topographical seamounts or ridges that rise to within a few thousand meters of the surface usually results in elevated phytoplankton biomass in the UML and no DCM due to decreased solar irradiance in the TML. The effect of such enrichment of Fe in surface pelagic waters that results from upwelling processes is most pronounced in the Scotia Sea, in the Polar Frontal region downstream of South Georgia, over the Southwest Indian Ridge, over the Kerguelen Plateau, and over the Pacific Antarctic and Southeast Indian Ridges.

Holm-Hansen, O, Kahru M, Hewes CD, Kawaguchi S, Kameda T, Sushin VA, Krasovski I, Priddle J, Korb R, Hewitt RP, Mitchell BG.  2004.  Temporal and spatial distribution of chlorophyll-a in surface waters of the Scotia Sea as determined by both shipboard measurements and satellite data. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 51:1323-1331.   10.1016/j.dsr2.2004.06.004   AbstractWebsite

Chlorophyll-a (Chl-a) concentrations in surface waters were measured at 137 hydrographic stations occupied by four research vessels participating in the CCAMLR 2000 Survey and the values were compared to estimates from data acquired by the SeaWiFS satellite. The Chl-a concentrations measured on board ship ranged from 0.06 to 14.6 mg m(-3), a range that includes most surface Chl-a concentrations during mid-summer in the Southern Ocean. Owing to persistent cloud cover over much of the Southern Ocean, it was necessary to acquire multi-day composites of satellite data in order to obtain reliable estimates of Chl-a at each of the hydrographic stations. The correlation between the median value for the eight-day composites and the Chl-a concentrations measured on board ship had an R-2 value of 0.82, with the satellite data under-estimating the values obtained on board ship at high Chl-a concentrations and slightly overestimating the shipboard data at Chl-a concentrations of < 0.2 mg m(-3). For Chl-a concentrations of < 1.0 mg m(-3), the ratio of the satellite estimates divided by the shipboard values was 0.89 +/- 0.45 (n = 50). As the mean Chl-a concentration in most pelagic Antarctic waters is close to 0.5 mg m(-3), satellite estimates for Chl-a concentrations in surface waters are thus close to shipboard measurements, and offer the advantage of providing synoptic maps of Chl-a distribution over extensive areas of the Southern Ocean. Satellite Chl-a images for the months preceding (December 1999) and following (February 2000) the CCAMLR 2000 Survey cruises showed that the general pattern of Chl-a concentration in the Scotia Sea and adjoining waters was similar in all three months, but that the phytoplankton biomass was generally lowest in December, reached maximal values in January, and started to decline in February. in contrast, Chl-a concentrations in Drake Passage declined progressively from early December through February. Published by Elsevier Ltd.

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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.

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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.

Kahru, M, Elmgren R, Di Lorenzo E, Savchuk O.  2018.  Unexplained interannual oscillations of cyanobacterial blooms in the Baltic Sea. Scientific Reports. 8   10.1038/s41598-018-24829-7   AbstractWebsite

Population oscillations in multi-species or even single species systems are well-known but have rarely been detected at the lower trophic levels in marine systems. Nitrogen fixing cyanobacteria are a major component of the Baltic Sea ecosystem and sometimes form huge surface accumulations covering most of the sea surface. By analysing a satellite-derived 39-year (1979-2017) data archive of surface cyanobacteria concentrations we have found evidence of strikingly regular interannual oscillations in cyanobacteria concentrations in the northern Baltic Sea. These oscillations have a period of similar to 3 years with a high-concentration year generally followed by one or two low-concentration years. Changes in abiotic factors known to influence the growth and survival of cyanobacteria could not provide an explanation for the oscillations. We therefore assume that these oscillations are intrinsic to the marine system, caused by an unknown, probably mainly biological mechanism that may be triggered by a combination of environmental factors. Interactions between different life cycle stages of cyanobacteria as well as between predator-prey or host-parasite are possible candidates for causing the oscillations.

Kahru, M, Fiedler PC, Gille ST, Manzano M, Mitchell BG.  2007.  Sea level anomalies control phytoplankton biomass in the Costa Rica Dome area. Geophysical Research Letters. 34   10.1029/2007gl031631   AbstractWebsite

Satellite data show that chlorophyll-a concentration (Chl-a) in the northeastern tropical Pacific is well correlated with sea level anomaly (SLA). This correlation spans a wide spectrum of scales from large-scale phenomena like ENSO to mesoscale cyclonic and anticyclonic eddies. Negative SLA (e. g. during La Ni (n) over tildea events and in cyclonic eddies) is associated with the lifting of isopycnals in the nutricline and increased Chl-a due to enhanced phytoplankton growth, while positive SLA (e. g. during El Ni (n) over tildeo events and in anticyclonic eddies) is associated with a deeper nutricline and reduced Chl-a due to decreased phytoplankton growth. The coupling between SLA and Chl-a anomaly in the Costa Rica Dome (CRD) area is tighter than has previously been recorded anywhere in the world ocean. 70% of the interannual variations in Chl-a anomaly in the CRD area is explained by a combination of the positive and negative effects of SLA.

Kahru, M, Horstmann U, Rud O.  1994.  Satellite Detection of Increased Cyanobacteria Blooms in the Baltic Sea - Natural Fluctuation or Ecosystem Change. Ambio. 23:469-472. AbstractWebsite

Using data from the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA series of satellites, an increase in the area covered by cyanobacteria blooms in the Baltic Sea was detected. The time series of satellite data covers a period of 12 years from 1982 to 1993. The total area covered by surface-floating cyanobacteria (bluegreen algae) has increased in the 1990s, reaching over 62 000 km in 1992. From 1992, visible accumulations appeared for the first time in the Gulf of Riga and reappeared, in the western Gulf of Finland, after being absent from 1984. Conspicuous surface blooms were also present in the early 1980s, coincident with a period of sunny and calm summers. However, when the influence of variable sunshine duration is taken into account, the increase in 1991-1993 is still distinct, indicating significant changes in the Baltic environment. The causal factors for the increased cyanobacteria blooms are still not clear.

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.

Kahru, M, Aitsam A, Elken J.  1981.  Coarse-Scale Spatial Structure of Phytoplankton Standing Crop in Relation to Hydrography in the Open Baltic Sea. Marine Ecology-Progress Series. 5:311-318.   10.3354/meps005311   AbstractWebsite

During an interdisciplinary study of the BOSEX site in the Gotland Basin, central Baltic Sea, a number of mappings of the chlorophyll and hydrographic fields were made by vertical profilings on a regular grid of 20 X 25 nautical miles with a 5-mile station spacing. Spatial variability of the chlorophyll and density fields was partitioned into the fine-scale [- 100 m) and coarse-scale [- 10 km) components. The chlorophyll coarse-scale variance dominates over the fine-scale variance by an order of magnitude in the upper layer. The coarse-scale pattern of the phytoplankton standing crop, measured by the vertically integrated chlorophyll a concentration, shows 2.4-fold variations and coupling to the pattern of the intermediate layer thickness. The intermediate layer separates the upper layer, suitable for plant growth, and the deep layer, the source of nutrients. It is suggested that above a thin intermediate layer, by increased vertical current shear and breaking internal waves, the upward flux of nutrients is intensified causing an enhanced phytoplankton growth and the increased standing crop.

Kahru, M, Leppanen JM, Nommann S, Passow U, Postel L, Schulz S.  1990.  Spatio-temporal Mosaic of the Phytoplankton Spring Bloom in the Open Baltic Sea in 1986. Marine Ecology-Progress Series. 66:301-309.   10.3354/meps066301   AbstractWebsite

The spatio-temporal variability of the phytoplankton spring bloom in the open Baltic Sea during an international joint study (PEX'86) is described in relation to the physical and chemical environment. Simultaneous participation of 14 research vessels enabled highly synoptic sampling of the study area of 20 X 40 n mile. Onset of the bloom was controlled by mesoscale hydrodynamic features - a pair of cyclonic and anticyclonic eddies; water masses with different salinities. The eddies supported an exceptionally high amount of chlorophyll (> 700 mg m-' in the cyclonic eddy). The temporal development of the bloom was different in each of the mesoscale features. Chlorophyll levels in the eddies started to decrease due to segmentation before formation of the thermocline and the depletion of inorganic nutrients. Variability of the chlorophyll distribution was highest just before the peak in the spatially averaged concentrahon was attained.

Kahru, M.  1997.  Using satellites to monitor large-scale environmental change: A case study of cyanobacteria blooms in the Baltic Sea. Monitoring algal blooms : new techniques for detecting large-scale environmental change. ( Kahru M, Brown CW, Eds.).:43-61., Berlin ; New York: Springer Abstract
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Kahru, M, Mitchell BG.  2010.  Blending of ocean colour algorithms applied to the Southern Ocean. Remote Sensing Letters. 1:119-124.   10.1080/01431160903547940   AbstractWebsite

A method of blending of a specific satellite ocean colour algorithm in the Southern Ocean (SO) with a generic algorithm elsewhere is proposed. The SO is known to have bio-optical properties that require a different bio-optical algorithm for retrieving chlorophyll-a concentration. Merging data retrieved with a specific algorithm in one area with data retrieved with another algorithm elsewhere has remained a problem. We use a blending scheme that uses both bio-optical properties and the location of the pixel relative to the mean position of the Subtropical Front to create a smooth transition from one algorithm to another. The method can be applied to other regions or variables after modification.

Kahru, M, Mitchell BG.  2001.  Seasonal and nonseasonal variability of satellite-derived chlorophyll and colored dissolved organic matter concentration in the California Current. Journal of Geophysical Research-Oceans. 106:2517-2529.   10.1029/1999jc000094   AbstractWebsite

Time series of surface chlorophyll a concentration (Chl) and colored dissolved organic matter (CDOM) derived from the Ocean Color and Temperature Sensor and Sea-Viewing Wide Field-of-View Sensor were evaluated for the California Current area using regional algorithms. Satellite data composited for 8-day periods provide the ability to describe large-scale changes in surface parameters. These changes are difficult to detect based on in situ observations alone that suffer from undersampling the large temporal and spatial variability, especially in Chi. We detected no significant bias in satellite Chi estimates compared with ship-based measurements. The variability in CDOM concentration was significantly smaller than that in Chi, both spatially and temporally. While being subject to large interannual and short-term variations, offshore waters (100-1000 km from the shore) have an annual cycle of Chi and CDOM with a maximum in winter-spring (December-March) and a minimum in late summer. For inshore waters the maximum is more likely in spring (April-May). We detect significant increase in both Chi and CDOM off central and southern California during the La Nina year of 1999. The trend of increasing Chi and CDOM from October 1996 to June 2000 is statistically significant in many areas.