Publications

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2018
2016
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.

2015
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.

2013
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.

2011
Kahru, M, Brotas V, Manzano-Sarabia M, Mitchell BG.  2011.  Are phytoplankton blooms occurring earlier in the Arctic? Global Change Biology. 17:1733-1739.   10.1111/j.1365-2486.2010.02312.x   AbstractWebsite

Time series of satellite-derived surface chlorophyll-a concentration (Chl) in 1997-2009 were used to examine for trends in the timing of the annual phytoplankton bloom maximum. Significant trends towards earlier phytoplankton blooms were detected in about 11% of the area of the Arctic Ocean with valid Chl data, e.g. in the Hudson Bay, Foxe Basin, Baffin Sea, off the coasts of Greenland, in the Kara Sea and around Novaya Zemlya. These areas roughly coincide with areas where ice concentration has decreased in early summer (June), thus making the earlier blooms possible. In the selected areas, the annual phytoplankton bloom maximum has advanced by up to 50 days which may have consequences for the Arctic food chain and carbon cycling. Outside the Arctic, the annual Chl maximum has become earlier in boreal North Pacific but later in the North Atlantic.

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.

2010
Kahru, M, Gille ST, Murtugudde R, Strutton PG, Manzano-Sarabia M, Wang H, Mitchell BG.  2010.  Global correlations between winds and ocean chlorophyll. Journal of Geophysical Research-Oceans. 115   10.1029/2010jc006500   AbstractWebsite

Global time series of satellite-derived winds and surface chlorophyll concentration (Chl-a) show patterns of coherent areas with either positive or negative correlations. The correlation between Chl-a and wind speed is generally negative in areas with deep mixed layers and positive in areas with shallow mixed layers. These patterns are interpreted in terms of the main limiting factors that control phytoplankton growth, i.e., either nutrients that control phytoplankton biomass in areas with positive correlation between Chl-a and wind speed or light that controls phytoplankton biomass in areas with negative correlation between Chl-a and wind speed. More complex patterns are observed in the equatorial regions due to regional specificities in physical-biological interactions. These correlation patterns can be used to map out the biogeochemical provinces of the world ocean in an objective way.

2008
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.

2007
Kahru, M, Mitchell BG, Gille ST, Hewes CD, Holm-Hansen O.  2007.  Eddies enhance biological production in the Weddell-Scotia confluence of the southern ocean. Geophysical Research Letters. 34   10.1029/2007gl030430   AbstractWebsite

[1] Satellite data show that oceanic eddies generated in the Southern Antarctic Circumpolar Current Front ( SACCF) are associated with increased phytoplankton biomass. Cyclonic eddies with high chlorophyll a concentration ( Chl-a) retain phytoplankton within the eddy cores and increase the light available for photosynthesis in the upper mixed layer by limiting vertical mixing and lifting of the isopycnal surfaces. Anticyclonic eddies have low Chl- a in the core but increased Chl- a in the periphery. Cross- frontal mixing mediated by eddies transports nutrients ( e. g., Fe and Si) to the north and contributes to the increased Chl- a in the frontal zone. Interannual variations in the cyclonic eddy activity are positively correlated with variations in Chl- a during the spring bloom in regions of the Antarctic Circumpolar Current around South Georgia.

2004
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.

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.

1991
Kahru, M, Leeben A.  1991.  Size Structure of Planktonic Particles in Relation to Hydrographic Structure in the Skagerrak. Marine Ecology-Progress Series. 76:159-166.   10.3354/meps076159   AbstractWebsite

Horizontal distributions of the particle size spectrum and chlorophyll a concentration in near-surface water in relation to hydrographic structure were studied along a transect across the Skagerrak as part of the international SKAGEX-I exercise in May-June, 1990. Results are compared with those of Kiorboe et al. (Mar. Ecol. Prog. Ser. 59: 19-32, 1990) who explained changes in the plankton size structure and the food web as resulting from differences in water column stratification. It was confirmed that the particle size fraction corresponding to diatoms was most abundant along the Skagerrak periphery where stronger vertical mixing is assumed to take place. The smallest (1 to 4-mu-m), chlorophyll-correlated particulate fraction was most abundant in the central Skagerrak with the dome-like near-surface stratification and the presumably reduced vertical mixing. However, distributions along the transect changed considerably over 1 to 3 d due to advection. In addition to the basin-wide changes, correlations of the different particle size fractions to chlorophyll concentration changed over much shorter length scales, often being in opposite phase to each other. An equivalent spherical diameter of 4-mu-m was the size at which the most significant shift in the horizontal distributions occurred.

Kahru, M, Kaasik E, Leeben A.  1991.  Annual Cycle of Particle-Size Fractions and Phytoplankton Biomass in the Northern Baltic Proper. Marine Ecology-Progress Series. 69:117-124.   10.3354/meps069117   AbstractWebsite

A large data set of particle concentrations in the range 1 to 1000-mu-m, chlorophyll a, and associated measurements from the surface waters of the northern Baltic proper during 1985 to 1989 was analyzed to construct spatially and temporally averaged annual time curves of particle size fractions and phytoplankton biomass. Chlorophyll a concentration was well correlated with the 28-73-mu-m size fraction and had 2 seasonal peaks: in April-May, corresponding to the diatom spring bloom, and in August, corresponding to the late-summer bloom of blue-green algae. The 2 maxima were separated by the early summer minimum when the 1-10-mu-m particle fractions corresponding to a mixed assemblage of small phytoplankton and microheterotrophs reached their maxima. The largest particles (163-1000-mu-m), corresponding to smaller mesozooplankton, reached their maximum in July-August before the blue-green maximum. Automated particle counting is well suited for routine monitoring of pelagic systems as it reveals all major shifts in the planktonic community.