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Book Chapter
Kahru, M, Nommann S, Simm M, Vilbaste K.  1986.  Plankton distributions and the processes in the Baltic boundary zones. Marine interfaces ecohydrodynamics. ( Nihoul JCJ, Ed.).:270-274., Amsterdam; New York; New York, NY, U.S.A.: Elsevier ; Distributors for the U.S. and Canada, Elsevier Science Pub. Co. Abstract
Journal Article
Kahru, M, Nommann S, Zeitzschel B.  1991.  Particle (Plankton) Size Structure across the Azores Front (Joint Global Ocean Flux Study North-Atlantic Bloom Experiment). Journal of Geophysical Research-Oceans. 96:7083-7088.   10.1029/90jc02693   AbstractWebsite

The horizontal distributions of particle size spectrum and chlorophyll a concentration across the northern edge of the North Atlantic subtropical gyre along 21-degrees-W were studied as part of the Joint Global Ocean Flux Study North Atlantic Bloom Experiment in March-April, 1989. An abrupt change in the particle size spectrum at 32-degrees-40'N was located within the broad thermohaline front between the central gyre water in the south and the Azores Current in the north. Whereas the small particulate fraction (1-10-mu-m) and chlorophyll a concentration were similar (within a factor of 2), the abundance of the "diatom" size fraction (28-72-mu-m) showed a drastic increase by more than an order of magnitude in the Azores Current. The diatom fraction seemed to contribute little to the chlorophyll pool, indicating a postbloom condition. Different trophic pathways, i.e., grazing of the picoplankton in the gyre versus the diatom bloom with the following detrital loop in the Azores Current and northward, can be conjectured. The increased scattering versus absorption due to the different particle size spectrum and composition could bias the algorithms of pigment retrieval from remote sensing data.

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.

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.

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.

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.

Kahru, M.  1983.  Phytoplankton Patchiness Generated by Long Internal Waves - a Model. Marine Ecology-Progress Series. 10:111-117.   10.3354/meps010111   AbstractWebsite

A simple computer model is used to investigate the interaction between long internal waves (period ~ 13 d) and processes of primary productivity in the Baltic Sea. The periodic vertical displacements in or near the photic zone affect the availability of light and nutrients to primary producers, resulting in integral vertical fluxes of nutrients and biomass. If the nitrogen flux is parameterized implicitly by a diffusive flux across the halocline, the apparent coefficients of eddy diffusivity are more often negative and exceed in absolute value by 2 orders of magnitude the experimentally determined values. Differences in the depth integrals of daily primary production between wave crests and troughs amount to ~ 50 % and the vertical flux of the nitrate nitrogen may locally reach ~ 50 mg N m-2d-1 The outlined mechanism might be a significant additional source for the formation of mesoscale horizontal heterogeneities in phytoplankton biomass.

Kahru, M, Nommann S.  1990.  The Phytoplankton Spring Bloom in the Baltic Sea in 1985, 1986 - Multitude of Spatiotemporal Scales. Continental Shelf Research. 10:329-354.   10.1016/0278-4343(90)90055-q   AbstractWebsite

The spatio-temporal development of the phytoplankton spring bloom in the Baltic Sea for two consecutive years is analysed. Quasi-continuous, on-track measurements of particle concentration, fluorescence, temperature and salinity with a resolution of the spatial scales from ≈400 m to basin-wide were supplemented with quantitative samples of the phytoplankton abundance, pigments, and vertical CTD/fluorescence profiles. The improved spatial and temporal resolution allowed us to distinguish variability on different time and space scales. Year-to-year differences were found that include not only the timing of the bloom but also the size distribution of the plankton (composition of the phytoplankton assemblage). Contrary to conventional understanding, the bloom does not start due to the establishment of the vertical thermal stratification as the vertical density profile is controlled by the salinity stratification. The well-known massive diatom bloom is preceded by an initial growth of unidentified small-sized (1–4 μm) phytoplankton. As the bloom usually starts when the surface temperature is still below the temperature of maximum density (about 2.4°C for the salinity in the central Baltic), warming of the surface layer during that period has in fact a destabilizing effect on the stratification. The expansion of the bloom does not appear as a smooth, wave-like propulsion in the northeastern direction but rather as centripetal movements in the form of eddies and filaments from the more stratified coastal areas towards the center; hence, in the northern Baltic proper the progression is roughly in the southerly direction. The central eastern Gotland Basin with the least likelihood of vertical stratification in the photic layer is the last where the bloom commences. There the bloom starts in a mosaic of filaments and eddies that provide the vertical stability. The filaments with reduced salinity and increased temperature probably originate from the coastal areas and represent transformed coastal water.

Nommann, S, Sildam J, Noges T, Kahru M.  1991.  Plankton Distribution During a Coastal Upwelling Event Off Hiiumaa, Baltic Sea - Impact of Short-Term Flow Field Variability. Continental Shelf Research. 11:95-108.   10.1016/0278-4343(91)90037-7   AbstractWebsite

The evolution of an upwelling event and the associated plankton distribution off Hiiumaa island, northeastern Baltic Sea, is traced through continuous registration of wind and currents, consecutive CTD and chlorophyll fluorescence surveys and underway shipboard measurements of near-surface temperature and particle concentration over a 10-day period in June 1986. The earlier mesoscale pattern of warmer (13-degrees-C) near-shore waters containing higher chlorophyll concentration was drastically changed as the wind turned from SW to NNW and increased up to 12 m s-1, which resulted in the offshore Ekman transport of the warmer coastal water and upwelling of cold (6-7-degrees-C) phytoplankton-poor deeper water along the coastal slope. A relatively fast biological response to the upwelling resulted in the form of enhanced primary production and 4-7-fold increase of the standing crop of some phytoplankton populations (mainly dinoflagellates) within 4 days (at the upwelling frontal boundary). It is shown that a persistent, moderate wind is favourable to sustain a local phytoplankton bloom while keeping the vertical transport of the deeper nutrient-rich water still going but being not powerful enough to stir away the growing phytoplankton. Our measurements confirm the importance of coastal upwellings for the productivity of the Baltic near-shore ecosystems in the summer stage.

Kahru, M, Elken J, Kotta I, Simm M, Vilbaste K.  1984.  Plankton Distributions and Processes across a Front in the Open Baltic Sea. Marine Ecology-Progress Series. 20:101-111.   10.3354/meps020101   AbstractWebsite

Sections of the temperature-salinity structure in the southeastern Gotland Basin, central Baltic Sea, revealed the existence of a pronounced salinity front. A water mass with anomalously low salinity extended vertically across the horizontally uniform thermocline. Plankton distribution and primary productivity showed consistent features with the frontal structure: the near-surface primary productivity increased ?-fold adjacent to the front and levelled down farther away from it; the column chlorophyll had a broader and less distinct maximum on the high-salinity side of the front; the zooplankton community was similar in composition but its biomass more than doubled in the higher- salinity water. It is suggested that upwelling of nutrient-rich water in the frontal zone had triggered the productivity peak, whereas the subsequent advection and diffusion were instrumental for the broader chlorophyll maximum. Owing to the lower phytoplankton/zooplankton ratio, the nutrient cycling had to be faster in the higher-salinity water mass.

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

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.

Vasilkov, AP, Herman J, Krotkov NA, Kahru M, Mitchell BG, Hsu C.  2002.  Problems in assessment of the ultraviolet penetration into natural waters from space-based measurements. Optical Engineering. 41:3019-3027.   10.1117/1.1516822   AbstractWebsite

Satellite instruments currently provide global maps of surface UV irradiance by combining backscattered radiance data with radiative transfer models. The models are often limited by uncertainties in physical input parameters of the atmosphere and surface. Global mapping of the underwater UV irradiance creates further challenges for the models. The uncertainties in physical input parameters become more serious because of the presence of absorbing and scattering quantities caused by biological processes within the oceans. We summarize the problems encountered in the assessment of the underwater UV irradiance from space-based measurements, and propose approaches to resolve the problems. We have developed a radiative transfer scheme for computation of the UV irradiance in the atmosphere-ocean system. The scheme makes use of input parameters derived from satellite instruments such as the total ozone mapping spectrometer (TOMS) and sea-viewing wide field-of-view sensor (SeaWiFS). The major problem in assessment of the surface UV irradiance is to accurately quantify the effects of clouds. Unlike the standard TOMS UV algorithm, we use the cloud fraction products available from SeaWiFS and MODIS to calculate instantaneous surface flux at the ocean surface. Daily UV doses can be calculated by assuming a model of constant cloudiness throughout the day. Both SeaWiFS and a moderate resolution imaging spectroradiometer (MODIS) provide some estimates of seawater optical properties in the visible. To calculate the underwater UV flux, the seawater optical properties must be extrapolated down to shorter wavelengths. Currently, the problem of accurate extrapolation of visible data down to the UV spectral range is not solved completely, and there are few available measurements. The major difficulty is insufficient correlation between photosynthetic and photoprotective pigments of phytoplankton absorbing in the visible and UV, respectively. We propose to empirically parameterize seawater absorption in the UV on a basis of available datasets of bio-optical measurements from a variety of ocean waters. Another problem is the lack of reliable data on pure seawater absorption in the UV. Laboratory measurements of the UV absorption of both pure water and pure seawater are required. (C) 2002 Society of Photo-Optical Instrumentation Engineers.