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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.

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.

Kahru, M, Elmgren R, Savchuk OP.  2016.  Changing seasonality of the Baltic Sea. Biogeosciences. 13:1009-1018.   10.5194/bg-13-1009-2016   AbstractWebsite

Changes in the phenology of physical and ecological variables associated with climate change are likely to have significant effect on many aspects of the Baltic ecosystem. We apply a set of phenological indicators to multiple environmental variables measured by satellite sensors for 17-36 years to detect possible changes in the seasonality in the Baltic Sea environment. We detect significant temporal changes, such as earlier start of the summer season and prolongation of the productive season, in several variables ranging from basic physical drivers to ecological status indicators. While increasing trends in the absolute values of variables like sea-surface temperature (SST), diffuse attenuation of light (Ked490) and satellite-detected chlorophyll concentration (CHL) are detectable, the corresponding changes in their seasonal cycles are more dramatic. For example, the cumulative sum of 30 000 W m(-2) of surface incoming short-wave irradiance (SIS) was reached 23 days earlier in 2014 compared to the beginning of the time series in 1983. The period of the year with SST of at least 17 degrees C has almost doubled (from 29 days in 1982 to 56 days in 2014), and the period with Ked490 over 0.4 m(1) has increased from about 60 days in 1998 to 240 days in 2013 -i.e., quadrupled. The period with satellite-estimated CHL of at least 3 mg m(-3) has doubled from approximately 110 days in 1998 to 220 days in 2013. While the timing of both the phytoplankton spring and summer blooms have advanced, the annual CHL maximum that in the 1980s corresponded to the spring diatom bloom in May has now shifted to the summer cyanobacteria bloom in July.

Kahru, M, Aitsam A.  1985.  Chlorophyll Variability in the Baltic Sea - a Pitfall for Monitoring. Journal Du Conseil. 42:111-115. AbstractWebsite

Within the time and space scales of quasi-synoptic areal surveys (—1 day, —50 km)the fluorescence, measured by an in situ profiler, is shown to be a reliable measure of the chlorophyll a concentration, although significant areal and temporal differences in the fluorescence yield were observed. Analysis of variance is used to partition the total chlorophyll variance into effects due to differences between large-scale areas in different basins, to time differences between surveys, and to synoptic-scale (—10 km) and fine-scale (~ 100 m) spatial variability. It is shown that the large-scale areal differences in chlorophyll concentration are inevitably missed, being over-shadowed bythe other components of variance. The dominant sources of variance are due to the time and to the synoptic-scale space variability. With respect to the variance structure, the total water column chlorophyll is preferable for monitoring purposes. The results point up weaknesses in the conventional practice of monitoring a highly variable parameter via a sparse grid a few times a year.

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.

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.

Valente, A, Sathyendranath S, Brotas V, Groom S, Grant M, Taberner M, Antoine D, Arnone R, Balch WM, Barker K, Barlow R, Belanger S, Berthon JF, Besiktepe S, Brando V, Canuti E, Chavez F, Claustre H, Crout R, Frouin R, Garcia-Soto C, Gibb S, Gould R, Hooker S, Kahru M, Klein H, Kratzer S, Loisel H, McKee D, Mitchell BG, Moisan T, Muller-Karger F, O'Dowd L, Ondrusek M, Poulton AJ, Repecaud M, Smyth T, Sosik HM, Twardowski M, Voss K, Werdell J, Wernand M, Zibordi G.  2016.  A compilation of global bio-optical in situ data for ocean-colour satellite applications. Earth System Science Data. 8:235-252.   10.5194/essd-8-235-2016   AbstractWebsite

A compiled set of in situ data is important to evaluate the quality of ocean-colour satellite-data records. Here we describe the data compiled for the validation of the ocean-colour products from the ESA Ocean Colour Climate Change Initiative (OC-CCI). The data were acquired from several sources (MOBY, BOUSSOLE, AERONET-OC, SeaBASS, NOMAD, MERMAID, AMT, ICES, HOT, GeP&CO), span between 1997 and 2012, and have a global distribution. Observations of the following variables were compiled: spectral remote-sensing reflectances, concentrations of chlorophyll a, spectral inherent optical properties and spectral diffuse attenuation coefficients. The data were from multi-project archives acquired via the open internet services or from individual projects, acquired directly from data providers. Methodologies were implemented for homogenisation, quality control and merging of all data. No changes were made to the original data, other than averaging of observations that were close in time and space, elimination of some points after quality control and conversion to a standard format. The final result is a merged table designed for validation of satellite-derived ocean-colour products and available in text format. Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) were preserved throughout the work and made available in the final table. Using all the data in a validation exercise increases the number of matchups and enhances the representativeness of different marine regimes. By making available the metadata, it is also possible to analyse each set of data separately. The compiled data are available at doi: 10.1594/PANGAEA.854832 (Valente et al., 2015).

Kahru, M, Lee ZP, Mitchell BG.  2017.  Contemporaneous disequilibrium of bio-optical properties in the Southern Ocean. Geophysical Research Letters. 44:2835-2842.   10.1002/2016gl072453   AbstractWebsite

Significant changes in satellite-detected net primary production (NPP, mgCm(-2)d(-1)) were observed in the Southern Ocean during 2011-2016: an increase in the Pacific sector and a decrease in the Atlantic sector. While no clear physical forcing was identified, we hypothesize that the changes in NPP were associated with changes in the phytoplankton community and reflected in the concomitant bio-optical properties. Satellite algorithms for chlorophyll a concentration (Chl a, mgm(-3)) use a combination of estimates of the remote sensing reflectance Rrs() that are statistically fitted to a global reference data set. In any particular region or point in space/time the estimate produced by the global mean algorithm can deviate from the true value. Reflectance anomaly (RA) is supposed to remove the first-order variability in Rrs() associated with Chl a and reveal bio-optical properties that are due to the composition of phytoplankton and associated materials. Time series of RA showed variability at multiple scales, including the life span of the sensor, multiyear and annual. Models of plankton functional types using estimated Chl a as input cannot be expected to correctly resolve regional and seasonal anomalies due to biases in the Chl a estimate that they are based on. While a statistical model using RA() time series can predict the times series of NPP with high accuracy (R-2=0.82) in both Pacific and Atlantic regions, the underlying mechanisms in terms of phytoplankton groups and the associated materials remain elusive.

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.

Kahru, M, Leppanen JM, Rud O, Savchuk OP.  2000.  Cyanobacteria blooms in the Gulf of Finland triggered by saltwater inflow into the Baltic Sea. Marine Ecology-Progress Series. 207:13-18.   10.3354/meps207013   AbstractWebsite

In the 1980s and 1990s prior to 1995, massive blooms of the diazotrophic cyanobacterium Nodularia spumigena occurred in the Baltic Sea Proper but never extended into the central and eastern Gulf of Finland. The absence of nitrogen-fixing cyanobacteria blooms in parts of the Baltic Sea with a high N:P ratio (e.g. Gulf of Finland) has been explained by their reduced competitive advantage in conditions of P limitation. Starting with the summer of 1995, massive blooms of N. spumigena occurred in the central and eastern Gulf of Finland, as detected by both satellite sensors and in situ monitoring. We propose that the eastward expansion of N. spumigena blooms was triggered by the 1993 saltwater inflow into the Baltic. With the arrival of the saline and oxygen-depleted waters in the Gulf of Finland in 1995, stratification in the bottom layers increased, oxygen concentrations decreased, and increased amounts of phosphate were released from the sediments. The subsequent decrease in the N:P ratio may have caused the reoccurring N, spumigena blooms.

Kahru, M, Leppanen JM, Rud O.  1993.  Cyanobacterial Blooms Cause Heating of the Sea-Surface. Marine Ecology-Progress Series. 101:1-7.   10.3354/meps101001   AbstractWebsite

A series of AVHRR (Advanced Very High Resolution Radiometer) satellite images and simultaneous ship transects in July 1992 were used to show that surface accumulations of cyanobacteria (blue-green algae) in the southern Baltic Sea can cause local increases in the satellite-derived sea surface temperature (SST) by up to 1.5-degrees-C. The warmer SST is attributed to increased absorption of sunlight due to increased phytoplankton pigment concentration. The distribution of surface cyanobacterial accumulations detected as increased reflectance in the visible channel of the AVHRR satellite sensor was correlated with chlorophyll concentration at 5 m depth. Warm SST anomalies ('hot spots') appeared both in accumulations of surface-floating cyanobacteria and in areas of high chlorophyll concentration (detected by shipboard measurements). The 'hot spots' followed the detailed boundaries of the cyanobacterial plumes and probably represented a shallow, diurnally heated top layer that appeared by afternoon in conditions of low wind (2 m s-1) and weak mixing, disappeared during the night due to thermal convection and were hardly detectable on days with wind speed of 6 to 8 m s-1. The vertical extension of the top diurnally heated layer was probably less than 1 m and definitely less than 5 m, at which depth no temperature increase was detected. It is suggested that the day/night SST difference in low-wind conditions may be an indicator of near-surface phytoplankton pigment concentration.