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Hiromi Kasai - One of the best experts on this subject based on the ideXlab platform.
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dynamics of dissolved and particulate organic matter during the Spring Bloom in the oyashio region of the western subarctic pacific ocean
Aquatic Microbial Ecology, 2010Co-Authors: Toru Hasegawa, Hiromi Kasai, Atsushi Tsuda, Tsuneo Ono, Hiroshi OgawaAbstract:We evaluated the dynamics of dissolved organic matter (DOM) and particulate organic matter (POM) at various stages of the Spring Bloom in the Oyashio region of the western subarctic Pacific Ocean by means of samples obtained during 3 cruises in April, May, and June 2003. In April, the average concentration (±1 SD) of nitrate plus nitrite ions was 23.3 ± 0.39 µmol l -1 in surface waters, and this decreased to below the detection limit (<0.05 µmol l -1 ) during the Bloom period. About 80% of newly accumulated organic carbon was partitioned into POM. Whereas the C:N ratios of bulk DOM ranged from 13 to 20, the average C:N ratio of newly accumulated DOM was 7.3 ± 1.5. The lability of DOM was assessed by seawater culture-type incubation experiments on surface sea- water samples from each observation site. Although the newly accumulated DOM was relatively rich in nitrogen, most of it escaped rapid (d to wk) bacterial consumption in the seawater culture experi- ments.
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nutrient and plankton dynamics in the oyashio region of the western subarctic pacific ocean
Deep-sea Research Part Ii-topical Studies in Oceanography, 2002Co-Authors: Hiroaki Saito, Atsushi Tsuda, Hiromi KasaiAbstract:Abstract Nutrients and plankton dynamics were investigated in the Oyashio region of the western subarctic Pacific. Observations were carried out between 1990 and 1998 along A-line, which crosses the Oyashio, a cold western boundary current of the North Pacific. Nutrient concentrations were highest in March and decreased with the Spring phytoplankton Bloom, which was observed in April and/or May. Nitrate and silicic acid concentrations at the sea surface declined to near-depletion by August. Compared with the eastern subarctic Pacific, the Oyashio region is characterized by large seasonal variability in nutrients and plankton biomass. The magnitudes of seasonal changes in nitrate and silicic acid in the Oyashio region were 2.6 and 2.8 times those of Station P in the eastern subarctic Pacific, respectively. Phytoplankton and zooplankton were 53 and 7.9 times higher in the Oyashio than at Station P, respectively. These differences were suggested to be a result of higher new production and higher ecological efficiency of the Oyashio ecosystem than in the eastern subarctic Pacific. The Spring Bloom ended by June–July when both nitrate and silicic acid were still replete at half of the stations. Thus, high-nutrient low-chlorophyll water occurred in the Oyashio region during late Spring to summer. This result indicated that factors other than macronutrient depletion were important in terminating the Spring Bloom. The concentration ratio of silicic acid to nitrate (Rsw) at the sea surface was 1.65 before the Spring Bloom. Rsw was almost constant at the early-Bloom period. During the mid- to late-Bloom period, the ratio first increased, then decreased. The net consumption ratio of silicic acid to nitrate (ΔSi(OH)4:ΔNO3) at the sea surface was 1.28 during March and April, and ΔSi(OH)4:ΔNO3 increased to 2.59 during April and May. The preferential uptake of silicic acid suggests diatoms were stressed during mid- to late-Bloom period and the stress influenced nutrient and plankton dynamics in the Oyashio region. In order to understand the controlling factors of nutrient and plankton dynamics during the Spring Bloom, mathematical simulations with a silicon–nitrogen cycle model were carried out. The model showed that light and silicic acid limitation depressed phytoplankton growth, and cell sinking and zooplankton grazing decreased phytoplankton biomass. Changes in silicic acid and nitrate uptake by diatoms during the Spring Bloom was essential in reproducing the temporal change in nutrient dynamics in the Oyashio region. Although zooplankton grazing did not prevent the initiation of the Spring Bloom, grazing pressure was an important factor controlling the magnitude and the duration of the Spring Bloom. The observed large fluctuations in the biomass and timing of zooplankton recruitment suggested that zooplankton play an important role in determining the interannual variations of the Spring Bloom in the Oyashio region.
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estimation of standing stock of chlorophyll a and primary production from remote sensed ocean color in the oyashio region the western subarctic pacific during the Spring Bloom in 1997
Journal of Oceanography, 1998Co-Authors: Hiromi Kasai, Hiroaki Saito, Atsushi TsudaAbstract:In order to examine the applicability of remotely-sensed ocean color for the estimation of phytoplankton biomass and primary production in the Oyashio region, the western subarctic Pacific, vertical distributions of chlorophylla concentration and primary production were observed in April and May 1997. Spring Bloom was observed in both April and May, and the surface concentration of chlorophylla exceeded 40 mg m−3. The relationship between the standing stocks of chlorophylla within the layer from the sea surface to one optical depth (0–1/k layer) and the surface chlorophylla concentration is expressed as a Michaelis-Menten equation. The mean ratio of the standing stock of chlorophylla in the euphotic layer to that in the 0–1/k layer was 4.41, this ratio did not significantly differ from 4.61 which was obtained at homogeneous distribution of chlorophylla within the euphotic layer. These facts suggest that the distribution of chlorophylla could be assumed to be homogeneous in the euphotic layer during the Spring Bloom. Results of primary production measurements by simulatedin situ method were compared with those by an algorithm with two variables; chlorphylla and non-spectral PAR. Daily primary production in the euphotic layer estimated by the algorithm varied in a range of 38–274% of that estimated by incubation, although the primary productions by the algorithm agreed with those by the incubation at a half of stations. Primary production within the euphotic layer calculated using simply the surface data was the same as that estimated using vertical distribution of chlorophylla. These results show that the primary production in the euphotic layer may be estimated from the remote sensed measurements during the Spring Bloom in the Oyashio region.
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variability in timing and magnitude of Spring Bloom in the oyashio region the western subarctic pacific off hokkaido japan
Fisheries Oceanography, 1997Co-Authors: Hiromi Kasai, Hiroaki Saito, Akira Yoshimori, Satoru TaguchiAbstract:The Spring Bloom of phytoplankton is a well-established, regular, seasonal event in the western subarctic Pacific and is considered one of the most important conditions of massive production of pelagic fishes. A series of 12 cruises was conducted from 1990 to 1992 to examine the timing and magnitude of the Spring phytoplankton Bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan. An interannual variability in the Bloom events was also analysed. On the basis of hydrographical characteristics, the study area was divided into three water masses: the Oyashio Water Mass, the Mixed Water Mass, and the Coastal Water Mass. Spring Blooms were observed first in April in the Oyashio and the Coastal Water Masses, and continued to May in 1991 and 1992. However, no Bloom was recorded in the Mixed Water Mass. High nutrient supply into the surface mixed layer during winter is likely to be one of the factors supporting an intense Spring Bloom in the Oyashio Water Mass. A significant positive relationship between log-transformed surface chlorophyll a concentration and maximum density gradient (MDG) within the euphotic layer was obtained in April, indicating the importance of vertical stability of the water column in the initiation of Spring Blooms in the Oyashio and the Coastal Water Masses. The Spring Blooms in 1991 were much more extensive and lasted longer than in 1990. It is suggested that meteorological conditions and abundance of grazers were responsible for this interannual difference.
Hiroaki Saito - One of the best experts on this subject based on the ideXlab platform.
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grazing impact of the copepod community in the oyashio region of the western subarctic pacific ocean
Progress in Oceanography, 2008Co-Authors: Kazutaka Takahashi, Hiroaki Saito, Akira Kuwata, Keiichiro IdeAbstract:Abstract The role of copepod grazing on the ecosystem dynamics in the Oyashio region, western subarctic Pacific was investigated during six cruises from June 2001 to June 2002. In situ grazing rates of the copepod community (CGR) were measured by the gut fluorescence method in respect to developmental stages of dominant species. In terms of biomass, more than 80% of the copepod community was dominated by six large calanoid species (Neocalanus cristatus, Neocalanus flemingeri, Neocalanus plumchrus, Eucalanus bungii, Metridia pacifica and Metridia okhotensis) throughout the year. Resulting from the observed pattern of the interzonal migrating copepods, the CGR in the Oyashio region was divided into three phases, i.e. Spring (Bloom), summer (post-Bloom) and autumn–winter phase. During the Spring Bloom, late copepodites of the interzonal migrating species, N. cristatus, N. flemingeri and E. bungii appeared in the surface layer (0–50 m) to consume the production of the Bloom, resulting in a high grazing rate of the copepod community (7.9 mg Chl m−2 d−1), though its impact on phytoplankton community was low due to the high primary productivity. During the post-Bloom period, although the copepod community which was dominated by N. cristatus, N. plumchrus, M. pacifica and newly recruited E. bungii still maintained a high biomass, the CGR was generally lower (1.8–2.6 mg Chl m−2 d−1 for June and August 2001), probably due to the lower availability of phytoplankton. Nevertheless, the highest CGR was also observed during this period (10.5 mg Chl m−2 d−1 in June 2002). The high CGR on autotrophic carbon accounted for 69% of the primary production, suggesting that the copepod community in the Oyashio region potentially terminates the phytoplankton Bloom. Abundant occurrence of young E. bungii, which is a characteristic phenomenon in the Oyashio region, was largely responsible for the high grazing pressure in June 2002 suggesting that success of reproduction, growth, and survival in E. bungii during the Spring Bloom is an important factor in controlling phytoplankton abundance during the post-Bloom season. During autumn and winter, CGR was the lowest in the year (0.29–0.38 mg Chl. m−2 d−1) due to the disappearance of the interzonal migrating copepods from the surface layer. Diel migrant M. pacifica was the most important grazer during this period. The annual ingestion of the copepod community is estimated as 37.7 gC m−2 on autotrophic carbon (converted using C:Chl ratio of 30) or 137.9 gC m−2 on suspended particles (using C:Chl ratio of in situ value, 58–191), accounting for 13% and 46% of annual primary production, respectively. This study confirms that copepod grazing is an important pathway in carbon flow in the Oyashio region and in particular their role in the phytoplankton dynamics is significant for the termination of the Spring Bloom.
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nutrient and plankton dynamics in the oyashio region of the western subarctic pacific ocean
Deep-sea Research Part Ii-topical Studies in Oceanography, 2002Co-Authors: Hiroaki Saito, Atsushi Tsuda, Hiromi KasaiAbstract:Abstract Nutrients and plankton dynamics were investigated in the Oyashio region of the western subarctic Pacific. Observations were carried out between 1990 and 1998 along A-line, which crosses the Oyashio, a cold western boundary current of the North Pacific. Nutrient concentrations were highest in March and decreased with the Spring phytoplankton Bloom, which was observed in April and/or May. Nitrate and silicic acid concentrations at the sea surface declined to near-depletion by August. Compared with the eastern subarctic Pacific, the Oyashio region is characterized by large seasonal variability in nutrients and plankton biomass. The magnitudes of seasonal changes in nitrate and silicic acid in the Oyashio region were 2.6 and 2.8 times those of Station P in the eastern subarctic Pacific, respectively. Phytoplankton and zooplankton were 53 and 7.9 times higher in the Oyashio than at Station P, respectively. These differences were suggested to be a result of higher new production and higher ecological efficiency of the Oyashio ecosystem than in the eastern subarctic Pacific. The Spring Bloom ended by June–July when both nitrate and silicic acid were still replete at half of the stations. Thus, high-nutrient low-chlorophyll water occurred in the Oyashio region during late Spring to summer. This result indicated that factors other than macronutrient depletion were important in terminating the Spring Bloom. The concentration ratio of silicic acid to nitrate (Rsw) at the sea surface was 1.65 before the Spring Bloom. Rsw was almost constant at the early-Bloom period. During the mid- to late-Bloom period, the ratio first increased, then decreased. The net consumption ratio of silicic acid to nitrate (ΔSi(OH)4:ΔNO3) at the sea surface was 1.28 during March and April, and ΔSi(OH)4:ΔNO3 increased to 2.59 during April and May. The preferential uptake of silicic acid suggests diatoms were stressed during mid- to late-Bloom period and the stress influenced nutrient and plankton dynamics in the Oyashio region. In order to understand the controlling factors of nutrient and plankton dynamics during the Spring Bloom, mathematical simulations with a silicon–nitrogen cycle model were carried out. The model showed that light and silicic acid limitation depressed phytoplankton growth, and cell sinking and zooplankton grazing decreased phytoplankton biomass. Changes in silicic acid and nitrate uptake by diatoms during the Spring Bloom was essential in reproducing the temporal change in nutrient dynamics in the Oyashio region. Although zooplankton grazing did not prevent the initiation of the Spring Bloom, grazing pressure was an important factor controlling the magnitude and the duration of the Spring Bloom. The observed large fluctuations in the biomass and timing of zooplankton recruitment suggested that zooplankton play an important role in determining the interannual variations of the Spring Bloom in the Oyashio region.
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estimation of standing stock of chlorophyll a and primary production from remote sensed ocean color in the oyashio region the western subarctic pacific during the Spring Bloom in 1997
Journal of Oceanography, 1998Co-Authors: Hiromi Kasai, Hiroaki Saito, Atsushi TsudaAbstract:In order to examine the applicability of remotely-sensed ocean color for the estimation of phytoplankton biomass and primary production in the Oyashio region, the western subarctic Pacific, vertical distributions of chlorophylla concentration and primary production were observed in April and May 1997. Spring Bloom was observed in both April and May, and the surface concentration of chlorophylla exceeded 40 mg m−3. The relationship between the standing stocks of chlorophylla within the layer from the sea surface to one optical depth (0–1/k layer) and the surface chlorophylla concentration is expressed as a Michaelis-Menten equation. The mean ratio of the standing stock of chlorophylla in the euphotic layer to that in the 0–1/k layer was 4.41, this ratio did not significantly differ from 4.61 which was obtained at homogeneous distribution of chlorophylla within the euphotic layer. These facts suggest that the distribution of chlorophylla could be assumed to be homogeneous in the euphotic layer during the Spring Bloom. Results of primary production measurements by simulatedin situ method were compared with those by an algorithm with two variables; chlorphylla and non-spectral PAR. Daily primary production in the euphotic layer estimated by the algorithm varied in a range of 38–274% of that estimated by incubation, although the primary productions by the algorithm agreed with those by the incubation at a half of stations. Primary production within the euphotic layer calculated using simply the surface data was the same as that estimated using vertical distribution of chlorophylla. These results show that the primary production in the euphotic layer may be estimated from the remote sensed measurements during the Spring Bloom in the Oyashio region.
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variability in timing and magnitude of Spring Bloom in the oyashio region the western subarctic pacific off hokkaido japan
Fisheries Oceanography, 1997Co-Authors: Hiromi Kasai, Hiroaki Saito, Akira Yoshimori, Satoru TaguchiAbstract:The Spring Bloom of phytoplankton is a well-established, regular, seasonal event in the western subarctic Pacific and is considered one of the most important conditions of massive production of pelagic fishes. A series of 12 cruises was conducted from 1990 to 1992 to examine the timing and magnitude of the Spring phytoplankton Bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan. An interannual variability in the Bloom events was also analysed. On the basis of hydrographical characteristics, the study area was divided into three water masses: the Oyashio Water Mass, the Mixed Water Mass, and the Coastal Water Mass. Spring Blooms were observed first in April in the Oyashio and the Coastal Water Masses, and continued to May in 1991 and 1992. However, no Bloom was recorded in the Mixed Water Mass. High nutrient supply into the surface mixed layer during winter is likely to be one of the factors supporting an intense Spring Bloom in the Oyashio Water Mass. A significant positive relationship between log-transformed surface chlorophyll a concentration and maximum density gradient (MDG) within the euphotic layer was obtained in April, indicating the importance of vertical stability of the water column in the initiation of Spring Blooms in the Oyashio and the Coastal Water Masses. The Spring Blooms in 1991 were much more extensive and lasted longer than in 1990. It is suggested that meteorological conditions and abundance of grazers were responsible for this interannual difference.
Laurent Seuront - One of the best experts on this subject based on the ideXlab platform.
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temporal patterns of phytoplankton assemblages size spectra and diversity during the wane of a phaeocystis globosa Spring Bloom in hydrologically contrasted coastal waters
Journal of the Marine Biological Association of the United Kingdom, 2008Co-Authors: Mathilde Schapira, Dorothee Vincent, Valerie Gentilhomme, Laurent SeurontAbstract:The space–time dynamic of phytoplankton diversity and succession was investigated during the wane of a Phaeocystis globosa Spring Bloom in four distinct hydrological sub-systems of the eastern English Channel. Nutrients, chlorophyll-a concentrations, and phytoplankton composition, standing stocks, size spectra and diversity were monitored during three key periods in 2003: late Spring, early summer and summer. Two consecutive diatom assemblages were observed, respectively dominated by: (i) small colonial species ( μ m; Melosira sp., Diploneis sp. and Navicula transitans ) in April; and (ii) large fine-walled cells (>200 μ m; Guinardia striata and Rhizosolenia imbricata ) in May and July. This shift in diatom composition appeared to be related to the potentially limitating silicic acid in early summer. Specific phytoplankton assemblages identified in distinct water masses have evolved from a mature/senescent community towards a relatively homogeneous aestival structure of dominant species that might have been triggered by the wane of the P. globosa Bloom. Our results also identified a strong heterogeneity in the distribution of secondary species between distinct water masses during the summer period, suggesting that the magnitude of the observed patterns was intrinsically related to the hydrological properties prevailing in each sub-system. The identification of distinct temporal patterns in phytoplankton species diversity and succession following the wane of a Spring Bloom at relatively small spatial scales (i.e. P. globosa Blooms in particular and phytoplankton Blooms in general and is suggested to have potentially strong consequences on food web dynamics and the carbon cycle in coastal ecosystems.
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increased seawater viscosity phaeocystis globosa Spring Bloom and temora longicornis feeding and swimming behaviours
Marine Ecology Progress Series, 2008Co-Authors: Laurent Seuront, Dorothee VincentAbstract:The suggested influence of increased seawater viscosity on the feeding and swimming behaviours of adult females of the calanoid copepod Temora longicornis was investigated during a Phaeocystis globosa Spring Bloom in the coastal waters of the eastern English Channel. Adult female gut contents did not exhibit any significant correlation with chlorophyll concentration or seawater excess viscosity over the course of the Bloom. Instead, the highest gut contents were observed when the seawater viscosity was maximum (up to 4.6 centipoise (cP)), after a 5-fold decrease in chlorophyll concentration related to the formation of foam. This demonstrates that even high viscosity did not mechanically hamper zooplankton grazing. Gut contents were controlled by the taxonomic availabil- ity rather than the quantitative availability of phytoplankton-based food. This is consistent with the observed sustained egg production rates despite drastic changes in the composition of protist resource over the course of the Bloom. Before and after the Bloom (in the absence of P. globosa), T. longicornis exhibited similar swimming paths characterized by their large spatial extent and low curviness. In contrast, during the Bloom their movements were spatially more localised, significantly slower and more convoluted. This behaviour is suggested as an adaptive strategy to optimise forag- ing activity during P. globosa Blooms, which have been recently shown to generate high level of phytoplankton patchiness.
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biologically induced modification of seawater viscosity in the eastern english channel during a phaeocystis globosa Spring Bloom
Journal of Marine Systems, 2006Co-Authors: Laurent Seuront, Dorothee Vincent, James G MitchellAbstract:To identify the potential relationship between Pheaocystis globosa Bloom conditions and seawater properties, a hydrobiological survey was performed in the inshore waters of the Eastern English Channel over the course of the phytoplankton Spring Bloom. Chlorophyll concentration, auto- and hetero/mixotrophic composition of protists and standing stock, and seawater viscosity were measured weekly from March to June 2004. The decline of the Bloom is characterized by a massive foam formation in the turbulent surf zone. Before foam formation, seawater viscosity significantly increased, showing a significant positive correlation with chlorophyll concentration. In contrast, after foam formation this correlation was negative, seawater viscosity kept increasing despite a sharp decrease in chlorophyll concentrations. No significant correlation has been found between seawater viscosity and the composition of the phytoplankton assemblages observed during the survey. However, significant positive correlations have been found between seawater viscosity and both the size and the abundance of P. globosa colonies. From the correlation patterns observed between chlorophyll concentration and seawater viscosity, we suggest that the rheological properties of seawater are mainly driven by extracellular materials associated with colony formation and maintenance rather than by cell composition and standing stock.
Nicholas A Bond - One of the best experts on this subject based on the ideXlab platform.
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climate change and control of the southeastern bering sea pelagic ecosystem
Deep-sea Research Part Ii-topical Studies in Oceanography, 2002Co-Authors: George L. Hunt, Phyllis J Stabeno, Gary E Walters, Elizabeth Sinclair, Jeffery M Napp, Richard D Brodeur, Nicholas A BondAbstract:We propose a new hypothesis, the Oscillating Control Hypothesis (OCH), which predicts that pelagic ecosystem function in the southeastern Bering Sea will alternate between primarily bottom-up control in cold regimes and primarily top-down control in warm regimes. The timing of Spring primary production is determined predominately by the timing of ice retreat. Late ice retreat (late March or later) leads to an early, ice-associated Bloom in cold water (e.g., 1995, 1997, 1999), whereas no ice, or early ice retreat before mid-March, leads to an open-water Bloom in May or June in warm water (e.g., 1996, 1998, 2000). Zooplankton populations are not closely coupled to the Spring Bloom, but are sensitive to water temperature. In years when the Spring Bloom occurs in cold water, low temperatures limit the production of zooplankton, the survival of larval/juvenile fish, and their recruitment into the populations of species of large piscivorous fish, such as walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus) and arrowtooth flounder (Atheresthes stomias). When continued over decadal scales, this will lead to bottom-up limitation and a decreased biomass of piscivorous fish. Alternatively, in periods when the Bloom occurs in warm water, zooplankton populations should grow rapidly, providing plentiful prey for larval and juvenile fish. Abundant zooplankton will support strong recruitment of fish and will lead to abundant predatory fish that control forage fish, including, in the case of pollock, their own juveniles. Piscivorous marine birds and pinnipeds may achieve higher production of young and survival in cold regimes, when there is less competition from large piscivorous fish for coldwater forage fish such as capelin (Mallotus villosus). Piscivorous seabirds and pinnipeds also may be expected to have high productivity in periods of transition from cold regimes to warm regimes, when young of large predatory species of fish are numerous enough to provide forage. The OCH predicts that the ability of large predatory fish populations to sustain fishing pressure will vary between warm and cold regimes. The OCH points to the importance of the timing of ice retreat and water temperatures during the Spring Bloom for the productivity of zooplankton, and the degree and direction of coupling between zooplankton and forage fish. Forage fish (e.g., juvenile pollock, capelin, Pacific herring [Clupea pallasii]) are key prey for adult pollock and other apex predators. In the southeastern Bering Sea, important changes in the biota since the mid-1970s include a marked increase
Ragnar Elmgren - One of the best experts on this subject based on the ideXlab platform.
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satellite measurements of cyanobacterial Bloom frequency in the baltic sea interannual and spatial variability
Marine Ecology Progress Series, 2007Co-Authors: Matti Kahru, Oleg P Savchuk, Ragnar ElmgrenAbstract:Owing to the potentially harmful character of nitrogen-fixing filamentous cyanobacter- ial Blooms in the Baltic Sea, a capacity to predict their occurrence is of interest. We quantified the sur- face accumulations of cyanobacteria, mainly Nodularia spumigena, using ocean colour satellite data. The spatial and temporal frequency of the accumulations was mapped with an automated detection algorithm, using their high reflectance in the 670 or 555 nm bands. Coastal Zone Color Scanner (CZCS) data were used for a first period (1979 to 1984), and combined SeaWiFS and MODIS-Aqua data for a second period (1998 to 2006). The frequency of cyanobacterial accumulations (FCA) for each 1 km 2 pixel was calculated as the ratio of scenes with detected cyanobacteria to the number of valid cloud-free scenes in July-August. FCA varied greatly among years and the basins of the Baltic Sea. Mean FCA was 39% higher in the second period, but the increase was not statistically significant. The mean FCA for the whole Baltic was positively correlated with the residual phosphate (RP) concentration after the Spring Bloom in May-June. RP was a useful predictor of FCA in the coming summer, but could not predict the spatial pattern of the Bloom. This was better explained by prevailing winds, with high FCA in the southwestern Baltic after northerly winds in July. It seems that, at the time of writing, useful FCA predictions can be made after the Spring Bloom, but only for the whole Baltic.
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phytoplankton pigments in baltic sea seston and sediments seasonal variability fluxes and transformations
Estuarine Coastal and Shelf Science, 2002Co-Authors: Thomas S Bianchi, Carl Rolff, B Widbom, Ragnar ElmgrenAbstract:Abstract We studied the temporal variability in the composition of plant pigments in water-column particulate matter and in surface sediments, with specific emphasis on Bloom events. The pigment concentration of sinking and suspended particles was used to evaluate pre- and post-depositional decomposition of Bloom-derived phytodetritus in the Baltic proper. Water, sediment, and sediment trap samples were collected in 1994 at a long-term coastal monitoring station with fine sediments and a water depth of 36 m, situated near the Asko Laboratory in the north-western part of the Baltic Sea proper. Annual phytoplankton succession showed an early Spring Bloom dominated by diatoms, followed by dinoflagellates in late Spring and early summer. Later in summer, this dinoflagellate community was gradually replaced by a filamentous, nitrogen-fixing cyanobacterial Bloom; late summer-early autumn was characterized by diatoms. The highest chlorophyll a flux occurred in Spring during peak diatom Bloom conditions, which resulted in peak concentrations of chlorophyll a in sediments. Assuming that all chlorophyll a in sediments in early May was derived from the current Bloom, it was estimated that approximately half (48%) the Bloom-derived chlorophyll a was decomposed in two months. The highest total phaeopigment/chlorophyll a ratios (an index of the amount of chlorophyll a decay), were found in water column particulate matter (PM) during the Spring Bloom and occasional high-energy winter periods. The latter higher ratios were likely due to wind-induced resuspension of surface sediments into the water column and trap materials. The settling rate of pigments in the water column indicated rapid sedimentation of diatoms and slow sedimentation of dinoflagellates during the Spring Bloom. In summer, unicellular cyanobacteria, including picocyanobacteria, appeared to have higher sedimentation rates than filamentous nitrogen-fixing cyanobacteria, which seemed to decompose largely in the water column. Thus, this study demonstrated that plant pigments were useful biomarkers of the taxonomic composition of phytoplankton Blooms and their sedimentation and subsequent decomposition in the Baltic Sea.
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seasonal dynamics of sublittoral meiobenthos in relation to phytoplankton sedimentation in the baltic sea
Estuarine Coastal and Shelf Science, 1997Co-Authors: Emil Olafsson, Ragnar ElmgrenAbstract:Meiobenthic metazoans (40–500) μm were sampled monthly at a 37 m deep station in the north-western Baltic Sea proper. Nematodes dominated the meiofauna, ranging from 67% of total abundance in February to 91% in September. Harpacticoid copepods were the second most common group, ranging from 2% in September to 15% in February. Total meiofauna shell-free dry weight biomass was lowest in winter (0·9 mg 10 cm−2in January), and increased rapidly following the Spring Bloom, to high values in May–July (peak 1·7 mg 10 cm−2in July). As an annual average, ostracods contributed most to biomass, 38%, while nematodes and harpacticoids made up 24 and 15%, respectively. Only nematodes were common below 2 cm depth in the sediment, and few nematodes penetrated below 4 cm. Of Wieser’s morphologically based nematode feeding groups, epistrate feeders dominated the surface sediment, and non-selective deposit feeders dominated the deeper layer in May. Total nematode abundance was significantly different among dates, with lowest numbers in winter and Spring (October–April), and almost doubled within about 2 months after the Spring phytoplankton Bloom in March. There was a significant increase in selective deposit feeders and epistrate feeders after the Spring Bloom. Harpacticoid copepods were almost all of two species,Pseudobradyasp. andMicroarthridion littorale, both of which differed significantly in abundance among months, and displayed continuous reproduction throughout the year, with a peak in pairs in precopula in winter forPseudobradyasp. and in ovigerous females inM. littoraleafter the Spring Bloom.Pseudobradyawas significantly more numerous in winter than in other seasons.Microarthridion littoralehad its highest abundance from July to October. Three species of ostracods were common throughout the year and all differed significantly in numbers among months. Turbellaria, Kinorhyncha were found in lowest numbers during winter and peaked in summer. The peak of newly settledMacoma balthicaspat in June disappeared rapidly, as predicted from laboratory experiments showing they are eaten by amphipods. The results support the hypothesis that meiobenthic animals react in two ways to phytoplankton sedimentation, with surface feeders directly assimilating sedimented phytoplankton, and increasing markedly following the Spring Bloom, while subsurface feeders experience a more stable food supply, and rely only indirectly on sedimented phytoplankton.
