Subsurface Phytoplankton Research Articles

Simulation of annual plankton productivity cycle in the Black Sea by a one‐dimensional physical‐biological model

The annual cycle of the plankton dynamics in the central Black Sea is studied by a one‐dimensional vertically resolved physical‐biological upper ocean model, coupled with the Mellor‐Yamada level 2.5 turbulence closure scheme. The biological model involves interactions between the inorganic nitrogen (nitrate, ammonium), phytoplankton and herbivorous zooplankton biomasses, and detritus. Given a knowledge of physical forcing, the model simulates main observed seasonal and vertical characteristic features, in particular, formation of the cold intermediate water mass and yearly evolution of the upper layer stratification, the annual cycle of production with the fall and the spring blooms, and the subsurface phytoplankton maximum layer in summer, as well as realistic patterns of particulate organic carbon and nitrogen. The computed seasonal cycles of the chlorophyll and primary production distributions over the euphotic layer compare reasonably well with the data. Initiation of the spring bloom is shown to be critically dependent on the water column stability. It commences as soon as the convective mixing process weakens and before the seasonal stratification of surface waters begins to develop. It is followed by a weaker phytoplankton production at the time of establishment of the seasonal thermocline in April. While summer nutrient concentrations in the mixed layer are low enough to limit production, the layer between the thermocline and the base of the euphotic zone provides sufficient light and nutrient to support subsurface phytoplankton development. The autumn bloom takes place sometime between October and December depending on environmental conditions. In the case of weaker grazing pressure to control the growth rate, the autumn bloom shifts to December–January and emerges as the winter bloom, or, in some cases, is connected with the spring bloom to form one unified continuous bloom structure during the January–March period. These bloom structures are similar to the year‐to‐year variabilities present in the data.

Patterns of macromolecular synthesis by natural phytoplankton assemblages under changing upwelling regimes: in situ observations and microcosm experiments

A coastal station located in the southern Bay of Biscay was sampled on six occasions between 12 July and 10 September 1991. The vertical distribution of the patterns of photosynthetic carbon incorporation into proteins, polysaccharides, lipids and low molecular weight metabolites, as well as the biochemical composition of particulate matter were determined on each cruise. In addition, the same variables were monitored during microcosm experiments in which sub-surface phytoplankton assemblages were incubated under different rates of increasing irradiance simulating differences in upwelling intensity. During the first three experiments, phytoplankton cells did not show any growth response to the simulated upwelling conditions. When phytoplankton growth was slow or absent and nutrients were still available, the relative synthesis of proteins was high, suggesting that phytoplankton cells tended to maintain the synthesis of proteins rather than storage products under adverse growth-limiting conditions. Sub-surface phytoplankton assemblages had the potential for growth in response to upwelling. Several diatom blooms developed during the last two experiments showing enhanced levels of protein and polysaccharide specific synthesis rates (SSR) and a marked increase in the protein to carbohydrate (P:C) compositional ratio. Parallel sea-truth observations carried out during an upwelling pulse indicated that phytoplankton assemblages under natural conditions underwent similar physiological changes to those found in the experimental microcosms under simulated upwelling. In general, the most remarkable increases in chlorophyll a (Chl a) concentration and macromolecular SSR took place in those microcosms showing higher rates of increasing irradiance. Variations in the patterns of photosynthate partitioning and the P:C ratio were also related to the intensity of the advective process. These results emphasize the importance of the fine variability of the physical field in modulating the physiological responses of phytoplankton to upwelling.

Comparison of the sedimentation of a diatom spring bloom and of a subsurface chlorophyll maximum

The seasonal pattern of sedimentation was determined over a 8-mo investigation period covering the productive season at a permanent station in the Southern Kattegat (Denmark) in 1990. The phytoplankton succession was characterised by a 4-mo long subsurface maximum associated with the pycnocline which was entirely dominated by the dinoflagellate Gyrodinium aureolum. The bulk sedimentation of organic matter took place during this period and accounted for >60% of the annual particulate organic carbon (POC) and particulate organic nitrogen (PON) sedimentation. The spring bloom period contributed 60% of the sedimentation of intact phytoplankton cells, but only 20% of the POC and PON sedimentation. A minor fraction of the sedimenting matter from the subsurface phytoplankton maximum consisted of intact phytoplankton (<20%), suggesting that the phytoplankton was processed by heterotrophs and that it was mainly products from this activity which contributed to thevertical flux of organic matter. The variation in oxygen concentration below the pycnocline coincided with the pattern of sedimentation with a delay of 3 to 6 wk.

Fronts in the Kattegat: The Hydrodynamic Regulating Factor for Biology

The recurrence since 1981 of oxygen depletion in the southern part of the Kattegat (the transition area between the Baltic Sea and the North Sea) initiated an interdisciplinary project in biological oceanography. More than 100 years of observations from light-vessels in the region are at our disposal for a hydrographic description. Nevertheless, it was first after the analysis of satellite images and the intensive in situ measurements that the great importance of the complicated front-dynamics in the region was appreciated. A phenomenological description of the interaction between the major physical phenomena encountered in the southern Kattegat such as fronts, leakage (detrainment), baroclinic circulation, wind mixing, entrainment, upwelling, and intrusions is given. The importance of these estuarine events to phytoplankton production is discussed. The importance of the hitherto unnoticed subsurface phytoplankton productions in connection with intrusions at the primary halocline is underlined.

Meridional zonation of the Barents Sea ecosystem inferred from satellite remote sensing and in situ bio-optical observations

The Barents Sea is a productive, shallow, high-latitude marine ecosystem with complex hydrographic conditions. Zonal hydrographic bands defined by a coastal current. North Atlantic Water, the Polar Front, and the seasonally variable marginal ice edge zone create a meridional zonation of the ecosystem during the spring-summer transition. The features reveal themselves in satellite imagery and by high-resolution (vertical and horizontal) physical-optical-biological sampling. Surprisingly, the long-term (7-year) mean of Coastal Zone Color Scanner (CZCS) imagery reveals the Barents Sea as an anomalous “blue-water” regime at high latitudes that are otherwise dominated by satellite-observed surface blooms. A combination of satellite imagery and in situ bio-optical analyses indicate that this pattern is caused by strong stratification in summer with surface nutrient depletion. The onset of stratification of the entire region is linked to the extent of the winter ice edge: cold years with extensive sea ice apparently stratify early due to ice melt; warm years stratify later, perhaps due to weaker thermal stratification of the Atlantic waters (e.g. Skjoldal et al. 1987). The apparent “low chlorophyll” indicated by the CZCS 7-year mean is partly due to sampling error whereby the mean is dominated by images taken later in the summer. In fact, massive blooms of subsurface phytoplankton embedded in the pycnocline persist throughout the summer and maintain substantial rates of primary production. Further, these subsurface blooms that are not observed by satellite are responsible for dramatic gradients in the beam (c1) and spectral diffuse (k) attenuation coefficients. The Barents Sea exemplifies the need to couple satellite observations with spatially and temporally resolved biogeographic ecosystem models in order to estimate the integrated water column primary production, mass flux or spectral light attenuation coefficients.

Seasonal distribution and production of phytoplankton in the southern Kattegat

The seasonal distributions of chlorophyll a and primary production throughout the water column were examined at a fixed station in the southern Kattegat. The most intensive data collection was in 1989 when the station was visited 55 times. Less intensive data collection was also carried out in 1988 and 1990. Surface chlorophyll distributions indicate the presence of a spring and autumn-winter 'bloom' in surface waters and relatively low chlorophyll a concentrations (< 2 pg I-') in the surface mixed layer during summer. Phytoplankton in surface waters during summer (mid-May to ca 1 October) appear to be nutrient limited. From mid July until ca 1 October, the entire euphotic zone appears to be nutrient depleted. The highest chlorophyll concentrations observed during the spring bloom are associated with frontal waters separating the Kattegat and waters of the Baltic outflow. From the period immediately following the spring bloom until ca 1 September, the highest chlorophyll concentrations were observed in association with the pycnocline (nutriciline). During late spring and early summer, the pycnocline usually occurs at a depth of between ca 10 and 20 m. The phytoplankton associated with the pycnocline during this period are usually photosynthetically active even at the photon flux densities encountered at the pycnocline. Thus, significant primary production occurs in this subsurface layer In 1989, total annual primary production at the study station was calculated to be ca 290 g C m-' Approximately 19 % of this production is estimated to occur during the spring bloom while on the order of 30 O/ O of the total is estimated to take place in subsurface phytoplankton populations during summer.

Produção primária e características fotossintéticas do fitoplâncton na região sueste do Brasil

Taxas de producao primaria do fitoplâncton (C-14) foram obtidas em 14 estacoes da plataforma intermediaria e areas oceânicas da regiao sueste do Brasil, no inverno de 1982 e outono de 1983.0 calculo da producao na zona euforica foi feito utilizando-se o metodo in situ simulado. Em duas ocasioes, a taxa de producao foi estimada com base nos dados de clorofila-a e radiacao solar diaria, e os resultados obtidos foram comparados com os obtidos pelo metodo in situ simulado. A taxa de producao variou de 0,04-0,49 gC/m²/d. Os resultados obtidos com o metodo da clorofila foram cerca de 35% menores. Alem dos experimentos de producao, foram estudadas as caracteristicas fotossinteticas do fitoplâncton atraves da relacao fotossintese x luz. O numero de assimilacao cresceu com a profundidade indicando uma fotoadaptacao das populacoes fitoplanctonicas dos estratos intermediarios da zona euforica (30-60% de luz), em relacao as populacoes de superficie.

Single-species dominance in a subsurface phytoplankton concentration at a Mediterranean Sea front

A narrow band of high Chl a (23.1 µg liter−1) was observed at a salinity front in the western Mediterranean Sea in late November 1987. The biomass peak was found deep in the photic zone, at 54 m, in a region of low light. A single diatom species, Thalassiosira partheneia Schrader in gelatinous colonies, represented 98% of the total phytoplankton biomass in the layer and achieved abundances >9.8 × 106 cells liter−1. Although lack of temporal sampling precludes precise determination of the processes responsible, the high biomass accumulation and dominance by this species was likely due to its preference for low light coupled with turbulence-induced high nutrient levels. Interleaving patterns in the temperature, salinity, and nutrient profiles suggest increased horizontal advection at the front. High shear at the boundaries of the interleavings might confine the biomass to a thin band, as well as generate turbulence to mix nutrients in from surrounding layers. Alternatively, stabilization of the water column following a brief, pulsed upwelling event could have reduced dispersion of the biomass, thereby confining it to the thin layer we observed.

Thermocline characteristics and phytoplankton dynamics in Agulhas Bank waters

Eastern and central Agulhas Bank waters are characterized by strong, shallow thermoclines and well developed subsurface maxima of phytoplankton biomass. The thermoclines are advectively maintained and as such are dynamically stable, but they do exhibit short-term variations in absolute temperature gradients. There are corresponding variations in subsurface phytoplankton biomass maxima. The water-column phytoplankton production maxima are generally situated in the biomass maxima because these are sufficiently shallow to ensure adequate light. Self-shading becomes a limiting factor with the development of high phytoplankton biomasses. Western Agulhas Bank waters are characterized by deeper thermoclines and less intense subsurface maxima of phytoplankton biomass. Due to the depth of the thermoclines the biomass maxima are generally situated below the depth of the 1-per-cent surface light intensity and, consequently, phytoplankton production is limited. Possible mechanisms for the maintenance of these deep ch...

Observations On Subsurface Distributions Of Chlorophyll a And Phytoplankton Carbon In The Northeast Pacific Ocean

Measurements of subsurface concentrations of chlorophyll a and phytoplankton carbon were made between Vancouver Island and Hawaii during summer 1971. In general, chlorophyll a concentrations increased and concentrations of phytoplankton carbon decreased from 10 m to depths varying between 100 and 150 m. These distributions indicated that cellular contents of chlorophyll a increased as cells were exposed to diminishing daily irradiations. Daily compensation irradiations for net photosynthesis of subsurface phytoplankton in the northeast Pacific Ocean were estimated in summer 1973. Values ranged between 0.5 and 0.7 cal cm−2 but variability among replicates was large giving a range of 0 and 1.2 cal cm−2. It was not possible to correlate compensation irradiations with taxonomic composition of phytoplankton populations.

Nitrate distribution in the subarctic Northeast Pacific Ocean

Data are presented on the distribution of nitrate in the subarctic Northeast Pacific Ocean. Over most of the area nitrate is present at the surface throughout the year at a concentration above that limiting phytoplankton growth. However, within coastal regions and south of about 45°N, nitrate may become depleted during the spring and summer. It is suggested that the maintenance of high nitrate concentrations over much of the area is due to relatively intensive entrainment of deep water into the upper zone coupled with a slow rate of removal of nitrate by the primary producers. Nitrate depletion in the southern area may be explained by less intensive entrainment of deep water and the utilization of nutrients by a subsurface phytoplankton maximum beneath the seasonal thermocline.