Winter Phytoplankton Research Articles

동해 왕돌초 주변 해역의 동계와 하계 식물플랑크톤 군집 분포

Wangdol-cho, located 23 km offshore of Hupo in southwest of East Sea, is underwater rock floor, called to Wangdol-Am or Wangdol-Jam and has three tops as Mat-Jam, Middle-Jam and Set-Jam. The composition, abundance, diversity and community structure were investigated in winter and summer in 2002 around Wangdol-cho. The temperature around the Northwest and Southeast part of Wangdol-cho was influenced by the North Korea Cold Current (NKCC) and East Korea Warm Current (EKWC), respectively. Nutrient and chlorophyll-a concentration were higher at the top of Wangdol-cho than other area. A total of 41 genera and 78 species of phytoplankton were identified. The average cell abundance of phytoplankton in winter and summer were <TEX>$286{\times}10^3\;cells/m^3,\;432{\times}10^3\;cells/m^3$</TEX> respectively. The largest community was Bacillariophyta containing 52 taxa. The dominant species were Lauderia anulata and Coscinodiscus spp. which preferred cold water in winter. In contrast, warm water species such as Rhizosolenia stolterfothii and Ceratium spp. were dominant in summer. The average species diversity index of phytoplankton in winter was higher than that in summer. According to dominant species and standing crops, phytoplankton community resulted in a clear separation. One group was western area, which showed low density, and the other was eastern area, which showed the higher density. The abundance and species composition of phytoplankton. were affected by topological characteristics around Wangdol-cho.

Sedimentary records of reduction in resting egg production of Daphnia galeata in Lake Biwa during the 20th century: a possible effect of winter warming

To clarify long-term variations in the resting egg production of Daphnia galeata in Lake Biwa during the 20th century, we examined an abundance of plankton remains and ephippia in a 26-cm sediment core with a time resolution of approximately 2–6 years. Historical changes shown by these plankton remains indicated that the Daphnia population has not produced resting eggs since the 1980s, but it has remained the most abundant zooplankton species in the lake. Plankton monitoring data collected from 1966 to 2000 revealed that the overwintering individuals (January–March) of D. galeata plankters showed a significant increasing trend in recent years, such increase being negatively correlated with ephippial abundance in the sediment samples. Further analyses showed that the dominant phytoplankton in winter has changed from large inedible diatoms to edible flagellated algae, probably due to changes in vertical mixing regimes resulting from winter warming. These changes that occurred in the last several decades suggest that global warming has improved winter food conditions and thus enabled the Daphnia plankton to maintain its population without producing resting eggs in Lake Biwa.

Winter phytoplankton communities in different depths of three mesotrophic lakes (Łęczna-Włodawa Lakeland, Eastern Poland)

AbstractPhytoplankton samples were collected from three mesotrophic lakes: Piaseczno, Rogóźno and Krasne during winter seasons (from January to March). The samples were analyzed for species analysis and abundance of planktonic algae in relation to different depths of water column (0–7 m). Selected water physical-chemical parameters were also measured. Abundance of phytoplankton depended strongly on the thickness of snow and ice cover or mixing conditions. The maximal phytoplankton total number reached about 5 × 106 ind. L−1 beneath the clear ice in the Krasne Lake, minimal numbers were recorded under the thick snow and ice layers in the Piaseczno Lake (2 × 103 ind. L−1). The winter phytoplankton communities were dominated by flagellates principally cryptomonads (Cryptomonas spp. Rhodomonas minuta), euglenophytes (Trachelomonas volvocina, T. volvocinopsis), dinoflagellates (Peridinium bipes, Gymnodinium helveticum) and chrysophytes (Mallomonas elongata, M. akrokomos, Dinobryon sociale) or non-motile small species of blue-green algae (e.g. Rhabdoderma lineare, Limnothrix redekei), diatoms (Stephanodiscus spp., Asterionella formosa), and green algae (e.g. Scenedesmus spp., Monoraphidium spp.). Phytoplankton abundance and structure showed differentiation during the winter season and along the water column as well.

From climate regime shifts to lower-trophic level phenology: Synthesis of recent progress in retrospective studies of the western North Pacific

Pelagic ecosystems of the western North Pacific have experienced dramatic changes in recent decades. Prior retrospective analysis of several regions has shown that ecosystem or population changes have occurred coincident with or shortly following significant changes in ocean climate and atmospheric forcing. Here, we summarize these changes and attempt to identify specific mechanisms responsible for the changes in three regions in the western North Pacific: the Oyashio (OY), the subtropical water (ST), and the southern Japan/East Sea (JES). A cooling condition after the climate regime shift of 1976–1977 deepened the winter mixed layer depth (MLD) in both the OY and ST, but influenced lower-trophic level productivity differently between these regions. The deep MLD reduced winter phytoplankton and zooplankton biomass in the subarctic OY, presumably due to a decrease in wintertime light availability. Concurrently, it increased spring plankton biomass in the ST, presumably due to the replenishment of springtime nutrients. When a warming condition became prevalent after the 1988–1989 regime shift, the shallow winter MLD increased the winter plankton biomass in the OY, but decreased the spring plankton biomass in the ST through the same mechanism controlling light and nutrient availability. In the JES, with a complex water column structure consisting of the surface warm current and dense subsurface cold water, the hydrographic conditions and ecosystem responses could not be explained by the mechanisms for the OY and ST. We also detected cooling and warming phase shifts induced a phenological change in the OY. A model hindcast for the OY estimated an average 5-day (max. 20-day) delay in the timing of the primary production peak during the cool phase after the mid-1970s. This timing became earlier after the mid-1990s, several years after the 1988–1989 regime shift, reaching the level before the mid-1970s. The observed increase and decrease in winter and spring phytoplankton biomass, respectively, during the warm phase in the 1990s, indicate a moderate but prolonged productive season, which differs from the conventional seasonal variation pattern of the OY with an extensive but short spring bloom. This alternate phytoplankton seasonality coupled with the direct influence of a warm temperature anomaly might dramatically alter the zooplankton community structure in the OY after the 1990s, as suggested by the marked increase in the occurrence of warm-water copepods in spring. Based on these findings, we warn that retrospective analyses based on annual averages and lacking careful consideration of seasonal variation may result in erroneous findings of causes and consequences.

Ecophysiological growth characteristics and modeling of the onset of the spring bloom in the Baltic Sea

The onset of spring bloom in temperate areas is a transition period where the low productive, winter phytoplankton community is transformed into a high productive spring community. Downwelling irradiance, mixing depth and the ability of the phytoplankton community to utilize the light, are key parameters determining the timing of the onset of the spring bloom. Knowing these parameters would thus provide tools for modeling the spring bloom and enhance our knowledge of ecophysiological processes during this period. Our main objective with this study was to provide data for the growth characteristics of some key species forming the spring bloom in the Gulf of Finland, and to apply those results in a simple dynamic model for the onset of the spring bloom, in order to test if the timing of the spring bloom predicted by the models corresponds to field observations. We investigated the photosynthetic characteristics of three diatoms and two dinoflagellates ( Chaetoceros wighamii, Melosira arctica, Thalassiosira baltica, Scrippsiella hangoei and Woloszynskia halophila), at low temperatures (4–5 °C). All of these species are common during spring bloom in the Baltic Sea. Cultures of these species were acclimated to different irradiance regimes prior to measurements of photosynthesis, respiration, pigment concentration and light absorption. We did not find a positive relationship between respiration and growth rate, and we hypothesize that this relationship, which is well established at higher temperatures, is negligible or absent at low temperatures (< 10 °C). Photosynthetic maximum ( P m), and maximum light utilization coefficient ( α) was lowest and respiration ( R) highest in the dinoflagellates. We made a model of the onset of the spring bloom in the western part of Gulf of Finland, using the obtained data together with monitoring data of mixing depth and water transparency from this area. Model results were compared to field observations of chlorophyll- a (Chl- a) concentration. There was a good agreement between the model predictions and the observed onset of the spring bloom for the diatoms. S. hangoei, however, was not able to reach positive production in the model, and W. halophila had the similar growth characteristics as S. hangoei. Consequently, these species must have other competition strategies enabling them to exist and grow during spring bloom.

Typology of environmental conditions at the onset of winter phytoplankton blooms in a shallow macrotidal coastal ecosystem, Arcachon Bay (France)

Phytoplankton dynamics were assessed in the macrotidal ecosystem of Arcachon Bay through highfrequency surveys over a 5-year period in order to characterize typology of environmental conditions at the onset of the productive period. Temporal variations of hydrological and biological parameters were examined in external and internal waters of the lagoon, during the winter–spring periods from 1999 to 2003. An additional survey was performed during winter–spring 2005 in order to study the vertical structure of the water column. The occurrence of winter phytoplankton blooms between January and March emerged as a recurrent event. The early onset of the productive period is influenced by the biological functioning of adjacent Bay of Biscay oceanic waters. It is hypothesized that under a propitious hydrodynamic regime, phytoplankton inocula from the Bay of Biscay enter in the Arcachon Bay where cells presumably find favourable conditions for their fast development. The timing of the onset of those winter blooms in Arcachon Bay seems to be mainly influenced by the presence of anticyclonic weather conditions (associated with an increase in incident irradiance) during late winter (i.e. by February), while the water column does not show any particular stabilization nor stratification liable to facilitate the onset of these blooms. Moreover, these winter blooms dominated by diatoms led to an early nutrient depletion which could have inevitable consequences on the structuration of the food web during spring and summer.

Winter phytoplankton assemblages of coastal Yellow Sea connected to Jiaozhou Bay, China

The daily species variation of phytoplankton assemblage of coastal Yellow Sea connected to Jiaozhou Bay in the seawater around the Xiaoqingdao Island was studied for the three months of winter, 2003. A total of 79 taxa from four phyla, Bacillariophyta (52 species), Pyrrophyta (25 species), Chrysophyta (1 species), and Cyanophyta (1species) were determined. In general, the most important groups were Bacillariophyta and Pyrrophyta on cell abundance and species richness. In January, the dominant species was mainly composed of cosmopolitan species such as Skeletonema costatum, then it changed to cosmopolitan species Thalassiosira nordenskiodii in February, and finally, it recovered to Skeletonema costatum again in March. Cell abundance ranged from 1.02 to 130.71 × 103 cells L−1 with a single peak on 6th Feb., and the average abundance was 28.11 ± 26.01 × 103 cells L−1 (n=90) during the winter time. The trends of Shannon-Weiner diversity index and Pielou’s evenness index were contrary to cell abundance, whereas the species diversity and evenness were the lowest in February. Temperature and salinity were closely correlated to phytoplankton species composition and cell abundance. According to the variations of temperature and salinity, species sequence was obvious at the studied site. This implies that the phytoplankton community of inner Jiaozhou Bay would be influenced heavily by the coastal Yellow Sea phytoplankton assemblages.

An analysis of the characteristics of chlorophyll in the Sulu Sea

The seasonal and spatial variations of chlorophyll concentrations, Sea Surface Temperature (SST), wind fields and wind-induced Ekman pumping in the Sulu Sea are investigated using a set of new remote sensing measurements from October 1997 to December 2004. The results show the seasonality of chlorophyll, wind fields and SST and reveal the phytoplankton blooming events in the Sulu Sea basin during the northeast monsoon season. In summer, chlorophyll concentrations were relatively low (< 0.2 mg/m 3) and distributed uniformly throughout the basin with a narrow belt of high chlorophyll concentrations along the coastal waters, particularly the coasts of Borneo and of the Sulu Archipelago. In winter, chlorophyll concentrations increased (> 0.2 mg/m 3) throughout the entire basin, and phytoplankton bloomed southward to the central basin, while chlorophyll concentrations reached high levels (1 mg/m 3) in the center of the blooms. One peak was observed during the northeast monsoon season each year. SSTs have significant negative correlations with chlorophyll concentrations; i.e., high and uniformly distributed in summer but lower with an obvious tongue of cold waters southward to the central basin in winter. The seasonal variation of chlorophyll concentrations and SST distribution were associated with the seasonally reversing monsoon. The winter phytoplankton blooming and the tongue of the cold waters were correlated to the vertical upwelling cold and nutrient-rich waters drawn by the northeast wind, with the center of the blooms and the location of cold tongues coinciding with the maximum of the wind speeds and the Ekman pumping velocities.

Winter phytoplankton blooms in the shallow mixed layer of the South China Sea enhanced by upwelling

The South China Sea (SCS) is the largest marginal sea in the world. Previous studies, including recent intensive paleo-oceanographic studies, suggest that the SCS is sensitive to many types of physical forcing on the short-term (e.g., internal waves and tides, mesoscale eddies, typhoons, etc.), annual (e.g., monsoon), inter-annual (e.g., El Niño), and very long-term (e.g., climate change) time scales. To better understand how various types of physical forcing influence biogeochemical cycles in the water column, a time-series study was initiated. Bimonthly hydrographic surveys occupied stations in the subtropical–tropic SCS at 19°N, 118.5°E. Results suggest that the Southeast Asian monsoons, northeasterly from October to April and southwesterly from May to September, have important effects on biogeochemical cycles in the upper water column. Hydrographic data showed that the mixed layer depth was much shallower in winter than in other seasons. During the winter monsoon period, the nitricline became shallower and upwelling sustained an elevated phytoplankton standing stock. Mean chlorophyll concentrations (0.65 mg Chl m − 3) in winter were 8 times higher than in summer, and the integrated primary productivity over the euphotic zone reached as high as ca. 684 mg C m − 2 day − 1 in winter. The upwelling is produced by convergence of currents in the cyclonic gyre near the Luzon Strait, where the Kuroshio intrudes. In summer the current reverses following the wind change. The nitricline is depressed as downwelling occurs off northwest Luzon, resulting in strong nutrient limitation and very low chlorophyll concentrations.

Phytoplankton and bacterial alkaline phosphatase activities in relation to phosphate and DOP availability within the Gironde plume waters (Bay of Biscay)

Previous studies conducted on the continental shelf in the Southeast Bay of Biscay influenced by Gironde waters (one of the two largest rivers on the French Atlantic coast) showed the occurrence of late winter phytoplankton blooms and phosphorus limitation of algal growth thereafter. In this context, the importance of dissolved organic phosphorus (DOP) for both algae and bacteria was investigated in 1998 and 1999 in terms of stocks and fluxes. Within the mixed layer, although phosphate decreased until exhaustion from winter to spring, DOP remained high and phosphate monoesters made up between 11 to 65% of this pool. Total alkaline phosphatase activity (APA, V max) rose gradually from winter (2–8 nM h −1) to late spring (100–400 nM h −1), which was mainly due to an increase in specific phytoplankton (from 0.02 to 3.0 nmol μgC −1 h −1) and bacterial APA (from 0.04 to 4.0 nmol μgC −1 h −1), a strategy to compensate for the lack of phosphate. At each season, both communities had equal competitive abilities to exploit DOP but, taking into account biomass, the phytoplankton community activity always dominated (57–63% of total APA) that of bacterial community (9–11%). The dissolved APA represented a significant contribution. In situ regulation of phytoplanktonic APA by phosphate (induction or inversely repression of enzyme synthesis) was confirmed by simultaneously conducted phosphate-enrichment bioassays. Such changes recorded at a time scale of a few days could partly explain the seasonal response of phytoplankton communities to phosphate depletion.

Seasonal variation of crude digestive protease activity in Antarctic krill Euphausia superba

Crude krill digestive protease (CKP) was extracted on board a fishing vessel from fresh whole krill caught around the Antarctic Peninsula, and its protease activity was measured by using the azocasein method. CKP activity was high from December to February (in summer) and was low from March to August (in winter). CKP activity and mean feed intake index had a high correlation coefficient by Pearson’s correlation coefficient test (P<0.05). These results suggest that the seasonal variation of CKP activity coincides with the seasonal prevalence of some phytoplanktons as feed for krill. In summer, phytoplankton is plentiful so that krill feed on them and the krill protease activity remains high, but in winter phytoplankton abundance is low and krill do not have to keep their proteolytic activity high.

Plankton succession and assemblage structure in two neighbouring littoral ecosystems in the north-west Mediterranean Sea

We studied seasonal variations in bacterial abundance and succession in phyto- and zooplankton assemblages (particularly small taxa) in two neighbouring shallow bays (near Toulon, Mediterranean Sea, France): Little Bay (polluted, eutrophic), and Niel Bay (less polluted, oligotrophic). In Little Bay, bacteria developed in northern spring and phytoplankton (Dinophyceae &gt; 20 µm) in late northern winter–early spring. Zooplankton levels peaked at the end of northern spring and in autumn; this community was dominated by Oithona nana. In Niel Bay, bacterial levels peaked during northern spring and autumn. Phytoplankton (Dinophyceae, Bacillariophyceae) abundance was low and only peaked in June. Zooplankton levels peaked in northern mid-summer. Little Bay was influenced more by the land and by human activities than by the sea. Seasonal factors (e.g. water temperature) and sudden influences (e.g. rain and, indirectly, Mistral wind) may have modified the succession of the plankton communities in this bay. Successions did not follow Margalef’s model and the classical scheme for zooplankton. Conversely, Niel Bay functioning and plankton assemblages were most influenced by the physical environment of the sea than by the land or by human activities. Successions were closely related to the classical scheme of the Mediterranean Sea.

Patterns and mechanisms of phytoplankton variability in Lake Washington (USA).

Temporal variability in lake phytoplankton is controlled largely by a complex interplay between hydrodynamic and chemical factors, and food web interactions. We explored mechanisms underlying phytoplankton interannual variability in Lake Washington (USA), using a 25-yr time series of water quality data (1975-1999). Time-series analysis and PCA were used to decompose chlorophyll data into modes of variability. We found that phytoplankton dynamics in Lake Washington were characterized by four seasonal modes, each of which was associated with different ecological processes. The first mode coincided with the period when the system was light limited (January-March) and phytoplankton patterns were driven by the amount of available solar radiation. The second mode (April-June) coincided with the peak of the spring bloom and the subsequent decline of phytoplankton biomass, and was largely controlled by total phosphorus levels and grazing pressure from cladoceran zooplankton. Evidence of co-dependence and tight relationship between phytoplankton and cladoceran dynamics were also found from July to October when a large portion of the phosphorus supply in the mixed layer was provided by zooplankton excretion. The fourth mode (November-December) was associated with the transition to thermal and chemical homogeneity and the winter phytoplankton minima (2-2.5 microg/l). Finally, we examined the effects of meteorological forcing and large-scale oceanic climate fluctuations (ENSO and PDO) on phytoplankton dynamics and assessed the significance of their role on the interannual variability in the lake.

Satellite and in situ observations of a late winter phytoplankton bloom, in the northern Bay of Biscay

A phytoplankton bloom was observed in late winter 2000, on the continental shelf offshore of southern Brittany, in northwestern Bay of Biscay. This bloom appeared initially along the 120-m isobath, in stratified and clear waters, at the interface between the oceanic water and the plumes of southern Brittany rivers (mainly the Loire and Vilaine). The development of the bloom was triggered by favourable meteorological conditions, characterised by solar irradiance reaching the maximum level expected for that period of the year. Outside of the bloom area, the phytoplankton photosynthesis was irradiance limited: inshore, because of the stronger attenuation of the light; offshore, because of the weak stratification. The hydrological conditions at the onset of the bloom were observed in the field, during the oceanographic cruise MODYCOT. However, without SeaWiFS, the only observations related to this major event in the primary production would have been those of the coastal phytoplankton network (REPHY (REseau PHYtoplankton)). Observed initially offshore by SeaWiFS, as early as March 5, 2001, the phytoplankton bloom extended onshore and was observed at REPHY stations by mid-March. On, March 15, fishermen reported that they were handicapped in trawling, the presence of mucilage in their nets carried clogging by very heavy slime. This mucilage has been shown to be produced by the diatom Coscinodiscus and, especially, C. wailesii. These observations were made as the main bloom was declining. A simple calculation, based upon the SeaWiFS chlorophyll concentration maps observed from March 5–16, shows that phosphate was probably totally depleted offshore, by March 16. The former bloom area appeared very poor in chlorophyll on the SeaWiFS image of April 7, which was the first image available after 3 weeks of an overcast sky. A second cruise undertaken at the end of April confirmed the phosphate depletion and the low chlorophyll concentration in that area.

171 Light‐Induced Growth of Winter Phytoplankton Collected from the Benthic Boundary Layer in Coastal Oregon Waters

It has been speculated that overwintering phytoplankton in the benthic boundary layer (BBL) are seed stocks for spring blooms in coastal upwelling systems. Experiments were conducted in Jan/Feb 2003 to test the hypothesis that phytoplankton in the BBL are light limited. Water collected from the BBL at four sites across the northern Oregon shelf was incubated at in situ temperature and nutrient concentrations (∼1 μM phosphate, 10 μM nitrate, 15 μM silicate) and exposed to two different daylengths and four light levels. The daylengths were 9 h (winter) and 12 h (early spring), and the light levels (in μmol photons·m−1 ·s−1) were 0 (Control), 20 (L), 70 (M), and 160 (H). Light‐induced growth was observed in all four incubations. Exponential growth was reached in the 12 H, 12 M, and 9 H treatments after 4 to 6 days, and stationary growth was reached after 5 to 10 days. Maximum in vivo fluoresence was obtained in the 12 H, 12 M, and 9 H treatments, while maximum particulate organic carbon (POC) was obtained in the 12 H treatment. In vivo fluorescence increased in the L light treatments relative to the control, but POC did not. Blooms were terminated by nitrate or silicate depletion. These results demonstrate that growth of winter phytoplankton off Oregon is primarily light‐limited, and that input of light of sufficient intensity and constancy (i.e. water column stabilization) can lead to blooms on time scales typical of upwelling blooms (&lt;1 week).

Seasonal and spatial distribution of chlorophyll- a concentrations and water conditions in the Gulf of Tonkin, South China Sea

The Gulf of Tonkin is a semi-closed gulf northwest of the South China Sea, experiencing reversal seasonal monsoon. Previous studies of water conditions have been conducted in the western waters of the gulf, but very few studies of the Chlorophyll- a (Chl- a) distribution have been carried out for the entire gulf. The present study investigates seasonal and spatial distributions of Chl- a and water conditions in the Gulf of Tonkin by analyzing Sea-viewing Wide Field-of-View Scanner (SeaWiFS) derived Chlorophyll- a (Chl- a), in situ measurements, sea surface temperatures (SST), and other oceanographic data obtained in 1999 and 2000. The results show seasonality of Chl- a and SST variations in the Gulf of Tonkin, and reveal phytoplankton blooming events in the center part of the gulf during the northeast monsoon season. In summer, Chl- a concentrations were relatively low (<0.3 mg m −3) and distributed uniformly throughout most of the area, with a belt of higher Chl- a concentrations along the coast, particularly the coast of Qiongzhou Peninsula; in winter, Chl- a concentration increased (0.5 mg m −3) in the entire gulf, and phytoplankton blooms offshore-ward from the northeast coast to the center of the gulf, while Chl- a concentrations reached high levels (0.8–1 mg m −3) in the center of the blooms. One peak of Chl- a concentrations was observed during the northeast monsoon season in the year. SST were high (27–29 °C) and distributed uniformly in summer, but lower with a large gradient from northeast (17 °C) to southwest (25 °C) in winter, while strong northeast winds (8–10 m/s) were parallel to the east coast of the gulf. Comparison of Chl- a values shows that SeaWiFS derived Chl- a concentrations match well with in situ measurements in most parts of the gulf in May 1999, but SeaWiFS derived Chl- a are higher than in situ data in river mouth waters. The seasonal variation of Chl- a concentrations and SST distribution were associated with the seasonally reversing monsoon; the winter phytoplankton blooms were related to vertical mixing and upwelling nutrients drawn by the northeast wind.

Examination of silicate limitation of primary production in Jiaozhou Bay, China

Analysis and comparison of Jiaozhou Bay data collected from May 1991 to February 1994 revealed the spatiotemporal variations of the ambient Si(OH) 4∶NO 3 (Si∶N) concentration ratios and the seasonal variations of (Si∶N) ratios in Jiaozhou Bay and showed that the Si∶N ratios were 1 throughout Jiaozhou Bay in spring, autumn, and winter. These results provide further evidence that silicate limits the growth of phytoplankton (i.e. diatoms) in spring, autumn and winter. Moreover, comparison of the spatiotemporal variations of the Si∶N ratio and primary production in Jiaozhou Bay suggested their close relationship. The spatiotemporal pattern of dissolved silicate matched well that of primary production in Jiaozhou Bay. Along with the environmental change of Jiaozhou Bay in the last thirty years, the N and P concentrations tended to rise, whereas Si concentration showed cyclic seasonal variations. With the variation of nutrient Si limiting the primary production in mind, the authors found that the range of values of primary production is divided into three parts: the basic value of Si limited primary production, the extent of Si limited primary production and the critical value of Si limited primary production, which can be calculated for Jiaozhou Bay by Equations (1), (2) and (3), showing that the time of the critical value of Si limitation of phytoplankton growth in Jiaozhou Bay is around November 3 to November 13 in autumn; and that the time of the critical value of Si satisfaction of phytoplankton growth in Jiaozhou Bay is around May 22 to June 7 in spring. Moreover, the calculated critical value of Si satisfactory for phytoplankton growth is 2.15-0.76 μmol/L and the critical value of Si limitation of phytoplankton growth is 1.42-0.36 μmol/L; so that the time period of Si limitation of phytoplankton growth is around November 13 to May 22 in the next year; the time period of Si satisfactory for phytoplankton growth is around June 7 to November 3. This result also explains why critical values of nutrient silicon affect phytoplankton growth in spring and autumn are different in different waters of Jiaozhou Bay and also indicates how the silicate concentration affects the phytoplankton assemblage structure. The dilution of silicate concentration by seawater exchange affects the growth of phytoplankton so that the primary production of phytoplankton declines outside Jiaozhou Bay earlier than inside Jiaozhou Bay by one and half months. This study showed that Jiaozhou Bay phytoplankton badly need silicon and respond very sensitively and rapidly to the variation of silicon.

Modelling nitrogen fluxes in oligotrophic environments: NW Mediterranean and NE Atlantic

Stocks and fluxes of nitrogen in two oligotrophic ecosystems are evaluated using a vertically resolved turbulence-driven ecological model. The model simulates the intra-annual variability of the ecological processes in the upper 300 m of the water column in the Catalan Sea (CS), northwestern Mediterranean, and in the subtropical northeast Atlantic (NEA). Higher irradiance and daylight length making the euphotic layer thicker, together with a higher mixing layer depth explained that the phytoplankton maximum was deeper in NEA than in CS during summertime. In general, the amount of irradiance appears to control the chlorophyll maximum depth while the amount of nitrate transported from the bottom boundary is mainly constraining the phytoplankton stocks. The summer chlorophyll maximum was found at depths below the thermocline and close to the nitracline. In both locations, zooplankton grazing controlled the late winter phytoplankton bloom but reduced more than expected the subsurface phytoplankton concentration in summer. The nitrite maximum was successfully simulated to be close to the chlorophyll maximum, explained by the phytoplankton reduction of nitrate in the dark. The yearly estimates of upward fluxes of nitrate to the euphotic zone were variable depending on the horizon at which fluxes were computed. Just below the euphotic zone, the upward fluxes of nitrate were 0.64 and 0.22 mol N m −2 in the CS and NEA, respectively, more closely connected to the variation of the nitrogen gradient than to the density field.

Influence of the Plume of the River Po on the Distribution of Subtidal Microphytobenthos in the Northern Adriatic Sea

In order to assess the influence of the River Po outflow on microphytobenthos distribution, two areas of the northern Adriatic Sea, differently affected by the River Po outflow, were investigated. Sediment samples were collected in June 1996 and February 1997 from two grids of six stations (from 14 to 66 m deep) located across the River Po plume front. In the top 1 cm of sediment, chlorophyll-a ranged from 11.2 to 278.8 mg m - 2 , often exceeding integrated values in the water column, except during the winter phytoplankton bloom. Microphytobenthos densities (size >20 pm) ranged from 728 to 9278 cells cm - 3 . Diatoms largely dominated microphy-tobenthic assemblages with both epipelic motile (Pleurosigma spp., Gyrosigma spp., Navicula spp.) and non-motile forms (Paralia sulcata, Hyalodiscus laevis). Filamentous cyanobacteria (Oscillatoriales) were abundant in the summer. The vertical distribution of microphytobenthos revealed that a considerable fraction of the microalgal biomass (up to 40%) was located in the 3-6 cm sediment layer. Microphytobenthos did not display significant changes between sampling periods. Coastal sediments of the northern area displayed high densities of the marine planktonic Thalassiosira angulata, suggesting that high sedimentation rates occurred in the area facing the River Po delta. Conversely, in the southern area pennate epipelic diatoms dominated the microphytobenthic community. In both areas, offshore sediments displayed the dominance of the centric benthic diatom Paralia sulcata. The results of the present study indicate that the plume, which determined marked coast-offshore gradients, influenced both abundance and community structure.

Increased blooms of a dinoflagellate in the NW Atlantic

Sampling by the Continuous Plankton Recorder (CPR) over the NW Atlantic from 1960 to 2000 has enabled long-term studies of the larger components of the phytoplankton community, highlighting various changes, particularly during the 1990s. Analysis of an index of phytoplankton biomass, the Phytoplankton Colour Index (PCI) has revealed an increase over the past decade, most marked during the winter (December to February) months. Examination of the structure of the community using multiple linear-regression models indicates that the winter phytoplankton community composition has changed markedly in the 1990s compared to the 1960s. One phytoplankter, the dinoflagellate Ceratium arcticum (Cleve), has undergone dramatic changes in abundance during this period, with pronounced large winter blooms and decreased autumnal levels, and its contribution to the Phytoplankton Colour index values has increased significantly. Other dominant species in the phytoplankton community, both diatoms and dinoflagellates, did not show the same variations over the examined time period. It is suggested that the response of C. arcticum is probably a result of previously reported changes in stratification in the NW Atlantic, due to dynamic hydro-climatic (freshening and cooling) events.