Changes in phytoplankton biomass and primary production between 1991 and 2001 in the Westerschelde estuary (Belgium/The Netherlands)

The Westerschelde estuary is a very polluted and turbid estuary, but the last decade the waterquality improved. Dredging activity also increased in 1997 to allow bigger ships to enter the port of Antwerpen. This could potentially decrease the light conditions for the phytoplankton. Because of all these recent changes in the estuary we studied primary productivity in 2001 and compared it to values in 1991. The results show that due to a decrease in discharge in particulate and dissolved organic carbon the oxygen concentrations in general have increased in the upstream region, although in spring and summer low oxygen concentrations (10–30% saturation) can still be found. Phosphate and ammonia concentrations have decreased and the zone of nitrification which was very large in 1991 has become very small and is now located in the uppermost upstream region of the estuary. Si-concentrations have remained the same. All nutrient concentrations are still high enough not to limit phytoplankton growth. Turbidity remained unaltered as a result of the dredging works, and as a result phytoplankton biomass in most of the estuary did not show a decrease, although there were signs that in the upstream region phytoplankton biomass decreased, possible caused by increased grazing pressure. The relationship between phytoplankton biomass and primary productivity did not change, and from the data it can be concluded that the dredging activity will not influence the gross and net primary productivity of the phytoplankton.     

Secondary production of the brackish copepod communities and their contribution to the carbon fluxes in the Westerschelde estuary (The Netherlands)

The zooplankton community of the brackish part of the Westerschelde estuary (November 1989–October 1990) was dominated by two calanoid copepods, Eurytemora affinis and Acartia tonsa. Eurytemora was present during a longer period of the year and was much more important in terms of total abundances and biomasses than Acartia.The secondary production of these species was estimated by means of the growth rate method, using weight-specific growth rates obtained from laboratory cultures (Eurytemora) or from the literature (Acartia).Due to the substantially higher growth rates of Acartia compared to Eurytemora, total yearly productions of both communities were comparable, notwithstanding the large discrepancies in biomass. They amounted to about 5 and 6 g C m^–2 y^–1 by Acartia and Eurytemora respectively.The food needed to realise this production was estimated to be about 14 and 17 g C m^–2 y^–1 by Acartia and Eurytemora respectively. Provided that the copepods are able to selectively ingest the phytoplankton, in situ net production provides sufficient carbon for zooplankton demands for a short period of the year only. As phytoplankton standing stock is very low and net phytoplankton productivity is negative from late fall to early spring, nutritional demands of the copepods have to be fulfilled by other than algal food at least during this period of the year.Although the copepods in the brackish part can have an important impact on some food items, their contribution to total carbon fluxes in the brackish zone is negligible: each year some 6% of all consumed carbon in the brackish part of the estuary passes through the copepod food web.     

Carbon flows in the Westerschelde estuary (The Netherlands) evaluated by means of an ecosystem model (MOSES)

The autotrophic production and heterotrophic consumption of organic matter in the Westerschelde, a highly turbid and eutrophic estuary in the Southwest Netherlands is examined by means of a dynamic simulation model. The model describes the ecologically relevant processes in thirteen spatial compartments and adequately fits most observed data.Three autotrophic processes are included in the model. Net pelagic photosynthetic production is relatively low (average 41 gC m^–2 yr^–1) and three spatial compartments near the turbidity maximum zone are respiratory sinks of phytoplankton biomass. According to the model, net phytobenthic primary production is more important than pelagic primary production in the upstream half of the Westerschelde. On the scale of the entire estuary, benthic primary production amounts to about 60% of pelagic primary production. Water-column nitrification, which is very important in the nitrogen cycle, is most pronounced near the turbidity zone where it accounts for the major autotrophic fixation of carbon (up to 27 g C m^–2 yr^–1). Viewed on the scale of the total estuary, however, the process is not very important.Less than 20% of total organic carbon input to the estuary is primary produced, the remainder is imported from waste discharges and from the river.The degree of heterotrophy of the Westerschelde estuary proved to be one of the highest yet reported. On average 380 g carbon per square metre is net lost per year (range 200–1200 gC m^–2 yr^–1). The yearly community respiration (bacterial mineralization, respiration of higher trophic levels and sedimentation) is 4 to 35 times (estuarine mean of 6) higher than the net production. This degree of heterotrophy is highest near the turbidity maximum and generally decreases from the freshwater to the seaward boundary. About 75% of all carbon losses can be ascribed to pelagic heterotrophic processes; the sediment is only locally important.Mineralisation rates are highest in the turbidity region, but as only a fraction of total carbon resides here, less than 20% of all organic carbon is lost in this part of the estuary. This result is in contradiction with a previous budget of the estuary, based on data of the early seventies, where more than 80% of all carbon was estimated to be lost in the turbidity zone. Part of this discrepancy is probably caused by changes that have occurred in the estuary since that time.Due to the high heterotrophic activity, nearly all imported and in situ produced carbon is lost in the estuary itself and the Westerschelde is an insignificant source of organic matter to the coastal zone.The model estuary acts as a trap for reactive organic matter, both from the land, from the sea or in situ produced. Internal cycling, mainly in the water column, results in the removal of most of the carbon while the more refractory part is exported to the sea.     

Spatial and temporal variation in community composition and photosynthetic characteristics of phytoplankton in the upper Westerschelde estuary (Belgium,SW Netherlands)

Community composition, biomass and primary production of phytoplankton were studied in the east- ernmost section of the Westerschelde estuary in 1984. Photosynthetic characteristics were compared with distribution of some dominant phytoplankton species along a salinity gradient from 5 to 22 Spring phytoplankton, with Cyclotella meneghiniana (freshwater) and Skeletonema costatum (marine) as the dominant species grew faster than summer phytoplankton. In summer, biomass achieved its maximum, due to the riverine Scenedesmus species and the marine diatoms Thalassiosira levanderi and Ditylum brightwellii, as dominants. Growth conditions were more favourable to phytoplankton communities above 15%, than below this salinity. The data were compared with previous studies (1972) of species composition in the area.     

Nitrogen dynamics in the Westerschelde estuary (SW Netherlands) estimated by means of the ecosystem model MOSES

A tentative nitrogen budget for the Westerschelde (SW Netherlands) is constructed by means of a simulation model with thirteen spatial compartments. Biochemical and chemical processes in the water column are dynamically modeled; fluxes of dissolved constituents across the water-bottom interface are expressed by means of diagenetic equations.The model is calibrated on a large amount of observed variables in the estuary (1980–1986) with relatively fine temporal and spatial detail. Additional constraints are imposed by the stoichiometric coupling of carbon, nitrogen and oxygen flows and the required conservation of mass. The model is able to reproduce rather well the observed distributions of nitrate, ammonium, oxygen and Kjeldahl nitrogen both in time and space. Also, model output of biochemical oxygen demand and total organic carbon falls within observed ranges.By far the most pervasive process in the nitrogen cycle of the estuary is nitrification which mainly takes place in the water column of the upper estuarine part. On average about three times as much nitrate is leaving the estuary at the sea side compared to what enters from the river and from waste discharges. Ammonium on the other hand is consumed much faster (nitrification) than it is regenerated and only about one third of the total import leaves the estuary at the sea side. The budget for detrital nitrogen reveals import from the river, from wastes and from the sea. Phytoplankton uptake of inorganic nitrogen is negligible in the model.About 21% of total nitrogen, 33% of inorganic nitrogen, is removed from the estuary (mainly to the atmosphere through denitrification) and the load of nitrogen net exported to the sea amounts to about 51 000 tonnes per year. Total denitrification in our model is lower than what was estimated in the literature from the late seventies, where a nitrogen removal up to 40–50% of the total inorganic load was reported. Part of the differences could be methodological, but inspection of the nutrient profiles that led to these conclusions show them to be different to the ones used in our study. The oxygen deficient zone has moved upstream since the late seventies, entrailing the zone of denitrification into the riverine part of the Schelde. The nitrification process now starts immediately upon entering the estuary.     

The closure of the grevelingen estuary: Its influence on phytoplankton primary production and nutrient content

The Grevelingen estuary was cut off from the sea in May 1971, and changed into stagnat lake Grevelingen. After the closure nitrate concentrations decreased to extremely low values (less than 2 μgat NH₃-N/1). Ammonia concentrations varied between 10–30 μgat NH₃-N/1, comparable with the situation before the closure. Phosphate concentrations fluctuated between1–2 μgat PO₄-P/1 in the estuarine phase, and increased to μgat after the closure, presumably caused by decomposition of biological material and release of phosphates from the bottom. Phytoplankton primary production was not markedly affected by the damming up, and amounted to 175 g C/m² in 1971. Communication no. 147 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands     

Estimating estuarine residence times in the Westerschelde (The Netherlands) using a box model with fixed dispersion coefficients

The residence time of the water masses in the Westerschelde estuary was determined using a simple compartment-model that simulates the advective-diffusive transport of a conservative dissolved substance (chlorinity). The residence time of a water parcel in the upstream part of the estuary (i.e. the time needed for this water parcel to leave the estuary) varied from about 50 days in winter to about 70 days in summer. The most seaward compartment had residence times of about 10-15 days.Dispersive coefficients that are fixed in time were able to reproduce the observed salinity distributions very well in the Westerschelde. They were obtained by calibration on observed chlorinities. It is argued that the apparent relationship of dispersive coefficients with freshwater flow, which is observed in certain studies, could (partly) reflect the deviation from steady state conditions which are required assumptions to calculate these dispersive coefficients directly from salinity profiles.     

Short-term fluctuations in benthic diatom numbers on an intertidal sandflat in the Westerschelde estuary (Zeeland,The Netherlands)

During the period March–May 1991, sediment samples were taken every two or three days at one intertidal station in the brackish part of the Westerschelde estuary. Quantitative cell counts were made in order to investigate the short-term temporal changes in diatom numbers and assemblage structure.Throughout the whole sampling period, the diatom assemblage was dominated by epipsammic diatoms. Three species, Achnanthes delicatula, Opephora cf. perminuta and Catenula adhaerens on average accounted for almost 67% of all valves counted. The epipsammic diatom fraction showed no significant changes in absolute numbers; its species composition appeared relatively stable. In contrast, epipelic diatom densities significantly increased towards the end of the study period. Species composition within this fraction was less stable. Multivariate analysis (Principal Components Analysis), in combination with multiple regression, indicated that total sky irradiance (on the second and third day preceding sampling) and percentage organic matter were related to the short-term fluctuations of the epipelic diatom fraction.     

Nutrients and phytoplankton primary production in the marine tidal Oosterschelde estuary (The Netherlands)

In a shallow marine tidal area, the eastern part of Oosterschelde estuary in the S.W. Netherlands, phytoplankton primary production amounted to 176–338 g C.m^?2.y^?1 during the period 1981–1985. The influence of nutrient concentrations on the phytoplankton primary production is discussed. Phosphate and inorganic nitrogen generally were amply available. Import of inorganic nitrogen into the basin was shown and an intense delivery of ammonia by zoobenthos was suggested. Nitrate was considered to be slightly influenced by phytoplankton consumption and mainly by nitrate reduction at the bottom. Silicate may have played a limiting role in phytoplankton primary production. The first phytoplankton bloom in spring (diatom bloom) always terminated when silicate concentration decreased below K_s values. Further on in 1983 and 1984 both primary production and chlorophyll curves showed a dip when silicate was not available. The influence of available light on the primary production was demonstrated during situations with a low extinction coefficient when primary production reached maximum values. Further on during 1985 the spring bloom occurred already in March when winter extinction coefficients were lower than during preceding winters. Long term production studies are necessary to understand the extreme fluctuations of annual production patterns in relation to the prevailing environmental conditions.     

Physical and chemical characters,phytoplankton and primary production of Ezequiel Ramos Mexía Reservoir (Argentina)

We describe the distribution in space and time of a series of physical and chemical variables, phyto-plankton, and primary production in Ezequiel Ramos Mexía Reservoir (Argentina). Its waters are soft, poor in nutrients and with a low transparency that greatly depresses primary production. Phytoplankton data indicate the presence of 79 taxa with Bacillariophyceae, Cyanophyta and Chlorophyta alternatively dominant. Chlorophyll a was always low and never exceeded 3 mg m⁻³. Based on these results, the trophic status of this ecosystem is discussed.     

Influence of changes in salinity and light intensity on growth of phytoplankton communities from the Schelde river and estuary (Belgium/The Netherlands)

In the Schelde continuum, a succession in the phytoplankton community is observed along the transition from the river to the freshwater tidal reaches of the estuary and from the freshwater to brackish reaches of the estuary. The goal of this study was to experimentally evaluate the contribution of changes in salinity and light climate to this succession. In summer 2000 and in spring 2001, phytoplankton communities from the river, the freshwater tidal reaches and the brackish reaches of the estuary were incubated under high or low light intensities and exposed to a change in salinity. HPLC analysis was used to evaluate the response of different algal groups to changes in light intensity and salinity. When incubated at a light intensity corresponding to the mean underwater light intensity of the freshwater tidal reaches, growth of phytoplankton from the river as well as from freshwater tidal reaches was significantly lower than when incubated at a light intensity corresponding to the mean underwater light intensity of the river. The phytoplankton community from the freshwater tidal reaches did not appear to be better adapted to low light intensities than the phytoplankton community from the river. Although diatoms were expected to be less sensitive to a reduction in light intensity than green algae, the opposite response was observed. Freshwater and brackish water phytoplankton were negatively affected by respectively an increase or decrease in salinity. However, the effect of salinity was not strong enough to explain the disappearance of freshwater and brackish water phytoplankton between a salinity of 0.5 and 10 psu, suggesting that other factors also play a role. In the freshwater phytoplankton communities from the river and the freshwater tidal reaches, green algae and diatoms responded in a similar way to an increase in salinity. In the brackish water phytoplankton community, fucoxanthin displayed a different response to salinity than lutein and chlorophyll a.     

Underestimation of primary production as indicated by measurements with size-fractionated phytoplankton in Lake Maarsseveen I (The Netherlands)

Primary productivity of four size classes of phytoplankton (<150m, <50m, <20m and <5m) was measured from March through October 1986 in Lake Maarsseveen I with an incubator technique. The mean column production was approximately 400 mg C.m^–2.day^–1, with a range of values between 150 and 750 mg C.m^–2.day^–1. The mean contribution of the size fractions <50m, <20m and <5m to the size fraction<150m was 80%, 60% and 35%, respectively. During their appearance the grazing impact of small herbivorous zooplankton,e.g. rotifers, can give an underestimation of the size fraction <150m. An indication of this phenomenon is given.     

Seasonal change in the proportion of bacterial and phytoplankton production along a salinity gradient in a shallow estuary

We intended to evaluate the relative contribution of primary production versus allochthonous carbon in the production of bacterial biomass in a mesotrophic estuary. Different spatial and temporal ranges were observed in the values of bacterioplankton biomass (31–273 g C l^–1) and production (0.1–16.0 g C l^–1 h^–1, 1.5–36.8 mg C m^–2 h^–1) as well as in phytoplankton abundance (50–1700 g C l^–1) and primary production (0.1–512.9 g C l^–1 h^–1, 1.5–512.9 mg C m^–2 h^–1). Bacterial specific growth rate (0.10–1.68 d^–1) during the year did not fluctuate as much as phytoplankton specific growth rate (0.02–0.74 d^–1). Along the salinity gradient and towards the inner estuary, bacterio- and phytoplankton biomass and production increased steadily both in the warm and cold seasons. The maximum geographical increase observed in these variables was 12 times more for the bacterial community and 8 times more for the phytoplankton community. The warm to cold season ratios of the biological variables varied geographically and according to these variables. The increase at the warm season achieved its maximum in the biomass production, particularly in the marine zone and at high tide (20 and 112 times higher in bacterial and phytoplankton production, respectively). The seasonal variation in specific growth rate was most noticeable in phytoplankton, with seasonal ratios of 3–26. The bacterial community of the marine zone responded positively – generating seasonal ratios of 1–13 in bacterial specific growth rate – to the strong warm season increment in phytoplankton growth rate in this zone. In the brackish water zone where even during the warm season allochthonous carbon accounted for 41% (on average) of the bacterial carbon demand, the seasonal ratio of bacterial specific growth rate varied from about 1 to 2. During the warm season, an average of 21% of the primary production was potentially sufficient to support the whole bacterial production. During the cold months, however, the total primary production would be either required or even insufficient to support bacterial production. The estuary turned then into a mostly heterotrophic system. However, the calculated annual production of biomass by bacterio- and phytoplankton in the whole ecosystem showed that auto- and heterotrophic production was balanced in this estuary.     

Primary production of phytoplankton in Lake Valencia (Venezuela)

Lake Valencia is heavily polluted by waste water of domestic, agricultural and industrial origin. The high organic load may have produced important changes in the limnological properties. Cyanobacteria dominated in numbers and biomass (over 90% throughout the year). Chlorophyll-a content averaged 37.7 + 15 μg · 1⁻¹. Maximum concentrations of 50–80 μg · 1⁻¹ were found near the inflows affected by organically polluted affluents. There has been a 50% reduction in the euphotic zone in only 13 years. The maximum rate of gross photosynthesis per hour at light saturation was determined within the uppermost 1-meter layer. The highest value was 16,290 mg O₂ · m⁻³ · h⁻¹. Lake Valencia is among the most productive lakes in the world, with areal net photosynthesis averaging 7.5 g C · m⁻² · d⁻¹.     

3D-numerical modelling of cohesive suspended sediment in the Western Scheldt estuary (The Netherlands)

A cohesive sediment transport model considering the effects of flocculation, deposition and erosion is used in an attempt to simulate the suspended sediment distribution in a mesotidal estuary. The numerical model solves the three-dimensional (3D) advection-diffusion equation using a two-time level scheme, and a semi-implicit finite difference approach. The transport model is coupled to a 3D-barotropic hydrodynamic model for the simulation of the major tidal components reproducing the non-linear effects. An application of these models in the Western Scheldt estuary is described. The results of the different tests show that the adopted approach provides a useful basis for a good understanding of the physical processes involved in sediment transport and for the study of practical problems. The sensitivity of the model to key parameters controlling the simulation of bed sediment/water exchanges, shows the importance of a good definition of bottom sediment characteristics and the importance of further development of a consolidation algorithm.     

Microphytobenthic species composition,pigment concentration,and primary production in sublittoral sediments of the Trondheimsfjord (Norway)1

In spring 2005, monthly sampling was carried out at a sublittoral site near Tautra Island. Microphytobenthic identification, abundance (ABU), and biomass (BIOM), were performed by microscopic analyses. Bacillariophyceae accounted for 67% of the total ABU, and phytoflagellates constituted 30%. The diatom floristic list consisted of 38 genera and 94 species. Intact light‐harvesting pigments chl a, chl c, and fucoxanthin and their derivatives were identified and quantified by HPLC. Photoprotective carotenoids were also observed (only as diadinoxanthin; no diatoxanthin was detected). Average fucoxanthin content was 4.57 ± 0.45 μg fucoxanthin · g sediment dry mass^?1, while the mean chl a concentration was 2.48 ± 0.15 μg · g^?1 dry mass. Both the high fucoxanthin:chl a ratio (considering nondegraded forms) and low amounts of photoprotective carotenoids indicated that the benthic microalgal community was adapted to low light. Microphytobenthic primary production was estimated in situ (MPPs, from 0.15 to 1.28 mg C · m^?2 · h^?1) and in the laboratory (MPPp, from 6.79 to 34.70 mg C · m^?2 · h^?1 under light saturation) as ¹⁴C assimilation; in April it was additionally estimated from O₂‐microelectrode studies (MPP_O2) along with the community respiration. MPP_O2 and the community respiration equaled 22.9 ± 7.0 and 7.4 ± 1.8 mg C · m^?2 · h^?1, respectively. A doubling of BIOM from April to June in parallel with a decreasing photosynthetic activity per unit chl a led us to suggest that the microphytobenthic community was sustained by heterotrophic metabolism during this period.     

Dynamics of phytoplankton detritus in a shallow,eutrophic lake (Lake Loosdrecht,The Netherlands)

A study was made of the mortality and aerobic decomposition of light- and phosphorus-limited cultures of Oscillatoria limnetica, a dominant phytoplankton species in shallow, eutrophic Lake Loosdrecht (The Netherlands). When placed in the dark at 20 °C, most cells died and lysed within twelve days. The labile organic matter was completely decomposed within three weeks. Absorbance spectra indicated that blue green algae may contributed significantly to the refractory dissolved substances in the lake. Refractory particulate matter constituted from 7 to 24% of the biomass of O. limnetica, depending on the growth rate before incubation in the dark. The decomposition rate of this fraction was 0.005 d^–1. On a basis of a steady-state model of the dynamics of phytoplankton detritus, the areal organic dry weight concentration of the detritus in the lake is ca. 60 g m^–2. This means the quantities of detritus in the seston and epipelon are about equal.     

Phytoplankton growth rates in the freshwater tidal reaches of the Schelde estuary (Belgium) estimated using a simple light-limited primary production model

During the course of 1996, phytoplankton was monitored in the turbid, freshwater tidal reaches of the Schelde estuary. Using a simple light-limited primary production model, phytoplankton growth rates were estimated to evaluate whether phytoplankton could attain net positive growth rates and whether growth rates were high enough for a bloom to develop. Two phytoplankton blooms were observed in the freshwater tidal reaches. The first bloom occurred in March and was mainly situated in the most upstream reaches of the freshwater tidal zone, suggesting that it was imported from the tributary river Schelde. The second bloom occurred in July and August. This summer bloom was situated more downstream in the freshwater tidal reaches and appeared to have developed within the estuary. A comparison between phytoplankton growth rates estimated using a simple primary production model and flushing rate of the water indicated that no net increase in phytoplankton biomass was possible in March while phytoplankton could theoretically increase its biomass by 20% per day during summer. Chlorophyllaconcentrations at all times decreased strongly at salinities between 5–10 psu. This decline was ascribed to a combination of salinity stress and light limitation. Phytoplankton biomass and estimated annual net production were much higher in the freshwater tidal zone compared to the brackish reaches of the estuary (salinity > 10 psu) despite mixing depth to euphotic depth ratios being similar. Possible reasons for this high production include high nutrient concentrations, low zooplankton grazing pressure and import of phytoplankton blooms from the tributary rivers.     

Dynamics and distribution of microphytobenthic chlorophyll-a in the Western Scheldt estuary (SW Netherlands)

The temporal dynamics and spatial distribution of microphytobenthic chlorophyll-a in the layer 0–1 cm were determined in the Western Scheldt estuary over the period 1991–1992. Connections between the annually averaged benthic chlorophyll-a and station elevation and sediment composition (as a measure of the hydrodynamic energy caused by currents and waves) were also examined.Microphytobenthic chlorophyll-a showed one main peak in early summer and a smaller peak in autumn. The mean chlorophyll-a concentration of 113 mg Chl-a m^–2 in the upper centimeter is of the same order of magnitude as in other estuarine areas. The average annual primary production of the microphytobenthos has been estimated at 136 g C m^–2 y^–1 The primary production of sediment inhabiting microalgae is at least 17% of the total primary production in the estuary.Considerable differences in annually averaged chlorophyll-a emerges between the stations. These differences are related mainly to the interaction between station elevation and clay content of the sediment.     

Carbonate system of the Razdolnaya River estuary (Amur Bay, Sea of Japan)

Two methods, the total alkalinity measurement by Bruevich [4] and pH measurement in a cell without liquid junction [11], were suggested for study of the carbonate system of estuaries. Based on new measurements, the empirical equations were obtained for the first and second seawater concentration constants of carbonic acid for the ranges of salinity 0–40 and temperatures 0–30°C. Applying the constants and above methods, we studied the carbonate system of the Razdolnaya River-Amur Bay estuary in two expeditions of July 2001, the first in a period of average water level and the second after a flood. In the latter survey, extremely low values (60 µatm) of pCO₂ (carbon dioxide partial pressure) were recorded in the seaward part of the estuary and extremely high ( 13 300 µatm) were noted in the river. High pCO₂ in the surface water was caused by intense bacterial activity, and low levels were caused by phytoplankton bloom. The nonconservative behavior of the total alkalinity and dissolved inorganic carbon was revealed in the estuary. Based on the data of the carbonate system, the production/destruction of organic matter was assessed.Original Russian Text Copyright © 2005 by Biologiya Morya, Tishchenko, Wong, Volkova, Gramm-Osipov, Johnson, Dudarev, Zvalinskii, Nedashkovskii, Pavlova, Chichkin, Sagalaev, Shevtsova, Shkirnikova.