Fixation of dissolved silicate and sedimentation of biogenic silicate in the lower river Rhine during diatom blooms

Concentrations of dissolved silicate and particulate biogenic silicate were measured in three branches of the lower river Rhine in The Netherlands in order to analyse the role of this element in the eutrophication of the river basin. Particulate silicate followed the seasonal development of the phytoplankton, which was dominated by diatoms. The concentration of dissolved silicate fell during blooms (< 0.1 mg.l^–1 ), but the amounts of biogenic silicate measured ( 1 mg.l^–1) were insufficient to explain the seasonal decrease in the dissolved fraction; this indicates retention of silicate upstream. Some particulate biogenic silicate in river water settled in man-made sedimentation areas in the Rhine delta. The observations suggest that changes in silicate fixation in the Rhine may have contributed to the incidence of non-diatom phytoplankton blooms in receiving waters.     

Effects of phytoplankton on the elemental composition (C,N, P) of suspended particulate material in the lower river Rhine

The spatial and temporal distribution of element concentrations were monitored together with chlorophyll a as an indicator of algal density to assess the effect of phytoplankton on the elemental composition (C, N, P) of suspended materials in the lower Rhine. The high concentrations of particulate C, N and P in the river were found to decrease in the delta and to increase again in the estuarine turbidity zone. Phytoplankton blooms increased the concentrations of particulate C, N, and P significantly in the upstream part of the river. In summer 1989, 15–65% of the particulate C and 20–75% of the particulate N were attributable to phytoplankton. Together with published data these observations indicate that in eutrophic rivers, the input of organic materials from the catchment is strongly modified and supplemented by in situ growth of phytoplankton. During seaward transport the phytoplankton and the particulate elements disappeared from the river water concomitantly with the suspended matter, indicating an increased retention of these elements due to sedimentation. In contrast, soluble ammonia, nitrite and phosphate increased in the tidal reaches of the river because of local input in the harbour and city of Rotterdam and because of mineralization. Therefore the total nutrient load of the Rhine estimated at the German/Dutch border does not reflect the actual input into the sea.     

Phases in the development of riverine plankton: Examples from the rivers Rhine and Meuse

Observations on phyto and zooplankton in two hydrographically different rivers were compared in order to discriminate phases in plankton development. Along the longitudinal axis of the River Rhine a gradual increase in the development of phytoplankton was observed, which reached its maximumca. 100 km before the river flows into its artificial sedimentation area. The development of rotifer populations was slightly retarded as compared with that of phytoplankton and highest population densities were only reached in the sedimentation area. Crustaceans developed in significant numbers, not until the river water had entered the sedimentation area. Development of zooplankton coincided here with a strong decrease in the density of phytoplankton. A similar trend in plankton development was observed in the River Meuse, although in this river the highest densities of phyto and zooplankton already occurred in its middle reaches. The differences in the timing of plankton growth in the two rivers are probably caused by differences in flow regime between both rivers. The River Rhine, which is fed by rainwater and melting of glaciers in the Alps, has a relatively constant discharge. On the other hand, the low discharge of the rain-fed River Meuse combined with an increased residence time of the water as a consequence of large numbers of weirs, allows a full cycle of plankton development long before its discharge into the sea. This phenomenon was also reflected in the silicate cycle in the Meuse, where the consumption by planktonic diatoms and the regeneration of silicate of deposits seem to be important. In contrast, in the main branches of the River Rhine only the effects of silicate consumption were detectable.     

Seasonal variation in composition and production of planktonic communities in the lower River Rhine

1. The composition and activity of phytoplankton, zooplankton and bacterioplankton in the lower River Rhine were measured in 1990 as part of an international biological inventory of the river. A seasonal study was carried out on two stations: one in the river mouth (km 1019) and one at the German/Dutch border (km 863). 2. High densities of phytoplankton (with up to 140 μg chlorophyll a 1-^?1) and occasional depletion of dissolved silicate were observed at the upstream station. Phosphate concentrations were also lowered during blooms. 3. Phytoplankton blooms, dominated by a few species of centric diatoms, declined one order of magnitude during downstream transport. During non-bloom conditions (low) algal densities were maintained during transport, or increased slightly, indicating the suitability of the river reach for algal growth. 4. Bacterial cell number and production (measured by the ³H-thymidine method) showed a broad summer maximum with activity peaks (0.5 nK < M thymidine h^?1) coincident with declining phytoplankton blooms. Winter values of bacterial production (0.02–0.05 n < Mh^?1) were substantial, probably as a result of allochthonous input of organic matter. 5. Rotifers and crustaceans made up the greater part of the zooplankton biovolume, but at the upstream station the contribution of Dreissena larvae and rhizopods was also substantial. High zooplankton biovolumes, of over 500 × 10⁶μm³1-^?1, were observed only during the phytoplankton spring bloom. 6. Quantitative relationships between the high phytoplankton production (2.1–3.4 gCm^?2 day^?1), the high bacterial substrate uptake (0.5–1gCm^?2 day^?1), and grazing were analysed for the growing season 1990. Algal grazing by metazoan herbivores was substantial only during spring, while the role of phagotrophic microplankton and cell lysis were indicated as major factors responsible for the downstream decline of phytoplankton blooms in the lower Rhine.     

The role of the blue mussel Mytilus edulis in the cycling of nutrients in the Oosterschelde estuary (The Netherlands)

The fluxes of particulate and dissolved material between bivalve beds and the water column in the Oosterschelde estuary have been measured in situ with a Benthic Ecosystem Tunnel. On mussel beds uptake of POC, PON and POP was observed. POC and PON fluxes showed a significant positive correlation, and the average C:N ratio of the fluxes was 9.4. There was a high release of phosphate, nitrate, ammonium and silicate from the mussel bed into the water column. The effluxes of dissolved inorganic nitrogen and phosphate showed a significant correlation, with an average N:P ratio of 16.5. A comparison of the in situ measurements with individual nutrient excretion rates showed that excretion by the mussels contributed 31–85% to the total phosphate flux from the mussel bed. Ammonium excretion by the mussels accounted for 17–94% of the ammonium flux from the mussel bed. The mussels did not excrete silicate or nitrate. Mineralization of biodeposition on the mussel bed was probably the main source of the regenerated nutrients.From the in situ observations net budgets of N, P and Si for the mussel bed were calculated. A comparison between the uptake of particulate organic N and the release of dissolved inorganic N (ammonium + nitrate) showed that little N is retained by the mussel bed, and suggested that denitrification is a minor process in the mussel bed sediment. On average, only 2/3 of the particulate organic P, taken up by the mussel bed, was recycled as phosphate. A net Si uptake was observed during phytoplankton blooms, and a net release dominated during autumn. It is concluded that mussel beds increase the mineralization rate of phytoplankton and affect nutrient ratios in the water column. A comparison of N regeneration by mussels in the central part of the Oosterschelde estuary with model estimates of total N remineralization showed that mussels play a major role in the recycling of nitrogen.     

Seasonal distribution of phytoplankton in the Aby lagoon system,Ivory Coast,West Africa

Seasonal variability of phytoplankton species composition, abundance and physical and chemical factors influencing phytoplankton dynamics were investigated in the Aby lagoon system, south-eastern Ivory Coast, covering the main climatic seasons in 2006–2007. Seasonal and spatial variability of nutrient concentrations in the system were influenced by freshwater inflow from the Bia and Tanoé rivers. The decrease in the salinity gradient in Aby Lagoon, which is permanently stratified during the long dry season, increased the bottom inputs of soluble reactive phosphate and soluble reactive silicate which, in combination with good light penetration during the long dry season, enhanced phytoplankton production in the system. During the rainy seasons, water discharges into the system washed phytoplankton biomass out, preventing the development of blooms. Overall, 192 taxa from eight phyla were recorded: Bacillariophyta (32%), Chlorophyta (31%), Cyanobacteria (23%), Euglenophyta (12%), Dinophyta (0.5%), Xanthophyta (0.5%), Chrysophyta (0.5%) and Rhodophyta (0.5%). During the long dry season, Cyanobacteria cells comprised >50% of the phytoplankton abundance. The main phytoplankton taxa responsible for this high abundance were Microcystis aeruginosa, Oscillatoria princeps, Pseudanabaena limnetica, Aphanizomenon sp. 2 and Anabaena planctonica.     

Extracellular enzyme activity associated with suspended matter in the River Rhine

SUMMARY. 1. Spatial and temporal variations in exoenzymatic proteolysis and alkaline phosphatase activity in the River Rhine were measured fluorometrically using L-leucyi (ß-naphthylamide and methylumbelliferyl phosphate (MUFP), respectively, as test substances.
2. Extracellular proteolytic activity measured in the lower 150-km reach of the river during 1988 followed the changes in the concentration of particulate nitrogen associated with the occurrence of phytoplankton blooms. Alkaline phosphatase activity was not related to the concentration of particulate phosphate and correlated weakly with particulate nitrogen and phytoplankton concentrations.
3. Seasonal observations (1989) in the lower River Rhine and transport studies in a 660-km-long reach of the river (1990) indicated that the variations in the exoenzyme activities were unrelated (or not closely related) to fluctuating bacterial numbers. Exoproteolytic activity correlated well with thymidine incorporation and algal density, confirming indications in the literature that the exoenzyme activity is related to the overall metabolic rate of the bacterial populations.
4. The exoproteolytic activities in the Rhine are similar to the highest observed in eutrophic freshwaters, indicating a rapid turnover of proteins in the river. Bacterial phosphatase activities, measured with MUFP, were similar to those in the few marine and inland waters studied so far. However, phosphate-limited algal blooms, such as reported for the plume of the Rhine in the North Sea, are likely to contribute substantially to hydrolysis of natural organic phosphates.     

Recent changes in the contributions of river Rhine and North Sea to the eutrophication of the western Dutch Wadden Sea

From 1955 to the mid 1980s the loads of both nitrogen and phosphorus from the river Rhine to the Dutch coastal area, the Wadden Sea included, increased. Since 1985 the phosphorus loads has decreased significantly, while the nitrogen load remained about the same.Annual primary production in the western Dutch Wadden Sea has increased fromc. 40 g C m^–2 (1950) to 150 (mid 1960s) and over 500 g C m^–2 (1986). The biomass of macrozoobenthos has more than doubled since 1970. Simultaneously, the meat yield of cultured blue mussels (Mytilus edulis), has increased since the 1960s. Previously, it was indicated that the increase in primary production of the phytoplankton over the period 1950 to 1986 was stimulated by the load of dissolved inorganic phosphate from Lake IJssel, a reservoir supplied by Rhine water. Since 1990, however, primary production has been higher than was expected from decreased phosphate loads from Lake IJssel. It is argued that this lack of response may have been caused by increased concentrations of dissolved inorganic phosphate at sea originating from increased inflow from a.o. the Strait of Dover, which compensate for the decrease in phosphate from the rivers, possibly in combination with a significant improvement of the light conditions of the water in the Wadden Sea.     

Impact of hydrology on the chemistry and phytoplankton development in floodplain lakes along the Lower Rhine and Meuse

The impact of hydrology (floods, seepage) on the chemistry of water and sediment in floodplain lakes was studied by a multivariate analysis (PCA) of physico-chemical parameters in 100 lakes within the floodplains in the lower reaches of the rivers Rhine and Meuse. In addition, seasonal fluctuations in water chemistry and chlorophyll-a development in the main channel of the Lower Rhine and five floodplain lakes along a flooding gradient were monitored. The species composition of the summer phytoplankton in these lakes was studied as well.At present very high levels of chloride, sodium, sulphate, phosphate and nitrate are found in the main channels of the rivers Rhine and Meuse, resulting from industrial, agricultural and domestic sewage. Together with the actual concentrations of major ions and nutrients in the main channel, the annual flood duration determines the physico-chemistry of the floodplain lakes. The river water influences the water chemistry of these lakes not only via inundations, but also via seepage. A comparison of recent and historical chemical data shows an increase over the years in the levels of chloride both in the main channel of the Lower Rhine and in seepage lakes along this river. Levels of alkalinity in floodplain lakes showed an inverse relationship with annual flood duration, because sulphur retention and alkalinization occurred in seepage waters and rarely-flooded lakes. The input of large quantities of nutrients (N, P) from the main channel has resulted, especially in frequently flooded lakes, in an increase in algal biomass and a shift in phytoplankton composition from a diatom dominated community towards a community dominated by chlorophytes and cyanobacteria.     

Phytoplankton ecology of the Lake of Menteith,Scotland

The results discussed in this paper represent the first seasonal ecological study carried out on the phytoplankton of the Lake of Menteith. All measured nutrients reached maximum levels during the winter, with silicate showing particularly high concentrations (up to 85 µg at Si l^–1). During the summer period phosphate, nitrate and silicate showed almost complete exhaustion in surface waters. The lake water was consistently alkaline, never falling below pH 7, while the alkalinity ranged from 20 to 24 mg CaCO₃ l^–1. Generally, the nutrient status of the main inflow had a rapid effect on the water quality of the lake.The region of the lake under investigation showed no thermal stratification at any period of the year, although continuous thermal gradients were recorded in the winter. The continual circulation of the water mass probably prevented oxygen saturation from falling below 77% even following a large phytoplankton bloom and subsequent decomposition.From an examination of net phytoplankton samples the Lake of Menteith could be described as blue-green or blue-green/diatom in nature. From the quantitative study, large pulses of Melosira, Asterionella and Fragilaria were recorded in the spring. The disappearance of the species appears to be related to silicate limitation. The summer growth of Asterionella may have been promoted by a nitrogen source other than nitrate and nitrite, both of which were reduced to critical levels. This alternative source of combined nitrogen may have been contributed by nitrogen-fixing algae in the lake. Three species of Anabaena were recorded, all of which produced large populations during the year.Department of Botany, The University of GlasgowPresent Address: Department of Biology, College of Science, University of Sulaimaniyah, Sulaimaniyah, Iraq     

The Northeast Water polynya,Greenland Sea

The nutrient and phytoplankton distributions in the North East Water polynya (NEW) were determined in June 1991. At Norske Øer Ice Barrier (the polynya's southern boundary), water was upwelled, but vertical instability precluded the development of phytoplankton blooms. Along the length of the northward coastal current, part of the anticyclonic circulation in this area, the vertical stability increased to the north by the input of melt water and solar heating. This caused a gradual increase in phytoplankton biomass and a decrease in nutrient concentrations until, in the northernmost area, nitrate was depleted at the surface, and sub-surface maxima of chlorophyll a were observed. The band of high chlorophyll a concentrations extending from this area to the south along the eastern margin of the polynya was interpreted as the presence of phytoplankton advected by the local circulation. The phytoplankton communities, consisting mainly of flagellates and diatoms, were typical for the beginning of phytoplankton development in ice-covered areas. They seemed to be partially released from melting ice. Three communities were distinguished, which represented, firstly, the upwelled water and its northern extension, secondly, an area of high phytoplankton biomass in the northwestern part of the polynya, and thirdly, the pack-ice region. The major taxa co-occurred at all stations, with only their relative importance changed. The nutrient concentrations in the NEW were different from those in the adjacent areas. The low nitrate values of about 4 M in the upper 70 m, found to be representative for the beginning of the growth season, imposed limitations on the overall phytoplankton production. Therefore, fertilization mechanisms such as upwelling along the Norske Øer Ice Barrier are important for local nutrient replenishment during the period of active phytoplankton growth. Eventually, silicate and phosphate supplied in higher concentrations by jets of the Arctic outflow may also support phytoplankton production, although these nutrients were not limiting during this study. The high-nutrient jets were detected in the upper 100 m of the water column at the eastern boundary of the polynya.     

Plankton in the River Rhine: structural and functional changes observed during downstream transport

The growth dynamics of phytoplankton, zooplankton and bacterioplanktonin the River Rhine were analysed simultaneously with a numberof environmental factors in order to identify environmentalsteering factors and to describe some of their interrelations.Observations on the metabolic activity (for algae and bacteria)and density (for all organisms) were carried Out three timesin 1990 using successive sampling of the same water parcel duringits transport in the lower 660 km reach of the river. High algaldensities (up to 170.5 µg chlorophyll a l^–1), rotifers(up to 1728 l^–1), crustaceans (up to 65 l^–1) andbacteria (up to 16x10⁹ l^–1) were found. Algae and rotifersshowed a rapid successive development during transport, whilecrustaceans were only abundant in the tidal reach of the river.In May, a vigorous growth of phytoplankton, zooplankton andbacteria was found. The diatom-dominated phytoplankton depletedthe dissolved silicate in the river water and this led to acollapse of the populations, indicated by a decreased specificrate of photosynthesis (measured by the ¹⁴C method) and vigorousbacterial growth (measured by [³H]thymidine incorporation).Subsequently, the remaining phytoplankton diminished to verylow levels near the river mouth. In July and September, it seemedthat biological interactions within the plankton populationsor between plankton and benthos balance the population densitiesso that separate developmental stages, as in spring, were lessprominent. Estimates of the growth rates and loss rates of thephytoplankton were made. Phytoplankton exerted a substantialinfluence on the partitioning of nutrients (nitrogen, phosphorus,silicate) over water and suspended matter (as analysed by elementanalysis). It seems likely that only the reduction of phosphate,as planned under the Rhine Action Programme, and not that ofnitrogen, may restrict the peaks of plankton growth describedhere.     

Abstracts of papers read at the winter meeting at the University of Bristol

Summaries of changes since 1965 in the phytoplankton fora of Lake Mälren, the third largest lake in Sweden, are presented. The phosphorus load was reduced by 90% in 1970–1973 and a fertilizer factory on the lake decreased its N-outlet by 80%. The phosphorus concentrations in the water decreased by up to 40% and the total nitrogen concentration by 50% between 1970 and 1983. But in one part of the lake the N-concentration increased by 50% due to drainage from surrounding land and atmospheric deposition. With changes in the ratios of N/P and Si/P as well as the turbidity of the water there have been drastic alterations in species composition and total phytoplankton biomasses.

There was a rise in the number of species over the period of study at all times of the year. Colonial species decreased while small and large flagellates of chrysophytes, cryptomonads and dinoflagellates increased. In the most polluted basins cyanobacterial blooms have halved their peak biomasses or have almost disappeared.

At times during the 19 years of the study the summer phytoplankton communities have been tested to fits of a log-normal distribution, assuming such a distribution to be valid in well-balanced ecosystems. The general trend indicates an increasing adjustment to a log-normal distribution. Phytoplankton changes in the lake are associated with effects known to cause reversed eutrophication, although climatically induced variations may also be identified.     

Effects of Mississippi River water on phytoplankton growth and composition in the upper Barataria estuary,Louisiana

Diversion of river waters to adjacent estuaries may occur during wetland restoration, navigation channel development, or storms. We proposed that diversions of nitrogen- and phosphorus-enriched waters from the river to estuarine waters would result in increased phytoplankton biomass and shifts to noxious or harmful algal blooms. We tested this hypothesis by conducting four seasonal microcosm experiments in which Mississippi River water was mixed at different volume ratios with ambient estuarine waters of three lakes in the upper Barataria Basin, Louisiana, USA. These lakes included two brackish lakes that were in the path of diverted Mississippi River water, and a freshwater lake that was not. The results from the 3- to 8-day experiments yielded a predictable increase in phytoplankton biomass related to nutrient additions from Mississippi River water. The subsequent decreases in the dissolved nitrate + nitrite, soluble reactive phosphorus, and silicate concentrations explained 76 to 86% of the increase in chlorophyll a concentrations in the microcosms. Our experiments showed that cyanobacteria can successfully compete with diatoms for N and P resources even under non-limiting Si conditions and that toxic cyanobacteria densities can increase to bloom levels with increased Mississippi River water inputs to ambient waters in the microcosms. Diversions of Mississippi River into adjacent estuarine waters should be considered in relation to expected and, possibly, unexpected changes in phytoplankton communities to the receiving waters and coastal ecosystems.

     

Effect of silicate on the growth and arsenate uptake by rice (Oryza sativa L.) seedlings in solution culture

A solution culture experiment was conducted to investigate the effect of silicate on the yield and arsenate uptake by rice. Rice seedlings (Oryza sativaL. cv. Weiyou 77) were cultured in modified Hoagland nutrient solution containing three arsenate levels (0, 0.5 and 1.0 mg L ^–1 As) and four silicate levels (0, 14, 28 and 56 mg L ^–1 Si). Addition of Si significantly increased shoot dry weight (P=0.001) but had little effect on root dry weight (P=0.43). Addition of As had no significant effect on shoot dry weight (P=0.43) but significantly increased root dry weight (P=0.01). Silicon concentrations in shoots and roots increased proportionally to increasing amounts of externally supplied Si (P < 0.001). The presence of As in the nutrient solution had little effect on shoot Si concentration (P=0.16) but significantly decreased root Si concentration (P=0.005). Increasing external Si concentration significantly decreased shoot and root As concentrations and total As uptake by rice seedlings (P <0.001). In addition, Si significantly decreased shoot P concentration and shoot P uptake (P <0.001). The data clearly demonstrate a beneficial effect of Si on the growth of rice seedlings. Addition of Si to the growth medium also inhibited the uptake of arsenate and phosphate by the rice seedlings.     

Abiotic drivers of interannual phytoplankton variability and a 1999–2000 regime shift in the North Sea examined by multivariate statistics

The Dutch coastal zone is a region of the North Sea with a marked interannual and long‐term abiotic and phytoplankton variability. To investigate the relationship between abiotic variability and phytoplankton composition, two routine water monitoring data sets (1991–2005) were examined. Multivariate statistics revealed two significant partitions in the data. The first consisted of interannual abiotic fluctuations that were correlated to Rhine discharge that affected the abundance of summer and autumn diatom species. The second partition was caused by a shift in the abiotic data from 1998 to 1999 that was followed by a shift in phytoplankton composition from 1999 to 2000. Important factors in the abiotic shift were decreases in suspended matter (SPM) and phosphate (DIP) concentrations, as well as in pH. The decrease in SPM was caused by a reduction in wind speed. The increase in water column daily irradiance from the decrease in SPM led to increases in the abundance of winter–spring species, notably the prymnesiophyte Phaeocystis globosa. Because wind speed is related to the North Atlantic Oscillation (NAO) index it was possible to correlate NAO index and P. globosa abundance. Only five abiotic variables representing interannual and long‐term variability, including Rhine discharge and NAO index, were needed to model the observed partitions in phytoplankton composition. It was concluded that interannual variability in the coastal phytoplankton composition was related to year‐to‐year changes in river discharge while the long‐term shift was caused by an alternating large‐scale meteorological phenomenon.     

A seasonal study of the distributions of surface state variables in Liverpool Bay. II. Nutrients

Distributions of dissolved inorganic nitrate, phosphate, and silicate observed in the surface waters of Liverpool Bay during a seasonal cycle of cruises in 1975 are presented. Winter distributions were influenced by the proportion of waters from various sources, their chemistry, and the mode of circulation, all of which were temporally variable. Non-winter distributions were additionally influenced by in situ biological activity. Principal component analysis provided a preliminary means of separating and assessing the relative importance of these factors and depicting the centres of biological activity during the non-winter cruises. Different circulation modes in January and December produced distributions with little visual similarity, and patchy nutrient distributions accompanied the spring and autumn phytoplankton blooms. Nitrogen-rich industrial and domestic effluents modified the nutrient availability ratios in the receiving waters so that silicon limited diatom growth over large areas of the Bay in spring. Nitrogen became ‘bio-limiting’ in summer and autumn. The direction of the surface residual circulation before the spring bloom is proposed as the major influence governing subsequent nutrient distributions and phytoplankton populations.     

The seasonal cycle of the phytoplankton in the coastal waters of Ghana

Seasonal changes in the phytoplankton at four depths off Tema, Ghana were investigated between September 1973 and November 1974. The physico-chemical factors show that there are two marine seasons, the season of major upwelling (July–October), characterized by low water temperatures (< 25°C), high salinity (> 35) and high nutrient levels, and a non-upwelling period (November–June) when water temperatures are higher and salinity and nutrients are lower. The latter marine season is broken by a small, unpredictable upwelling (December-January). Phytoplankton cell counts are high (> 1000 × 103 cells/1) during the major upwelling period and can be very low (< 2 × 10³ cells/1) during the non-upwelling period. Dinoflagellates form the main components of the phytoplankton population during the nonupwelling period and diatoms form the dominant components at other times. There is a close relationship between the physicochemical factors and the phytoplankton population especially during the major upwelling period. For example there is a good correspondence between the peaks in phytoplankton numbers and low levels of nutrients such as silicate, nitrate and phosphate with the reverse taking place at other times.     

Seasonal variation in marine phytoplankton and ice algae at a shallow antarctic coastal site

The phytoplankton population near Davis, Vestfold Hills, Antarctica was monitored throughout 1982. Chlorophyll-a determinations and counts of living cells in both the water column and sea ice demonstrated a marked seasonality in phytoplankton abundane and species composition. From April to October nanoplanktonic organisms contributed most of the chlorophyll-a in both the sea ice and water column. Blooms of diatoms occurred in May, November and December in the bottom of the sea-ice and in January and February in the water column. Phaeocystis pouchetii was dominant during December in the water column. Large numbers of dead diatoms were found in winter. The concentrations of nitrate, dissolved inorganic phosphate and dissolved silicate increased throughout the year until December, when the concentrations of nitrate and silicate fell sharply, followed a month later by a reduction in phosphate concentration. The diversity of phytoplankton was greatest during the summer months.     

Nutrient pulsing as a regulator of phytoplankton abundance and community composition in Galveston Bay, Texas

Galveston Bay, Texas, is a large shallow estuary with a watershed that includes 60% of the major industrial facilities of Texas. However, the system exhibits low to moderate (2-20 μg l⁻¹) microalgal biomass with sporadic phytoplankton blooms. Both nitrogen (N) and phosphate (P) limitation of phytoplankton growth have been proposed for the estuary. However, shifts between N and P limitation of algae growth may occur due to annual fluctuations in nutrient concentrations. The primary goal of this work was to determine the primary limiting nutrient for phytoplankton in Galveston Bay. Nutrient addition bioassays were used to assess short-term (1-2 days) phytoplankton responses (both biomass and community composition) to potentially limiting nutrients. The experimental bioassays were conducted over an annual cycle using natural water collected from the center to lower part of the estuary. Total phytoplankton biomass increased in the nitrate (10 μM) additions in 11 of the 13 bioassays, but no significant increases were detected in the phosphate (3 μM)-only additions. Bioassay results suggest that the phytoplankton community was usually not phosphate limited. All major groups increased in biomass following nitrate additions but diatoms increased in biomass at a faster rate than other groups, shifting the community composition toward higher relative abundance of diatoms. The results of this study suggest that pulsed N input events preferentially favor increases in diatom biomass in this estuary. The broader implications of this study are that N pulsing events, primarily due to river discharge, play an important role in structuring the phytoplankton community in the Galveston Bay estuary.