Zooplankton species composition in the Oosterschelde (SW Netherlands) before,during and after the construction of a storm-surge barrier

During the period of construction of a storm-surge barrier current velocities decreased strongly and the Eastern compartment of the basin obtained a lagoon-like character. The rotifer Synchaeta spp., already abundant in the neighbouring salt and brackish lakes, profited from this condition. Higher and less fluctuating salinities caused the estuarine character of this compartment to disappear in the post-barrier years. This was reflected in the obscured succession of some Acartia species: the estuarine A. tonsa dominated in pre-barrier- and barrier years during summer, while the marine A. clausi was abundant in the post-barrier Oosterschelde during that time. Also the changed food conditions (Bakker & Vink, 1993) may have played a role in this phenomenon.Further changes in species composition were hardly observed. The main changes were of a quantitative nature (Bakker & Van Rijswijk, 1993; Tackx et al., 1993). A list of the commonly occurring species is given.     

Nutrient concentrations and planktonic diatom-flagellate relations in the Oosterschelde (SW Netherlands) during and after the construction of a storm-surge barrier

Nutrients The inflow of Rhine water into the Oosterschelde was strongly reduced from 1987 onwards. This caused the winter concentrations of silicate and nitrate to decrease in the Eastern compartment, while those in the deeper Western compartment, more dependent on North Sea concentrations, hardly changed. The result was a levelling of the former East-West gradients for these nutrients. In East, summer concentrations of nitrate reached limiting levels in the post-barrier period and molar nitrate/ammonium ratios became < 1, indicating that any release of nitrogen must be important to stimulate phytoplankton growth in this area. Silicate summer concentrations in East, on the other hand, were higher in the new situation. In West, differences in summer nutrient concentrations between the old and new situation were smaller than in East, due to the still continuing exchange with the North Sea. Phytoplankton diatoms and flagellates In East during summer, N-depletion and longer residence times caused the phytoplankton to become strongly dependent on nutrient regeneration processes and increased zooplankton grazing. Average diatom biomass declined, but flagellate biomass rose during summer. Spring conditions for phytoplankton development in this area improved due to the increased water transparency, nutrients being present in excess, and this resulted in a higher new production of diatoms than before.In West, summer biomass of diatoms decreased, probably due to increased consumption by mussels under conditions of longer residence times; nutrients were not limiting, due to important benthic mineralization processes and exchange with the North Sea. The previously existing West-East biomass gradients disappeared, or sometimes reversed.Experimental (mesocosm studies) as well as field data, reported in the literature, give evidence for the given explanations.     

Consumption of benthic invertebrates by waterbirds in the Oosterschelde estuary,SW Netherlands

The number of waders in the Oosterschelde, S.W. Netherlands, declined after a reduction in intertidal area due to the construction of a storm surge barrier and secondary dams, suggesting that the carrying capacity had been reached (Schekkerman et al., 1993). In this paper we present data on consumption and predation pressure by birds to explore whether the reduction in their numbers is due to prey depletion or to other factors.The total annual consumption of benthic invertebrates by birds in the Oosterschelde amounted to 1573 × 10³ g ADW y^–1 in the period before the coastal engineering works (pre-barrier) and 1500 × 10³ kg ADW y^–1 in the post-barrier period. More than half of the total amount of biomass is eaten by the Oystercatcher, and only seven (pre-barrier) or even six (post-barrier) bird species together take 90% of the total.Although the consumption by individual species may vary considerably among years, the total consumption was remarkably stable, with a CV of only 3–4% of the mean, especially compared to the variability of the prey populations. In the pre-barrier period, consumption was lowest in mid summer, increased sharply from August onwards until a peak was reached in January. A sharp decrease took place in March. In the post-barrier period, consumption peaked in October.The total consumption per unit area per year does not differ much between different sectors of the Oosterschelde, apart from a distinctly lower value in the eastern part. Of the total amount of food taken by birds, only 0.1–0.4% is taken in the subtidal compartment. In several study plots on an individual tidal flat, there was a clear relation between consumption and benthic biomass.The predation pressure was 13 and 23% of the standing stock, in the post- and pre-barrier period respectively. When cockles, mussels and their main predator, the Oystercatcher, are excluded from the calculations, the predation pressure of the other species was 30 and 37% of the biomass, respectively.Predation pressure of Oystercatchers in individual study plots varied from less than 10% to more than 70% of the standing stock. On cockle beds the predation pressure was positively related to the average length of the cockles present.Based on these results and a comparison with the literature we conclude that, at least for several species that feed intertidally, carrying capacity could be limited by the stocks of food. This does not mean that birds face food shortage each season. As the variability of the benthos populations is much higher than that of the bird densities it is likely that at some times food is not limiting, at other times it is. On the other hand, consumption is very low in the subtidal compartment and species feeding here could potentially increase substantially in numbers in the Oosterschelde.     

A new trend in the development of the phytoplankton in the Oosterschelde (SW Netherlands) during and after the construction of a storm-surge barrier

During the pre-barrier period (1982–83), the Oosterschelde phytoplankton were a diatom-dominated community, comprising a species-rich assemblage throughout the year. Assemblages of spring, early summer and summer, developed in response to a gradually evolving turbidity-light gradient during the course of the year.During the barrier-construction period (1984–87), characterized by decreasing current velocities, increasing sedimentation of suspended matter, increasing water transparencies and unchanged nutrient conditions, the growth season for the phytoplankton started earlier and lasted longer. Some flagellate species responded by much higher biomass than before. The impact of short-term climatic factors during this period, notably severe winters, could be illustrated with examples of clear responses of some species (e.g. Biddulphia aurita).In the post-barrier years (1987–90) a changed light-nutrient-salinity regime (i.e. much light, limitation of nitrate, high salinity) was demonstrated and an extended summerseason developed, without the original gradual transitions. This was reflected in an a-seasonal trend of the phytoplankton assemblage, where summer species were already observed in spring and spring species decreased in abundance. In summer small flagellates increased and some weakly silicified diatom species made their appearance. In the eastern compartment no colony formation of Phaeocystis occurred in summer and this was thought to be due to nitrate limitation. Changes in abundance of some species (Phaeocystis, Ditylum brightwellii, Skeletonema costatum), occurring during the entire period of investigation (1982–90), could be explained using field observations compared with experimental evidence from the literature.The relationship between species composition and biomass on the one hand and environmental variables on the other hand, was analysed in a Canonical Correspondence Analysis, for both compartments separately.     

Field observations in the Oosterschelde (The Netherlands) onCoscinodiscus concinnus andCoscinodiscus granii (Bacillariophyceae) infected by the marine fungusLagenisma coscinodisci (Oomycetes)

During 1985–1990Coscinodiscus concinnus andCoscinodiscus granii from the Oosterschelde were infected by the marine fungusLagenisma coscinodisci, although not every year with the same intensity. Infected cells were only observed during the period July–October at water temperatures between 13.2 and 20.2°C. In 1986 and 1987 the course of the infection withL. coscinodisci inC. concinnus andC. granii, was recorded at three stations. The highest infection percentages varied between 22.2 and 58.3% inC. concinnus and between 7.1 and 41.9% inC. granii. It is concluded that the water temperature may play an important role in the appearance ofL. coscinodisci inC. concinnus andC. granii and that silicate limitation is of minor importance. Finally it is discussed that the infection was not of great importance for the phytoplankton community in the Oosterschelde.     

Seasonal variation in phytoplankton composition and physical-chemical features of the shallow Lake Doïrani,Macedonia, Greece

Phytoplankton species composition, seasonal dynamics and spatial distribution in the shallow Lake Doïrani were studied during the growth season of 1996 along with key physical and chemical variables of the water. Weak thermal stratification developed in the lake during the warm period of 1996. The low N:P ratio suggests that nitrogen was the potential limiting nutrient of phytoplankton in the lake. In the phytoplankton of the lake, Chlorophyceae were the most species-rich group followed by Cyanophyceae. The monthly fluctuations of the total phytoplankton biomass presented high levels of summer algal biomass resembling that of other eutrophic lakes. Dinophyceae was the group most represented in the phytoplankton followed by Cyanophyceae. Diatomophyceae dominated in spring and autumn. Nanoplankton comprised around 90% of the total biomass in early spring and less than 10% in summer. The seasonal dynamics of phytoplankton generally followed the typical pattern outlined for other eutrophic lakes. R-species (small diatoms), dominant in the early phase of succession, were replaced by S-species (Microcystis, Anabaena, Ceratium) in summer. With cooling of the water in September, the biomass of diatoms (R-species) increased. The summer algal maxima consisted of a combination of H and M species associations (sensu Reynolds). Phytoplankton development in 1996 was subject to the combined effect of the thermal regime, the small depth of mixing and the increased sediment-water interactions in the lake, which caused changes in the underwater light conditions and nutrient concentrations.     

Seasonal and long term changes of the phytoplankton in the lake Tortum in relation to environmental factors, Erzurum, Turkey

Seasonal changes in phytoplankton community structure of the lake Tortum were studied over one year period, from March 2002 to February 2003. The collected data were compared with the data collected 21 years ago. Chlamydomonas microsphaerella, Cyclotella krammeri, C. glomerata, and Ceratium hirundinella were identified to be dominant several times during the study period. Species diversity and biomass of the phytoplankton were very low in spite of sufficient and high levels of nutrient concentrations. Maximum phytoplankton density levels were observed during summer and late autumn. Phytoplankton density was positively correlated with nutrients, temperature and pH, and it was negatively correlated with Secchi depth and dissolved oxygen. Phytoplankton growths were negatively affected from water transparency and high levels of water mass transport (circulation) and velocity in the lake.     

International study on Artemia LXIII. Field study of the Artemia urmiana (Günther, 1890) population in Lake Urmiah, Iran

Lake Urmiah is a large (total surface 4750–6100 km² in recent times) thalassohaline hypersaline lake (150–180 g l^–1 in the period 1994–1996), located in northwestern Iran. It is the habitat of the endemic Artemia urmiana. Over the period July 1994–January 1996 a sampling campaign was organized: 36 fixed sampling stations, distributed over the entire lake's area, were sampled weekly to determine water temperature, salinity and transparency. At each occasion a filter net was dragged over a distance of 400 m in the superficial water layer to assess the density and composition of the Artemia population. A more limited sampling campaign focused on the annual fluctuations in chlorophyll concentration and on the reproductive behaviour of the brine shrimp population. Several stages of brine shrimp survived during winter months (water temperature 3°C) at low densities. Compared to available data for the Great Salt Lake, USA, Lake Urmiah shows a low algal biomass and overall low Artemia density. The increasing grazing pressure of the developing brine shrimp population in spring seems to prevent the phytoplankton from reaching high blooming concentrations, and oviparity is the dominant reproductive mode throughout the reproductive season.     

Turbidity,nutrients and phytoplankton primary production in the Oosterschelde (The Netherlands) before,during and after a large-scale coastal engineering project (1980–1990)

Turbidity, nutrient concentrations and phytoplankton primary production were monitored in the Oosterschelde before, during and after the construction of a storm-surge barrier and two compartment dams.Flow velocities and suspended matter concentrations decreased severely, causing an increased transparency of the watercolumn. In the eastern and northern compartments, the previously pronounced seasonal variation disappeared.Reduction of the freshwater load and decreasing nutrient concentrations in the adjacent North Sea coastal waters resulted in lower nitrite + nitrate and silicate concentrations. Autumn phosphate concentrations remained at the same level as before the nutrient reduction. Silicate was a limiting nutrient during the pre-barrier period and nitrogen and silicate were limiting during the post-barrier period.Annual patterns in chlorophyll-a concentrations in the western and central compartments showed no obvious trend; in the eastern and northern compartments higher values were measured from 1985 onwards.Primary production during the period 1980–1990 varied between 176 and 550 g C m^–2 yr^–1. The annual primary production in the western compartment had decreased, while in the central and eastern compartments annual primary production did not change: the formerly existing gradient disappeared. In the northern compartment higher chlorophyll-a concentrations and high annual production suggest that the phytoplankton could benefit from the increased transparency while nutrient concentrations were still high enough to support phytoplankton growth.Changes in photosynthetic physiological parameters were observed which suggested shade adaptation. This is in contrast to improved light conditions and reduced nutrient availability. The apparent incoherence with light-shade adaptation theory may be explained by the species shift that occurred.As a result of the opposite effects of a more favourable light climate and a reduced nutrient availability, together with the resulting species shift, the annual primary production showed a large degree of homeostasis.     

Disturbance events affecting phytoplankton biomass,composition and species diversity in a shallow,eutrophic, temperate lake

The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm³ l^–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads).     

Primary production and phytoplankton in two small,polyhumic forest lakes in southern Finland

Primary production and phytoplankton in polyhumic lakes showed a very distinct seasonal succession. A vigorous spring maximum produced by Chlamydomonas green algae at the beginning of the growing season and two summer maxima composed mainly of Mallomonas caudata Iwanoff were typical. The annual primary production was ca. 6 g org. C · m^–2 in both lakes. The mean epilimnetic biomass was 1.1 in the first lake and 2.2 g · m^–2 (ww) in the second one. The maximum phytoplankton biomass, 14 g · m^–2, was observed during the vernal peak in May.     

Long-term Changes and Seasonal Development of Phytoplankton in a Strongly Stratified,Hypertrophic Lake

Changes in the phytoplankton community of the hypertrophic, sharply stratified Lake Verevi have been studied over eight decades. Due to irregular discharge of urban wastewater, the trophic state of the lake has changed from moderately eutrophic to hypertrophic. We found that the trophic state in summer increased in the 1980s and remained at a hypertrophic level since then. Planktothrix agardhii was recorded first in the 1950s and became the dominant species in the 1980s, forming biomass maxima under the ice and in the metalimnion during the vegetation period. In summer 1989, P. agardhii contributed almost 100% of the phytoplankton biomass. Generally, the highest biomass values occurred in the metalimnion. In spring, when P. agardhii was less numerous, diatoms and cryptophytes prevailed. In springs 2000 and 2001 different diatoms dominated – Synedra acus var. angustissima (18.6 g m⁻³) and Cyclostephanos dubius (9.2 g m⁻³), respectively. In recent years, the spring overturn has been absent. In the conditions of strong thermal stratification sharp vertical gradients of light and nutrients caused a large number of vertically narrow niches in the water column. During a typical summer stage, the epilimnion, dominated by small flagellated chrysophytes, is nearly mesotrophic, and water transparency may reach 4 m. The lower part of the water column is hypertrophic with different species of cryptophytes and euglenophytes. A characteristic feature is the higher diversity of Chlorococcales. Often, species could form their peaks of biomass in very narrow layers, e.g. in August 2001 Ceratium hirundinella (18.6 g m⁻³) was found at a depth of 5 m (the lower part of the metalimnion with hypoxic conditions), Cryptomonas spp. (56 g m⁻³) at 6 m (with traces of oxygen and a relatively high content of dissolved organic matter) and euglenophytes (0.6 g m⁻³) at 7 m and deeper (without oxygen and a high content of dissolved organic matter).     

The relation between salinity and copper complexing capacity of natural estuarine waters and the uptake of dissolved 64Cu by Macoma balthica

Abstract

The radiotracer ⁶⁴Cu was used to assess the influence of natural organic ligands on the bioavailability of copper. Biological availability of the ⁶⁴Cu-complexes was measured by accumulation in the bivalve Macoma balthica. The experiments were carried out in April as well as in February with water from the relatively clean Oosterschelde Sea arm and the relatively polluted Westerschelde estuary. Adsorption onto shells, as well as uptake in tissues was assessed at salinities of 10‰ and 30‰. Simultaneously with the exposure experiments, ligand characteristics of the natural waters were assessed. High ligand concentrations, as occurring in the Westerschelde around February, reduced ⁶⁴Cu (320 nM) uptake by more than 50%, in spite of the much lower salinity in the Westerschelde water. At the low salinity, uptake was increased slightly in Westerschelde water, but considerably in Oosterschelde water. This implies that at low ambient ligand concentrations (during the whole year in Oosterschelde water and in the summer period also in Westerschelde water) the influence of salinity on ⁶⁴Cu uptake is more pronounced.     

Ceratophyllum demersum – phosphorus interactions in nutrient enriched aquaria

High macrophyte density in shallow lakes is often associated with clear water, especially when the non-rooted, submerged angiosperm Ceratophyllum demersum is dominant. Lack of true roots and high surface area:volume ratio suggest that nutrient uptake from the water column by C. demersum may be high. Therefore, possible competition for nutrients, including phosphorus (P), could contribute to phytoplankton inhibition. C. demersum ability to absorb and store P at four nutrient levels (unenriched + three enrichment treatments) was investigated in a 34-day laboratory experiment using agar-based nutrient diffusing substrates (NDSs). P uptake rates and abatement potential by C. demersum were assessed from total phosphorus concentration (TP) patterns in the water column. Changes in C. demersum biomass (wet weight) also were determined. C. demersum took up P quickly. Some P release occurred during the experiment, especially under high nutrient conditions. Initial net P uptake by C. demersum was high, but medium-term (five weeks) average uptake was relatively low. Projected long-term net P uptake approached zero. Plant biomass loss and production of macrodetritus (plant fragments >1 mm) were highest in unenriched aquaria. Biomass loss in the lower enriched treatments was equally divided between loss as macrodetritus and as dissolved organic matter (DOM), but loss as DOM was four times higher than loss as macrodetritus in the highest nutrient treatment. The results suggest that medium- and long-term low phytoplankton biomass in C. demersum-rich lakes is achieved via mechanisms other than direct competition for nutrients from the water column.     

Phytoplankton associations in a small hypertrophic fishpond in East Hungary during a change from bottom-up to top-down control

Phytoplankton species composition and abundance of a shallow hypertrophic fishpond (Mézeshegyi-tó, East Hungary) was studied for the period 1992–1995. The pond showed a pronounced algal periodicity. High-diversity phytoplankton assemblages occurred in spring and autumn. During the winter period, low diversity values were related either to stable community states, when K-strategist species dominated the plankton, or to a large bloom of r-strategist species. In summer, the stable environment led to low-diversity, high-biomass phytoplankton assemblages, dominated by Cylindrospermopsis raciborskii. At this time, the growth conditions for Cylindrospermopsiswere akin to those prevailing in a continuous fermentor. The overwhelming dominance of this species lasted for more than four months, during which time, the phytoplankton resembled that of one in the tropics. In August, 1993, an unsuccessful chemical treatment for reducing the algal bloom succeeded in killing the pond's entire population of fish. The large fish-stock comprised the planktivorous silver carp. Although the summer of 1994 was one of the warmest summers of this century, the expected Cylindrospermopsis bloom failed to develop probably because of a higher grazing pressure by large zooplankton. In spite of the fact that the temporal and spatial pattern of the phytoplankton is influenced principally by bottom-up effects, changes in cascading trophic interactions may also considerably influence the species composition and biomass.     

The benthic macro-algae of Georgian Bay,the North Channel and their drainage basin

Thirty-nine subgeneric taxa of macroalgae have been collected from 83 sites in Georgian Bay, the North Channel and their drainage basin. There were 15 species in the Bay and Channel and 32 species in streams, rivers and impoundments in the basin. Only 8 of the Bay and Channel species were also found in the watershed. Cladophora glomerata was the most important species in Georgian Bay and the North Channel, having an estimated 640 × 10³ m² cover and 19 × 10⁴ kg fresh weight standing crop. However, this species was largely concentrated on the southwestern shorelines of these water bodies. Its distribution along the northeastern shoreline appears to be limited by total ion and phosphorus levels. Chara globularis/vulgaris was the subdominant taxon in Georgian Bay and the North Channel with an estimated 70 × 10³ m² cover and 15 × 10³ kg fresh weight biomass. This species was more widely distributed than C. glomerata. No other taxon contributed significantly to the standing crop including the frequently occurring Ulothrix zonata, Zygnema spp. and Spirogyra spp. The maximum benthic macroalgal biomass was estimated to be approximately 10% of the phytoplankton biomass.     

Sedimentation in Arctic Canada: Species composition and biomass of phytoplankton contributed to the marine sediments in Frobisher Bay

Summary Sedimentation of phytoplankton provides food and energy for zoobenthic communities. In this study the rates, species composition and biomass of phytoplankton input to Frobisher Bay sediments were examined during ice (late November to July) and open water (late July to October) periods from 1982 to 1985. The rates were higher on the sea bed than at 20 m. The minimum rate (3x10⁵ cells·m^-2·day^-1) of sedimentation occurred during the early part of the ice period. It increased as the ice thickened and reached a maximum of 2.8x10⁸ cells·m^-2·day^-1 after the phytoplankton bloom at the beginning of the open water period in the first two weeks of August. The sedimented phytoplankton was dominated by diatoms, with a great majority of pennate species during the spring (April to June) and centric forms during the summer (July to August). Green flagellates, dinoflagellates and chrysophytes occurred as a low percentage of the total population in all seasons. Other indicators (chlorophyll a and phaeopigments) showed highest biomass levels in the deepest traps. They were consistently low during the winter (December to March) and reached their maxima during the open-water period of summer. Their abundance was correlated with the seasonal cycle of the phytoplankton in the water column.     

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.     

Spring development of a Chlamydomonas population in Lake Nimetön,a small humic forest lake in southern Finland

Biomass development and vertical distribution of a Chlamydomonas population in a small humic forest lake was followed by daily sampling in May-June, 1984. Chlamydomonas dominated the phytoplankton spring bloom, forming 71% of the maximum phytoplankton biomass on 18 May. In early May the outflow rate was high and during the 24 hour period when the maximum rate of surface runoff was recorded (8–9 May), 43% of the Chlamydomonas biomass was flushed out of the lake, which delayed the onset of biomass increase. When surface runoff had slowed down Chlamydomonas biomass started increasing and during wax of the population most cells were < 10 µm in diameter. Population maximum lasted for one day (18 May) and there-after Chlamydomonas biomass decreased towards the end of the study. During wane of the population most cells were > 10 µm in diameter.     

Growth of the seaweed Ulva rigida C. Agardh in relation to biomass densities, internal nutrient pools and external nutrient supply in the Sacca di Goro lagoon (Northern Italy)

Growth of the seaweed Ulva rigida C. Agardh was investigated in relation to biomass densities, internal nutrient pools and external nutrient supply. Research was carried out from 23 March to 5 July 1994 in the Sacca di Goro (Po Delta, Northern Italy), whose south-eastern part was covered by extensive mats of Ulva rigida. Two types of field experiments were conducted by incubating Ulva thalli inside large cages. In the first experiment, beginning on 23 March, 100 g of wet thalli were placed into the cages, allowed to grow for two weeks, then collected and replaced. This procedure was repeated 8 times over the study period. In the second experiment, Ulva thalli were left inside the cages and collected at selected time intervals (14, 27, 41, 64 and 76 days) in order to simulate the effects of increased density on growth and nutrient storage.We recorded specific growth rates (NGR) ranging from 0.025 to 0.081 d^–1 for a period up to two months in the repeated short-term experiments performed at relatively low initial algal densities (300–500 g AFDW m^–3). These NGR resulted significantly related to dissolved inorganic nitrogen (DIN) in the water column. Tissue concentrations of total Kjeldahl nitrogen (TN) were almost constant, while extractable nitrate decreased in a similar manner to DIN in the water column. Total phosphorus showed considerable variation, probably linked to pulsed freshwater inflow.In the long-term incubation experiment, NGR of Ulva was inversely related to density. Internal concentrations of both total P and TN reached maximum values after one month; thereafter P concentration remained almost constant, while TN decreased below 2% w/w (by dry weight). The TN decrease was also accompanied by an abrupt decrease in nitrate tissue concentration. The biomass incubated over the two month period suffered a progressive N limitation as shown by a decreasing NY ratio (49.4 to 14.6). The reciprocal control of Ulva against biogeochemical environment and viceversa is a key factor in explaining both resource competition and successional stages in primary producer communities dominated by Ulva. However, when the biomass exceeds a critical threshold level, approximately 1 kg AFDW m^–3, the macroalgal community switches from active production to rapid decomposition, probably as a result of selfshading, biomass density and development of anaerobic conditions within the macroalgal beds.