Nitrogen as a Factor Affecting Algal Growth Potential of an Oligotrophic Coastal Environment of Eastern Mediterranean Sea

Potential nitrogen limitation to chl a production in surface waters of Saronicos Gulf, Aegean Sea was assayed using the alga Pavlova lutheri as the test organism. The oligotrophic and eutrophic water types of this area were compared by in situ and in vitro chl a production estimations. Additions of ammonium alone as well as in combination with complete nutrient enrichment were made to the oligotrophic waters and the algal growth yield was determined and compared with the corresponding yield in unenriched water cultures. The results from routine nutrient analysis and bioassay experimentation support the view that nitrogen has a priority among the factors limiting phytoplankton growth in the Eastern Mediterranean Sea.     

Contrasting responses of phytoplankton and benthic algae to recent nutrient enrichment in Arctic tundra ponds

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Species-specific phytoplankton responses to nutrients and zooplankton manipulations in enclosure experiments

1. In situ enclosure experiments were performed in the mesotrophic Bermejales reservoir to evaluate the algal response to changes in the nutrient supply and in the zooplankton size structure and density in a 2 × 2 factorial design. The experiments were conducted during the spring bloom of nanoplanktonic diatoms in 1989. 2. Nutrient enrichment promoted a great increase of phytoplankton biomass indicating a strong nutrient limitation on phytoplankton growth. Total phytoplankton biomass was significantly lower in the Daphina-added enclosures at a given nutrient level and strong direct an indirect effect of zooplankton on phytoplankton community structure and nutrient availability were observed. 3. Most of the nanoplanktonic species were effectively grazed but species with protective coverings and large size colonies were favoured by grazers and small chlorococcales were unaffected probably because of their compensatory high growth rates. The decrease in total biomass imposed by grazers is attributable mainly to the decrease of Cyclotella ocellata, the most abundant species. This taxon suffers two net effects of zooplankton: direct grazing and the indirect decrease of Si availability caused by the growth of C. ocellata which was promoted by P excretion by zooplankton. Indirect effects of grazers on Si availability should, therefore, be taken into account in explaining phytoplankton succession and community structure. 4. In this experiment grazers affected considerably the nanoplanktonic community in Bermejales reservoir. The extent which they were affected, however, depended not only on the algal size as a determinant of edibility but also greatly on the specific nutrient requirements and taxonomic features of the algal species.     

Nutrient enrichment experiments in three central Florida lakes of different trophic states

Seasonal nutrient enrichment experiments (short-term bioassays) were conducted in three Florida lakes of different trophic states to determine the effects of addition of various nutrient combinations upon chlorophyll a and phytoplankton standing crops. Nutrient enriched surface water samples with crustacean zooplankton removed were incubated in situ in clear polyethylene bags for 3 to 6 days. The 2⁵ factorial design employed two levels (ambient and enriched) of each of five nutrients [NH₄ ⁺, PO _inf4 ^sup3− , Fe⁻ -EDTA, SiO _inf3 ^sup2− and a cation (Ca²⁺ or K⁺) or trace elements]. Ammonium produced significant increases in chlorophyll a and phytoplankton standing crops in all experiments. Phosphate produced similar results in the mesotrophic lake, but the eutrophic lakes had both positive and nonsignificant responses which varied seasonally between lakes. Iron increased chlorophyll a in most experiments but affected total phytoplankton standing crop only during the summer and fall. Silicon had negative effects in some experiments. Cations and trace elements produced marked differences between lakes for chlorophyll a, but total phytoplankton standing crop showed few significant responses. Synergistic responses to two- and three-factor interactions were observed in all lakes. Differences in the responses of phytoplankton taxonomic divisions to enrichment may be responsible for much of the between lake variation in chlorophyll a and total phytoplankton volume responses. Nutrient limitations in these lakes are discussed and related to limnological factors and predictive models.     

Nutrient addition effects on chlorophyll a,phytoplankton biomass,and heterocyte formation in Lake Erie’s central basin during 2014–2017: Insights into diazotrophic blooms in high nitrogen water

1. Phosphorus (P) usually is the primary limiting nutrient of phytoplankton biomass, but attention towards nitrogen (N) and trace nutrients, such as iron (Fe), has surfaced. Additionally, N-fixing cyanobacterial blooms have been documented to occur in N-rich, P-poor waters, which is counterintuitive from the paradigm that low N and high P promotes blooms. For example, Lake Erie's central basin has Dolichospermum blooms when nitrate concentrations are high, which raises questions about which nutrient(s) are selecting for Dolichospermum over other phytoplankton and why an N-fixer is present in high N waters?
2. We conducted a 4-year (2014–2017) study in Lake Erie's central basin to determine which nutrient (P, N, or trace nutrients such as Fe, molybdenum [Mo], and boron [B]) constrained chlorophyll concentration, phytoplankton biovolume, and nitrate assimilation using nutrient enrichment bioassays. The enriched lake water was incubated in 1-L bottles in a growth chamber programmed at light and temperatures of in situ conditions for 4–7 days. We also quantified heterocytes when N-fixing cyanobacteria were present.
3. Compared to the non-enriched control, the P-enriched (+P) treatment had significantly higher chlorophyll and phytoplankton biovolume in c. 75% of experiments. Combination enrichments of P with ammonium-N, nitrate-N, Fe, Mo, and B were compared to the +P treatment to determine secondary limitations. +P and ammonium-N and +P nitrate-N resulted in higher chlorophyll in 50% of experiments but higher phytoplankton biovolume in only 25% of experiments. These results show that P was the primary limiting nutrient, but there were times when N was secondarily limiting.
4. Chlorophyll concentration indicated N secondary limitation in half of the experiments, but biovolume indicated only N secondary limitation in 25% of the experiments. To make robust conclusions from nutrient enrichment bioassays, both chlorophyll and phytoplankton biovolume should be measured.
5. The secondary effects of Fe, Mo, and B on chlorophyll were low (<26% of experiments), and no secondary effects were observed on phytoplankton biovolume and nitrate assimilation. However, +P and Fe resulted in more chlorophyll than +P in experiments conducted during Dolichospermum blooms, and +P and B significantly increased the number of heterocytes in Dolichospermum. These results indicate that low Fe availability might select for Dolichospermum, and low B constrains heterocyte formation in the central basin of Lake Erie. Furthermore, these results could apply to other lakes with high N and low P where diazotrophic cyanobacterial blooms occur.

     

Nutrient regeneration by zooplankton: effects on nutrient limitation of phytoplankton in a eutrophic lake

We tested the hypothesis that excretion of nutrients by zooplanktoncan reduce the severity of nutrient limitation of phytoplankton,and determine whether the phytoplankton community is limitedby nitrogen or phosphorus. In situ experiments were conductedin eutrophic Lake Mendota (Wisconsin, USA) during the summerof 1988, where phytoplankton were limited by N and P, but periodsof nutrient limitation were transitory Increased zooplanktonbiomass and the consequent increased excretion of nutrientsby zooplankton reduced P limitation (as measured by specificalkaline phosphatase activity) in all experiments Excretionof nutrients also reduced N limitation (as measured by ammoniumenhancement response) in one of three experiments. In additionalexperiments in the more highly eutrophic Lake Wingra, excretionof nutrients by zooplankton reduced both N and P limitationThese results support the hypothesis that zooplankton have potentiallyimportant indirect effects on phytoplankton communities throughrecycling of nutrients     

NUTRIENT LIMITATION OF BENTHIC ALGAE IN LAKE MICHIGAN: THE ROLE OF SILICA1

In the Laurentian Great Lakes, phytoplankton growth and biomass are secondarily limited by silica (Si), as a result of phosphorus (P) enrichment. Even modest levels of P enrichment can induce secondary Silimitation, which, in turn, promotes a shift from the native diatom phytoplankton flora to chlorophyte and cyanobacteria species. However, very little is known about the nutritional status of benthic populations and their response to nutrient enrichment. Two experiments were performed in the littoral zone of Lake Michigan where nutrients were delivered to in situ benthic algal (episammic and epilithic) assemblages using nutrient‐diffusing substrata. In order to test the hypothesis that benthic algae in Lake Michigan are Si limited, a 2 × 3 factorial experiment was used to deliver all combinations of Si, N, and P to resident assemblages growing on artificial substrata composed of natural (Si rich) versus calcium carbonate (Si poor) sand. A second experiment utilized a serial enrichment to evaluate the role of Si in mediating changes in taxonomic composition. These findings indicate that benthic algae in Lake Michigan exhibit signs of secondary Si limitation, and that their response to enrichment is similar to the phytoplankton. Moreover, natural sand substrata may provide a source of Si to resident benthic algae.     

Influence of phytoplankton on the response of bacterioplankton growth to nutrient enrichment

1. Laboratory experiments were conducted to test the effect of nutrient enrichment on bacterioplankton growth in the presence and absence of phytoplankton. 2. In one series of experiments, bacterioplankton growth in terms of specific activity [³H-thymidine incorporation (cell number)^?1] was greater in whole lake water samples than in samples from which phytoplankton had been removed by filtration (1.0 μm), regardless of the nutrient enrichments (control, NH⁺₄ plus PO^3-₄ and mannitol). Organic C enhanced bacterioplankton growth in both whole and filtered lake water. 3. In another series of experiments (with the same nutrient enrichments as in the first experiment except that glucose replaced mannitol), bacterioplankton growth in whole lake water enriched with PO^3-₄ plus NH⁺₄ and incubated in the light was greater than in two treatments designed to inhibit photosynthetic activity (+DCMU and dark). Bacterioplankton response to nutrient addition was greatest in the PO^3-₄ plus NH⁺₄ enrichment under all three conditions (light +DCMU, and dark). 4. These results indicate that bacterioplankton growth could be directly limited by inorganic P and N when these elements are in short supply. Enhancement of bacterioplankton growth by phytoplankton occurs only under PO^3-₄ and NH⁺₄ replete environments.     

Community resistance and change to nutrient enrichment and fish manipulation in a vegetated lake littoral

1. High biomass of macrophytes is considered important in the maintenance of a clear‐water state in shallow eutrophic lakes. Therefore, rehabilitation and protection of aquatic vegetation is crucial to the management of shallow lakes. 2. We conducted field mesocosm experiments in 1998 and 1999 to study community responses in the plant‐dominated littoral zone of a lake to nutrient enrichment at different fish densities. We aimed to find the threshold fish biomass for the different nutrient enrichment levels below which large herbivorous zooplankton escapes control by fish. The experiments took place in the littoral of Lake Vesijärvi in southern Finland and were part of a series of parallel studies carried out jointly at six sites across Europe. 3. In 1998, when macrophyte growth was poor, a clear‐water state with low phytoplankton biomass occurred only in unenriched mesocosms without fish or with low fish biomass (4 g fresh mass m^?2). Both nutrient enrichment and high fish biomass (20 g fresh mass m^?2) provoked a turbid water state with high planktonic and periphytic algal biomass. The zooplankton community was dominated by rotifers and failed to control the biomass of algae in nutrient enriched mesocosms. The littoral community thus had low buffer capacity against nutrient enrichment. 4. In 1999, macrophytes, especially free‐floating Lemna trisulca L., grew well and the zooplankton community was dominated by filter‐feeding cladocerans. The buffer capacity of the littoral community against nutrient enrichment was high; a clear‐water state with low phytoplankton biomass prevailed even under the highest nutrient enrichment. High grazing rates by cladocerans, together with reduced light penetration into the water caused by L. trisulca, were apparently the main mechanisms behind the low algal biomass. 5. Effects of fish manipulations were less pronounced than effects of nutrient enrichment. In 1999, clearance rates of cladocerans were similar in fish‐free and low‐fish treatments but decreased in the high‐fish treatment. This suggests that the threshold fish biomass was between the low‐ and high‐fish treatments. In 1998, such a threshold was found only between fish‐free and low‐fish treatments. 6. The pronounced difference in the observed responses to nutrient enrichment and fish additions in two successive years suggests that under similar nutrient conditions and fish feeding pressure either clear or turbid water may result depending on the initial community structure and on weather.     

Effects of nutrient loading,temperature regime and grazing pressure on nutrient limitation of periphyton in experimental ponds

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Influence of sediment organic enrichment and water alkalinity on growth of aquatic isoetid and elodeid plants

1. Lake eutrophication has increased phytoplankton blooms and sediment organic matter. Among higher plants, small, oligotrophic rosette species (isoetids) have disappeared, while a few tall, eutrophic species (elodeids) may have persisted. Despite recent reduction of nutrient loading in restored lakes, the vegetation has rarely regained its former composition and coverage. Patterns of recovery may depend on local alkalinity because HCO₃^? stimulates photosynthesis of elodeids and not of isoetids. In laboratory growth experiments with two isoetids (Lobelia dortmanna and Littorella uniflora) and two elodeids (Potamogeton crispus and P. perfoliatus), we test whether organic enrichment of lake sediments has a long‐lasting influence by: (i) reducing plant growth because of oxygen stress on plant roots and (ii) inhibiting growth more for isoetids than elodeids. We also test whether (iii) increasing alkalinity (from 0.17 to 3.20 meq. L^?1) enhances growth and reduces inhibition of organic sediment enrichment for elodeids but not for isoetids. 2. In low organic sediments, higher oxygen release from roots of isoetids than elodeids generated oxic conditions to greater sediment depth for Lobelia (4.3 cm) and Littorella (3.0 cm) than for Potamogeton species (1.6–2.2 cm). Sediment oxygen penetration depth fell rapidly to 0.4–1.0 cm for all four species at even modest organic enrichment and oxygen consumption in the sediments. Roots became shorter and isoetid roots became thicker to better supply oxygen to apical meristems. 3. Growth of elodeids was strongly inhibited across all levels of organic enrichment of sediments being eight‐fold lower at the highest enrichment compared to the unenriched control. Leaf biomass of isoetids increased three‐fold by moderate organic enrichment presumably because of greater CO₂ supply from sediments being their main CO₂ source. At higher organic enrichment, isoetid biomass was reduced, leaf chlorophyll declined up to 10‐fold, root length declined from 7 to <2 cm and mortality rose (up to 50%) signalling high plant stress. 4. Lobelia was not affected by HCO₃^? addition in accordance with its use of sediment CO₂. Biomass of elodeids increased severalfold by rising alkalinity from 0.17 to 3.20 meq. L^?1 in accordance with their use of HCO₃^? for photosynthesis, while the negative impact of organically enriched sediments remained. 5. Overall, root development of all four species was so strongly restricted in sediments enriched with labile organic matter that plants if growing in situ may lose root anchorage. Other experiments demonstrate that this risk is enhanced by greater water content and reduced consolidation in organically rich sediments. Therefore, formation of more muddy and oxygen‐demanding sediments during eutrophication will impede plant recovery in restored lakes while high local alkalinity will help elodeid recovery.     

Role of phosphorus and nitrogen for bacteria and phytopankton development in a large shallow lake

Nutrient (P and N) enrichment experiments in small enclosures (20 l) were carried out to determine P and/or N limitation of bacterioplankton in Lake Võrtsjärv. The specific interest of the study was to test if it is possible to detect nutrient `physiological' or growth (rate) limitation of bacterioplankton and competition for nutrients (N and P) with phytoplankton in generally nutrient rich lake. Thymidine and leucine incorporation; leucine aminopeptidase, -D-glucosidase and alkaline phosphatase activity, total count of bacteria, chlorophyll a concentration and primary production as well as the concentrations of different chemical forms of N and P were followed during 4–5 days of the experiment. To address the question of the interactions between nutrients, bacterio- and phytoplankton, experimental and seasonal data sets were included in the analyses. Phosphorus (P) had a positive effect on bacterioplankton in enclosure experiments in June 1997; no effects of nutrients were found in September 1996, while in May 1996, P affected mainly the phytoplankton. On the seasonal scale, the development of bacterioplankton was connected to primary production, total phosphorus and temperature. In enrichment experiments, bacterioplankton was mainly related with primary productivity but the possible importance of bacterial grazers could be presumed. Thus, no evidence was found for nutrient growth limitation and/or competition for N and/or P, rather bacterioplankton depended on organic food supply originating from phytoplankton.     

Variation in nutrient limitation of lotic and lentic algal communities in a Texas (USA) river

Nutrient limitation of periphyton and phytoplankton was assessed in the Upper Guadalupe River, Texas USA. Nutrient-diffusing substrates with added nitrogen (N) and phosphorus (P) were used to identify the limiting nutrient for lotic algae at three river sites in summer, fall, and winter. Pots enriched with P had significantly higher chlorophyll a concentrations for 7 of 9 trials. Added N alone did not significantly increase algal standing crops, although it was found to be secondarily limiting on one (and possibly two) occasions. Flow-through enrichment experiments were conducted in order to quantify the concentration of P needed to significantly increase algal standing crops. Response to enrichment was rapid when ambient P concentration was low (< 0.010 mg L^–1), but more moderate when ambient P levels were higher (0.015–0.025 mg L^–1). Nutrient limitation of phytoplankton in small surface-release reservoirs varied throughout the study, but N was either primarily or secondarily limiting in 6 of 8 trials; shifts in the limiting nutrient were correlated with fluctuations in flow into the reservoirs. Our enrichment studies show that algal response to nutrient addition was unpredictable as phytoplankton tended to be N-limited while periphyton was mainly P-limited. Further, while discharge apparently dictated the nutrient-biomass relationship for phytoplankton in reservoirs, ambient nutrient level is an important determinant of lotic periphyton response to enrichment.     

Differences in nutrient limitation and grazer suppression of phytoplankton in seepage and drainage lakes of the Adirondack region,NY, U.S.A

1. For seepage and drainage lakes of the Adirondack mountain region (NY, U.S.A) hydrologic regime is correlated with physical and chemical differences that can affect phytoplankton and planktonic food webs (e.g. presence and influence of wetlands, dissolved organic carbon concentration, anoxia, nutrient cycling). We conducted short‐term (48 h), in situ enclosure experiments to evaluate the relative importance of macrozooplankton grazing and nutrient limitation of phytoplankton biomass in small Adirondack seepage and drainage lakes (N = 18, 1–137 ha). Epilimnetic dissolved organic carbon (DOC) concentrations and pH values represented the diversity of the region. We measured chlorophyll a changes in response to grazer removal (> 120 μm) and nutrient addition (～ 10× ambient N, P, or N + P), and evaluated changes with respect to in situ light, temperature, NO₃, NH₄, SRP, and crustacean assemblage characters. 2. Nutrient addition stimulated significant increase in chlorophyll a concentration at 11 of 18 sites (GLM, Tukey–Kramer). Phytoplankton of clearwater drainage lakes were P‐limited, whereas clearwater and brownwater seepage lakes responded to additions of N and/or N + P. Relative light availability explained half the variance in response to nutrient addition in drainage (r2 = 0.48), but not seepage lake experiments (P > 0.05). 3. We observed responses to grazer removal at eight of 18 sites, usually clearwater drainage lakes. Crustacean grazing may be as significant as nutrient limitation of [chl a] for many drainage lake phytoplankton assemblages. Responses were related to in situ density of zooplankton only in drainage lakes. Light explained some variability in response to grazer removal for drainage (r2 = 0.35) and seepage lake experiments (r2 = 0.35). 4. These experiments provide evidence that hydrology may ultimately play an important role in determining nutrient and grazer regulation of phytoplankton. Proximate mechanisms affecting our results may be associated with differences in wetland vegetation, [DOC], and nutrient cycling.     

Nutrients and chlorophyll-a dynamics in a temperate reservoir influenced by Asian monsoon along with in situ nutrient enrichment bioassays

Long-term nutrients and chlorophyll-a dynamics during 1993–2000 were analyzed in a temperate reservoir influenced by the Asian monsoon. Nonparametric Mann–Kendall tests and seasonal trend analyses indicated that there were no long-term annual increasing or decreasing trends in major trophic parameters over 8 years, but the monsoon seasonality was evident. Seasonality in chlorophyll (CHL) and total phosphorus (TP) showed a mono-modal pattern, which was closely associated with the monsoon season of July–August, and the magnitude of the mono-modal peak was greater in the headwater zone than in the downlake zone. Such temporal patterns fluctuated interannually over the study period, and the magnitude of the variation was directly controlled by the intensity of the monsoon rain. Empirical models of annual mean CHL–TP were developed supporting the view that phytoplankton in lentic ecosystems responds to P enrichment and that annual mean TP may provide a reliable basis for predicting the average algal abundance. Ambient nutrient analyses, N:P ratios and in situ nutrient enrichment bioassay experiments (NEBs) in premonsoon and postmonsoon supported the P limitation for phytoplankton growth. Ambient nutrients and non-volatile suspended solid (NVSS) data on CHL in the intense monsoon year, however, showed the possibility of light limitation, even though the NEBs did not show the direct evidence. These findings were confirmed by two-dimensional graphic approaches of trophic state index deviations (TSIDs).     

The effects of nutrient addition and pH manipulation in bag experiments on the phytoplankton of a small acidic lake in West Virginia,USA

In situ bag experiments were performed during summer and autumn in a small acidic lake, Tibbs Run Lake, West Virginia, USA. The objective was to evaluate phytoplankton responses to pH manipulation and nutrient addition. Increasing the pH from below 4.5 to over 6.3 resulted in great declines in phytoplankton biovolume. There was also a succession from dinoflagellates (Peridinium inconspicuum to small chlorophytes. The trend was more rapid where phosphorus (P) additions were made along with pH enhancement. During summer, P limitation was indicated, while nitrogen (N) appeared to limit production in autumn. In both seasons, nutrient additions greatly altered the phytoplankton composition in high pH treatments, but had no discernable effects at (the natural) low pH. A low pH, P addition treatment in autumn was the single exception. When N was subsequently added, phytoplankton composition changed dramatically, probably because the proceeding P additions caused severe secondary N-limitation. In general, however, the results supported the view that phytoplankton compositional responses to nutrient additions are suppressed in low pH, relative to high pH lake water.     

In situ nutrient enrichment experiment in the Bohai and Yellow Sea

Nutrient concentrations and N : P ratios have changed significantlyin the past 40 years in the Bohai and Yellow Sea. How do thesechanges influence or contribute to the growth of phytoplankton?Nutrient enrichment experiments were conducted in 1998 and 1999to shed light on which was the first nutrient to limit algalgrowth and uptake rates of nitrogen and phosphorus. Significantvariance analysis, together with nutrient concentration andratio, demonstrated that phosphorus was the first nutrient tolimit the growth of phytoplankton in the Laizhou Bay (SouthBohai); nitrogen was the first, whilst phosphate might be thepotential, nutrient to limit the growth of phytoplankton inthe West Yellow Sea; the Central Yellow Sea was oligotrophicand any one of nitrogen, phosphorus and silicon would limitthe growth of phytoplankton; and silicon was confirmed not tolimit the growth of phytoplankton, although the silicate concentrationdecreased acutely, in the Laizhou Bay. Moreover, the ratio ofnitrogen to phosphorus in phytoplankton uptake was smaller thanthat in seawater, which suggested that phosphorus was preferentiallyused before nitrogen by the phytoplankton. The preference ofphosphorus over nitrogen indicates a further limitation of phosphorusin the Laizhou Bay, and increases the high possibility thatphosphorus, rather than nitrogen, is the first nutrient to limitthe growth of phytoplankton in the West Yellow Sea. Half-saturationconstants (K_s) of dissolved inorganic nitrogen (DIN) and phosphatein the Bohai and Yellow Sea were 1.80 µM and 0.13 µM,respectively. Compared with other sea areas, the K_s value ofDIN in the Bohai and Yellow Sea was located at the high endof the spectrum.     

Allelopathic effect of the aquatic macrophyte, Stratiotes aloides, on natural phytoplankton

1. A survey of different Dutch Stratiotes stands showed that the density of phytoplankton (except cyanobacteria) was always higher outside S. aloides than between the rosettes of S. aloides. Analyses of water samples revealed that nutrient limitation was unlikely to have caused the lower phytoplankton biomass in the vicinity of S. aloides. 2. An in situ incubation experiment in the Danube Delta, Romania, indicated allelopathic activity against phytoplankton in S. aloides stands. The growth rate of natural phytoplankton populations exposed to water from S. aloides stands was significantly lower than that of populations that had not been in contact with S. aloides exudates. 3. A laboratory microcosm experiment showed a significantly lower phytoplankton biomass in treatments with S. aloides exudates. Nutrient concentrations and the light intensity were high enough that the lower phytoplankton biomass could not be explained by nutrient or light limitation.     

Contrasting effects of low‐level phosphorus and nitrogen enrichment on growth of the mat‐forming alga Didymosphenia geminata in an oligotrophic river

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Influence of nutrient availability on phytoplankton growth and community structure in the Port Adelaide River,Australia: [2pt] bioassay assessment of potential nutrient limitation

We investigated the influence of nutrient availability, specifically nitrogen, phosphorus and silicon on growth and community structure of phytoplankton from the Port Adelaide River estuary, South Australia. Two bioassay experiments were conducted. The first, Nutrich1, involved addition of nutrients in vitro to samples of the natural phytoplankton community from a single location in the upper estuary. The second, Nutrich2, involved nutrient addition and incubation of water from five locations in the estuary following inoculation with a `standardised' phytoplankton assemblage derived from laboratory cultures. In Nutrich1, enrichment with silicon led to greatly enhanced phytoplankton biomass due to increased growth of diatoms. Addition of nitrogen or phosphorus had little effect on phytoplankton growth. In Nutrich2, addition of nitrogen resulted in enhanced growth of phytoplankton in water collected from near the mouth the estuary, but there were no differences in growth among nutrient treatments for the remaining locations. Comparison of phytoplankton growth rate among locations revealed a trend of decreasing growth in moving towards the mouth of the estuary. This trend was unaffected by enrichment with nitrate, phosphate or silicate. We suggest that spatial variation in growth potential within the Port Adelaide River estuary may relate to variation in the concentration of nitrogen as ammonium.