Surface nutrients,chlorophyll a and phaeopigment in some Scottish sea lochs

The distribution of several surface properties, both physical and chemical, of three Scottish sea lochs were studied during the spring phytoplankton bloom.Concentrations of nitrate, phosphate, and silicate have been determined and related to fluctuations in the freshwater input and uptake by the phytoplankton population. The ratios nitrate: phosphate and silicate: phosphate (on atom bases) in Loch Creran and Loch Linnhe were similar, but there were certain marked differences from those in Loch Etive.Nitrate, rather than phosphate, was a critical factor in the productivity of Loch Creran and Loch Linnhe. In Loch Etive the limitation of these nutrients was directly affected by the amount of fresh-water entering into the Loch.Patchiness of chlorophyll a and phaeopigments in Loch Creran and Loch Etive reflected those factors (e.g., freshwater/sea-water inflow, turbulence produced by the loch topography etc.) which affect the surface circulation in the two lochs.The delay in the development of the phytoplankton bloom in Loch Linnhe may be attributed to a poor stability of the surface layers due to low run-off.     

The Loch Eil project: Planktonic pigments in sediments from Loch Eil and the firth of Lorne

Photosynthetic pigments and their derivatives were measured in sediments in the fjordic Loch Eil and the Firth of Lome, Scotland, between November 1975 and November 1976. After acetone extraction from the top 10 mm of sediment cores, pigments were crudely separated, by fluorescence change on acidification, into (chlorophyll a + chlorophyllide a) and phaeopigments. The greatest pigment concentrations (mean 73 μg · g sediment dry wt^?1) were found in the most reducing sediments which also had a high average proportion (23%) of chlorophyll. The least mean pigment concentration (23 μg · g^?1) and proportion of chlorophyll (17%) were found in the most oxidizing sediments in the Firth of Lorne where there was a clear seasonal cycle, with a peak in sediment pigment concentration and chlorophyll proportion in May and June, just after the planktonic spring increase. The Loch Eil stations showed a less clear or no seasonal cycle; the station most affected by organic input was the most variable from month to month. It was concluded that redox status was the most obvious control of sediment pigment content, whereas the effect of sedimentation of phytoplankton was complex.     

Soluble fractions of phosphorus compounds and alkaline phosphatase activity in loch creran and loch etive,Scotland

Reactive phosphorus, polyphosphate, monophosphate ester, dissolved organic phosphorus, and alkaline phosphatase activity were measured in Loch Creran and Loch Etive. Argyll, Scotland during the spring outburst of 1976. In addition, data on chlorophyll a, phacopigment, and salinity were obtained on each occasion.The mean levels of reactive phosphorus initially present in Loch Creran and Loch Etive were 0.50 and 0.22 μg-at. PO₄?P/I, respectively. As the diatom bloom progressed, it fell below the limit of detection. The mean values of polyphosphate were between 0.05–0.08 μg-at. PO₄?P/l in Loch Creran and 0.04 μg-at. PO₄?P/l in Loch Etive, and although present at the initiation and decline of the bloom, it was absent when chlorophyll a was high and the reactive phosphorus was low or undetectable. Monophosphate ester, was found in Loch Creran only, and then in the same concentration as that of the polyphosphate fraction. The mean levels of the dissolved organic phosphorus in Loch Creran and Loch Etive ranged between 0.08–0.18 μg-at. PO₄-P/l and 0.05–0.08 μg-at. PO₄?P/l, respectively. Alkaline phosphatase activity was detected when there was no reactive phosphorus in the water: the highest values in Loch Creran and Loch Etive were 0.003 and 0.005 I.U. of enzyme/4 ml of sample, respectively. The highest mean values of chlorophyll a were 8.0 μg/l in Loch Creran, and 6.0 μg/l in Loch Etive. The mean percentages of phaeopigment fluctuated between 20–55% in Loch Creran and 30–65% in Loch Etive. The highest values were found at the end of the bloom. The mean surface salinities ranged from 22.0 to 31.0‰ in Loch Creran and 3.7 to 17.0‰ in Loch Etive. The lowest values were recorded at the end of the bloom. The statistical treatment of the data, suggests a strong relationship between zooplankton grazing and levels of dissolved phosphorus compounds and phaeopigment.     

Studies on the phytoplankton of the River Ganges,Varanasi, India,Part II “The seasonal growth and succession of the plankton algae in the River Ganges”

Summary An attempt has been made to find out the productivity and periodicity of the phytoplankton in relation to the physical and chemical characteristics of the river Ganges, between University Ghat and Raj Ghat at Varanasi. The observations were taken during a single year (i.e., March 1957 to March 1958). The chemical analyses of the water and estimates of the number of phytoplankton were made on samples collected fortnightly.The phytoplankton has proved to be eutrophic and polymixic in nature. Diatoms formed the largest bulk, blue-greens being the second major constituent while the green algae formed the third in order of abundance with, however, large number of species. A definite phytoplankton periodicity has been noticed and found to be seasonal in character.Correlations between the phytoplankton periodicity and population maxima, with the habitat factors revealed depletion of nutrients like, nitrogen, phosphate, silica, carbon, magnesium and sulphates.Changes in the phytoplankton populations were clearly evident more in relation to physical than to chemical conditions of the water. Changes in water-level, transparency and temperature affected the growth of the phytoplankton.High alkalinity and buffering capacity resulted in preferential growth of diatoms and blue-green algae. Rich phosphates and silicates coupled with moderate nitrogen contents were responsible for high phytoplankton yields in summer and winter seasons.The abundance of blue-greens may be due to the higher values of pH, temperature, dissolved organic matter, phosphate, nitrogen, and relatively high values of dissolved oxygen. Green algae showed a wide range of adaptability but could not develop in any abundance.     

Sea trout in North Argyll sea lochs: II. diet

The diet of sea trout in some of the sea lochs of the west coast of Scotland was investigated. The contents of 986 sea trout stomachs from the Loch Etive area (1970–1973), and 291 stomachs from the Loch Eil area (1964–1973), were examined and the composition of the diet, seasonal changes and the effect of trout size were analysed using frequency of occurrence, dry weight and number of organisms methods. Benthic feeding (crustacea and annelids) was more important in winter while midwater and surface organisms (young fish and insects) were preferred in summer. Young fish (mainly clupeids and sand eels) featured more in the diet of larger trout (≤21 cm) than in the smaller size range (≥21 cm). Surveys of 24-h were completed in June and September, 1972, and indicated that availability of food was the main factor influencing the presence or absence of trout. Bottom feeding was greatest during the day while the amount of midwater and surface feeding tended to increase between sunset and sunrise.     

Optical and hydrographic consequences of freshwater run-off during spring phytoplankton growth in a Scottish fjord

A combination of in situ measurements and radiative transfermodelling were used to study optical conditions in the innerbasin of Loch Etive, a Scottish fjord, in March and April 2000.The basin was strongly stratified with three layers separatedby marked pycnoclines. The surface layer averaged 5 m in depthand was heavily stained with coloured dissolved organic matter(CDOM) which reduced the euphotic depth to between 7 and 10m. Approximately 20% of the photosynthetically available radiation(PAR) in the water column was absorbed by phytoplankton, 44%by CDOM and 36% by sea water. Detectable concentrations of themajor inorganic nutrients (nitrate, phosphate and silicate)occurred at all depths, but significant phytoplankton populations(averaging 6 mg chlorophyll a m^–3) were found only inthe reduced-salinity surface layer. The freshwater input thereforeacted both as a source of buoyancy which promoted phytoplanktongrowth near the surface and as an attenuator of visible lightwhich inhibited growth deeper in the water column.     

Spatial distribution of phytoplankton in Lake Baikal, Spring 1991

• 1 Studies carried out in Lake Baikal in late spring (late May-early June) 1991 showed marked differences in the species composition and abundance of phytoplankton in different regions of the lake. The south and north basins were characterized by small forms of algae. The middle basin, Maloe More and the shallow waters of the Selenga had species with large cells including Aulacoseira islandica subsp. helvetica, Dinobryon cylindricum and D. divergens.
• 2 Areas of high biomass were correlated with shallow waters and river inputs. This was especially apparent in the region of the Selenga delta. The vertical distribution of phytoplankton indicated the non-synchronous start of the spring homothermy throughout the lake. Phytoplankton were concentrated in the upper 100 m layer with subsurface maxima resulting from the sinking of large algae.
• 3 The concentration of phytoplankton biomass in general at this time characterized the lake as moderately productive.

     

Biological availability and turnover rate of acetate in marine and estuarine sediments in relation to dissimilatory sulphate reduction

Abstract Acetate turnover rates were determined using ¹⁴C acetate in sediment slurries from two Scottish sea lochs and an estuary which had different rates of oxygen uptake and sulphate reduction. Turnover rates in Loch Etive and Loch Eil were 0.504 and 0.651 μMh⁻¹ respectively, but in the River Tay Estuary there was substantially higher acetate turnover (12.22 μMh⁻¹). The addition of 20 mM sodium molybdate, a specific metabolic inhibitor of sulphate-reducing bacteria (SRB), resulted in a complete inhibition of acetate turnover. These data suggest that SRB were solely responsible for acetate oxidation in these sediments. A comparison of acetate turnover rates in the absence of molybdate and accumulation rate in the presence of the inhibitor demonstrated that at least two pools of acetate with different biological availabilities existed. In Loch Etive only 19% of chemically measured acetate was available with corresponding values of 48% and 65% for Loch Eil and the Tay Estuary respectively.     

The fish populations in inshore waters of the West Coast of Scotland. The distribution, abundance and growth of the whiting (Merlangius merlangus L.)

The whiting ( Merlangius merlangus ) is an important commercial species from the West Coast of Scotland. This paper describes the distribution, abundance, age composition and growth of juvenile whiting which are found in the inshore areas and sea lochs. No spawning was observed in the areas and it is assumed there was an active migration of whiting from the open sea. They were first caught by the bottom trawl in July, reached maximum abundance in November and December and thereafter declined in numbers until few remain by early summer. An exception to this occurs in Loch Etive and possible explanations for the fact that whiting tend to remain for longer periods and grow at a slower rate in this loch are discussed.     

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.     

Assessing the potential impact of salmon fisheries management on the conservation status of harbour seals (Phoca vitulina) in north-east Scotland

Conservation efforts are often constrained by uncertainty over the factors driving declines in marine mammal populations. In Scotland, there is concern over the potential impact of unrecorded shooting of seals, particularly where this occurs near Special Areas of Conservation. Here, we show that the abundance of harbour seals Phoca vitulina in the Moray Firth, north-east Scotland, declined by 2–5% per annum between 1993 and 2004. Records from local salmon fisheries and aquaculture sites indicated that 66–327 harbour seals were shot each year between 1994 and 2002. Matrix models and estimates of potential biological removal indicate that this level of shooting is sufficient to explain observed declines. Nevertheless, uncertainty over the number and identity of seals shot means that other factors such as changes in food availability may be contributing. Recent conservation measures markedly reduced the recorded levels of shooting in 2003 and 2004. In 2005, a coordinated management plan was developed to protect salmon fisheries interests while minimizing impacts on local seal populations. Comprehensive monitoring of future population trends and improved regulation of culls are now required to provide more robust assessments of the impact of human persecution on harbour seal populations in the Moray Firth and in other parts of the UK.     

Three species of Mytilus and their hybrids identified in a Scottish Loch: natives, relicts and invaders?

Three species of the mussel, Mytilus, occur in the North Atlantic region, M. edulis, M. galloprovincialis and M. trossulus, and hybrid zones are present where their distributions overlap. M. edulis is a native species in the UK. M. galloprovincialis originated in the Mediterranean and its distribution extends northwards along the Atlantic seaboard to Scotland. Baltic Sea mussels have a M. trossulus ancestry but are highly introgressed by M. edulis. In recent decades, farming of mussels on long-line rope culture systems has been introduced into Scotland. On farms in Loch Etive, a form of mussel with a fragile shell and a different shape to either M. edulis or M. galloprovincialis has been increasing in frequency over recent years. Samples of fragile shelled, normal strong shelled and intermediate mussel types were sampled from two farms in 2006 and compared with samples of M. edulis, M. galloprovincialis and M. trossulus from other sources where their species identity is well established. Abundance relative to depth, shell strength, condition index and shell morphology were analysed together with 5 allozyme loci and one nuclear DNA genetic marker (Me 15/16). The fragile shelled mussels, and many of those classed as intermediate, were identified as a mixture of M. trossulus and M. trossulus x M. edulis hybrids. This identification was strongly supported by both morphological and genetic data and is the first record of the presence of M. trossulus in UK waters. M. trossulus in Loch Etive are most likely to be a post-glacial relict population restricted to the low salinity area of the Loch that has recently increased in abundance due to commercial mussel growing activity. In addition, individual mussels of all three species and their hybrids were detected amongst Loch Etive mussels. This is the first genetic demonstration of all three species and their hybrids occurring together in one location in the Atlantic region and provides a unique opportunity to study the processes of speciation, divergence, and introgression in the genus Mytilus.     

The biology of Fries' goby, Lesueurigobius friesii (Malm), in the Firth of Clyde, Scotland, and a comparison with other stocks

The general biology of a population of Lesueurigobius friesii in the Firth of Clyde, Scotland, was studied between October 1977 and December 1979. In this population the fish attained 95 mm and lived a maximum of nine years. The population growth characteristics from von Bertalanffy growth curves showed that the Clyde stock of L. friesii attained a greater maximum theoretical length (L∞) than stocks in the Lynn of Lorne or Biscay and had a lower growth coefficient ( K ). In all three stocks, the majority of the growth in length ( A _0.95) was achieved by the end of the fourth year of life. In general the sex ratio was 1: 1 throughout the season. Spawning occurred from May to August, approximately the same time as the Lynn of Lorne stock. Sexual maturity was first reached at a smaller size in males than females. The abundance of L. friesii in the Clyde fluctuated considerably with reduced numbers of fish caught just after spawning.     

Changes in phytoplankton over various time scales in a shallow, eutrophic: the Loch Leven experience with special reference to the influence of flushing rate

SUMMARY. 1. The pattern of fluctuations in the total biomass and species composition of phytoplankton in the shallow, eutrophic Loch Leven exhibits considerable inter-annual and within-year variability. Nevertheless, studies over the 18-year period reviewed here (1968–85) show that many of the observed changes can be explained in -terms of light regimes and the concentrations and fluxes of nutrients. On occasion, the incidence of fungal parasitism and of protozoan, rotiferan and crustacean grazing is also important. 2. Changes in annual mean algal biomass from very high levels in the late 1960s and early 1970s to somewhat lower levels in the late 1970s, followed by occasional high peaks in the present decade, are attributed to shifts in phosphorus loading-particularly from a P-rich industrial source. 3. In spite of complex and erratic sequences of algal species, seasonal patterns in the size distribution of the phytoplankton assemblage have been identified. However, these also changed. During the 4 years prior to 1971, Daphnia hyalina was not recorded in the plankton, and small algae have been abundant only in late winter or early spring. 4. The potential importance of the weather is highlighted as one of the possible causes of the irregular appearance of algal species. The effect of the characteristically variable, oceanicclimate of Northern Britain on this large (13.3 km²), shallow (z?= 3.9 m) loch is thought to be of particular importance. 5. The influence of the weather on phytoplankton sequences is explored by comparing records of monthly flushing rate values with time-series data on aspects of the aquatic environment and plankton populations. The preliminary assessment suggests that variation in flushing rate ( p ) has a considerable effect on temperature regimes and the supplies and in-loch dynamics of nutrients; through such changes, p controls major features of phytoplankton succession such as the temporal abundance of diatoms, as well as detailed sequences of events relating to the development and collapse of particular algal species - and. as a consequence, in some cases, of the animals preying on them. 6. Concluding remarks emphasize how little would have been understood about the functioning of shallow lakes, had the long-term commitment to Loch Leven research been abandoned. The view is also expressed that the findings are of considerably greater relevance to research on ‘classic’ stratifying lake systems, than the focus on the shallow waterbody might hitherto have suggested.     

Interactions of light, temperature and inorganic nitrogen in controlling planktonic nitrogen utilisation in the Tagus estuary

The effect of light, temperature and ammonium on inorganic nitrogen uptake by phytoplankton was investigated from June 1994 through December 1995 at three sites in the Tagus estuary (Portugal), during high tide of neap tides. Ammonium concentrations higher than 10 M reduced nitrate uptake down to 24% but never prevented it. Below this threshold concentration, nitrate uptake was neither inhibited nor changed. Uptake of both nitrate and ammonium as a function of light intensity exhibited a saturation response. Uptake reduction occurred in the near bottom phytoplankton populations, particularly for nitrate. The ammonium uptake system was less limited by light than the nitrate uptake system, indicating the importance of ammonium as a nitrogen source for the phytoplankton which is likely to experience high changes in light in the well-mixed water column of this estuarine environment. Ammonium uptake was exponentially related to temperature in the upper estuary whereas in the mid and lower estuary this relationship was linear. The effect of temperature on nitrate uptake was linear but far less marked than for ammonium uptake.     

Light limitation of phytoplankton development in an oligotrophic lake - Loch Ness, Scotland

• 1 The underwater light climate in Loch Ness is described in terms of mixing depth (Z_m) and depth of the euphoric zone (Z_eu). During periods of complete mixing, Z_m equates with the mean depth of the loch (132 m), but even during summer stratification the morphometry of the loch and the strong prevailing winds produce a deep thermocline and an epilimnetic mixed layer of about 30 m or greater. Hence, throughout the year the quotient Z_m/Z_eu is exceptionally high and the underwater light climate particularly unfavourable for phytoplankton production and growth.
• 2 Phytoplankton biomass expressed as chlorophyll a is very low in Loch Ness, with a late summer maximum of less than 1.5 mg chlorophyll a m^-3 in the upper 30 m of the water column. This low biomass and the resulting very low photosynthetic carbon fixation within the water column are evidence that a severe restraint is imposed on the rate at which phytoplankton can grow in the loch.
• 3 The chlorophyll a content per unit of phytoplankton biovolume and the maximum, light-saturated specific rate of photosynthesis are both parameters which might be influenced by the light climate under which the phytoplankton have grown. However, values obtained from Loch Ness for both chlorophyll a content (mean 0.0045 mg mm^-3) and maximum photosynthetic rate (1–4 mg C mg Chla^-1 h^-1) are within the range reported from other lakes.
• 4 Laboratory bioassays with the natural phytoplankton community from Loch Ness on two occasions in late summer when the light climate in the loch is at its most favourable, suggest that even then limitation of phytoplankton growth is finely balanced between light and phosphorus limitation. Hence, for most of the year, when the light climate is less favourable, phytoplankton growth will be light limited.
• 5 Quotients relating mean annual algal biomass as chlorophyll a (c. 0.5 mg Chla m^-3) and the probable annual specific areal loading of total phosphorus (0.4–1.7 g TP m^-2 yr^-1) suggest that the efficiency with which phytoplankton is produced in Loch Ness per unit of TP loading is extremely low when compared with values from other Scottish lochs for which such an index has been calculated. This apparent inefficiency can be attributed to suppression of photosynthetic productivity in the water column due to the unfavourable underwater light climate.
• 6 These several independent sources of evidence lead to the conclusion that phytoplankton development in Loch Ness is constrained by light rather than by nutrients. Loch Ness thus appears to provide an exception to the generally accepted paradigm that phytoplankton development in lakes of an oligotrophic character is constrained by nutrient availability.

     

Effect of pollution on phytoplankton in the Ashar Canal,a highly polluted canal of the Shatt al-Arab Estuary at Basrah,Iraq

The effect of pollution on the phytoplankton in the Ashar Canal was studied during March and June 1976. Higher temperatures were recorded in June. The canal water was more turbid than that of the adjoining Shatt al-Arab estuary due to high algal populations and sewage disposal. The chlorosity values were remarkably high in the canal, due to sewage disposal. The flood waters in June lowered the chlorosity in both estuary and canal. Sewage disposal into the canal decreased its dissolved oxygen content. The lower oxygen values in the estuary in June were accompanied with higher temperatures. The pH values were generally higher in the estuary and were also higher in June than in March.Eighty-six algal taxa were recorded, 45 of which were diatoms. The phytoplankton in the estuary was more abundant in March, due to the increase in currents and turbidity in June. The Chlorophyta, Conjugatophyta, Myxophyta and Bacillariophyta in the estuary showed reverse seasonal trends from those of the total phytoplankton. These algal seemed to flourish during periods of high temperatures. The dense blue-green algae recorded in June is attributed to the high temperature and dissolved organic matter. The Chrysophyta recorded high populations in the estuary in March and were totally absent in June. Lower temperatures and high transparency in March might have led to their relative abundance.The total phytoplankton in the Ashar Canal followed two different seasonal trends, one resembling and the other differing from those in the estuary. The disposal of domestic sewage and agricultural wastes in the canal seemed to lead to such adverse seasonal variations. The diatoms only dominated the phytoplankton at one station in both months. The sewage and agricultural wastes disposal seemed to favour the abundance of the other phyla than the diatoms. Increased organic load in the canal has led to a considerable increase in the Myxophyta in June. Blooms of Chlamydomonas sp. and Euglena acus increased the phytoplankton densities in March at stations II and IV, respectively. Benthic or periphytic algae were found in the phytoplankton populations. Raised salinity seemed to restrict the distribution of the planktonic diatom Bacillaria paradoxa. Among the noticeable widespread planktonic algae in the study area are Dinobryon sertularia, Pediastrum duplex and P. simplex.     

Light penetrance in lake kinneret

The characteristics of light penetrance in Lake Kinneret, Israel, were observed over the years 1970 to 1973. Light measurements were made concurrently with those of algal speciation and biomass, chlorophyll concentrations and primary production. Vertical extinction coefficients of green light (filter VG9), the most penetrating spectral component, ranged from 0.15 (August 1970) to 0.93 In units m^–1 (April 1970), reflecting the large differences between algal standing crops in non-bloom and bloom seasons. During the dinoflagellate bloom (Peridinium cinctum fa westii) from February through June, the increment of extinction coefficient per unit increase of chlorophyll concentration was 0.006 ln units mg^–1 m². The uneven vertical distribution of algae at this period caused irregularities in the depth curves of light penetrance. At other times, when the phytoplankton cells were more homogeneously dispersed with depth, regular light penetrance curves were observed; however, as previously noted (Rodhe, 1972), attenuation of algal photosynthetic activity often appeared to be regulated by the blue spectral component (filter BG 12). Ratios of absorbed to scattered light in the upper water column ranged from 85:15 to 75:25.     

The sensitivity of phytoplankton in Loch Leven (U.K.) to changes in nutrient load and water temperature

1. Loch Leven is a shallow, eutrophic lake in Scotland, U.K. It has experienced much change over the 30 years that it has been studied; this has primarily been due to reduced nutrient loads to the lake through active catchment management. Its recovery has been slow and, therefore, we used a phytoplankton community model (PROTECH) to test its sensitivity to changing nutrient loads and water temperature.
2. PROTECH was initialized to simulate the observed phytoplankton community in 1995 and was then repeatedly run through a combination of step-wise changes in water temperature and nutrient load (two treatments were simulated for nutrient load: one changing both nitrate and phosphorus, and one changing just phosphorus). The effect on total chlorophyll- a concentration, cyanobacteria abundance and phytoplankton diversity was examined.
3. Whilst changes in temperature had little effect, variations in the nutrient load produced a range of responses. Increasing only the phosphorus load caused a large increase in Anabaena abundance and total chlorophyll- a concentration. However, the opposite response was recorded when nitrate load was changed as well, with Anabaena increasing its biomass under reduced nutrient load scenarios.
4. The key factor determining the type of response appeared to be nitrogen availability. Anabaena , a nitrogen fixer, could exploit the phosphorus resource of Loch Leven under limiting nitrogen conditions, allowing it to dominate under most of the scenarios tested apart from those supplying extra nitrogen to the lake. The model predictions agree with the observed data, which show that Anabaena continues to dominate the summer phytoplankton bloom in Loch Leven despite the considerable reduction in phosphorus supply from the catchment. This research provides a possible explanation for this.