Anabaena Blooms Research Articles

Testing the Sensitivity of Phytoplankton Communities to Changes in Water Temperature and Nutrient Load, in a Temperate Lake

Freshwater lakes are biologically sensitive to changes in the surrounding environment and the impacts that such changes have on their water quality are of considerable ecological, recreational and economic importance. In this study the phytoplankton community model, PROTECH, was used to experiment with the effects of elevated temperatures and increased nutrient load on phytoplankton succession and productivity. The response of a phytoplankton community to combined incremental changes in these drivers was analysed, in order to elucidate the resulting ecological changes. Annual mean phytoplankton biomass increased with increases in temperature and nutrient loading, although the latter had the larger effect. The phenology of the dominant phytoplankton taxa changed with increasing water temperature; the three spring blooming species all peaked earlier in the year. The simulated summer bloom of Anabaena became earlier in the year and the Chlorella bloom later. The increased phytoplankton biomass was largely dominated by the cyanobacterium Anabaena, which was especially prevalent during the summer bloom. This resulted in a progressive loss of phytoplankton biodiversity with increasing water temperature and nutrient supply. Model experimentation showed that whilst both factors greatly affected the community, the changes to nutrient loading generally had the greater effect and that at low nutrient levels the effect of water temperature change was reduced considerably. Finally, the model predicted that cyanobacteria have the potential to dominate the phytoplankton community, with clear consequences for water quality, and that this dominance was at its greatest when high water temperatures were combined with high nutrient loads.

Effects of salinity and source of inocula on germination of Anabaena akinetes from a tidally influenced lake

Summary1. Sedimentary akinetes (resting stages) may represent significant potential inocula for nuisance blooms of cyanobacteria. We studied the effects of salinity and sediment source on the germination and subsequent growth of Anabaena flos‐aquae akinetes from a shallow, tidally influenced lake.2. Surface sediments collected from littoral and open‐water sites were used as inocula to culture A. flos‐aquae akinetes in four salinities (0.1, 2.2, 4.4 and 6.5) over 22 days. Akinete germination and development was followed by counting developmental stages every second day.3. Filament growth, but not akinete germination, was inhibited by salinity and there were significantly fewer filaments at 6.5 than at 0.1 and 2.2. Cultures inoculated with littoral sediment had more akinetes, germlings and filaments than those inoculated with open‐water sediment.4. Sediment is a potential source of inocula for Anabaena blooms in the lake, which potentially could develop solely from this source because germination and subsequent filament growth do not depend on the existence of an initial pelagic Anabaena population.

Development of cyanobacterial blooms in Valencian rice fields

Seasonal variation of biomass, photosynthetic pigments and C and N contents of the cyanobacterial blooms developed during the crop cycles of 1998, 1999 and 2000 at two locations in Valencian rice fields (Spain) were studied to find their potential contribution to soil fertility. Blooms of Gloeotrichia sp., Gloeocapsa sp., Microchaete sp., and Nostoc sp. were small and dispersed and appeared only for a few weeks in the experimental fields of the Tancat de Malta location during the crop season of 1998. The biomass of these blooms ranged from 4.4 to 12.8 mg dry wt cm−2. A bloom of Anabaena sp. was found in the same location during most of the crop cycles of 1998 and 1999, covering up to 80% of the experimental fields. The biomass of this bloom ranged from 2.7 to 11.4 mg dry wt cm−2. In the moment of its maximal extension (July 1998) we estimate a total biomass of 2,100 kg dry wt for this bloom, equivalent to 420 kg dry wt ha−1. In the Sueca location a bloom of Microchaete sp. was observed in August 1999 and during most of the crop cycle of 2000, when it covered a wide area (60–70%) of the experimental field. The biomass of this bloom ranged from 0.9 to 5.6 mg dry wt cm−2. Carbon contents of the blooms ranged from 150 to 310 μg mg−1 dry wt and N contents ranged from 11 to 30.2 μg mg−1 dry wt. Taking into account their N content and their extension we estimate that the incorporation of cyanobacterial blooms could add 4–12 kg N ha−1 to soil. From an agronomical point of view this means that, in spite of their impressive aspect, cyanobacterial blooms can only fulfill a small proportion of the N requirements of rice plants.

Reduction of germination frequency in Anabaena akinetes by sediment drying: a possible method by which to inhibit bloom formation

The effect of desiccation on germination frequency of Anabaena akinetes was investigated to evaluate the practical usefulness of sediment drying in reservoirs as a measure of inhibition of Anabaena bloom formation. Sediment samples taken from a small reservoir, where an abundance of Anabaena blooms occur every summer were desiccated in the outdoors and in an incubator at 5, 10, 15, and 25 degrees C. As a result, the germination frequency of Anabaena akinetes was strongly inhibited in comparison with that of Anabaena akinetes preserved in a refrigerator without desiccation as a control. Greater inhibition of Anabaena germination found at a higher temperature was examined, and almost all akinetes lost the ability of germination at 25 degrees C after 3 days of the desiccation treatment. Therefore, sediment drying is considered to be a useful measure to reduce periods and scales of Anabaena blooms, and its effect will be enhanced by performance during the warmer seasons.

Development of Anabaena blooms in a small reservoir with dense sediment akinete population, with special reference to temperature and irradiance

The development of Anabaena ucrainica blooms in a small agricultural reservoir was monitored in 1998 and 1999. In the reservoir, numerous Anabaena akinetes were found in all regions of the sediment analyzed, with an average cell density in the uppermost layer (0–2 cm) of 1.5 × 104 cm−3. Anabaena ucrainica filament numbers began to increase exponentially in mid-May 1998 and in late April 1999, when the water temperature exceeded 15°C. The average in situ net growth rate was 0.18 day−1 as measured by filament numbers. The effect of temperature on germination of the akinetes was investigated using Anabaena akinetes taken from the reservoir sediment. High germination percentages were observed at temperatures between 14 and 23°C; however, the Anabaena akinetes did not germinate without irradiance. Growth experiments using an axenic culture of A. ucrainica isolated from the reservoir showed that an increase in incubation temperature to 26°C resulted in a rise in the specific growth rate. Consequently, it was hypothesized that temperature increases could similarly enhance the growth rate of A. ucrainica during bloom development. Furthermore, judging from the in situ growth rate of A. ucrainica, initial inocula arising from dense akinete populations in the sediment would advance bloom formation and could enhance the relative probability of Anabaena bloom formation.

Cyanobacterial Neurotoxins from Southern Brazilian Freshwaters

Blooms of Cylindrospermopsis raciborskii and Anabaena spiroides were studied in relation to their toxins composition, geographical locations, and other characteristics of the waters in the southern region of Brazil. All forms of Cylindrospermopsis were paralytic shellfish toxin producers, with a similar profile of the toxins. Anabaena blooms were studied in relation to the production of anatoxin-a(S). In all samples containing Anabaena spiroides, a positive result was found when the AChE inhibition technique was used. Methods applied for both studies are very convenient for monitoring this large region and give a reasonable view of the present situation of water reservoirs in southern Brazil.

Variations in environmental factors and their effects on biological characteristics of meromictic Lake Abashiri

To clarify the variations in environmental factors and biological characteristics of eutrophic, brackish, meromictic Lake Abashiri, Japan, we studied long-term variations in concentrations of chloride and nutrients in lake water and characteristics of their supply. This lake has an anoxic layer containing much chloride, nutrients, and sulfide. Although a dense Anabaena bloom appeared in the lake every summer before 1986, the bloom did not appear after upwelling from the bottom layer (a blue tide) occurred in spring 1987. A dense layer of photosynthetic bacteria (Chlorobium sp.) formed at the top of the anoxic layer from 1989 to May 1992. The concentrations of nutrients and chloride in the oxygenic layer were determined by the balance between nutrients from influent rivers, including dissolved inorganic nitrogen, and matter supplied from the anoxic layer, including chloride and dissolved inorganic phosphorus. These influences controlled the dominant phytoplankton species and their biomass. The appearance of an Anabaena bloom in Lake Abashiri is promoted by nitrogen limitation caused by decreased loading from influent rivers and/or by a stable supply of phosphorus from the anoxic layer (no formation of photosynthetic bacterial layer and small upwelling), but the appearance is inhibited by a high chloride ion concentration with large upwelling. The main reason for the formation of a photosynthetic bacterial layer from 1989 to May 1992 was that sunlight reached the top of the anoxic layer because of the increase in transparency due to inhibition of the Anabaena bloom with upwelling. The dense layer of photosynthetic bacteria blocked nutrient diffusion from the anoxic layer to the oxygenic layer by uptake, and decreased the biomass of phytoplankton in the oxygenic layer.

A note on the factors limiting Anabaena bloom in a temperate eutrophic lake

The population ofAnabaena in Rostherne Mere (Cheshire, UK) was monitored in summer 1995 using both traditional methods and X-ray microanalysis. The numbers increased in June, reaching an epilimnion density of 1700 filaments per ml, but rapidly declined in July The demise of the Anabaena population could have been related to the concurrent dramatic decrease in the lake water P and intracellular K levels, a decrease in transparency and light availability, and an increase in fungal and bacterial parasitism. The relative importance of these factors for the determination of the timing and magnitude of Anabaena blooms in Rostherne Mere should be clarified by further research.

Effect of temperature on growth of the cyanobacterium Aphanizomenon flos-aquae in Lake Biwa and Lake Yogo

SUMMARY The water bloom-forming cyanobacterium Aphanizomenon flos-aquae Ralfs ex Bornet et Flahault (Nos-tocales, Cyanophyceae) appeared in Lake Biwa and Lake Yogo in 1999 for the first time. The morphological characteristics were described using natural samples. In contrast to the other water bloom-forming cyanobacteria such as Microcystis and Anabaena in Lake Biwa and Lake Yogo, the small summer population of A. flos-aquae is apt to grow in winter, suggesting the low temperature preference or tolerance of this species. In order to clarify the effect of temperature on the growth, culture experiments were conducted using an axenic strain isolated from Lake Biwa. The strain could grow at above 8°C with an optimum temperature ranging from 23 to 29°C, and survived even at 5°C for at least 25days under low light conditions. Although these results confirmed the ability of the bloom formation during late autumn and winter, it is still unclear why the Aphanizomenon bloom occurred at temperatures of ca 10°C in December and not immediately after the disappearance of Microcystis and/or Anabaena bloom during autumn.

Effect of cyanobacterial blooms on thermal stratification

Enclosure experiments were performed at Akanoi Bay, Lake Biwa, in 1995 to determine whether the blooms of cyanobacterial algae changed thermal stratification in the lake. We used four rectangular enclosures, each 10 m × 10 m, with a volume of 200 m3, which were open to the sediments. Two enclosures, A and B, were mixed artificially by aquatic pumps from 1000 to 1400 every day, and the other two enclosures, C and D, were controls with no mixing. The experiment was conducted during late summer from August 3 to September 27. Chlorophyll a concentrations were highest in enclosure D, followed by enclosure C, both of which were controls without mixing. Enclosure A had lower concentrations than enclosures C and D, and enclosure B had the lowest concentrations. No large cyanobacterial algae blooms of Anabaena sp. and Microcystis sp. were seen in the mixed enclosures A and B. In enclosures C and D, blooms of Anabaena sp. occurred in the middle of August, and Microcystis sp. later became dominant in enclosure D at the end of August. In enclosure D, the water temperature changed over the diel cycle before August 17, with thermal stratification during the day and complete mixing at night. After August 17, as Anabaena sp. and Microcystis sp. became dominant, the temperature at the bottom of the enclosure did not change clearly over the 24-h cycle. The APE (available potential energy) density (a measure of water column stability) in the enclosures increased by almost 100% when the biovolume of Anabaena sp. + Microcystis sp. exceeded 20 mm3 l−1. These results indicate that blooms of Anabaena sp. and Microcystis sp. can increase the available potential energy in the water column and create more stable stratification for their growth.

ANABAENA AND MICROCYSTIS IN THE PACIFIC NORTHWEST: A TALE OF PLASTICITY

The molecular marker, cpcBA‐IGS, was used to examine the biogeography of representatives of Anabaena and Microcystis collected from Pacific Northwest lakes and their phylogenetic relationship to isolates from the UTEX and CCAP culture collections. cpcBA‐IGS contains portions of the alpha and beta subunit of cyanophycocyanin with an intergenic spacer (IGS) of ∼100 base pairs situated between the subunits. The use of this cyanobacteria specific marker and primers allowed us to amplify cpcBA‐IGS DNA from uni‐cyanobacterial field samples. The morphology and habit of specimens were documented by photomicroscopy. Single filaments or colonies of field material and 3–4 filaments of culture material were then amplified using the polymerase chain reaction (PCR). Results include analysis of non‐toxic summer and toxic winter blooms of Anabaena from 1989–1997 in American Lake, WA and a comparison of non‐toxic and toxic Microcystis from Lake Sammamish, WA. A preliminary phylogenetics analysis of Pacific Northwest and UTEX isolates also was performed using parsimony, maximum likelihood and distance methods. The results of this analysis suggest that revision of Anabaena and Microcystis taxonomy may be needed.

Management strategies for cyanobacterial blooms in an impounded lowland river

The incidence of blooms of the toxic cyanobacterium Anabaena circinalis in the major rivers of the Murray–Darling Basin has been strongly related to river discharge, because they are more prevalent during times of low flows. This study into the causes of cyanobacterial blooms in Maude Weir pool on the Murrumbidgee River demonstrates that for Anabaena blooms to form, the water column needs to be persistently stratified, a condition that occurs only during times of low discharge. Based on the relationship between discharge, stratification, and bloom formation, four strategies that might be implemented to minimize the occurrence or impacts of cyanobacterial blooms in weir pools are suggested. These strategies include setting a minimum discharge, pulsing the discharge, changing the discharge height, and altering the depth of water withdrawal. These strategies are evaluated in turn using results from the field study and from the predictions of a numerical model of the weir pool system. Copyright © 2000 John Wiley & Sons, Ltd.

Factors affecting the seasonality and distribution of cyanobacteria and chlorophytes: a case study from the large lakes south of the Alps, with special reference to Lake Garda

Chlorophytes and cyanobacteria are among the most typical algal groups, contributing to the aesthetic appearance and quality of the epilimnetic waters of the deep (251–410 m) and large (6.5–49×109 m3) lakes located on the southern edge of the Alps (from oligo-mesotrophy to meso-eutrophy: Maggiore, Garda, Como, Iseo and Lugano). The results obtained from monthly surveys carried out in the largest of these lakes (Garda) have been reported in detail. The thermal stability of the water column and silica depletion were the main factors responsible for the decline of the great spring diatoms. The successive growth of Mougeotia sp. was favoured by its lower sinking velocity and resistance to the increasing grazing pressure. During summer, the maximum stability of the water column, with high vertical nutrient concentration gradients, determined a major algal differentiation with a typical increase, among others, of Chlorococcales at the surface and metalimnetic stratification of various Oscillatoriales. The development of oligotrophic blooms, caused by a rapid thickening at the surface of Anabaena in the eastern, sheltered basin, was also discussed in light of the trophic characterisation of Lake Garda. From autumn to spring, the decreasing light, the increasing mixing depth and nutrient availability favoured a progressive dominance of vertical homogeneous populations of Planktothrix (autumn) and colonial diatoms. The same functional groups of chlorophytes and cyanobacteria have been recognised in the studies published so far on the phytoplankton of deep southern subalpine lakes. In this respect, their morphometric and physical properties appear to constitute a sort of standardising factor, reducing the range of possible dominants. With increasing TP concentrations and biomass, filaments of Planktothrix and Planktolyngbya, along with the Chlorococcales, became important. The dominance of Mougeotia (one of the most characteristic features of these large lakes) appears restricted to a medium trophic range, whereas the distribution of the Chroococcales and other filaments ascribed to Pseudanabaena and/or Limnothrix is more irregular. The conspicuous presence of Aphanizomenon in Lake Lugano is typical and characteristic of a more productive lake. Among the Nostocales, a clear interpretation of the Anabaena blooms in lakes Garda and Iseo is complicated by the peculiar behaviour of filaments concentrating at the surface, which is apparently restricted, within the medium trophic range, to stable water columns.

Changes in the plankton community following introduction of filter‐feeding planktivorous fish

1. We conducted enclosure experiments in a shallow eutrophic lake, in which a biomass gradient of the filter‐feeding planktivore, silver carp, Hypophthalmichthys molitrix Valenciennes, was created, and subsequent community changes in both zooplankton and phytoplankton were examined.2. During a summer experiment, a bloom of Anabaena flos‐aquae developed (≈ 8000 cells mL−1) solely in an enclosure without silver carp. Concurrent with, or slightly preceding the Anabaena bloom, the number of rotifer species and their abundance increased from seven to twelve species (1700–14 400 organisms L−1) after the bloom in this fish‐free enclosure. Protozoans and bacteria were generally insensitive to the gradient of silver carp biomass.3. During an autumn experiment, on the other hand, large herbivorous crustaceans were more efficient than silver carp in suppressing the algae, partly because the lower water temperature (≈ 24 °C) inhibited active feeding of this warm‐water fish and also formation of algal colonies. Heterotrophic nanoflagellate and bacterial densities were also influenced negatively by the crustaceans.4. Correspondence analysis (CA) was applied to the weekly community data of zooplankton and phytoplankton. A major effect detected in the zooplankton community was the presence/absence of silver carp rather than the biomass of silver carp, whereas that in the phytoplankton community was the fish biomass before the Anabaena bloom, but shifted to the presence/absence of the fish after the bloom.

Horizontal Distribution of Geosmin in a Reservoir before and after Copper Treatment

Extracellular geosmin, chlorophyll a, and Anabaena circinalis filament density were measured at several stations on the surface of Lake Perris, a 1.62 × 108 m3 (131,450 acre-ft) reservoir in southern California, before and after the reservoir was treated with copper sulfate. Samples were collected from 22 stations within three hours the day before copper was applied by helicopter to the reservoir, which was undergoing an odorous Anabaena bloom. The day after the copper application, 19 stations were sampled over the same short period of time to see how the copper application affected parameter distribution on the reservoir surface. The presence of higher geosmin concentrations in some areas of the lake after copper treatment suggests that additional geosmin was released into the water when Anabaena cells were lysed by the copper.

Horizontal distribution of geosmin in a reservoir before and after copper treatment

Extracellular geosmin, chlorophyll a, and Anabaena circinalis filament density were measured at several stations on the surface of Lake Penis, a 1.62 × 108 m3 (131,450 acre-ft) reservoir in southern California, before and after the reservoir was treated with copper sulfate. Samples were collected from 22 stations within three hours the day before copper was applied by helicopter to the reservoir, which was undergoing an odorous Anabaena bloom. The day after the copper application, 19 stations were sampled over the same short period of time to see how the copper application affected parameter distribution on the reservoir surface. The presence of higher geosmin concentrations in some areas of the lake after copper treatment suggests that additional geosmin was released into the water when Anabaena cells were lysed by the copper.

Assessment of Environmental Conditions That Favor Hepatotoxic and Neurotoxic Anabaena spp. Strains Cultured under Light Limitation at Different Temperatures

Toxic cyanobacterial mass occurrences have caused animal poisonings worldwide and may pose a health hazard for humans. Strains of the genus Anabaena are either non-toxic or produce hepatotoxins, microcystins (MCYST), or neurotoxins (such as anatoxin-a). In order to study which growth conditions favor hepatotoxic vs neurotoxic strains and how production of toxins varies, we compared the responses of two microcystin- and two anatoxin-a-producing Anabaena strains in continuous turbidostat cultures, at different temperatures, under growth-limiting light levels. Growth rates consistently remained <0.8 divisions per 24 h. Differences were strain-specific and not associated with hepatotoxicity or neurotoxicity. Thus, differential adaptation of strains to temperature and to growth-limiting light levels cannot explain why, in some cyanobacterial water blooms, hepatotoxic strains, and in others, neurotoxic ones become dominant. A statistical analysis of field data showed that the most significant discriminating factors between different types of blooms were the concentrations of dissolved PO4-phosphorus and NO3-nitrogen. Anabaena blooms with unknown neurotoxicity associated with low PO4-phosphorus and high NO3-nitrogen concentrations. Among other Anabaena blooms, the hepatotoxic ones associated with the lowest, and most of the non-toxic ones with higher concentrations of PO4-phosphorus. Anabaena blooms that contained anatoxin-a and hepatotoxic Microcystis blooms showed tendencies towards the highest concentrations of PO4-phosphorus. Non-toxic blooms dominated by genera other than Anabaena occurred over a wide range of growth conditions. In turbidostat cultures, maximal production of microcystins correlated with maximal growth rates. Light regulated the production of MCYST-LR variants, and temperature affected the production of MCYST-RR variants. Anatoxin-a seemed to be produced most under temperatures and light levels slightly suboptimal for growth. Under low light, considerable amounts of extracellular anatoxin-a were detected while microcystins consistently remained intracellular.

ESTIMATION OF CHLOROPHYLL a FROM TIME SERIES MEASUREMENTS OF HIGH SPECTRAL RESOLUTION REFLECTANCE IN AN EUTROPHIC LAKE

We acquired high spectral resolution reflectance data in Carter Lake, a eutrophic oxbow on the Iowa–Nebraska border, from April 1995 to April 1996. Chlorophyll a, total seston, sestonic organic matter, Secchi depth, and nephelometric turbidity were determined for each respective spectral measurement. Changes in algal taxonomic structure and abundance coincided with the development and senescence of a midsummer through autumn bloom of Anabaena. Taxonomic structure was more diverse in late winter and spring when Synedra sp. (diatom) and several chlorophytes and dinoflagellates were present. Overall, chlorophyll a varied from about 20 to 280 μg·L−1, Secchi transparency from 18 to 74 cm, and seston dry weight from 11 to 48 mg·L−1 in February and September, respectively. Particulate matter completely dominated lake water light attenuation. Dissolved organic matter had low optical activity. The most sensitive spectral feature to variation in chlorophyll a concentration was the magnitude of the scattering peak near 700 nm. The 700‐nm peak correlated to chlorophyll concentration through the relationships between algal pigment absorption near 670 nm and the cell biomass and surface‐related scattering signal in the near infrared. An algorithm relating the height of the 700‐nm reflectance peak above a reference baseline between 670 and 850 nm to chlorophyll a was accurate and robust despite large variations in optical constituents caused by both strong seasonality in the algal system and short‐term variations in seston from wind‐induced sediment resuspension. The present algorithms were successfully used in other systems with different seasonality and productivity patterns. The coefficients of the models relating chlorophyll a and spectral reflectance variables appeared to be ecosystem specific: both the intercept and slope for the models in this study were moderately lower than for several other recently published results. We validated our algorithm coefficients with a second, independent dataset. The standard error for chlorophyll a prediction was ±28 μg·L−1.

Detection of an anatoxin-a(s)-like anticholinesterase in natural blooms and cultures of cyanobacteria/blue-green algae from danish lakes and in the stomach contents of poisoned birds

Ten natural bloom samples of cyanobacteria from the Danish lakes Knud sø (5), Ravn sø (4), and Salten Langsø (1) collected during 1993–1995 were assayed for toxicity by mouse bioassay, for acetylcholinesterase inhibiting activity by a colorimetric method, and for microcystins by enzyme-linked immunosorbent assay. In the mouse bioassay, seven samples were neurotoxic, two were non-toxic and one gave a protracted toxic response. One of the non-toxic and the single protracted toxic sample both contained anticholinesterase activity equivalent to 4 μg anatoxin-a(s) g −1. The neurotoxic samples contained equivalents to 20–3300 μg anatoxin-a(s) g −1. The highest anticholinesterase activities (equivalent to 2300 and 3300 μg anatoxin-a(s) g −1, respectively) were found in samples collected from Lake Knud sø in connection with bird-kills in 1993 and 1994. Small amounts of microcystins (0.1–0.9 μg g −1) were detected in all samples but one. All Lake Knud sø and Lake Ravn sø samples were dominated by Anabaena lemmermannii, and the Lake Salten Langsø sample by several species of Anabaena. Gel filtration profiles indicated similarity between the toxic component from the Lake Knud sø 1994 bloom with registered bird-kills and anatoxin-a(s) isolated from Anabaena flos-aquae NRC-525-17. Anti-cholinesterase-producing cultures of A. lemmermannii were isolated from the Lake Knud sø 1993 bloom. These laboratory cultures produced anatoxin-a(s) equivalents of 29–743 μg g −1. Other cultures of A. lemmermanii isolated from Lake Knud sø and Lake Ravn sø were hepatotoxic or non-toxic. Dead birds collected from Lake Knud sø during the neurotoxic 1993 Anabaena bloom possibly died from cyanobacterial toxicosis. The stomach contents contained colonies and single trichomes of Anabaena, and anticholinesterase activities equivalent to 21 89.7 μg anatoxin-a(s) kg g −1 body weight and microcystins (53–95 ng kg/g −1) were also detected.

Modelling cyanobacteria (blue-green algae) in the River Murray using artificial neural networks

In recent times, an apparent increase in the frequency and intensity of blooms of cyanobacteria (blue-green algae) in the River Murray (Australia) has caused widespread concern. When present in large numbers, they can cause serious problems for domestic, industrial, agricultural and recreational users of water, as they can produce toxins and impart undesirable tastes and odours to water. It is important to understand the relationship between the incidence of algal populations and the prevailing environmental conditions in order to prevent algal blooms from occurring. In this paper, artificial neural networks (ANNs) are used to model the incidence of a specific genus of cyanobacteria ( Anabaena sp. ) in the River Murray at Morgan, with the dual objectives of forecasting algal concentrations to give prior warning of impending blooms and to identify the factors that affect the blooms of Anabaena . The model inputs include weekly values of turbidity, colour, temperature, flow and the concentrations of total nitrogen, as well as soluble and total phosphorus. The results obtained are very promising as the model was able to forecast most major variations in Anabaena concentrations (timing and magnitude) for an eight-year period two weeks in advance. A sensitivity analysis carried out on the model inputs indicated that all input variables are important, with no one variable being dominant.