Phosphorus inactivation in a eutrophic lake by the direct addition of ferric aluminium sulphate changes in phytoplankton populations

SUMMARY. 1. Reduced total phosphorus concentrations in the summer which followed the addition of iron aluminium sulphate to White Lough failed to reduce significantly the phytoplankton. which continued to be dominated by Oscitlatoria agardhii var. isothrix Skuja.
This species was present throughout the 4 years studied, forming over 50% of the algal volume in 80% of samples. In contrast, species which occurred principally in the summer months were found to be severely curtailed when sediment release of phosphorus was suppressed.
2. The phosphorus: carotenoid ratio was used to assess the extent of phosphorus limitation because laboratory studies on O. agardhii var. isothrix showed that this ratio was a much beter indicator of cell phosphorus content than the phosphorus: chlorophyll a ratio. Reduced summer phosphorus concentrations in White Lough caused a transition from intermittent to continuous phosphorus limitation rather than a proportional reduction in the summer phytoplankton.
3. Reduced autumn grazing pressure by Daphnia hyatina Leydig allowed large algal populations to develop in the winters following phosphorus reduction despite a 50% decline in total phosphorus. The combination of increased winter phytoplankton and lower total phosphorus reduced soluble reactive phosphorus concentrations to less than 5,μg P l⁻¹ which in turn curtailed the spring diatom pulse.     

The importance of temperature conditioning to the respiration of natural phytoplankton communities

For natural phytoplankton communities from Kinnego Bay, Lough Neagh, the respiration rate at each of five laboratory temperatures was not constant through a season but showed changes which could be related to the lake temperature at the time of sampling. This effect was more marked when respiration rates were expressed per unit of chlorophyll a rather than per unit of algal volume, and provides one explanation for some improbably high respiratory Q₁₀ values that have been reported from field studies. Other factors besides temperature may be involved in producing the observed effect, and the ecological implications are not clear.     

Decomposition of aquatic biota and sediment formation: lipid components of two blue-green algal species and of detritus resulting from microbial attack

To simulate early stages in the diagenesis of algal material in lakes, microbial attack was allowed to proceed on natural populations of two blue-green algal species, Gloeotrichia echinulata (J. E. Smith) P. Richter and Oscillatoria agardhii var. isothrix Skuja. Changes in composition and abundance of the major lipid fractions were related to differences in the ease of microbial attack and to the effect of oxygen on the decay process. The loss of labile unsaturated compounds during diagenesis is consistent with the small amount of these compounds in most lake sediments.     

The significance of alkaline phosphatase activity in Lough Neagh

SUMMARY. A study of the eutrophic Lough Neagh was undertaken to ascertain the use of alkaline phosphatase activity as an indicator of phosphorus-limited algal growth. Lough Neagh and its surrounding rivers were monitored during 1974 and 1975. Alkaline phosphatase activity increased in the lough when the algal crop increased and depleted orthophosphate to a growth-limiting concentration. Enzyme activity was appreciable, however, at all other times of the year and may on occasions have been associated with zooplankton. The enzyme activity in the inflowing rivers was much greater than in the lough itself. Depending on enzyme stability the enzyme inputs from rivers could contribute significantly to the lough activity. Sewage disposal works effluent appeared to be a major source of enzyme activity in river water. As the alkaline phosphate activity in Lough Neagh resulted from several sources, increased activity could not be used as a reliable indicator of phosphorus limited algal growth.     

Predicting the spring algal biomass in Lough Neagh using time series analysis

1. Lough Neagh is a large eutrophic lake covering 387 km² with a mean depth of 8.9 m. It is an important natural resource, being the largest single source of potable water for Belfast, Northern Ireland. 2. This report examines the causes of the year-to-year variation in the April–June (spring) algal biomass, measured as chlorophyll a, for the period 1974–92. 3. The spring chlorophyll a declined following the introduction of a phosphorus (P) reduction programme at major sewage treatment works in 1981. However, since 1990 the chlorophyll a concentrations in the spring have increased. 4. Time series methodology was employed to develop a model which explained 76% of the annual variation in spring chlorophyll a concentrations. 5. The independent variables used in the multiple regression model were the previous year’s spring chlorophyll a concentration, soluble reactive P inputs for April–June and the particulate P concentration in the Lough during the previous summer.     

The past and future of phytoplankton in the UK's largest lake,Lough Neagh

Lough Neagh is the largest lake in the UK and has been extensively monitored since 1974. It has suffered from considerable eutrophication and toxic algal blooms. The lake continues to endure many of the symptoms of nutrient enrichment despite improvements in nutrient management throughout the catchment, in particular a permanently dominant crop of the cyanobacterium Planktothrix agardhii. This study examines the historical changes in the Lough, and uses the PROTECH lake model to predict how the phytoplankton community may adapt in response to potential future changes in air temperature and nutrient load. PROTECH was calibrated against 2008 observations, with a restriction on the maximum simulated mixed depth to reflect the shallow nature of the lake and the addition of sediment released phosphorus throughout the mixed water column between 1 May and 1 October (with an equivalent in-lake concentration of 2.0 mg m⁻³). The historical analysis showed that phytoplankton biomass (total chlorophyll a) experienced a steady decline since the mid-1990s. During the same period the key nutrients for phytoplankton growth in the lake have shown contrasting trends, with increases in phosphorus concentrations and declines in nitrate concentrations. The modelled future scenarios which simulated a temperature increase of up to 3 °C showed a continuation of those trends, i.e. total chlorophyll a and nitrate concentrations declined in the surface water, while phosphorus concentrations increased and P. agardhii dominated. However, scenarios which simulated a 4 °C increase in air temperature showed a switch in dominance to the cyanobacteria, Dolichospermum spp. (formerly Anabaena spp.). This change was caused by a temperature related increase in growth driving nutrient consumption to a point where nitrate was limiting, allowing the nitrogen-fixing Dolichospermum spp. to gain sufficient advantage. These results suggest that in the long term, one nuisance cyanobacteria bloom may only be replaced by another unless the in-lake phosphorus concentration can be greatly reduced.     

Changes in phytoplankton physiology and morphology in response to dissolved nutrients in Lough Neagh, N. Ireland

SUMMARY. Cell phosphorus and anthrone-reactive carbohydrate contents were measured over a 6-month period in the phytoplankton of Lough Neagh, N. Ireland. When all the soluble reactive phosphorus had disappeared from the water column, cell phosphorus contents began to decrease, reaching a minimum value of 0.4% ash-free dry weight. At this time, cell carbohydrate contents greatly increased and phosphorus sorption assays showed rapid uptake of orthophosphate. Following a presumed sediment phosphorus release, cell carbohydrate and phosphorus uptake rate decreased greatly and there was a subsequent increase in cell phosphorus content to 1.6% ash-free dry weight. As the cell phosphorus content declined, blue-green algal filaments became progressively shorter. On the basis of these responses it is likely that phosphorus limitation occurred over a 4-week period.     

Environmental factors influencing chlorophyll v. nutrient relationships in lakes

SUMMARY. 1. A model relating log chlorophyll a concentration to log epilimnetic total phosphorus (TP) concentration was re-examined based on: (a) comparative and temporal studies of four stratifying Wisconsin and other highly eutrophic temperate lakes; (b) comparative summer lake surveys from Iowa and Alberta.
2. Although P-limited, deeper lakes with long hydraulic residence times and low external and internal nutrient loading in summer had summer chlorophyll a yields below model predictions based on spring and summer epilimnetic TP concentrations.
3. For lakes with summer epilimnetic TP between 30 and 80 mg m⁻³, chlorophyll a concentrations exceeded model predictions based on summer TP. This relationship held even for Lake Delavan, Wisconsin, where the ratio of available N to P was unfavourably low during spring turnover, and where the trans-thermocline N:P flux ratio was sub-optimal for algal needs in early summer.
4. With increasing summer TP concentrations and/or increasing epilimnetic circulation depth (>5m), chlorophyll a concentrations fell below model predictions—independent of the potential for N-limitation. This plateauing in chlorophyll a response occurred at lower epilimnetic TP content (−2) in lakes with elevated non-algal light extinction coefficients. Using Tailing's algorithm for the'column compensation point' (algal photosynthesis = algal respiration over diel cycle), light limitation best explains this fall-off in chlorophyll a yield.
5. The failure of the Dillon & Rigler (1974) spring TP v . summer chlorophyll a model for these Wisconsin lakes is unrelated to N-limitation. Instead, it reflects internal adjustment in take TP in response to stratification and seasonal external P loading.     

A silica budget for Lough Neagh 1970–1972

In this study of the Lough Neagh catchment a relationship has been sought between the loads of dissolved and total silica and the flow of each of the six inflowing rivers. Two of these rivers-the Main and Six Mile Water-carry a higher ratio of dissolved to total silica than do the remaining four rivers due to the geology of the catchment. The multiple regressions of loads in tonnes against river flows in cubic metres show a linear relationship for dissolved but not for total silica. The explanation for this non-linearity has been sought in the river proaies, where the fall for the last 30 m varies from 5 to 25 km depending on the river. The rivers Main and Six Mile Water are the steepest, and linear relationships were obtained in their regressions while the shallowest profile was found in the Blackwater where non-linearity in the multiple regression was obtained. In Lough Neagh the dissolved silica content was gradually reduced from early winter through to springtime by diatom growth and a balance has been drawn up for this utilization. This reduction also compares with the quantity in the diatom crop at maximum based on cell volume measurements. The summer build up of the dissolved silica content of the lough has been shown to be greater than the quantity contributed by the inflowing rivers from late spring to autumn and the data show that the recycling of silica from some 65% of the frustules from the spring crop after its collapse would account for the summer increase.     

The life cycle of Chironomus anthracinus in Lough Neagh

Chironomus anthracinus generally had a one year life cycle at 8 m depth in Lough Neagh, during the study period. In spring 1975 however only a proportion of the population emerged as usual, the rest remaining a second year in the benthos to emerge in 1976. The mean dry weight of larvae just before emergence was 2.8 mg in 1971 and 2.6 mg in 1974 but only 1.6 mg in 1975, and it seems that larvae must reach a weight of at least 1.0 mg in the early spring to emerge that year. Considering temperature, oxygen saturation and standing crop of phytoplankton as factors influencing the life cycle, mean monthly chlorophyll a (phytoplankton) was weakly correlated with mean monthly increase in weight and % monthly weight increase could be related to temperature.     

Notes on the growth and sporulation of a natural population of Aphanizomenon flos-aquae

Aphanizomenon flos-aquae contributed regularly to the summer phytoplankton in Kinnego Bay, a highly eutrophic part of Lough Neagh. Growth of the alga was variable in different years, and was apparently related to the duration of depletion of dissolved nitrate-nitrogen in the water. Although four growth pulses occurred during the study period, sporulation was only observed during one of these, when the population was very dense although evenly distributed down the water column. It seems that Aphanizomenon overwintered in Kinnego Bay as vegetative filaments and that production of akinetes was not necessary for perennation of the species. The observed sporulation in 1973 may have been induced by the high pH values caused by dense phytoplankton crops; no other measured environmental factors were particularly unfavourable to growth of Aphanizomenon at this time.     

Seasonal changes in sedimenting material collected by high aspect ratio sediment traps operated in a holomictic eutrophic lake

Cylindrical sediment traps with an aspect ratio (height (60 cm): diameter (5.1 cm)) of 11.8 were located 1 m above the surface sediment by a rigid metal framework support. Traps were exposed in Lough Neagh for one year, from May 1978. Each trap collected between 11 and 12 cm of faintly laminated sediment. One 12 cm sediment column was examined using conventional palaeolimnological techniques of core extrusion and analysis. The algal record in trapped sediment is shown to correspond with successional changes in phytoplankton abundance in the lake during the year of study. The sediment accumulation rate measured by the traps is an order of magnitude greater than that measured in dated sediment cores and redeposited and inwashed sediment formed the bulk of trapped material. However, the value of these high aspect ratio traps is that they provide a continuous but qualitative account of compositional changes in sedimenting material through time. Their potential as long term biological monitoring devices is emphasized.     

An Evaluation of Phosphorus-Chlorophyll-Phytoplankton Relationships for Lakes

There is considerable variation in published phosphorus-chlorophyll relationships, some of which can be accounted for by variation in sampling and analytical techniques. As well, there can be significant inherent variability in a single phosphorus-chlorophyll relationship under even ideal sampling and analytical conditions. A survey of the literature shows that the cellular chlorophyll content of algae ranges over two orders of magnitude (0.1—9.7% of fresh weight). Average daily radiation intensity and nutrient availability (especially N) appear to be the major factors controlling chlorophyll content of algal cells. The relationship between total phosphorus and phytoplankton biomass for a group of Ontario lakes is better defined if summer average cell volume is used instead of average chlorophyll a concentration.     

Floating meadow epiphyton: biological and chemical features of epiphytic material in an Amazon floodplain lake

SUMMARY. 1. Quantities and the chemical composition of epiphyton on the roots of floating aquatic macrophytes were measured in Lake Calado, an Amazon floodplain lake. Growth of epiphytic algae following physical disturbance and losses of epiphyton due to grazing and storms were investigated.
2. Deposition of silt from invading river water decreased chlorophyll and nutrient content (%C, %N, %P) of epiphyton during rising water. N:P ratios of epiphyton indicated that proximity to the river increased supplies of phosphorus. Attached algal biomass per unit root tissue was higher overall during the falling water period, when light was greater, storms less frequent, and new host plant tissue produced more slowly.
3. Epiphytic algal biomass at the margins of floating meadows exceeded that of the phytoplankton in the open water on a per unit area basis. Increases in attached algal chlorophyll ranged from two- to ten-fold over 1 week. Artificial denudation of roots was followed by rapid regrowth of attached algae, leading, after I week, to four-fold increases in chlorophyll over the pre-denuded state.
4. Wind-blown macrophytes experienced an episodic loss of 70% of epiphytic material in less than 1 h. Particulate material lost from roots grazed by snails included root tissue and contained significantly more carbon than material lost from ungrazed roots.     

Estimation of the in-situ settling velocity of particles in lakes using a time series sediment trap

SUMMARY 1. We outline a simple method for estimating the in-situ settling velocity of particles in wind disturbed lakes. The total sedimentation rate (primary and secondary sedimentation) was measured during storm events in two lakes using a time series sediment trap.
2. The sediment trap had 12 sample bottles which were programmed to open for periods of 3–4 h, giving total deployment times ranging from 36 to 48 h.
3. Wave mixed layer theory was used to infer if and when sediment resuspension occurred and the first order rate equation was used to estimate the settling velocity of resuspended particles.
4. The settling velocity during three resuspension events in Lough Neagh and one in Lough Macnean were 34.3, 29.5, 24.1 and 77.3 m day⁻¹, respectively. These are similar to values obtained using recent in-situ video imaging in marine environments, but much larger than previous lake sediment trap studies.     

The influence of daylength,light intensity and temperature on the growth rates of planktonic blue-green algae

The in vitro growth rates under continuous light of the four dominant blue-green algae in Lough Neagh, Anabaena flos-aquae Bréb., Aphanizomenon flos-aquae Ralfs fa. gracile Lemm., Oscillatoria agardhii Gom. and Oscillatoria redekei van Goor were slower than in situ rates from Lough Neagh that had been corrected for hours of light received by the algae. However, by culturing on a 6: 18 light-dark cycle in vitro growth rates were obtained that were similar to the in situ rates. Under continuous light small species showed the fastest growth with Oscillatoria redekei the dominant species. However, this pattern was almost completely reversed under the light-dark cycle with Oscillatoria redekei only exhibiting the fastest growth rate under low light conditions. This observation showed agreement with Lough Neagh field data which showed that Oscillatoria redekei reached its maximum crop in April while the other three species were dominant during the summer months. Compared to the generally assumed high thermal tendency of blue-green algae the temperature maxima of the four species were low. No growth was observed at 35°C for any species while Anabaena flos-aquae was severely inhibited at 25°C.     

Contribution of grazing in phytoplankton overall losses in a shallow French lake

SUMMARY 1. Based on data for ¹⁴C-primary production and biomass changes in a small and shallow lake (Créteil Lake, France), overall phytoplankton losses were calculated through an annual cycle (November 1985-October 1986). The summer period in 1986 is compared with two other summer periods in 1985 and 1980, these two years corresponding to extreme levels of algal biomass.
2. Independent from the trophic state of the lake, phytoplankton populations were dominated by small-sized species (<20 μm); their high growth rate (maximal in May and June: 0–8 day⁻¹) was characteristic of nanoplanktonic natural populations.
3. The positive correlation between phytoplankton losses and production indicates a close coupling between growth and loss processes.
4. With a high filtering rate (0.22 day⁻¹ as an annual average), zoo-plankton impact is considerable at any time of the year but especially in late summer, when grazing losses exceeded primary production.
5. Despite the uncertainty concerning the meaning of ¹⁴C-primary production, the persistence of small algae throughout the year implies that a great part of the phytoplankton production was harvested by grazers which led to a recycling of organic matter within the water column.     

Lake age and water level affect the turbidity of floodplain lakes along the lower Rhine

SUMMARY 1. We sampled a set of 93 lakes situated in the floodplains of the lower River Rhine in search for morphometric and other factors that explain their variation in clarity.
2. Lakes with a drop in summer water level were less turbid at the time of sampling, mainly because of a lower concentration of inorganic suspended solids (ISS).
3. We also found that older lakes were more turbid than younger lakes and that this was largely because of an increase in phytoplankton.
4. Water clarity was positively related to lake depth and the presence of vegetation.
5. Model calculations indicated that the underwater light climate was strongly affected by chlorophyll and ISS, the latter being the dominant factor affecting Secchi depth. Dissolved organic carbon (DOC) was less important.
6. The high concentration of ISS suggests that intensive resuspension occurs in most of the lakes. Using a simple wave model, and assuming that vegetation protects sediments against resuspension, we could eliminate wind resuspension as an important process in 90% of the lakes, leaving resuspension by benthivorous fish as probably the most important factor determining transparency.
7. Chlorophyll a concentration showed a strong positive correlation to ISS concentration, suggesting that resuspension may also have a positive effect on phytoplankton biomass in these lakes.
8. In conclusion, in-lake processes, rather than river dynamics, seem to be driving the turbidity of floodplain lakes along the lower River Rhine.     

Temporal variability in algal biomass and primary productivity in Florida lakes relative to latitudinal gradients,organic color and trophic state

Seasonal patterns in primary productivity and algal biomass in subtropical Florida lakes along increasing gradients of both dissolved organic color and phytoplankton biomass are presented. Chlorophyll a concentrations and gross primary productivity generally reached maxima during the summer and were most depressed in winter months, regardless of color or trophic classification. Primary productivity was more strongly correlated with chlorophyll a, nutrient concentrations and water clarity in clearwater (< 75 Pt units) than in colored (> 75 Pt units) systems. Amplitudes in algal biomass were considerably smaller than temperate lakes. Variability in primary production in Florida lakes was intermediate to patterns in the temperate zone and tropics, but was more closely aligned to northern latitudes. Within the Florida peninsula, variability of primary productivity decreased from north to south and corresponded to latitudinal gradients in climatic regimes.     

Preliminary results on phosphorus reduction in Lough Neagh - Assessing the effect against a background of change

Lough Neagh is a large (387 km²), shallow (mean depth 8.9 m) eutrophic lake. The phosphorus budget is described in order to illustrate that 75% of the particulate phosphorus input sediments to the bottom and that there is little retention of SRP in the lake. 54% of the SRP loading comes from sewage works and this is being reduced by phosphorus reduction at the major sewage works. There has been a downturn in the maximum algal crops, particularly of blue-green algae. However, there is an underlying upward trend in SRP loading and the paper stresses the importance of analysing trends in nutrient budgets to get an accurate evaluation of reduction strategies.