Seasonal dynamics of phytoplankton and phytobenthos,and associated chemical interactions,in a shallow upland lake (Malham Tarn,northern England)

Seasonal changes of phytoplankton were followed over 3 years (1985–87) in a shallow, unstratified and calcareous upland lake.The phytoplankton was of low to moderate abundance and generally dominated by phytoflagellates. Seasonality involved a winter minimum of abundance, a spring maximum of diatoms, and often brief increases in summer that included blue-greens, especially the colonial Gloeotrichia echinulata. Some components were of benthic origin. Seasonal growth of the main component of the phytobenthos, Chara globularisvar. virgata, caused a regular summer depletion in lake water of Ca²⁺ and HCO₃ ^- (alkalinity) by associated CaCO₃ deposition, and a more extreme (and unusual) depletion of K⁺. Chemical analysis of Chara biomass and of underlying sediments indicated a large benthic nutrient stock, much surpassing that represented by the phytoplankton. Growth in this biomass, and the magnitude of water-borne inputs, influenced the removals of Ca²⁺, K⁺ and inorganic N. The phytoplankton was probably limited by a low-P medium, to which co-precipitation of phosphate with CaCO₃ may have contributed. A vernal depletion of Si was probably limiting to diatom growth, and appeared to be mainly induced by benthic rather than planktonic diatoms. Examples of long-term change in composition of the phytoplankton and phytobenthos are noted and discussed in relation to the interaction of these components, nutrient enrichment, and possible alternative stable states.     

Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications

Diatoms are a major group of phytoplankton that account for approximately 40% of the ocean carbon fixation and the vast majority of biogenic silica production through the construction of their cell walls (termed frustules). These frustules accumulate and are partially preserved in the ocean sediments. Diatom growth and nutrient utilization in high‐nitrate, low‐chlorophyll regions of the world’s oceans are mostly regulated by iron availability. Diatoms acclimate to iron limitation by decreasing cell size. The associated increase in surface area‐to‐volume ratio and decrease in diffusive boundary layer thickness may improve nutrient uptake kinetics. In parallel, cellular silicon (Si) contents are elevated in iron‐limited diatoms relative to nitrogen (N) and carbon (C). Variations in degree of silicification and nutritional requirements of iron‐limited diatoms have been hypothesized to account for higher cellular Si and/or lower cellular N and C, respectively. However, in some diatoms, frustule silicification does not significantly change when cells are iron‐limited. Instead, changes in the Si‐containing valve surface area relative to volume within these diatoms is hypothesized to be responsible for the variations in the cellular Si : N and Si : C ratios. In particular, some examined iron‐limited pennate diatoms have reduced widths relative to their lengths (i.e. lower length‐normalized widths, LNW) compared to iron‐replete cells. In the pennate diatom Fragilariopsis kerguelensis, the mean LNWs of valves preserved in sediments throughout the Southern Ocean (a well‐characterized iron‐limited region) is positively correlated with satellite‐derived, climatological net primary productivity in the overlying waters. Because of the specific morphological changes in pennate diatom frustules in response to iron availability, the valve morphometerics (e.g. LNWs) can potentially be used as a diagnostic tool for iron‐limited diatom growth and relative changes in the Si : N (and Si : C) ratios in extant diatom assemblages as well as those preserved in the sediments.     

The first decade of oligotrophication of Lake Constance

In Lake Constance, after several decades of cutrophication, a decrease in phosphorus loading over the last decade has lead to a partial recovery from eutrophication. Here we analyse the shift in the taxonomic composition of phytoplankton during the first decade of oligotrophication in Lake Constance. During the 1980s, spring total P concentrations decreased from ca. 130 to less than 50 ·l^–1. This decrease was reflected by an approximately proportional decrease in summer phytoplankton biomass while spring phytoplankton biomass seemed unresponsive. Major taxonomic changes occured during both growth seasons. In spring, the proportion of diatoms, green algae and Chrysophyta increased while the proportion of Cryptophyta decreased. The summer trend was very different: the relative importance of diatoms decreased and Cryptophyta and Chrysophyta increased, while Chlorophyta reached their peak around 1985. These trends are also analysed at the genus level. Comparison with taxonomic trends during the eutrophication period shows the expected reversals in most cases. Comparison with other lakes shows general similarities, with the notable exception that Planktothrix rubescens has never been important in Lake Constance. The increase of diatoms during spring is attributed to their improved competitive performance with increasing Si:P ratios. Their decrease during summer is explained by the increasing silicate removal from the epilimnion by increasing spring populations.     

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.     

Origin and succession of phytoplankton in a river-lake system (Spree,Germany)

The River Spree (Germany) flows through an impoundment and several shallow lakes in its middle and lower course. In this river-lake system, the seasonal and longitudinal dynamics of dominant phytoplankton populations were studied in relation to retention time of water, mixing conditions and nutrient supply from 1988–92. Some phytoplankton species populated the same river section for weeks or months each year at their season. Such stable populations have to origin from river zones functioning like mixed reactors. In the Spree system, centric diatoms originated from an impoundment and filamentous cyanobacteria from a flushed lake with longer retention time of water. Downstream, biomass and composition of phytoplankton altered nearly simultaneously along the system.The fate of planktonic organisms washed from mixed reactors into the flow depended on the conditions at the zones of origin. During spring, populations dominating phytoplankton communities of the well-mixed lakes grew further under river conditions. However the biomass of summer species, adapted to intermittent stratification, was halved along the river course. These seasonal differences were probably caused by lower maximum growth rates of summer species and enhanced losses (photorespiration, sedimentation or grazing of benthic filter feeders, but not of zooplankton) of algal populations under river conditions in summer.Phytoplankton assimilation, settlement of diatoms, or denitrification caused declining (probably growth limiting) concentrations of dissolved inorganic phosphorus (spring), silicon (early summer) or nitrogen (summer) along the river course, respectively. The minimum content of DRP was often followed by a clear-water phase. Reduced DSi supply selected against diatoms and additional DIN shortage favoured N₂-fixing cyanobacteria in the last lake of the system.R-strategists (sensu Reynolds) were selected in both the flushed, shallow lakes and the lowland river. In general, the biomass of cyanobacteria increased within the lakes and declined along the river course. Some diatom populations grew in the river, but were grazed or settled down in the lakes. Beside this general picture, different populations from the same phylogenetic group did not necessarily perform in similar ways.     

Polymixis of a shallow lake (Groβer Müggelsee,Berlin) and its influence on seasonal phytoplankton dynamics

The polymictic properties of Lake Müggelsee, a eutrophic shallow lake in Berlin, are described by the water column stability (N ²) and gradients in saturation of oxygen at the deepest site of the lake (7.5 m). Mixing and stratification changed irregularly up to 7 times during the vegetation season (April to September), as was indicated by all of the stratification parameters. Thermally stable conditions generally lasted 1–2 weeks. A maximum of 5 weeks stratification was observed in 1982.In order to investigate the response of algal development, the internal rates of change of the dominant algal species in the lake during the vegetation period were estimated from weekly measurements of phytoplankton biomass from 1980 to 1990. The necessity taking a mixed sample in a shallow lake is discussed. The polymictic properties favoured the development of specific blue-green algal species; there dominance was also favoured by the trophic conditions. Among the dominant blue-greens the growth of Limnothrix redekei was independent of polymixis whereas stratification supported the starting conditions for the summer blue-greens Aphanizomenon flos-aquae and Planktothrix agardhii. After these algae reached a distinct level of biomass, they grew under mixing as well as under stratified conditions.For the development of solitary centric diatoms during summer regulation by growth restriction through nutrient limitation, esp. dissolved silicon was more important. However, Melosira sp. developed well under stratified conditions but collapsed due to increased sinking losses when the water column became too stable.An attempt is made to apply Reynolds' possibility matrix of the most likely phytoplankton assemblages as a function of nutrients and mixing in the shallow Lake Müggelsee.     

Causation of reversal simultaneity for diatom biomass and density of <Emphasis Type="Italic">Phormidium tenue</Emphasis> during the warm season in eutrophic Lake Barato,Japan

The diatom biomass of Lake Barato, as measured from July to September, decreased simultaneously with an increase in filament density of Phormidium tenue after 1997. There was a high negative correlation between the diatom biomass and the densities of P. tenue (r² = 0.928). Although total nitrogen (TN) and total phosphorus (TP) concentrations decreased from 1996, TN:TP ratio increased from 1997 because the TP concentration became markedly lower. The decrease in diatom biomass might have been due to the loss in phosphorus available for algae. Because the increase in density of P. tenue might have been due to the decrease in diatom biomass, experiments using a growth inhibitor for diatoms were performed to examine whether the density of cyanobacteria increases without diatom growth. Samples of the lake water collected in three seasons (August and October 1998, May 1999) were incubated with and without germanium (Ge) as a growth inhibitor of diatoms. The increase in density of P. tenue was inhibited concurrent with the increase in diatom biomass in the first and middle stages of incubation without the addition of Ge in August 1998 and May 1999. In contrast, a higher density of P. tenue was observed in the incubation with diatom growth inhibited by Ge over the same period. These results suggest that diatoms have an effect in restraining the growth of P. tenue.     

Response of a shallow boreal lake to recent nutrient enrichment – implications for diatom-based phosphorus reconstructions

Water quality of the shallow, mesotrophic, and macrophyte-dominated Lake Kaljasjärvi has been monitored at three to four year intervals since 1978. During the monitoring period, surface-water total phosphorus (TP) concentrations have typically varied between 20 and 25 g P l^–1. However, elevated total phosphorus concentrations were measured in 1987, 1991, and 1999. Diatom-based reconstruction of the historical lake-water TP concentrations was therefore employed to study the recent development of the lake. However, the diatom-TP model did not predict the high measured phosphorus concentrations despite the changes observable in diatom assemblages. In addition, the ratio of sedimentary diatom remains to chrysophycean stomatocysts declined towards the top of the sediment core, indicating decreasing trophy rather than eutrophication. Analysis of sedimentary pigments and phosphorus fractions, used to examine further the changes, also produced results that contradicted the simple eutrophication hypothesis. In particular, the proportion of chlorophyll derivatives instead of carotenoids increased and there was a rise in the concentration of refractory instead of NaOH-extracted phosphorus. These features appear to be related to the extensive littoral areas of the lake since enhanced littoral production can explain both the observed changes in sediment chemistry and the low diatom-inferred TP (DI-TP). Littoral primary producers are suggested to have benefited from the increased phosphorus inputs to the lake, transferring some of the phosphorus to the detrital pool and contributing to the increased pigment concentrations of sedimentary organic matter. High proportions of non-planktonic diatoms in the samples lower DI-TP because periphytic taxa are assigned low TP optima in the inference models used. Abundant aquatic macrophytes may also have made the lake resistant to eutrophication by assimilating nutrients, providing refuge for zooplankton, and having an allelopathic effect on phytoplankton. Since 1980, however, the sedimentary diatom assemblages also indicate increasingly eutrophic conditions. Additional loading from numerous cottages during the last 20 years seems to have caused observable changes in the phytoplankton communities.     

Sediment diatom assemblages and composition of pore-water dissolved organic matter reflect recent eutrophication history of Lake Peipsi (Estonia/Russia)

A paleolimnological approach was used for the assessment of the recent eutrophication history and identification of possible reference conditions in the large, shallow, eutrophic Lake Peipsi. Lake Peipsi is the fourth largest lake by area, and the largest transboundary lake in Europe, being shared between Estonia and Russia. Lake Peipsi has been anthropogenically impacted over a longer time-scale than that covered by instrumental limnological monitoring. The ²¹⁰Pb record and down-core distribution of fly-ash particles in the 40-cm core from the middle part of the lake suggest 130 years of sediment accumulation. Diatom assemblages indicate alkaline mesotrophic conditions and a well-illuminated water column, sediment pore-water fluorescence index values suggest low autochthonous productivity and a stable aquatic ecosystem similar to natural reference conditions during the second half of 19th and early 20th century. Near-synchronous stratigraphic changes including the expansion of the eutrophic planktonic diatom Stephanodiscus parvus, the appearance of new species associated with eutrophic lakes and the decrease in the relative abundance of littoral diatoms, together with changes in the fluorescence properties of sediment pore-water dissolved organic matter, imply increased nutrient availability, enlarged phytoplankton crops, reduced water-column transparency and the onset of human-induced disturbances in the lake since the mid-20th century. The most conspicuous expansion of eutrophic planktonic diatoms and maximum concentration of siliceous microfossils occur simultaneously with changes in the fluorescence indexes of pore-water dissolved organic matter, indicating a pronounced increase in the contribution of autochthonous organic matter to the lake sediment. This implies that nutrient loading and anthropogenic impact was at a maximum during the 1970s and 1980s. Sedimentary diatom flora may reflect a reduction of phosphorus loading since the 1990s. However, the absolute abundance of planktonic diatoms and sediment pore-water fluorescence index values vary greatly implying that the lake ecosystem is still rather unstable.     

Phytoplankton and its dynamics in two tropical lakes: a tropical and temperate zone comparison

Temporal patterns of phytoplankton biomass and community structure are described for two Kenyan lakes and subsequently compared with patterns reported in other tropical and temperate lakes. Lake Naivasha had a lower and more seasonally variable (10×) biomass, with a seasonal shift between diatoms and blue-greens, while the L. Oloidien biomass was less variable (3.7×) and dominated by blue-greens. Biomass and chlorophyll a were strongly correlated and in turn were coupled to the level of total phosphorus. A total of 143 and 94 taxa were described for L. Naivasha and L. Oloidien, respectively.The comparative analysis showed: a) a paucity of exclusively tropical species; b) that more than 30 percent of the species in two highly saline Kenyan lakes were also present in the two freshwater lakes; c) no evidence for a postulated decline of phytoplankton species abundance with latitude from the temperate zone to the tropics; d) that the low fraction of chrysophyte biomass in tropical lakes is a function of trophy rather than of latitude; e) that the fraction of chlorophyte biomass in tropical lakes is generally higher than in temperate lakes; f) that the proportion of nannoplankton in the two Kenyan freshwater lakes is not different from that in temperate lakes of the same trophy; g) that seasonal or annual biomass oscillations in the tropics are not systematically lower than in the temperate zone; h) evidence for large inter-year difference in the max.:min. biomass ratio in the only tropical lake (L. Naivasha) for which such data are available; i) that an average biomass ratio appears predictable for tropical lakes from the proportion of the sediment surface in contact with epilimnetic water. Overall, no evidence was found that the freshwater tropical phytoplankton composition or dynamics differ in any fundamental fashion from that observed in the temperate lakes during the summer.Contribution number 147 of the Limnology Research Centre, McGill University.     

Resource Ratios and Phytoplankton Species Composition in a Strongly Stratified Lake

The epilimnetic phytoplankton and its relations to nutrient content in Lake Verevi through the whole vegetation period in 2000 were studied. Lake Verevi (surface 12.6 ha, mean depth 3.6 m, maximum depth 11 m) is a hypertrophic hard-water lake, where the so-called spring meromixis occurs due to an extremely warm spring. Most dissolved nutrients in the epilimnion were low already in spring, and their concentrations were quite stable during the study period. The concentration of total silicon was very low in spring but increased rapidly in summer. Total phosphorus followed the pattern for stratified eutrophic lakes, and total nitrogen was quite high. The stoichiometric N:P ratio fluctuated between 25 and 81. The dynamics of phytoplankton biomass with a spring peak from April to May and a late summer peak from July to August is typical of Estonian eutrophic lakes. Green algae and chrysophytes occurred in the phytoplankton throughout the vegetation period. The spring peak was dominated by diatoms (Synedra ulna and Synedra acus var. angustissima) and the summer peak was caused by Aphanizomenon klebahnii and Ceratium hirundinella. The study showed that in physically stratified systems, the total concentration of limiting resources and plain physical factors (light and temperature) may be more important in the determination of phytoplankton dominants than different resource ratios. A combination of light and temperature optimum, along with nutrient utilization and transport capacity, effectively segregates phytoplankton species and can be used for the explanation of seasonal succession pattern.     

Seasonal changes in silicon content of diatoms estimated from the ratio of particulate silicon to diatom volume under silicon sufficiency in diatom-rich Lake Barato

To clarify the changes in Si content of diatoms, the particulate silicon (PSi) concentration and total diatom volume (TDV) were determined in Lake Barato, Japan, from April to July 1998–2000. The soluble reactive silicon (SRSi) concentration decreased markedly with the rapid increase in TDV in May and June in all three years, although the value did not fall below that at which diatom growth might be limited. The proliferation of small discoid diatoms contributed to the decrease in SRSi concentration each year. The Si content of diatoms may not be constant as indicated by the changes in PSi:TDV ratio. The low PSi:TDV ratio and the fact that PSi concentration was lower than diatom PSi concentration (calculated from the volume of diatom species) accompanying the decrease in TDV suggests the possibility of a disturbance in the silicification in May and June 1999. These parameter changes accompanying the increase in TDV suggest that the silicification did not catch up with the cell division in early April 1998, early May 1999, and mid-June 2000. In addition, the PSi:TDV ratio increased rapidly and showed large fluctuations in July 1998 and 1999. This may have been caused by a change in dominant species from small discoid diatoms to Aulacoseira granulata because of the differences in Si content per unit cell volume.     

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 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.     

The recovery of a very shallow eutrophic lake, 20 years after the control of effluent derived phosphorus

1. Monitoring at fortnightly to monthly intervals of a very shallow, lowland lake over 24 years has enabled the time course of recovery from nutrient enrichment to be investigated after high external P loading of the lake (>10 g P m^?2 year^?1) was reduced between 1977 and 1980. 2. The lake showed a relatively rapid response during the spring and early summer, with a reduction in phytoplankton biomass occurring after 5 years when soluble reactive phosphorus concentration was <10 μg L^?1. 3. However, during the later summer the response was delayed for 15 years because of sustained remobilisation of phosphorus from the sediment. The greater water clarity in spring and a gradual shift from planktonic to benthic algal growth may be related to the reduction in internal loading after 15 years. 4. Changes in the phytoplankton community composition were also observed. Centric diatoms became less dominant in the spring, and the summer cyanobacteria populations originally dominated by non‐heterocystous species (Limnothrix/Planktothrix spp.) almost disappeared. Heterocystous species (Anabaena spp. and Aphanizomenon flos‐aquae) were slower to decline, but after 20 years the phytoplankton community was no longer dominated by cyanobacteria. 5. There were no substantial changes in food web structure following re‐oligotrophication. Total zooplankton biomass decreased but body size of Daphnia hyalina, the largest zooplankton species in the lake, remained unchanged, suggesting that the fish population remained dominated by planktivorous species. 6. Macrophyte growth was still largely absent after 20 years, although during the spring water clarity may have become sufficient for macrophytes to re‐establish.     

The diversity of relationships between Antarctic sponges and diatoms: the case of Mycale acerata Kirkpatrick, 1907 (Porifera,Demospongiae)

The diatom assemblage associated with the Antarctic sponge Mycale acerata was studied through an analysis of the diatom frustule and pigment concentrations in both the sponge ectosome and choanosome. Sponges were sampled weekly from November 2001 to February 2002 at Terra Nova Bay, Antarctica, at a depth of 25–35 m. The most abundant diatoms were Porannulus contentus, Fragilariopsis curta, Thalassiosira cf. gracilis, T. perpusilla and Plagiotropis sp. High abundances of P. contentus were found on the sponge ectosome up to the beginning of November, before the ice melted, while later frustules were incorporated inside, indicating that P. contentus lives epibiontically on M. acerata and represents a potential food source for the sponge. The presence of other diatom species was mainly related to the summer phytoplankton bloom. The sponge incorporates diatoms from the water column and utilises them as a food source, accumulating frustules inside the choanosome. The lack of planktonic diatom frustules at the beginning of the summer indicates that they are expelled or dissolved during the cold season.     

Seasonal variation in the biochemical composition of particulate material collected by sediment traps at Signy Island, Antarctica

Particulate material recovered over an 18-month period from sediment traps deployed at a shallow-water nearshore Antarctic site was analysed for photosynthetic pigments, aliphatic hydrocarbons and fatty acids. All components showed a distinct seasonal variation, with high recovery rates during the summer open-water phytoplankton bloom and low rates under winter fast ice. The amount of trapped material differed between the two summers, indicating inter-annual variability of vertical flux associated with differences in the intensity of the summer phytoplankton bloom. Particulate material trapped in summer was dominated by that which originated in diatoms. High recoveries of chlorophyll a, fucoxanthin, n-C_21:6 hydrocarbon, 20:5(n-3) fatty acid and shorter chain (C₁₅–C₂₄) aliphatic hydrocarbons all pointed to a significant summer flux of ungrazed diatoms. There were, however, also signals of zooplankton grazing activity (notably pyrophaeophorbide a), and the presence of C18:4(n-3) and C22:6(n-3) fatty acids suggested a small flux of material from flagellates and other sources. Longer chain n-alkanes (C₂₅–C₃₄) indicative of nanoplankton were detected all year, but there was no significant deposition of zooplankton material in any sample. The major recovery rate of photosynthetic pigments was in late summer (February to April), and the major grazing signal occurred after the peak of the summer diatom bloom. Most of the diatom bloom appeared to settle out from the water column without being grazed. The major seasonal contrast in the biochemistry of the trapped material was the dominance of the diatom signature in summer, and in winter the predominance (but at much lower recovery rates) of material from nanoplankton. Received: 2 March 1998 / Accepted: 12 June 1998     

Climate forcing of diatom productivity in a lowland,eutrophic lake: White Lough revisited

1. In cultural landscapes, lake response to climate can be masked by land‐use change and nutrient loss from their catchments. Palaeolimnological methods were used to reconstruct the ecological response of diatoms in a eutrophic lowland lake (White Lough, Co. Tyrone, Northern Ireland) to altered nutrient P loading and precipitation variability over c. 100 years. 2. ²¹⁰Pb‐dated sediment cores were analysed to determine diatom assemblage variability, biogenic silica concentration, geochemical phosphorus concentration and accumulation rate. Manure P and agricultural N surplus data were collated from documentary sources. Long‐term trends in annual temperature and precipitation were derived from the Armagh Observatory. 3. Diatom community turnover from 1890 until c. 1960 was limited, and assemblages were dominated by Aulacoseira subarctica; after this date, changes primarily reflected a eutrophication sequence owing to increased diffuse nutrient inputs associated with intensification of land use (external P loading increased by a factor of three). 4. Diatom and biogenic Si profiles were compared with North Atlantic Oscillation (NAO) records, an index of regional weather patterns. Biogenic Si exhibited a c. 7‐year cycle, which tracked a cycle of similar timescale in the Armagh climate record for dry summers. In turn, this cycle was related to the variation in the NAO. 5. Monitoring data from 1971 to 2007 of nitrate exports from the Blackwater River showed that these too followed a roughly 7‐year cycle at least up to 2000, in which dry summers were followed by sharp increases in nitrate export. It is argued that diatom production in White Lough reflects the cyclic behaviour in nitrate loading and the constraints that nitrogen availability places on the spring diatom bloom in a lake that is dominated by cyanobacteria.     

The eutrophication of Cayuga Lake: a historical analysis of the phytoplankton's response to phosphate detergents

SUMMARY. Phytoplankton surveys of Cayuga Lake over the past 60 years indicate that summer algal crops have doubled and spring algal crops have increased 20-fold. The pattern of algal seasonal succession has changed from a summer maximum comprised of a mixture of diatoms, greens and blue-greens to a spring maximum comprised of diatoms followed by diminished summer levels with dominance by greens and blue-greens. Summer algal standing crop and the shift to a spring pulse are shown to be related phenomena resulting from the pattern of phosphorus loading to the lake. The combination of data on algal biomass, chlorophyll and Secchi disc transparency serve to describe the long-term trend in the lake's trophic state and demonstrate a large increase in algal standing crop in the period roughly coincidental with the advent of phosphorus-based detergents. The increase in algal standing crop from pre-1940 levels to present levels closely corresponds with that predicted by the Oglesby—Schaffner regression model from the increase in phosphorus loading due to phosphorus-based detergents.     

Identifying from recent sediment records the effects of nutrients and climate on diatom dynamics in Loch Leven

1. Changes in nutrients and climate have occurred over approximately the same timescales in many European lake catchments. Here, we attempt to interpret the sedimentary diatom record of a large shallow lake, Loch Leven, in relation to these pressures using information gained from analysis of long‐term data sets of water quality, climate and planktonic diatoms. 2. The core data indicate the enrichment of Loch Leven starting in c. 1800–1850, most likely from agricultural practices in the catchment, with a more marked phase since c. 1940–1950 caused by increased phosphorus inputs from sewage treatment works, land drainage and a woollen mill. 3. While the recent diatom plankton remains are dominated by taxa associated with nutrient‐rich conditions, an increase in Aulacoseira subarctica relative to Stephanodiscus taxa since the mid‐1980s suggests that reductions in external catchment sources of nutrients (since 1985) may have resulted in partial recovery. This observation accords well with the long‐term monitoring series of water chemistry and phytoplankton. 4. On a decadal‐centennial scale, the eutrophication signal in the sediment record outweighs any evidence of climate as a control on the diatom community. However, at an inter‐annual scale, while the diatom data exhibit high variability, there are several changes in species composition in the recent fossil record that may be attributed to climatic controls. 5. The study highlights the value of a palaeolimnological approach, particularly when coupled with long‐term data sets, for developing our understanding of environmental change at a range of temporal scales. The diatom record in the sediment can be used effectively to track recovery from eutrophication, but requires greater understanding of contemporary ecology to fully interpret climate impacts. 6. The study illustrates the complexity of ecosystem response to synchronous changes in nutrients and climate, and the difficulty of disentangling the effects of these multiple, interacting pressures.