Long-term trends and regional differences of phytoplankton in large Finnish lakes

Major nutrients (N and P) and phytoplankton from 19 large lakes from southern (61°) to northern (69°) Finland were analyzed to detect long-term trends and regional differences. The data sets from June, July and August cover the period from the early 1980s to the present. Altogether >700 phytoplankton and >4000 N, P and Chl a results were used for the study. In 40% of the lakes, the total phosphorus (TP) concentration decreased significantly and in >25% of the lakes a significant reduction was found in the total nitrogen (TN) concentration. At the same time, the phytoplankton biomass declined only in 15% of the lakes and the long-term trends in chlorophyll a more often increased than decreased. A clear gradient from southern to northern Finland and western to eastern Finland was found in the phytoplankton biomass. During the study period, the biomasses of cyanobacteria and centrales (diatoms) decreased whilst there was an increase in the biomass of pennales (diatoms) in one-third of the lakes. The proportion of chlorophytes in the total biomass also increased in >20% of the study lakes. In southern and western Finland, the total biomass and the contribution of cyanobacteria were higher. Centrales made a higher contribution to the total biomass in the north. Pennales and chlorophytes were less abundant and chrysophytes more abundant in the east. Differences in the community composition reflected the gradients in the total nutrients, and particularly in TP. The observations support the assumed role of phosphorus as the key limiting nutrient in large Finnish lakes irrespective of lake′s location. The N:P ratio proved to be a poor predictor of cyanobacteria occurrence in the study lakes.     

Trophic control changes with season and nutrient loading in lakes

Experiments have revealed much about top‐down and bottom‐up control in ecosystems, but manipulative experiments are limited in spatial and temporal scale. To obtain a more nuanced understanding of trophic control over large scales, we explored long‐term time‐series data from 13 globally distributed lakes and used empirical dynamic modelling to quantify interaction strengths between zooplankton and phytoplankton over time within and across lakes. Across all lakes, top‐down effects were associated with nutrients, switching from negative in mesotrophic lakes to positive in oligotrophic lakes. This result suggests that zooplankton nutrient recycling exceeds grazing pressure in nutrient‐limited systems. Within individual lakes, results were consistent with a ‘seasonal reset’ hypothesis in which top‐down and bottom‐up interactions varied seasonally and were both strongest at the beginning of the growing season. Thus, trophic control is not static, but varies with abiotic conditions – dynamics that only become evident when observing changes over large spatial and temporal scales.     

Responses of phytoplankton to fish predation and nutrient loading in shallow lakes: a pan-European mesocosm experiment

1. The impacts of nutrients (phosphorus and nitrogen) and planktivorous fish on phytoplankton composition and biomass were studied in six shallow, macrophyte‐dominated lakes across Europe using mesocosm experiments. 2. Phytoplankton biomass was more influenced by nutrients than by densities of planktivorous fish. Nutrient addition resulted in increased algal biomass at all locations. In some experiments, a decrease was noted at the highest nutrient loadings, corresponding to added concentrations of 1 mg L^?1 P and 10 mg L^?1 N. 3. Chlorophyll a was a more precise parameter to quantify phytoplankton biomass than algal biovolume, with lower within‐treatment variability. 4. Higher densities of planktivorous fish shifted phytoplankton composition toward smaller algae (GALD < 50 μm). High nutrient loadings selected in favour of chlorophytes and cyanobacteria, while biovolumes of diatoms and dinophytes decreased. High temperatures also may increase the contribution of cyanobacteria to total phytoplankton biovolume in shallow lakes.     

Increased nutrient loading and rapid changes in phytoplankton expected with climate change in stratified South European lakes: sensitivity of lakes with different trophic state and catchment properties

We hypothesised that increasing winter affluence and summer temperatures, anticipated in southern Europe with climate change, will deteriorate the ecological status of lakes, especially in those with shorter retention time. We tested these hypotheses analysing weekly phytoplankton and chemistry data collected over 2 years of contrasting weather from two adjacent stratified lakes in North Italy, differing from each other by trophic state and water retention time. Dissolved oxygen concentrations were higher in colder hypolimnia of both lakes in the second year following the cold winter, despite the second summer was warmer and the lakes more strongly stratified. Higher loading during the rainy winter and spring increased nutrient (N, P, Si) concentrations, and a phytoplankton based trophic state index, whilst the N/P ratio decreased in both lakes. The weakened Si limitation in the second year enabled an increase of diatom biovolumes in spring in both lakes. Chlorophyll a concentration increased in the oligo-mesotrophic lake, but dropped markedly in the eutrophic lake where the series of commonly occurring cyanobacteria blooms was interrupted. The projected increase of winter precipitation in southern Europe is likely to increase the nutrient loadings to lakes and contribute to their eutrophication. The impact is proportional to the runoff/in-lake concentration ratio of nutrients rather than to the retention time, and is more pronounced in lakes with lower trophy.     

Responses of epilimnetic phytoplankton to experimental nutrient enrichment in three small seepage lakes

This paper describes the responses of three epilimnetic phytoplanktoncommunities to experimental nitrogen and phosphorus enrichmentas compared to the phytoplankton community in a fourth, unmanipulated,lake. Increased nutrient inputs increased total phytoplanktonbiomass, primary productivity, chlorophytes, cryptomonads andspecies turnover rates in all three enriched lakes; cyanobacteriaincreased in two of the three enriched lakes. However, nutrientaddition also led to declines in previously dominant dinoflagellatesand chrysophytes, and in species diversity. At the species level,there were large changes in community composition from yearto year in both enriched and reference lakes, suggesting thatphytoplankton community composition is highly dynamic even inthe absence of enrichment. Overall, changes in total biomass,productivity and species diversity were consistent among theenriched lakes, while changes in species composition differeddue to variation in the physical, chemical and biotic environmentof each lake. This suggests that aggregated variates are moreuseful for quantitative prediction of nutrient effects, whilespecies responses can be used to signal qualitative differencesin environmental conditions among lakes. ³Present address: Department of Biological Sciences, DartmouthCollege, 6044 Gilman Laboratory, Hanover, NH 03755-3576, USA     

Light and nutrient control phytoplankton biomass responses to global change in northern lakes

Global change affects terrestrial loadings of colored dissolved organic carbon (DOC) and nutrients to northern lakes. Still, little is known about how phytoplankton respond to changes in light and nutrient availability across gradients in lake DOC. In this study, we used results from whole‐lake studies in northern Sweden to show that annual mean phytoplankton biomass expressed unimodal curved relationships across lake DOC gradients, peaking at threshold DOC levels of around 11 mg/L. Whole‐lake single nutrient enrichment in selected lakes caused elevated biomass, with most pronounced effect at the threshold DOC level. These patterns give support to the suggested dual control by DOC on phytoplankton via nutrient (positively) and light (negatively) availability and imply that the lakes' location along the DOC axis is critical in determining to what extent phytoplankton respond to changes in DOC and/or nutrient loadings. By using data from the large Swedish Lake Monitoring Survey, we further estimated that 80% of northern Swedish lakes are below the DOC threshold, potentially experiencing increased phytoplankton biomass with browning alone, and/or combined with nutrient enrichment. The results support the previous model results on effects of browning and eutrophication on lake phytoplankton, and provide important understanding of how northern lakes may respond to future global changes.     

Sedimentary records of accelerated nutrient loading in Florida lakes

Transfer functions relating trophic state (Carlson's TSI_{chlorophyll-a}) to present day accumulation rate of (1) nutrients, (2) cations, and (3) organic sediment, are computed using Binford's ²¹⁰Pb-dilution method. As computed from surficial sediments of 27 lakes, former trophic states are reconstructed for recent (²¹⁰Pb-dated) sedimentary histories of 14 lakes. Of the three kinds of models potentially available, model (3) (TSI vs. organic accumulation) is the weakest statistically, and may be unduly influenced by exceptional deposition and/or preservation of allochthonous organic matter. At present, however, it is the only model applicable to all 14 of the histories tested. Results are encouraging in that model accurately predicts observed TSI's in several mesotrophic and eutrophic lakes. Clearly significant increases (accelerations) are inferred only for a of the most eutrophic lakes of the set, while the model consistently overpredicts TSI's of the 7 most oligotrophic lakes. As Whitmore's diatom-assemblage index is a better predictor of TSI than is model (3) in the one eutrophic lake in which it has been tested, we expect more persuasive results when models (1) and (2) can be tested within a more complete set of analytical data. We were surprised to find 3 severely disturbed lakes among the 12 that show little or no acceleration in rate of eutrophication in recent decades, but we defer attempts at explanation until former nutrient loading can be tested by model (1).     

Physiological indicators of nutrient deficiency in phytoplankton in southern Chilean lakes

We assessed the nutrient status of phytoplankton in 28 lakes in southern Chile using two types of physiological indicators: specific alkaline phosphatase activity, and the elemental composition (carbon, nitrogen, and phosphorus) of seston. Alkaline phosphatase activity ranged from 0.001 to 0.11 mol P g chl^–1 h^–1, with P-deficiency indicated in about one-half the study lakes. C:N ranged from 3.9 to 24, C:P ranged from 86 to 919, and N:P ranged from 8.7 to 99. C:P and N:P ratios greater than the Redfield ratio were common, suggesting P deficiency in many of the lakes. C:N ratios were not generally indicative of N deficiency. Previous studies have suggested N may be the primary limiting nutrient in southern Chilean lakes, but our results indicate that P should not be discounted as a limiting nutrient.     

Primary productivity,phytoplankton and limiting nutrient factors in labrador lakes

The primary productivity of some lakes and reservoirs in western Labrador was measured by the ¹⁴C method in order to determine the range of productivities and the effects of impoundment. No primary productivity data previously existed for this part of Canada. Both the primary productivity and standing crops of phytoplankton were found to be low in a newly impounded lake but later rose to levels greater than in surrounding natural lakes. In nutrient enrichment experiments, carbon was never found to be limiting but phosphorus stimulated primary productivity when added alone or in combination with nitrogen.     

Response of submerged macrophytes in Danish lakes to nutrient loading reductions and biomanipulation

During the last two decades the nutrient loading to Danish lakes has been reduced with the aim to improve water quality. However, because of internal P-loading and biological resistance, the expected improvement has been delayed. Therefore, to reduce the duration of the recovery period and to accelerate recolonisation of submerged macrophytes, several lakes have been biomanipulated with the purpose of improving the top-down control by zooplankton. To elucidate the effects of these measures, we undertook an analysis of data on submerged macrophytes monitored annually in 17 lakes for 8 years. The results obtained show that the macrophyte coverage in non-biomanipulated lakes remained relatively stable following the external nutrient reduction. However, a small increase in macrophyte coverage occurred in a few lakes. In two of the four biomanipulated lakes, in contrast, macrophyte coverage increased from 0 up to 80% within 2–4 years following manipulation. In the other two lakes macrophyte colonisation failed. However, in lakes with a successful recolonisation, large inter-annual variations in macrophyte coverage, varying between 2 and 80% among growth seasons, occurred. We conclude that the potential of macrophyte recolonisation after nutrient loading reduction on the short term is higher in biomanipulated lakes than in lakes subjected to loading reduction only, although biomanipulation does not provide a guarantee for macrophyte recolonisation or a stable macrophyte community when colonisation occurs.

     

Trophic changes,without changes in the external nutrient loading

The impact of the fish population on trophic properties of lake water, was experimentally studied in an oligotrophic Swedish forest lake. Biotic changes following fish removal resulted in a development in oligotrophic direction as shown by the drop in limnetic primary production, pH, total phosphorus, total nitrogen and the increased transparency.     

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

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

Background nutrient concentration determines phytoplankton bloom response to marine heatwaves

Ocean temperature extreme events such as marine heatwaves are expected to intensify in coming decades due to anthropogenic global warming. Reported ecological and economic impacts of marine heatwaves include coral bleaching, local extinction of mangrove and kelp forests and elevated mortalities of invertebrates, fishes, seabirds and marine mammals. In contrast, little is known about the impacts of marine heatwaves on microbes that regulate biogeochemical processes in the ocean. Here we analyse the daily output of a near‐global ocean physical–biogeochemical model simulation to characterize the impacts of marine heatwaves on phytoplankton blooms in 23 tropical and temperate oceanographic regions from 1992 to 2014. The results reveal regionally coherent anomalies of shallower surface mixing layers and lower surface nitrate concentrations during marine heatwaves. These anomalies exert counteracting effects on phytoplankton growth through light and nutrient limitation. Consequently, the responses of phytoplankton blooms are mixed, but can be related to the background nutrient conditions of the study regions. The blooms are weaker during marine heatwaves in nutrient‐poor waters, whereas in nutrient‐rich waters, the heatwave blooms are stronger. The corresponding analyses of sea‐surface temperature, chlorophyll a and nitrate based on satellite observations and in situ climatology support this relationship between phytoplankton bloom anomalies and background nitrate concentration. Given that nutrient‐poor waters are projected to expand globally in the 21st century, this study suggests increased occurrence of weaker blooms during marine heatwaves in coming decades, with implications for higher trophic levels and biogeochemical cycling of key elements.     

Combining a regional climate model with a phytoplankton community model to predict future changes in phytoplankton in lakes

1. Linking a regional climate model (RCM) configured for contemporary atmospheric greenhouse gas concentrations, with a phytoplankton community model (PROTECH) produced realistic simulations of 20 years of recent phytoplankton data from Bassenthwaite Lake, in the North‐West of England. 2. Meteorological drivers were derived from the RCM to represent a future climate scenario involving a 1% per annum compound increase in atmospheric CO₂ concentrations until 2100. Using these drivers, PROTECH was run for another 20 year period representing the last two decades of the 21st century. 3. Comparison of these present and future simulations revealed likely impacts on the current seasonal phytoplankton development. Under future climate conditions, the simulated spring bloom showed an increase in cyanobacteria dominance caused by greater success of Planktothrix. Also, the summer cyanobacteria bloom declined earlier because of nutrient limitation caused by the increased spring growth. Overall productivity in the lake did not change. 4. Analysis showed that these predicted changes were driven by changes in water temperature, which were in turn triggered by the higher air temperatures predicted by the RCM.     

An experimental investigation of phytoplankton nutrient limitation in two contrasting low arctic lakes

We investigated whether phytoplankton communities in two lakes in SW Greenland were phosphorus or nitrogen limited. The study lakes have contrasting water chemistry (mean conductivities differ ten fold) and are located near Kangerlussuaq, SW Greenland (~67°N, 51°W). A microcosm nutrient enrichment experiment was performed in June 2003 to determine whether nitrate or phosphate addition stimulated phytoplankton growth. Samples were analysed for species composition, biomass, and alkaline phosphatase activity (APA). Initially, both lakes had extremely low total phosphorus but high total nitrogen concentrations and high APA, suggesting that the phytoplankton were phosphorus limited prior to the start of the experiment. The phytoplankton composition and biomass (mainly Ochromonas spp.) responded to phosphate but not to nitrate addition. In both lakes, chlorophyll a increased significantly when phosphate was added. Furthermore, APA was significantly lower in the two lakes when phosphate was added compared to the control and the nitrogen addition treatment. The dominance of mixotrophic phytoplankton and high DOC values suggest that these lakes may be regulated by microbial loop processes.     

Duckling response to changes in the trophic web of acidified lakes

We reared American Black Duck (Anas rubripes Brewster) and Common Goldeneye (Bucephala clangula Linnaeus) ducklings on two Quebec laurentian lakes in which we manipulated brook trout populations (Salvelinus fontinalis Mitchill), lake acidity and lake productivity to relate waterfowl foraging to trophic status of lakes. We developed a preliminary model to assess the effects of lake acidity and productivity, fish predation and interspecific fish/duck competition in relation to available food (aquatic invertebrates). We then validated the model using a factorial analysis of the relationships between the variables pertaining to the diet of the fish and ducklings, and the environmental characteristics of the lakes (acidity, biological production and fish predation).The first factorial axis can be interpreted in terms of biological productivity, while the second axis illustrates the effect that fish have on the quantity and type of food available to ducklings. Two different trends appear to occur depending on whether the carrying capacity of the lake is reduced by acidification of the water or increase through liming or fertilization. In the first case, fish predation appears to have a marked effect on available food, whereas in the second case, interspecific fish/duck competition is apparently to blame for changes in the diet of ducklings. In both instances, but to a lesser extent, fish compete increasingly (exploitation and/or interference) with the ducklings, forcing them to feed to a greater extent in riparian sites that are less accessible to fish.     

Long-term changes in phytoplankton, zooplankton and salmon related to climate

Recently, large‐scale changes in the biogeography of calanoid copepod crustaceans have been detected in the northeastern North Atlantic Ocean and adjacent seas. Strong biogeographical shifts in all copepod assemblages were found with a northward extension of more than ° in latitude of warm‐water species associated with a decrease in the number of colder‐water species. These changes were attributed to regional increase in sea surface temperature. Here, we have extended these studies to examine long‐term changes in phytoplankton, zooplankton and salmon in relation to hydro‐meteorological forcing in the northeast Atlantic Ocean and adjacent seas. We found highly significant relationships between (1) long‐term changes in all three trophic levels, (2) sea surface temperature in the northeastern Atlantic, (3) Northern Hemisphere temperature and (4) the North Atlantic Oscillation. The similarities detected between plankton, salmon, temperature and hydro‐climatic parameters are also seen in their cyclical variability and in a stepwise shift that started after a pronounced increase in Northern Hemisphere Temperature anomalies at the end of the 1970s. All biological variables show a pronounced change which started after circa 1982 for euphausiids (decline), 1984 for the total abundance of small copepods (increase), 1986 for phytoplankton biomass (increase) and Calanus finmarchicus (decrease) and 1988 for salmon (decrease). This cascade of biological events led to an exceptional period, which is identified after 1986 to present and followed another shift in large‐scale hydro‐climatic variables and sea surface temperature. This regional temperature increase therefore appears to be an important parameter that is at present governing the dynamic equilibrium of northeast Atlantic pelagic ecosystems with possible consequences for biogeochemical processes and fisheries.     

Microscale patchiness leads to large and important intraspecific internal nutrient heterogeneity in phytoplankton

Phytoplankton stoichiometry or nutrient content has been shown to vary in a number of dimensions (species, condition, time, space), but the heterogeneity within a species at a given time and location, and the underlying mechanisms and importance have not been explored. There are a number of mechanisms that can create intraspecific heterogeneity, and theory suggests it can affect the population growth rate. We studied heterogeneity in P content of the freshwater diatom Cyclotella meneghiniana in the Charles River in Boston. Single-cell observations using synchrotron-based X-ray fluorescence show that the nutrient status varies from P-starved to P-replete. We simulate individual cells using an agent-based model that accounts for a number of mechanisms that can create heterogeneity, including surface area–based uptake, mortality differentiation, stochastic biological variability in states and behavior, macroscale mixing, and microscale nutrient patch encounter. By performing a number of simulations with various mechanisms turned on/off and comparing to data, we conclude that the heterogeneity is mostly due to microscale patchiness (85%). We explore the importance of accounting for heterogeneity in models by performing a simulation with the growth rate based on the population-average internal nutrient, as is done in conventional population-level models. This shows that ignoring heterogeneity increases the population growth rate by a factor of 1.47. To account for different heterogeneity in the laboratory and field, population-level ecosystem models should reduce maximum growth rates. The magnitude of this correction depends on local conditions, and in our case, it is a factor of 0.72.     

Long-term trends and fine year-to-year tuning of phytoplankton in large lakes are ruled by eutrophication and atmospheric modes of variability

This study demonstrated how the impact of eutrophication in a deep lake at the southern border of the Alps (Lake Garda) was regulated by specific modes of atmospheric circulation relevant for the Mediterranean area. At the decadal scale, nutrients and phytoplankton increased concurrently since the 1970s. At the annual scale, year-to-year fluctuations in nutrients and phytoplankton were controlled through a chain of causal factors centred on deeply penetrative mixing events determining an upward transport of phosphorus from the hypolimnion to the trophogenic layers. The extent of mixing was in turn controlled by lake and air winter temperature, which were ultimately regulated by the winter fluctuations of the East Atlantic pattern (EA). In its negative state, the EA shows an intense high pressure over the West Atlantic, causing a north-easterly air flow bringing cold air from continental Europe to Mediterranean, thus favouring greater lake mixing and nutrient fertilisation. Cyanobacteria (mostly Planktothrix rubescens) were the organisms which greatly benefitted from the long-term increase in phosphorus concentrations and the year-to-year fluctuations in surface phosphorus availability controlled by the EA. Given the same availability of phosphorus in the water column, positive winter EA phases weakened the eutrophication effects and phytoplankton development.     

Seasonal changes in the spatial distribution of phytoplankton in small, temperate-zone lakes

Seasonal sampling across two small lakes shows that phytoplanktonpatchiness is greatly enhanced during winter ice-cover relativeto the open-water seasons of exposure to wind stress and rapidturbulent mixing.