Phytoplankton response to a changing climate

Phytoplankton are at the base of aquatic food webs and of global importance for ecosystem functioning and services. The dynamics of these photosynthetic cells are linked to annual fluctuations of temperature, water column mixing, resource availability, and consumption. Climate can modify these environmental factors and alter phytoplankton structure, seasonal dynamics, and taxonomic composition. Here, we review mechanistic links between climate alterations and factors limiting primary production, and highlight studies where climate change has had a clear impact on phytoplankton processes. Climate affects phytoplankton both directly through physiology and indirectly by changing water column stratification and resource availability, mainly nutrients and light, or intensified grazing by heterotrophs. These modifications affect various phytoplankton processes, and a widespread advance in phytoplankton spring bloom timing and changing bloom magnitudes have both been observed. Climate warming also affects phytoplankton species composition and size structure, and favors species traits best adapted to changing conditions associated with climate change. Shifts in phytoplankton can have far-reaching consequences for ecosystem structure and functioning. An improved understanding of the mechanistic links between climate and phytoplankton dynamics is important for predicting climate change impacts on aquatic ecosystems.     

Copepod life cycle adaptations and success in response to phytoplankton spring bloom phenology

In a seasonal environment, the timing of reproduction is usually scheduled to maximize the survival of offspring. Within deep water bodies, the phytoplankton spring bloom provides a short time window of high food quantity and quality for herbivores. The onset of algal bloom development, however, varies strongly from year to year due to interannual variability in meteorological conditions. Furthermore, the onset is predicted to change with global warming. Here, we use a long-term dataset to study (a) how a cyclopoid copepod, Cyclops vicinus , is dealing with the large variability in phytoplankton bloom phenology, and (b) if bloom phenology has an influence on offspring numbers. C. vicinus performed a two-phase dormancy, that is, the actual diapause of fourth copepodid stages at the lake bottom is followed by a delay in maturation, that is, a quiescence, within the fifth copepodid stage until the start of the spring bloom. This strategy seems to guarantee a high temporal match of the food requirements for successful offspring development, especially through the highly vulnerable naupliar stages, with the phytoplankton spring bloom. However, despite this match with food availability in all study years, offspring numbers, that is, offspring survival rates were higher in years with an early start of the phytoplankton bloom. In addition, the phenology of copepod development suggested that also within study years, early offspring seems to have lower mortality rates than late produced offspring. We suggest that this is due to a longer predator-free time period and/or reduced time stress for development. Hence, within the present climate variability, the copepod benefited from warmer spring temperatures resulting in an earlier phytoplankton spring bloom. Time will show if the copepod's strategy is flexible enough to cope with future warming.     

Water temperature and stratification depth independently shift cardinal events during plankton spring succession

In deep temperate lakes, the beginning of the growing season is triggered by thermal stratification, which alleviates light limitation of planktonic producers in the surface layer and prevents heat loss to deeper strata. The sequence of subsequent phenological events (phytoplankton spring bloom, grazer peak, clearwater phase) results in part from coupled phytoplankton–grazer interactions. Disentangling the separate, direct effects of correlated climatic drivers (stratification‐dependent underwater light climate vs. water temperature) from their indirect effects mediated through trophic feedbacks is impossible using observational field data, which challenges our understanding of global warming effects on seasonal plankton dynamics. We therefore manipulated water temperature and stratification depth independently in experimental field mesocosms containing ambient microplankton and inocula of the resident grazer Daphnia hyalina. Higher light availability in shallower surface layers accelerated primary production, warming accelerated consumption and growth of Daphnia, and both factors speeded up successional dynamics driven by trophic feedbacks. Specifically, phytoplankton peaked and decreased earlier and Daphnia populations increased and peaked earlier at both shallower stratification and higher temperature. The timing of ciliate dynamics was unrelated to both factors. Volumetric peak densities of phytoplankton, ciliates and Daphnia in the surface layer were also unaffected by temperature but declined with stratification depth in parallel with light availability. The latter relationship vanished, however, when population sizes were integrated over the entire water column. Overall our results suggest that, integrated over the entire water column of a deep lake, surface warming and shallower stratification independently speed up spring successional events, whereas the magnitudes of phytoplankton and zooplankton spring peaks are less sensitive to these factors. Therefore, accelerated dynamics under warming need not lead to a trophic mismatch (given similar grazer inocula at the time of stratification). We emphasize that entire water column dynamics must be studied to estimate global warming effects on lake ecosystems.     

Earlier onset of the spring phytoplankton bloom in lakes of the temperate zone in a warmer climate

The decoupling of trophic interactions is potentially one of the most severe consequences of climate warming. In lakes and oceans the timing of phytoplankton blooms affects competition within the plankton community as well as food–web interactions with zooplankton and fish. Using Upper Lake Constance as an example, we present a model‐based analysis that predicts that in a future warmer climate, the onset of the spring phytoplankton bloom will occur earlier in the year than it does at present. This is a result of the earlier occurrence of the transition from strong to weak vertical mixing in spring, and of the associated earlier onset of stratification. According to our simulations a shift in the timing of phytoplankton growth resulting from a consistently warmer climate will exceed that resulting from a single unusually warm year. The numerical simulations are complemented by a statistical analysis of long‐term data from Upper Lake Constance which demonstrates that oligotrophication has a negligible effect on the timing of phytoplankton growth in spring and that an early onset of the spring phytoplankton bloom is associated with high air temperatures and low wind speeds.     

Climate change and the phytoplankton spring bloom: warming and overwintering zooplankton have similar effects on phytoplankton

Indoor mesocosms were used to study the combined effect of warming and of different densities of overwintering mesozooplankton (mainly copepods) on the spring development of phytoplankton in shallow, coastal waters. Similar to previous studies, warming accelerated the spring phytoplankton peak by ca. 1 day °C^?1 whereas zooplankton did not significantly influence timing. Phytoplankton biomass during the experimental period decreased with warming and with higher densities of overwintering zooplankton. Similarly, average cell size and average effective particle size (here: colony size) decreased both with zooplankton density and warming. A decrease in phytoplankton particle size is generally considered at typical footprint of copepod grazing. We conclude that warming induced changes in the magnitude and structure of the phytoplankton spring bloom cannot be understood without considering grazing by overwintering zooplankton.     

Climate change and the timing, magnitude, and composition of the phytoplankton spring bloom

In this article, we show by mesocosm experiments that winter and spring warming will lead to substantial changes in the spring bloom of phytoplankton. The timing of the spring bloom shows only little response to warming as such, while light appears to play a more important role in its initiation. The daily light dose needed for the start of the phytoplankton spring bloom in our experiments agrees well with a recently published critical light intensity found in a field survey of the North Atlantic (around 1.3 mol photons m^?2 day^?1). Experimental temperature elevation had a strong effect on phytoplankton peak biomass (decreasing with temperature), mean cell size (decreasing with temperature) and on the share of microplankton diatoms (decreasing with temperature). All these changes will lead to poorer feeding conditions for copepod zooplankton and, thus, to a less efficient energy transfer from primary to fish production under a warmer climate.     

Effects of recent climate change on phytoplankton phenology in a temperate lake

1. A number of long-term studies have shown that spring biological events have advanced in recent decades and that this is a response to climate change. In lentic systems, changes in phytoplankton phenology have been attributed to various directly climate-related processes including changes in the onset and duration of thermal stratification, earlier ice-break up and increased water temperature. Both indirect climatic drivers and non-climate drivers such as elevated grazing pressure and nutrient enrichment can also affect phenology.
2. This study investigated whether phenological trends in phytoplankton could be detected in a relatively short time series in a shallow, ice-free, polymictic lake with a high annual discharge and whether any such trends could be causally explained.
3. It was found that the centre of gravity of the spring chlorophyll a bloom advanced significantly by 1.6 days per year over a 15-year period. This was accompanied by a significant increase in water temperature of 0.12 °C per year which is high compared to published rates of change over longer time series. No direct effects of ice cover, stratification or water discharge rates could be linked to the advancement of the spring bloom. Instead, the shift in timing was attributed to an advance in the timing of the dominant spring diatom, Aulacoseira spp., instigated by a temperature-driven increase in replication rate leading to an earlier onset of silica (SiO₂) limitation.     

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.     

Seasonal shifts in ice edge phytoplankton blooms in the Barents Sea related to the water column stability

The development of the phytoplankton bloom and its relation to water column stabilisation during the transition from early to high summer (of 1991) in the seasonally ice-covered zone of the Barents Sea were studied from a meridional transect of repeated hydrographic/biological stations. The water column stabilisation is described in detail with the aid of vertical profiles of the Brunt-Väisälä frequency squared (N²). The contributions of seasonal warming and ice melting to stabilisation are elucidated by determining the effects of temperature and salinity on N². The spring bloom in 1991 migrated poleward from June to July by about 400 km, associated with the retreat of the ice edge. The spring bloom culminated with maximum chlorophyll concentrations in the mixed layer about 100–300 km north of the centre of the meltwater lens, at its northern edge, where the ice cover was still substantial. From the distribution of N² it becomes obvious that the bloom starts at the very beginning of stabilisation, which results solely from the release of meltwater. The increase in temperature due to the seasonal warming does not contribute to the onset of vernal blooming; temperature starts to contribute to the stratification later, when the spring bloom has ceased due to the exhaustion of nutrients in the mixed layer. By that time a deep chlorophyll maximum has formed in the seasonal pycnocline, 20–30 m below the base of the mixed layer. The effect of the seasonal ice cover on the mean areal new primary production is discussed.     

Predicted warming and browning affect timing and magnitude of plankton phenological events in lakes: a mesocosm study

1. Aquatic ecosystems in Northern Europe are expected to face increases in temperature and water colour (TB) in future. While effects of these factors have been studied separately, it is unknown whether and how a combination of them might affect phenological events and trophic interactions. 2. In a mesocosm study, we combined both factors to create conditions expected to arise during the coming century. We focused on quantifying effects on timing and magnitude of plankton spring phenological events and identifying possible mismatches between resources (phytoplankton) and consumers (zooplankton). 3. We found that the increases in TB had important effects on timing and abundance of different plankton groups. While increased temperature led to an earlier peak in phytoplankton and zooplankton and a change in the relative timing of different zooplankton groups, increased water colour reduced chlorophyll‐a concentrations. 4. Increased TB together benefitted cladocerans and calanoid copepods and led to stronger top‐down control of algae by zooplankton. There was no sign of a mismatch between primary producers and grazers as reported from other studies. 5. Our results point towards an earlier onset of plankton spring growth in shallow lakes in future with a stronger top‐down control of phytoplankton by zooplankton grazers.     

Earlier and warmer springs increase cyanobacterial (Microcystis spp.) blooms in subtropical Lake Taihu,China

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Climatic effects on the phenology of lake processes

Populations living in seasonal environments are exposed to systematic changes in physical conditions that restrict the growth and reproduction of many species to only a short time window of the annual cycle. Several studies have shown that climate changes over the latter part of the 20th century affected the phenology and population dynamics of single species. However, the key limitation to forecasting the effects of changing climate on ecosystems lies in understanding how it will affect interactions among species. We investigated the effects of climatic and biotic drivers on physical and biological lake processes, using a historical dataset of 40 years from Lake Washington, USA, and dynamic time‐series models to explain changes in the phenological patterns among physical and biological components of pelagic ecosystems. Long‐term climate warming and variability because of large‐scale climatic patterns like Pacific decadal oscillation (PDO) and El Niño–southern oscillation (ENSO) extended the duration of the stratification period by 25 days over the last 40 years. This change was due mainly to earlier spring stratification (16 days) and less to later stratification termination in fall (9 days). The phytoplankton spring bloom advanced roughly in parallel to stratification onset and in 2002 it occurred about 19 days earlier than it did in 1962, indicating the tight connection of spring phytoplankton growth to turbulent conditions. In contrast, the timing of the clear‐water phase showed high variability and was mainly driven by biotic factors. Among the zooplankton species, the timing of spring peaks in the rotifer Keratella advanced strongly, whereas Leptodiaptomus and Daphnia showed slight or no changes. These changes have generated a growing time lag between the spring phytoplankton peak and zooplankton peak, which can be especially critical for the cladoceran Daphnia. Water temperature, PDO, and food availability affected the timing of the spring peak in zooplankton. Overall, the impact of PDO on the phenological processes were stronger compared with ENSO. Our results highlight that climate affects physical and biological processes differently, which can interrupt energy flow among trophic levels, making ecosystem responses to climate change difficult to forecast.     

Climate warming effects on the Olea europaea–Bactrocera oleae system in Mediterranean islands: Sardinia as an example

Global warming will affect all species but in largely unknown ways, with certain regions such as the Mediterranean Basin and its major islands including Sardinia being particularly vulnerable to desertification. Olive ( Olea europaea ) is of eco-social importance in the Mediterranean where it was domesticated. This drought-resistant crop and its major pest, the olive fly ( Bactrocera oleae ), have tight biological links that make them a suitable model system for climate change studies in the Mediterranean. Here a physiologically based weather-driven demographic model of olive and olive fly is used to analyze in detail this plant–pest system in Sardinia under observed weather (10 years of daily data from 48 locations), three climate warming scenarios (increases of 1, 2 and 3 °C in average daily temperature), and a 105-year climate model scenario for the Alghero location (e.g. 1951–2055). grass gis is used to map model predictions of olive bloom dates and yield, total season-long olive fly pupae, and percent fruit attacked by the fly. Island wide simulation data are summarized using multivariate regression. Model calibration with field bloom date data were performed to increase simulation accuracy of olive flowering predictions under climate change. As climate warms, the range of olive is predicted to expand to higher altitudes and consolidate elsewhere, especially in coastal areas. The range of olive fly will extend into previously unfavorable cold areas, but will contract in warm inland lowlands where temperatures approach its upper thermal limits. Consequently, many areas of current high risk are predicted to have decreased risk of fly damage with climate warming. Simulation using a 105-year climate model scenario for Alghero, Sardinia predicts changes in the olive–olive fly system expected to occur if climate continued to warm at the low rate observed during in the past half century.     

Biotic interactions may overrule direct climate effects on spring phytoplankton dynamics

To improve our mechanistic understanding and predictive capacities with respect to climate change effects on the spring phytoplankton bloom in temperate marine systems, we used a process‐driven dynamical model to disentangle the impact of potentially relevant factors which are often correlated in the field. The model was based on comprehensive indoor mesocosm experiments run at four temperature and three light regimes. It was driven by time‐series of water temperature and irradiance, considered edible and less edible phytoplankton separately, and accounted for density‐dependent grazing losses. It successfully reproduced the observed dynamics of well edible phytoplankton in the different temperature and light treatments. Four major factors influenced spring phytoplankton dynamics: temperature, light (cloudiness), grazing, and the success of overwintering phyto‐ and zooplankton providing the starting biomasses for spring growth. Our study predicts that increasing cloudiness as anticipated for warmer winters for the Baltic Sea region will retard phytoplankton net growth and reduce peak heights. Light had a strong direct effect in contrast to temperature. However, edible phytoplankton was indirectly strongly temperature‐sensitive via grazing which was already important in early spring at moderately high algal biomasses and counter‐intuitively provoked lower and later algal peaks at higher temperatures. Initial phyto‐ and zooplankton composition and biomass also had a strong effect on spring algal dynamics indicating a memory effect via the broadly under‐sampled overwintering plankton community. Unexpectedly, increased initial phytoplankton biomass did not necessarily lead to earlier or higher spring blooms since the effect was counteracted by subsequently enhanced grazing. Increasing temperature will likely exhibit complex indirect effects via changes in overwintering phytoplankton and grazer biomasses and current grazing pressure. Additionally, effects on the phytoplankton composition due to the species‐specific susceptibility to grazing are expected. Hence, we need to consider not only direct but also indirect effects, e.g. biotic interactions, when addressing climate change impacts.     

A seasonal study of the distribution of surface state variables in Liverpool Bay. IV. The spring bloom

Physical, chemical, and phytoplankton distributions observed in Liverpool Bay during early and terminal stages in development of the spring bloom in May, 1977, are described. Qualitative and quantitative changes in phytoplankton distributions are interpreted with respect to the contemporaneous chemical and physical hydrography and quantified changes in the hydrography between the periods of observation. In early May, a physical discontinuity demarcated the biological, physical, and most of the chemical characteristics of the offshore and coastal waters. Phytoplankton numbers were greatest in the inshore waters and decreased seaward. The diatom bloom was concentrated in coastal areas where the nutrient status of the waters was least favourable to sustain it. The waters bounded by the front and the coast were composed of a number of physically and chemically distinct water types and each had associated with it a qualitatively distinct phytoplankton population. During the early stage of the spring bloom no abnormal concentrations or accumulations of phytoplankton were associated with the front. By late May areas of diverse phytoplankton density had developed in the inshore waters, major differences in qualitative composition were associated with various density fields and the physical discontinuity, separating offshore and coastal waters, had developed into the site of greatest phytoplankton density. The spatial and temporal changes in the phytoplankton populations are shown to be related to the nutrient status of the waters both within and at the boundaries of the various density fields.     

A modelling investigation of the distribution of stratification and phytoplankton abundance in the Irish Sea

Thermal stratification and phytoplankton abundance are modelledon a 5 km grid covering the Irish Sea. The water column is approximatedby three layers. The top layer is uniformly mixed by wind stirringand the bottom by tidal energy, while linear gradients can occurin the middle layer. The model is forced with hourly meteorologicaldata and mean tidal energies. Primary production is representedby a model with a single nutrient and a single phytoplanktonpopulation. The results from the model show good agreement withdata collected on a Ministry of Agriculture, Fisheries and Food(MAFF) cruise in May 1992 and with historical data. When advectionis included, driven by depth-averaged currents, the surfacetemperature patterns are improved but bottom temperatures indeep water are raised and high concentrations of chlorophyllare carried offshore from coastal regions. This indicates alimitation of using depth-averaged currents and a need to accountfor differences in phytoplankton species composition in coastaland offshore waters. Calculations demonstrate the importanceof salinity variations to stratification and phytoplankton growth.Smoothing the wind mixing energy has the effect of delayingthe onset of the spring bloom in areas where wind mixing issignificant. Removing the diurnal cycle of solar heating alsodelays the spring bloom. The chlorophyll gradient in the middlelayer has a large impact on the response of the model to short-termvariability in the meteorological forcings.     

Detection of climate change‐driven trends in phytoplankton phenology

The timing of the annual phytoplankton spring bloom is likely to be altered in response to climate change. Quantifying that response has, however, been limited by the typically coarse temporal resolution (monthly) of global climate models. Here, we use higher resolution model output (maximum 5 days) to investigate how phytoplankton bloom timing changes in response to projected 21st century climate change, and how the temporal resolution of data influences the detection of long‐term trends. We find that bloom timing generally shifts later at mid‐latitudes and earlier at high and low latitudes by ~5 days per decade to 2100. The spatial patterns of bloom timing are similar in both low (monthly) and high (5 day) resolution data, although initiation dates are later at low resolution. The magnitude of the trends in bloom timing from 2006 to 2100 is very similar at high and low resolution, with the result that the number of years of data needed to detect a trend in phytoplankton phenology is relatively insensitive to data temporal resolution. We also investigate the influence of spatial scales on bloom timing and find that trends are generally more rapidly detectable after spatial averaging of data. Our results suggest that, if pinpointing the start date of the spring bloom is the priority, the highest possible temporal resolution data should be used. However, if the priority is detecting long‐term trends in bloom timing, data at a temporal resolution of 20 days are likely to be sufficient. Furthermore, our results suggest that data sources which allow for spatial averaging will promote more rapid trend detection.     

Hydrographic patterns in McMurdo Sound,Antarctica and their relationship to local benthic communities

Summary Measurements of hydrographic parameters (temperature, salinity, nitrate, nitrite, phosphate, chlorophyll a, phaeophytin, and oxygen) in McMurdo Sound, Antarctica during spring, 1984, before the regional phytoplankton bloom, and summer, 1984, after the peak of the bloom, indicate the several processes contribute to changes in the vertical and horizontal structure of the water column. Regional variation in the source of water masses within the Sound, ice cover patterns, and meltwater from the Ross Ice Shelf and nearby continental glaciers result in east-west and north-south gradients in the thermohaline, nutrient, and productivity characteristics of the Sound. These patterns are also related to the extremely variable structure and productivity of shallow water benthic macrofaunal communities in McMurdo Sound. Hydrographic patterns during Spring (November) were indicative of conditions at the end of winter prior to the spring phytoplankton bloom. The water column was nearly isothermal with temperatures near or below the surface freezing point of seawater with only a slight salinity increase with depth. Salinity was lower in the west Sound than in the east, probably in response to glacial meltwater input from the Ross Ice Shelf and/or terrestrial sources. Nutrient levels were high and nearly homogenous throughout the Sound. Chlorophyll a was low (<1.0 g/l) throughout most of the Sound, but was lowest in the western sound, as expected from the circulation pattern (Barry and Dayton 1988). Oxygen was uniformly low during spring. The summer hydrographic distributions, estimated from samples collected during the decline of the regional plankton bloom, were dramatically different than in during spring. Both the salinity and temperature were vertically stratified at all sites, particularly in the west Sound. Temperatures near the surface were well above the freezing point and occasionally near or above 0°C. Near surface salinity in the western Sound was nearly fresh (0.4 ppt) at some locations in the southwestern Sound. Chlorophyll a was high throughout the Sound relative to spring concentrations, and nutrient levels (NO₃, PO₄) were strongly depressed near the surface, due mainly to phytoplankton uptake rather than by dilution. Primary productivity estimates based on the summer nitrate and phosphate deficits over 90 days were 1.96–2.02 and 0.39–1.02 gCm^-2d^-1 for the east and west sound, respectively. Nutrient ratios indicated that glacial meltwater from the Ross Ice Shelf and/or nearby terrestrial sources may be an important component of the summer meltwater input to the western Sound. Enhanced water column stability due to this input may prolong the maintenance of high water column stability as this water mass flows northward and result in particularly high productivity in northern McMurdo Sound.     

Impact of climate warming on phenological asynchrony of plankton dynamics across Europe

Climate warming alters the seasonal timing of biological events. This raises concerns that species-specific responses to warming may de-synchronize co-evolved consumer-resource phenologies, resulting in trophic mismatch and altered ecosystem dynamics. We explored the effects of warming on the synchrony of two events: the onset of the phytoplankton spring bloom and the spring/summer maximum of the grazer Daphnia. Simulation of 16 lake types over 31 years at 1907 North African and European locations under 5 climate scenarios revealed that the current median phenological delay between the two events varies greatly (20–190 days) across lake types and geographic locations. Warming moves both events forward in time and can lengthen or shorten the delay between them by up to ±60 days. Our simulations suggest large geographic and lake-specific variations in phenological synchrony, provide quantitative predictions of its dependence on physical lake properties and geographic location and highlight research needs concerning its ecological consequences.     

太湖梅梁湾水华蓝藻复苏过程的研究

采用在底泥表面设置藻类细胞捕捉器的方法,测定其中的色素含量变化,并与水柱和底泥中的色素含量变化相比较.结果表明,藻类复苏与底泥环境中的温度、光照、溶解氧、氧化还原电位均有密切关系,叶绿素a、b和藻蓝素所表征的总藻类、绿藻以及蓝藻的上浮率分别为59.84%、76.83%和466.98%,3种藻的上浮量分别占相应浮游藻类最大生物量的7.18%、3.71%和9.33%.蓝藻复苏对太湖水华的形成具有很重要的意义.