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.     

Food web de‐synchronization in England's largest lake: an assessment based on multiple phenological metrics

Phenological changes have been observed globally for marine, freshwater and terrestrial species, and are an important element of the global biological ‘fingerprint’ of climate change. Differences in rates of change could desynchronize seasonal species interactions within a food web, threatening ecosystem functioning. Quantification of this risk is hampered by the rarity of long‐term data for multiple interacting species from the same ecosystem and by the diversity of possible phenological metrics, which vary in their ecological relevance to food web interactions. We compare phenological change for phytoplankton (chlorophyll a), zooplankton (Daphnia) and fish (perch, Perca fluviatilis) in two basins of Windermere over 40 years and determine whether change has differed among trophic levels, while explicitly accounting for among‐metric differences in rates of change. Though rates of change differed markedly among the nine metrics used, seasonal events shifted earlier for all metrics and trophic levels: zooplankton advanced most, and fish least, rapidly. Evidence of altered synchrony was found in both lake basins, when combining information from all phenological metrics. However, comparisons based on single metrics did not consistently detect this signal. A multimetric approach showed that across trophic levels, earlier phenological events have been associated with increasing water temperature. However, for phytoplankton and zooplankton, phenological change was also associated with changes in resource availability. Lower silicate, and higher phosphorus, concentrations were associated with earlier phytoplankton growth, and earlier phytoplankton growth was associated with earlier zooplankton growth. The developing trophic mismatch detected between the dominant fish species in Windermere and important zooplankton food resources may ultimately affect fish survival and portend significant impacts upon ecosystem functioning. We advocate that future studies on phenological synchrony combine data from multiple phenological metrics, to increase confidence in assessments of change and likely ecological consequences.     

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.     

Can overwintering versus diapausing strategy in <Emphasis Type="Italic">Daphnia</Emphasis> determine match–mismatch events in zooplankton–algae interactions?

Mismatches between predator and prey due to climate change have now been documented for a number of systems. Ultimately, a mismatch may have far-reaching consequences for ecosystem functioning as decoupling of trophic relationships results in trophic cascades. Here, we examine the potential for climate change induced mismatches between zooplankton and algae during spring succession, with a focus on Daphnia and its algal food. Whereas the development of an overwintering population of daphnids may parallel shifts in phytoplankton phenology due to climate warming, changes in the photoperiod–temperature interaction may cause the emerging population of daphnids to hatch too late and mismatch their phytoplankton prey. A decoupling of the trophic relationship between the keystone herbivore Daphnia and its algal prey can result in the absence of a spring clear water phase. We extended an existing minimal model of seasonal dynamics of Daphnia and algae and varied the way the Daphnia population is started in spring, i.e., from free swimming individuals or from hatching resting eggs. Our model results show that temperature affects the timing of peak abundance in Daphnia and algae, and subsequently the timing of the clear water phase. When a population is started from a small inoculum of hatching resting eggs, extreme climate warming (+6°C) results in a decoupling of trophic relationships and the clear water phase fails to occur. In the other scenarios, the trophic relationships between Daphnia and its algal food source remain intact. Analysis of 36 temperate lakes showed that shallow lakes have a higher potential for climate induced match–mismatches, as the probability of active overwintering daphnids decreases with lake depth. Future research should point out whether lake depth is a direct causal factor in determining the presence of active overwintering daphnids or merely indicative for underlying causal factors such as fish predation and macrophyte cover. Priority program of the German Research Foundation—contribution 5.     

Effects of decadal climate change on zooplankton over the last 50 years in the western subarctic North Pacific

Decadal‐ to multi‐decadal variations have been reported in many regional ecosystems in the North Pacific, resulting in an increasing demand to elucidate the link between long‐term climatic forcing and marine ecosystems. We detected phenological and quantitative changes in the copepod community in response to the decadal climatic variation in the western subarctic North Pacific by analyzing the extensive zooplankton collection taken since the 1950s, the Odate Collection. Copepod species were classified into five seasonal groups depending on the timing of the annual peak in abundance. The abundance of the spring community gradually increased for the period 1960–2002. The spring–summer community also showed an increasing trend in May, but a decadal oscillation pattern of quasi‐30‐year cycles in July. Phenological changes coincided with the climate regime shift in the mid‐1970s, indicated by the Pacific decadal oscillation index (PDO). After the regime shift, the timing of the peak abundance was delayed one month, from March–April to April–May, in the spring community, whereas it peaked earlier, from June–July to May–June, in the spring–summer community, resulting in an overlap of the high productivity period for the two communities in May. Wintertime cooling, followed by rapid summertime warming, was considered to be responsible for delayed initiation and early termination of the productive season after the mid‐1970s. Another phenological shift, quite different from the previous decade, was observed in the mid‐1990s, when warm winters followed by cool summers lengthened the productive season. The results suggest that climatic forcing with different decadal cycles may operate independently during winter–spring and spring–summer to create seasonal and interannual variations in hydrographic conditions; thus, combinations of these seasonal processes may determine the annual biological productivity.     

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.     

OCEAN CLIMATE CHANGE,PHYTOPLANKTON COMMUNITY RESPONSES,AND HARMFUL ALGAL BLOOMS: A FORMIDABLE PREDICTIVE CHALLENGE1

Prediction of the impact of global climate change on marine HABs is fraught with difficulties. However, we can learn important lessons from the fossil record of dinoflagellate cysts; long‐term monitoring programs, such as the Continuous Plankton Recorder surveys; and short‐term phytoplankton community responses to El Niño Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) episodes. Increasing temperature, enhanced surface stratification, alteration of ocean currents, intensification or weakening of local nutrient upwelling, stimulation of photosynthesis by elevated CO₂, reduced calcification through ocean acidification (“the other CO₂ problem”), and heavy precipitation and storm events causing changes in land runoff and micronutrient availability may all produce contradictory species‐ or even strain‐specific responses. Complex factor interactions exist, and simulated ecophysiological laboratory experiments rarely allow for sufficient acclimation and rarely take into account physiological plasticity and genetic strain diversity. We can expect: (i) range expansion of warm‐water species at the expense of cold‐water species, which are driven poleward; (ii) species‐specific changes in the abundance and seasonal window of growth of HAB taxa; (iii) earlier timing of peak production of some phytoplankton; and (iv) secondary effects for marine food webs, notably when individual zooplankton and fish grazers are differentially impacted (“match‐mismatch”) by climate change. Some species of harmful algae (e.g., toxic dinoflagellates benefitting from land runoff and/or water column stratification, tropical benthic dinoflagellates responding to increased water temperatures and coral reef disturbance) may become more successful, while others may diminish in areas currently impacted. Our limited understanding of marine ecosystem responses to multifactorial physicochemical climate drivers as well as our poor knowledge of the potential of marine microalgae to adapt genetically and phenotypically to the unprecedented pace of current climate change are emphasized. The greatest problems for human society will be caused by being unprepared for significant range expansions or the increase of algal biotoxin problems in currently poorly monitored areas, thus calling for increased vigilance in seafood‐biotoxin and HAB monitoring programs. Changes in phytoplankton communities provide a sensitive early warning for climate‐driven perturbations to marine 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 impacts on mismatches between phytoplankton blooms and fish spawning phenology

Substantial interannual variability in marine fish recruitment (i.e., the number of young fish entering a fishery each year) has been hypothesized to be related to whether the timing of fish spawning matches that of seasonal plankton blooms. Environmental processes that control the phenology of blooms, such as stratification, may differ from those that influence fish spawning, such as temperature‐linked reproductive maturation. These different controlling mechanisms could cause the timing of these events to diverge under climate change with negative consequences for fisheries. We use an earth system model to examine the impact of a high‐emissions, climate‐warming scenario (RCP8.5) on the future spawning time of two classes of temperate, epipelagic fishes: “geographic spawners” whose spawning grounds are defined by fixed geographic features (e.g., rivers, estuaries, reefs) and “environmental spawners” whose spawning grounds move responding to variations in environmental properties, such as temperature. By the century's end, our results indicate that projections of increased stratification cause spring and summer phytoplankton blooms to start 16 days earlier on average (±0.05 days SE) at latitudes >40°N. The temperature‐linked phenology of geographic spawners changes at a rate twice as fast as phytoplankton, causing these fishes to spawn before the bloom starts across >85% of this region. “Extreme events,” defined here as seasonal mismatches >30 days that could lead to fish recruitment failure, increase 10‐fold for geographic spawners in many areas under the RCP8.5 scenario. Mismatches between environmental spawners and phytoplankton were smaller and less widespread, although sizable mismatches still emerged in some regions. This indicates that range shifts undertaken by environmental spawners may increase the resiliency of fishes to climate change impacts associated with phenological mismatches, potentially buffering against declines in larval fish survival, recruitment, and fisheries. Our model results are supported by empirical evidence from ecosystems with multidecadal observations of both fish and phytoplankton phenology.     

The effects of global warming on Daphnia spp. population dynamics: a review

Various species of Daphnia usually play a key role in the food web of temperate freshwater systems. There is much evidence to show that climate change may influence Daphnia population dynamics, consequently altering both predator–prey interactions and the efficiency of algal biomass control in these ecosystems. This review will analyse and discuss the current knowledge on Daphnia responses to climate warming based on an analysis of selected papers. The presented results indicate that warming may have important direct and indirect effects on Daphnia biology and ecology via its influence on their life-history processes (metabolism, growth, reproduction) and the properties of their habitats. The plasticity of daphnids in terms of adaptive responses is generally high and includes phenotypic adaptations and changes in genotypes, although it also depends upon the strength of selection and the available genetic variation. The seasonal timing and magnitude of temperature increases are important for seasonal biomass fluctuations of Daphnia and similarly influence the potential synchrony of daphnids and phytoplankton succession (the timing hypothesis). In light of the most recent studies on this topic, even a minor warming during short but critical seasonal periods can cause factors that disturb Daphnia population dynamics to coincide, which may destabilize lake food webs by decoupling trophic interactions. Both winter and spring are important critical periods for determining future seasonal fluxes of Daphnia spp. and, consequently, the time of the clear-water phase and the occurrence and duration of Daphnia midsummer decline. Winter conditions may also affect the impact of fish predation on daphnids during summer months. However, the effects of global warming on Daphnia population dynamics and on ecosystem functioning are often difficult to predict due to their complexity and the presence of both antagonistic and synergistic drivers. Thus, the diverse responses of daphnids to climate anomalies depend on both biotic (predator abundance and seasonal phytoplankton succession) and abiotic factors (e.g. hydrodynamics, intensity and duration of thermal stratification, trophic state or geomorphology) of lakes, which are directly influenced by weather changes. The analysis and quantification of such complex interactions require the involvement of different kinds of specialists and the development of accurate research approaches, such as molecular genetic methods or mathematical modelling.     

Effects of climate and demography on reproductive phenology of a harvested marine fish population

Shifts in phenology are a well‐documented ecological response to changes in climate, which may or may not be adaptive for a species depending on the climate sensitivity of other ecosystem processes. Furthermore, phenology may be affected by factors in addition to climate, which may accentuate or dampen climate‐driven phenological responses. In this study, we investigate how climate and population demographic structure jointly affect spawning phenology of a fish species of major commercial importance: walleye pollock (Gadus chalcogrammus). We use 32 years of data from ichthyoplankton surveys to reconstruct timing of pollock reproduction in the Gulf of Alaska and find that the mean date of spawning has varied by over 3 weeks throughout the last >3 decades. Climate clearly drives variation in spawn timing, with warmer temperatures leading to an earlier and more protracted spawning period, consistent with expectations of advanced spring phenology under warming. However, the effects of temperature were nonlinear, such that additional warming above a threshold value had no additional effect on phenology. Population demographics were equally as important as temperature: An older and more age‐diverse spawning stock tended to spawn earlier and over a longer duration than a younger stock. Our models suggest that demographic shifts associated with sustainable harvest rates could shift the mean spawning date 7 days later and shorten the spawning season by 9 days relative to an unfished population, independent of thermal conditions. Projections under climate change suggest that spawn timing will become more stable for walleye pollock in the future, but it is unknown what the consequences of this stabilization will be for the synchrony of first‐feeding larvae with production of zooplankton prey in spring. With ongoing warming in the world’s oceans, knowledge of the mechanisms underlying reproductive phenology can improve our ability to monitor and manage species under changing climate conditions.     

The North Atlantic Oscillation and plankton dynamics in two European lakes –- two variations on a general theme

Long‐term data on water temperature, phytoplankton biovolume, Bosmina and Daphnia abundance and the timing of the clear‐water phase were compared and analysed with respect to the influence of the North Atlantic Oscillation (NAO) in two strongly contrasting lakes in central Europe. In small, shallow, hypertrophic Müggelsee, spring water temperatures and Daphnia abundance both increased more rapidly than in large, deep, meso/oligotrophic Lake Constance. Because of this, the clear‐water phase commenced approximately three weeks earlier in Müggelsee than in Lake Constance. In Müggelsee, the phytoplankton biovolume during late winter/early spring was related to the NAO index. In Lake Constance, where phytoplankton growth was inhibited by intense downward mixing during all years studied, this was not the case. However, in both lakes, interannual variability in water temperature, in Daphnia spring population dynamics and in the timing of the clear‐water phase, were all related to the interannual variability of the NAO index. The Daphnia spring population dynamics and the timing of the clear‐water phase appear to be synchronized by the NAO despite large differences between the lakes in morphometry, trophic status and flushing and mixis regimes, and despite the great distance between the lakes (～700 km). This suggests that a great variety of lakes in central Europe may possibly have exhibited similar interannual variability during the last 20 years.     

Changes in spring arrival of Nearctic-Neotropical migrants attributed to multiscalar climate

Climate-related changes associated with the California marine ecosystem have been documented; however, there are no studies assessing changes in terrestrial vertebrate phenology on the Pacific coast of western North America. We analyze the spring phenology of 21 Nearctic-Neotropical migratory songbird species in central and northern CA. Using observational and banding data at multiple sites, we evaluate evidence for a change in arrival timing being linked to either nonclimatic or multiscalar climatic explanations. Using correlation analysis, of the 13 species with a significant ( P <0.10) change in arrival, the arrival timing of 10 species (77%) is associated with both temperature and a large-scale climate oscillation index (El Niño Southern Oscillation, ENSO; North Atlantic Oscillation, NAO; and/or Pacific Decadal Oscillation, PDO) at least at one location. Eight of the 13 species (62%) are advancing their migratory timing. All species for which spring arrival is associated with climate at multiple locations are exhibiting changes ( n =5) and all species lacking evidence for association between migration phenology and climate ( n =3) exhibit no change. Migrants tend to arrive earlier in association with warmer temperatures, positive NAO indices, and stronger ENSO indices. Twelve species negatively correlate ( P ≤0.05) with local or regional temperature at least at one location; five species negatively correlate with ENSO. Eleven species' arrival is correlated ( P ≤0.05) with NAO; 10 are negatively associated. After an exhaustive literature search, this is apparently the first documentation of an association between NAO and migratory phenology in western North America.     

Changes in breeding phenology of eastern Ontario frogs over four decades

Global climate change has been implicated in phenological shifts for a variety of taxa. Amphibian species in particular are sensitive to changes in their environment due to their biphasic life history and restricted reproductive requirements. Previous research has shown that not all temperate amphibian species respond similarly to the same suite of climatic or environmental cues, nor are individual species necessarily uniform in their responses across their range. We examined both the timing of spring emergence and calling phenology of eight anuran species in southeastern Ontario, Canada, using an approximately 40‐year dataset of historical records of amphibian activity. Rana pipiens was the only species out of eight considered to emerge significantly earlier, by an estimated 22 days over four decades. Both R. pipiens and Bufo americanus have advanced initiation of calling over a four‐decade span significantly earlier by an estimated 37.2 and 19.2 days, respectively. Rana sylvatica showed a trend toward earlier emergence by 19 days, whereas we did not detect changes in emergence phenology for the remaining five species. This significant shift in breeding behavior for two species correlates to significant regional increases in spring temperatures of an estimated 2.7–2.8°C overall over four decades. Our study suggests that local temperature increases have affected the timing of emergence and the onset of calling activity in some Ontario anuran species. Global decline or range shifts ultimately may be related to changes in reproductive behavior and timing mediated by shifting climate.     

Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

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.     

Zooplankton species composition is linked to ocean transport in the Northern California Current

In the Northern California Current (NCC), zooplankton communities show interannual and multiyear shifts in species dominance that are tracked by survival of salmon populations. These zooplankton community changes correlate with the Pacific Decadal Oscillation (PDO) index: a ‘warm‐water’ copepod species group is more abundant during warm (positive) phases of the PDO and less abundant during cold (negative) phases; the reverse occurs for a ‘cold‐water’ species group. The observed relationship led to the hypothesis that the relative dominance of warm/cold‐water copepods in the NCC is driven by changes in the horizontal advection of surface water over different phases of the PDO. To test this hypothesis, variation in surface water advection to coastal regions of the NCC over the period of 1950–2008 was investigated using a Regional Ocean Modeling System (ROMS) and passive tracer experiments, then was compared with zooplankton collected off Oregon since 1996. Results showed that surface water advection varied with the phase of the PDO; the low‐frequency component of advection anomalies strongly correlated with copepod species composition (R>0.9). During positive phases of the PDO, current anomalies were northward and onshore, resulting in transport of warmer waters and the associated copepods into the region. During negatives phases, increased equatorward current anomalies led to a copepod community that was dominated by cold‐water taxa. Our results support the hypothesis that climate‐driven changes in basin‐scale circulation controls copepod community composition in the NCC, and demonstrate that large‐scale climate forcings downscale to influence local and regional ecosystem structure.     

Climate warming: a loss of variation in populations can accompany reproductive shifts

The most documented response of organisms to climate warming is a change in the average timing of seasonal activities (phenology). Although we know that these average changes can differ among species and populations, we do not know whether climate warming impacts within‐population variation in phenology. Using data from five study sites collected during a 13‐year survey, we found that the increase in spring temperatures is associated with a reproductive advance of 10 days in natural populations of common lizards (Zootoca vivipara). Interestingly, we show a correlated loss of variation in reproductive dates within populations. As illustrated by a model, this shortening of the reproductive period can have significant negative effects on population dynamics. Consequently, we encourage tests in other species to assess the generality of decreased variation in phenological responses to climate change.