Effects of a half a millennium winter on a deep lake – a shape of things to come?

Analyses of the effects of extreme climate periods have been used as a tool to predict ecosystem functioning and processes in a warmer world. The winter half‐year 2006/2007 (w06/07) has been extremely warm and was estimated to be a half‐a‐millennium event in central Europe. Here we analyse the consequences of w06/07 for the temperatures, mixing dynamics, phenologies and population developments of algae and daphnids (thereafter w06/07 limnology) in a deep central European lake and investigate to what extent analysis of w06/07 limnology can really be used as a predictive tool regarding future warming. Different approaches were used to put the observations during w06/07 into context: (1) a comparison of w06/07 limnology with long‐term data, (2) a comparison of w06/07 limnology with that of the preceding year, and (3) modelling of temperature and mixing dynamics using numerical experiments. These analyses revealed that w06/07 limnology in Lake Constance was indeed very special as the lake did not mix below 60 m depth throughout winter. Because of this, anomalies of variables associated strongly with mixing behaviour, e.g., Schmidt stability and a measure for phosphorus upward mixing during winter exceeded several standard deviations the long‐term mean of these variables. However, our modelling results suggest that this extreme hydrodynamical behaviour was only partially due to w06/07 meteorology per se, but depended also strongly on the large difference in air temperature to the previous cold winter which resulted in complete mixing and considerable cooling of the water column. Furthermore, modelling results demonstrated that with respect to absolute water temperatures, the model ‘w06/07’ most likely underestimates the increase in water temperature in a warmer world as one warm winter is not sufficient to rise water temperatures in a deep lake up to those expected under a future climate.     

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.     

Phosphorus decrease and climate variability: mediators of synchrony in phytoplankton changes among European peri-alpine lakes

1. In an attempt to discern long‐term regional patterns in phytoplankton community composition we analysed data from five deep peri‐alpine lake basins that have been included in long‐term monitoring programmes since the beginning of the 1970s. Local management measures have led to synchronous declines in phosphorus concentrations by more than 50% in all four lakes. Their trophic state now ranges from mesotrophic to oligotrophic. 2. No coherence in phytoplankton biomass was observed among lakes, or any significant decrease in response to phosphorus (P)‐reduction (oligotrophication), except in Lakes Constance and Walen. 3. Multivariate analyses identified long‐term changes in phytoplankton composition, which occurred coherently in all lakes despite the differing absolute phosphorus concentrations. 4. In all lakes, the phytoplankton species benefiting from oligotrophication included mixotrophic species and/or species indicative of oligo‐mesotrophic conditions. 5. A major change in community composition occurred in all lakes at the end of the 1980s. During this period there was also a major shift in climatic conditions during winter and early spring, suggesting an impact of climatic factors. 6. Our results provide evidence that synchronous long‐term changes in geographically separated phytoplankton communities may occur even when overall biomass changes are not synchronous.     

Use of ciliate and phytoplankton taxonomic composition for the estimation of eicosapentaenoic acid concentration in lakes

1. The polyunsaturated fatty acid eicosapentaenoic acid (EPA) plays an important role in aquatic food webs, in particular at the primary producer–consumer interface where keystone species such as daphnids may be constrained by its dietary availability. Such constraints and their seasonal and interannual changes may be detected by continuous measurements of EPA concentrations. However, such EPA measurements became common only during the last two decades, whereas long‐term data sets on plankton biomass are available for many well‐studied lakes. Here, we test whether it is possible to estimate EPA concentrations from abiotic variables (light and temperature) and the biomass of prey organisms (e.g. ciliates, diatoms and cryptophytes) that potentially provide EPA for consumers. 2. We used multiple linear regression to relate size‐ and taxonomically resolved plankton biomass data and measurements of temperature and light intensity to directly measured EPA concentrations in Lake Constance during a whole year. First, we tested the predictability of EPA concentrations from the biomass of EPA‐rich organisms (diatoms, cryptophytes and ciliates). Secondly, we included the variables mean temperature and mean light intensity over the sampling depth (0–20 m) and depth (0–8 and 8–20 m) as factors in our model to check for large‐scale seasonal‐ and depth‐dependent effects on EPA concentrations. In a third step, we included the deviations of light and temperature from mean values in our model to allow for their potential influence on the biochemical composition of plankton organisms. We used the Akaike Information Criterion to determine the best models. 3. All approaches supported our proposition that the biomasses of specific plankton groups are variables from which seston EPA concentrations can be derived. The importance of ciliates as an EPA source in the seston was emphasised by their high weight in our models, although ciliates are neglected in most studies that link fatty acids to seston taxonomic composition. The large‐scale seasonal variability of light intensity and its interaction with diatom biomass were significant predictors of EPA concentrations. The deviation of temperature from mean values, accounting for a depth‐dependent effect on EPA concentrations, and its interaction with ciliate biomass were also variables with high predictive power. 4. The best models from the first and second approaches were validated with measurements of EPA concentrations from another year (1997). The estimation with the best model including only biomass explained 80%, and the best model from the second approach including mean temperature and depth explained 87% of the variability in EPA concentrations in 1997. 5. We show that it is possible to predict EPA concentrations reliably from plankton biomass, while the inclusion of abiotic factors led to results that were only partly consistent with expectations from laboratory studies. Our approach of including biotic predictors should be transferable to other systems and allow checking for biochemical constraints on primary consumers.     

Lake responses to reduced nutrient loading – an analysis of contemporary long-term data from 35 case studies

1. This synthesis examines 35 long‐term (5–35 years, mean: 16 years) lake re‐oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L^?1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north‐temperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in‐lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in‐lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100–150 μg L^?1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re‐oligotrophication.     

Large-scale climatic signatures in lakes across Europe: a meta-analysis

Recent studies have highlighted the impact of the winter North Atlantic Oscillation (NAO) on water temperature, ice conditions, and spring plankton phenology in specific lakes and regions in Europe. Here, we use meta-analysis techniques to test whether 18 lakes in northern, western, and central Europe respond coherently to winter climate forcing, and to assess the persistence of the winter climate signal in physical, chemical, and biological variables during the year. A meta-analysis approach was chosen because we wished to emphasize the overall coherence pattern rather than individual lake responses. A particular strength of our approach is that time-series from each of the 18 lakes were subjected to the same robust statistical analysis covering the same 23-year period. Although the strongest overall coherence in response to the winter NAO was exhibited by lake water temperatures, a strong, coherent response was also exhibited by concentrations of soluble reactive phosphorus and soluble reactive silicate, most likely as a result of the coherent response exhibited by the spring phytoplankton bloom. Lake nitrate concentrations showed significant coherence in winter. With the exception of the cyanobacterial biomass in summer, phytoplankton biomass in all seasons was unrelated to the winter NAO. A strong coherence in the abundance of daphnids during spring can most likely be attributed to coherence in daphnid phenology. A strong coherence in the summer abundance of the cyclopoid copepods may have been related to a coherent change in their emergence from resting stages. We discuss the complex nature of the potential mechanisms that drive the observed changes.     

Flowering and fruiting phenology of Normanbya normanbyi (W. Hill) L. H. Bailey (Arecaceae), a palm endemic to the lowland tropical rainforest of north‐eastern Australia

Abstract Normanbya normanbyi (W. Hill) L. H. Bailey (Arecaceae) is a monoecious, arborescent palm with a very small distribution area within the Daintree rainforest in north‐eastern Australia. Our 2‐year study was focused on the reproductive phenology at the individual and population level. At the population level flowering peaked in the dry season, whereas fruiting was confined to the wet season. Each palm can bear up to three inflorescences/infructescences at the same time. Flowering of each inflorescence is separated from each other by a couple of weeks. A single inflorescence consists of about 1900 staminate and 800 pistillate flowers. The flowering of N. normanbyi is protandrous with a staminate phase lasting 40 days and a pistillate phase of approximately 2 weeks. Between both phases is a non‐flowering phase of about 9 days. Fruit ripening takes 21 weeks, with an average of about 280 ripe fruit per tree. Comparison of three study plots revealed a moderate synchrony of flowering and fruiting initiation in this species of palm. The male phase of flowering shows a higher degree of synchrony than the female phase at the population level. Seasonal regularity of flowering and fruiting peaks appears to be predictable. The general flowering and fruiting phenology of N. normanbyi follows a subannual pattern with a strong tendency towards a continual pattern.     

Mitochondrial Diversity and Distribution of African Green Monkeys (Chlorocebus Gray, 1870)

African green monkeys (Chlorocebus) represent a widely distributed and morphologically diverse primate genus in sub‐Saharan Africa. Little attention has been paid to their genetic diversity and phylogeny. Based on morphological data, six species are currently recognized, but their taxonomy remains disputed. Here, we aim to characterize the mitochondrial (mt) DNA diversity, biogeography and phylogeny of African green monkeys. We analyzed the complete mitochondrial cytochrome b gene of 126 samples using feces from wild individuals and material from zoo and museum specimens with clear geographical provenance, including several type specimens. We found evidence for nine major mtDNA clades that reflect geographic distributions rather than taxa, implying that the mtDNA diversity of African green monkeys does not conform to existing taxonomic classifications. Phylogenetic relationships among clades could not be resolved suggesting a rapid early divergence of lineages. Several discordances between mtDNA and phenotype indicate that hybridization may have occurred in contact zones among species, including the threatened Bale monkey (Chlorocebus djamdjamensis). Our results provide both valuable data on African green monkeys’ genetic diversity and evolution and a basis for further molecular studies on this genus. Am. J. Primatol. 75:350‐360, 2013. © 2013 Wiley Periodicals, Inc.     

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.