Comparing biological classifications of freshwater phytoplankton: a case study from South China

The use of ecological classification systems is becoming more and more widely used when studying phytoplankton. Grouping phytoplankton species into ecologically coherent groups allow to reduce redundancy and in this way, to handle a minor number of biological variables when investigating the ecological status of aquatic ecosystems. Three ecological classifications are mostly used when freshwater phytoplankton is studied: functional groups or coda, morpho-functional groups (MFGs) and morphology-based functional groups (MBFGs). In this study, these three ecological classifications were comparatively used along with two taxonomic classifications based on species and genera to analyse phytoplankton response to environmental variability in three sub-tropical Chinese reservoirs. Canonical correspondence analysis was performed to compare the five mentioned biological classifications. When ecological classifications were used, the percentage of variance explained in the biological groups?Cenvironmental variables was higher than that explained by the taxonomic classifications. Coda and MFGs showed a very high degree of overlapping, but since coda are associated to very detailed environmental templates, this method was more helpful in explaining phytoplankton variability in relation to environmental factors.     

Morpho-Functional Groups and phytoplankton development in two deep lakes (Lake Garda,Italy and Lake Stechlin,Germany)

Phylogenetic classifications of plants often do not reflect their ecological functions. In fact, the functional mechanisms of biological communities may be better understood if species are pooled into groups having similar characteristics. The objective of this work is to evaluate, with the use of multivariate methods, classifications based on the morphological and functional characteristics (size and form, mobility, potential mixotrophy, nutrient requirements, presence of gelatinous envelopes) of cyanobacteria and eukaryotic algae to explain the seasonal dynamic of the phytoplankton community. The analyses involve data from two deep lakes: Lake Garda, southern Alps, z _max = 350 m; biennium 2002–2003) and Lake Stechlin (north-east Germany, z _max = 67 m; 1995, 1998 and 2001). In both lakes, the temporal evolution of the phytoplankton communities within individual years followed a regular annual cycle, with the exception of Lake Stechlin in 1998, when an irregular phytoplankton pattern was caused by a sudden mass appearance of Planktothrix rubescens in the spring and summer months, resulting in a collapse of the whole community in autumn. Overall, the temporal developments of the phytoplankton communities obtained on the basis of patterns of the morpho-functional groups appeared highly comparable with those obtained, in the single years, on the basis of the original phytoplankton species matrices. The comparison of the morpho-functional groups of the lakes Garda and Stechlin showed important differences in the abundance and seasonality of the dominant phytoplankton types. The results obtained in this study underline that the use of classifications based on the adaptive strategies of the single species may represent a useful tool to investigate the community evolution and to compare phytoplankton assemblages of different lakes, overcoming problems related to possible differences of taxonomic accuracy and identification.     

Phytoplankton configuration in six deep lakes in the peri-Alpine region: are the key drivers related to eutrophication and climate?

The aim of this study was to draw a general picture of the phytoplankton community in peri-Alpine lakes, including for the first time a broad data set of six deep peri-Alpine lakes, belonging to the same geographical region. The objective was to define the main key drivers that influence the phytoplankton community composition in this particular vulnerable region, for which the impacts of climate change have been demonstrated to be stronger than on a global average. The phytoplankton was investigated with a particular focus on cyanobacteria and using a classification approach based on morpho-functional groups. We hypothesized that phytoplankton in peri-Alpine lakes is mainly driven by nutrient loads as well as by water temperatures, variables that are strongly influenced by climate change and eutrophication. Though different phytoplankton configurations among lakes were partly due to their geographical (altitude) position, assemblages were mostly linked to temperature and nutrients. Furthermore, the results confirmed the significant role of the spring fertilization on the seasonal phytoplankton development. Cyanobacteria were related to the increasing annual average of air and water temperature gradient and therefore might become more important under future warming scenario. Air temperatures have a significant impact on water temperature in the uppermost meters of the water column, with a stronger influence on warmer lakes.     

Equilibrium/steady-state concept in phytoplankton ecology

This paper summarises the outcomes of the 13^th Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP). The authors mostly addressed their contributions on the following topics: the effect of trophic state on the attainment of a steady-state; the establishment of equilibria in deep and shallow lakes; the role of spatial heterogeneity, disturbance, and stress in the establishment of equilibrium assemblages; the mechanisms leading to the steady state; the frequency and longevity of equilibrium phases, and the role of morphological and physiological plasticity of phytoplankton in maintaining the (apparently) same populations under different environmental conditions. The composition of steady-state assemblages is compared to that of phytoplankton functional groups (coda). Those functional associations recognised as steady-state assemblages appear to be strongly K-determined in many instances.     

Temperature modulated effects of nutrients on phytoplankton changes in a mountain lake

Piburger See, a dimictic mountain lake in Austria, experienced moderate cultural eutrophication in the 1950s. Lake restoration led to a re-oligotrophication in the 1990s with a decrease in seasonal phytoplankton biovolume until the late 1990s, but a reversed trend from the early 2000s onwards. We hypothesize that recent changes in phytoplankton biomass and functional structure are triggered by changes in lake nitrogen and silica concentrations, and we expect climate-related factors to modulate the trophic status of Piburger See. Phytoplankton data were analyzed by non-metric multidimensional scaling (NMDS) applied on biovolume of morpho-functional groups, combined with correlation analyses of environmental variables. Since the 2000s, short-term changes in phytoplankton of Piburger See were explained by varying concentrations and ratios of nitrogen and silica, while the inter-annual variability in phytoplankton species composition was rather attributed to superimposed rising water temperature and lake thermal stability. Our results underline the co-dominant role of phosphorus and nitrogen as phytoplankton drivers in lakes that experience periods of nitrogen limitation. The combined impact of nutrients and climate on phytoplankton development can thus mimic short-term increases in the trophic level of less productive lakes.     

Morphofunctional traits reflect differences in phytoplankton community between rivers of contrasting flow regime

A morphofunctional traits approach has been adopted to identify how environmental factors shape the phytoplankton community. This approach has been applied in two rivers where hydrodynamical conditions are expected to be the main factor acting on the phytoplankton community. Hence, morphological traits (motility, shape, size, mucilage, and silica) related to sedimentation resistance have been chosen. We have shown that differences in flow regulation through differences in flow velocities induce shifts in phytoplankton community. These shifts depend mostly on shape, buoyancy regulation, and motility of phytoplankton cells. Elongated shapes are the characteristic of unregulated sites with high flow velocities, while cells able to regulate actively their position or to reduce their density (mucilaginous colonies) are found in regulated sites with low flow velocities. Flattened shapes are also the characteristic of sites with rather low flow velocities. These results highlight the key role of flow velocity as a driving factor controlling the structure of phytoplankton community. In this study, flow velocity also structures phytoplankton community according to location rather than seasonality.     

Seasonal and spatial functional shifts in phytoplankton communities of five tropical reservoirs

Trait-based approaches have become increasingly important and valuable in understanding phytoplankton community assembly and composition. These approaches allow for comparisons between water bodies with different species composition. We hypothesize that similar changes in environmental conditions lead to similar responses with regard to functional traits of phytoplankton communities, regardless of trophic state or species composition. We studied the phytoplankton (species composition, community trait mean and diversity) of five reservoirs in Brazil along a trophic gradient from ultra-oligotrophic to meso-eutrophic. Samples at two seasons (summer/rainy and winter/dry) with a horizontal and vertical resolution were taken. Using multivariate analysis, the five reservoirs separated, despite some overlap, according to their environmental variables (mainly total phosphorus, conductivity, pH, chlorophyll a). However, between the seasonal periods, the reservoirs shifted in a similar direction in the multi-dimensional space. The seasonal response of the overall phytoplankton community trait mean differed between the ultra-oligotrophic and the other reservoirs, with three reservoirs exhibiting a very similar community trait mean despite considerable differences in species composition. Within-season differences between different water layers were low. The functional diversity was also unrelated to the trophic state of the reservoirs. Thus, seasonal environmental changes had strong influence on the functional characteristics of the phytoplankton community in reservoirs with distinct trophic condition and species composition. These results demonstrate that an ataxonomic trait-based approach is a relevant tool for comparative studies in phytoplankton ecology.     

Physical factors and dissolved reactive silica affect phytoplankton community structure and dynamics in a lowland eutrophic river (Po river,Italy)

We tested the hypothesis that species composition and persistence of phytoplankton communities in nutrient rich lowland rivers depends mainly on physical factors. The study aimed to analyse the effects of water discharge, temperature and chemistry on phytoplankton dynamic and species composition in the lowland reach of the eutrophic Po river (Italy). Both taxonomical and morpho-functional methods were used. True planktonic and tychoplanktic (i.e. detached taxa of benthic origin that remain in suspension) species were found, among which only a few taxa and functional groups prevailed. Diatoms were the most abundant, with a clear dominance of species either sensitive to the onset of water stratification or well adapted to turbid waters. Phytoplankton abundance, biomass and chlorophyll-a followed similar trends, attaining the highest values in summer, at low discharge rates. Correlation and multivariate analysis revealed that the development of a stable phytoplankton community was mainly controlled by water discharge rates. Namely, changes in water flow rates induced major variations in the community structure. The seasonal succession of phytoplankton assemblages was also related to water temperature and dissolved reactive silica availability to some extent overlapping flow effects.     

Hypertrophic phytoplankton and the Intermediate Disturbance Hypothesis

The provisions of Connell's Intermediate Disturbance Hypothesis (IDH) were investigated in relation to the behaviour of freshwater phytoplankton in a hypertrophic lagoon, paying special attention to the link between species-diversity and environmental disturbances. Phytoplankton diversity yielded different indices depending on the basic unit of calculations (cells, particles, phytoplankton units, biomass). Although their ranges were approximately equal, they did not covary so could not be considered mutually substitutable. For the purpose of IDH testing, biomass diversity was chosen.Equilibrium states were considered to obtain in those periods with a very high fraction of total phytoplankton biomass, shared by no more than three phytoplankton species. Disturbances were considered as counterparts of equilibrium states. Disturbance factors were mostly abiotic, environmental features of the lake operating on the phytoplankton community at different time scales (co-occurring and with 1- and 2-weeks' lags). These scales may relate to the time required to establish phytoplankton community structure.IDH could be suspected not to hold for the phytoplankton of this hypertrophic lake, which experienced seven near-equilibrium phases and six disturbance periods throughout the study. As a rule, equilibrium states lasted longer than disturbance periods. The expected relationships between both disturbance intensity or frequency were not shown. Furthermore, no relationship was demonstrated between diversity (and hence IDH) and the phytoplankton community change rate. Wind stress probably played a minor role in triggering disturbance events. Disturbances were shown partly to promote small-sized phytoplankton communities.Finally, a plea for studying hypertrophic phytoplankton in greater detail is stressed if its responses to disturbances are to be fully understood.