Lack of pattern among phytoplankton assemblages. Or,what does the exception to the rule mean?

In Science we cannot say that `the exception proves the rule'. We have been looking to define patterns in phytoplankton occurrence across trophic spectra where conspicuous covariations between algae and trophic states have been reported. We consider quite different phytoplankton communities observed under similar trophic conditions: we illustrate this point by considering five different phytoplankton communities living in five water bodies in the same wetland, along a TP gradient and over a period of 2 years. This system showed a remarkable dissimilarity of species representation, implying communities of uncorrelated species vary considerably over time. Despite the presence of some characteristic species, communities were not related to a given trophic state. However, coarser community attributes, such as clusters of taxonomic classes, appeared to be more useful in identifying patterns and assembly rules related to trophic spectra. Some ecological concepts can be related to this lack of pattern, e.g., nonconvergence, trajectories far from equilibrium and assembly rules of communities.     

Inferring the Relative Resilience of Alternative States

Ecological systems may occur in alternative states that differ in ecological structures, functions and processes. Resilience is the measure of disturbance an ecological system can absorb before changing states. However, how the intrinsic structures and processes of systems that characterize their states affects their resilience remains unclear. We analyzed time series of phytoplankton communities at three sites in a floodplain in central Spain to assess the dominant frequencies or “temporal scales” in community dynamics and compared the patterns between a wet and a dry alternative state. The identified frequencies and cross-scale structures are expected to arise from positive feedbacks that are thought to reinforce processes in alternative states of ecological systems and regulate emergent phenomena such as resilience. Our analyses show a higher species richness and diversity but lower evenness in the dry state. Time series modeling revealed a decrease in the importance of short-term variability in the communities, suggesting that community dynamics slowed down in the dry relative to the wet state. The number of temporal scales at which community dynamics manifested, and the explanatory power of time series models, was lower in the dry state. The higher diversity, reduced number of temporal scales and the lower explanatory power of time series models suggest that species dynamics tended to be more stochastic in the dry state. From a resilience perspective our results highlight a paradox: increasing species richness may not necessarily enhance resilience. The loss of cross-scale structure (i.e. the lower number of temporal scales) in community dynamics across sites suggests that resilience erodes during drought. Phytoplankton communities in the dry state are therefore likely less resilient than in the wet state. Our case study demonstrates the potential of time series modeling to assess attributes that mediate resilience. The approach is useful for assessing resilience of alternative states across ecological and other complex systems.     

The influence of Procambarus clarkii (Cambaridae,Decapoda) on water quality and sediment characteristics in a Spanish floodplain wetland

This study describes the effects of the American red swamp crayfish, Procambarus clarkii Girard, on water quality and sediment characteristics in the Spanish floodplain wetland, Las Tablas de Daimiel National Park. Our short term enclosure study during a summer drawdown revealed that crayfish acted as a nutrient pump that transformed and translocated sediment bound nutrients to the water column. Water quality impoverishment was mainly due to the increase of dissolved inorganic nutrients (soluble reactive phosphorus and ammonia), and a significant increase of total suspended solids occurred likely as a result of crayfish associated bioturbation. At the same time, crayfish reduced the content of organic matter in the sediment and we observed a slight increase of total sediment phosphorus and nitrogen content as a result of crayfish benthic activity. P. clarkii effects, in terms of internal nutrient loading (229.91 mg TP m^–2 d^–1), were shown to be important on a local scale, indicating the significance of internal nutrient supply to water column primary producers particularly under low external supply (summer). Extrapolations to the whole ecosystem, however, revealed a negligible crayfish contribution (0.06%) to total internal nutrient loading (0.035 mg TP m^–2 d^–1). Hence, crayfish spatial heterogeneity patterns are important in global and local matter fluxes and nutrient cycles in wetlands.     

Evaluating environmental conditions of a temporary pond complex using rotifer emergence from dry soils

In this study we evaluated the usefulness of rotifer emergence from dry soils to indicate the environmental status of a remnant temporary pond complex in an agricultural area in central Spain. Because the ponds did not flood during our project, emergence was studied in relation to environmental variables in outdoor microcosms. Redundancy analysis and nestedness analysis showed that salinity and total nitrogen concentrations shaped the emerged communities. Depauperate rotifer assemblages from pond microcosms with higher salinity and nitrogen levels were nested subsets of species-rich communities from microcosms of less enriched sites. Rotifer community structure also identified small ponds as reference sites that should receive priority in conservation and degradation mitigation programs. Results suggest that rotifer emergence from rewetted sediments using microcosms can be useful for evaluating the nutrient status during the dry phase of temporary wetlands. While results from this approach are conservative, requiring comparisons with field observations, their tentative value lies in alerting management and providing a basis for future research of poorly studied but threatened temporary habitats. Rotifer emergence could be a useful alternative to traditional biological indicators of nutrient status that depend on the presence of water.     

Local and landscape effects on temporary pond zooplankton egg banks: conservation implications

Understanding landscape correlates of local habitat integrity and community structure and the identification of spatial scales at which these associations operate are relevant for management and conservation of unique but globally threatened temporary ponds. We use a multivariate variance decomposition approach to determine taxon-specific associations of zooplankton communities banked in dry pond soils (rotifers, cladocerans, copepods) with local habitat features and landscape characteristics across four spatial scales (100 buffer strip, 1, 5, and 10 km scales). Results show similar degrees of correlation between rotifer and cladoceran communities with local habitat conditions (chiefly water quality). This is interpreted according to life-history traits of component species of wetland propagule banks. Associations with landscape features varied between communities with rotifers correlating with landscape structural features only at the buffer scale while cladocerans showed no significant correlations with landscape characteristics across all scales. Copepods were neither significantly associated with local nor landscape characteristics. The results of this study contrast strikingly with our previous population-based study, where populations of Triops cancriformis and Branchinecta orientales were significantly correlated with landuse features at the broadest scale. The combined results suggest that a three-way management scheme could be useful for conservation of zooplankton resting egg banks in this remnant wetland complex. These schemes could focus on the restoration of wetland water quality, the establishment of vegetated buffer strips around the ponds to counteract degradation resulting from runoff, and reforestation and/or the creation of hedgerows in agricultural catchments to avoid impacts resulting from broad-scale diffuse pollution fluxes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A plea for more ecology in phytoplankton ecology

When looking for a pattern of phytoplankton behaviour across trophic gradients, we need to cross the boundaries between different disciplinary areas, from autoecology to systems ecology, because eutrophication is a complex process which involves different time scales and different levels of community structure. Thus, we submit our observations to the muddled conceptual world of assemblage ecology. These inaccuracies arise, for example, from both species and community arguments; eutrophication as a fertilization or a metabolic phenomenon; and the notions frequently interwoven of pattern, process and rules. We suggest that it is advantageous to tackle this issue from the perspective of general ecology, rather than from a specifically planktonic orientation. In this way, useful general ecological tools, for example, time series and assembly-rule studies, can be used. Time-series study allows the dynamics of any variable to be described or to show that long term variable fluctuations may sometimes be unregulated, in response to some exogenous factor. Rules of assembly help us to resolve which traits are selectively involved during the eutrophication process. In this context, we advocate (1) the use of traits instead of morphospecies in phytoplankton studies, (2) looking for the dynamic patterns of phytoplankton with eutrophication, (3) the use of time series techniques to study phytoplankton trajectories, (4) the use of assembly rules to discern patterns in the formation of multispecies assemblages, (5) the consideration of the pelagic food-web in studies of phytoplankton dynamics and, as an overall suggestion, to borrow knowledge and inspiration from general ecology.     

A worldwide view of organic carbon export from catchments

Growing interest in the effects of global change on the metabolism, stoichiometry and cycling of carbon in aquatic ecosystems has motivated research on the export of organic carbon (OCE) from catchments. In this article, quantitative and functional features of the annual export rates of total, particulate and dissolved organic carbon (TOC, POC and DOC) were reviewed, and the stoichiometry of export (OC:N, OC:P and N:P) from 550 catchments worldwide was reported. TOC export ranged 2.1–92,474?kg?C?km^?2?year^?1, POC export ranged 0.4–73,979?kg?C?km^?2?year^?1 and DOC export ranged 1.2–56,946?kg?C?km^?2?year^?1. Exports of TOC and DOC were strongly linked, but POC export was unrelated to DOC. The DOC fraction comprised on average 73?±?21% of TOC export. The export rates of organic carbon were poorly related to those of total nitrogen and total phosphorus. Discrete and continuous environmental variables failed to predict TOC export, but DOC export was influenced by discharge and catchment area worldwide. Models of OCE in different catchment types were controlled by different environmental variables; hydrological variables were generally better predictors of OCE than anthropogenic and soil variables. Elemental ratios of carbon export in most catchments were above the Redfield ratio, suggesting that phosphorus may become the limiting nutrient for downstream plant growth. These ratios were marginally related to environmental data. More detailed hydrological data, consideration of in-stream processes and the use of quasi-empirical dynamical models are advocated to improve our knowledge of OCE rates and those of other nutrients.     

The significance of water inputs to plankton biomass and trophic relationships in a semi-arid freshwater wetland (central Spain)

This study attempts to describe changes in plankton biomass(including bacteria, phytoplankton, ciliates, rotifers, andcrustaceans) in the semi-arid, hypertrophic, freshwater wetland,Las Tablas de Daimiel National Park (central Spain), in relationto water inflow. An inter-annual comparison of 1996 and 1997,with contrasting rainfall, reveals that inflows in the formof flooding act as a bottom-up structuring force on total planktonbiomass. However, the responses of plankton biomass to floodingwere different in strength between the two years, which canbe attributed to shifts in plankton community structure. Crustaceanzooplankton conditioned total plankton biomass in 1996, basedon the relationship between the high individual biomass andcommunity development. Crust-aceans decreased in 1997, whilebacteria, phytoplankton and rotifers increased. The quantitativeincrease of the latter groups resulted in a higher total planktonbiomass in 1997. The key position of crustaceans in the wetlandplankton is also reflected in their capability for exertingsignificant top-down control, which was clearly evident in 1996but weaker in 1997, based on their numerical reduction. Statisticalanalyses provide evidence that the decline of trophic interactionsin 1997 is related to the strength of flooding in that year,thereby highlighting the significance of the disruptive actionof physical disturbance on biotic interactions in the plankton.Flooding and areal inundation were also shown to be significantfor spatial heterogeneity. In 1996, site-specific developmenttook place in the absence of prolonged flooding effects, resultingin high spatial heterogeneity. In 1997, however, remarkablehomogenization of plankton biomass occurred along the majorwater flow path. Thus, wetland landscape heterogeneity dependson inflows which condition areal inundation. This, in turn,influences plankton dynamics.     

Long-term changes in spatial patterns of emergent vegetation in a Mediterranean floodplain: natural versus anthropogenic constraints

This study describes the long-term changes of spatial patterns of cut-sedge (Cladium mariscus) and common reed (Phragmites australis) in the Spanish floodplain wetland Las Tablas de Daimiel. Using seven sets of aerial photographs, we determined changes in their spatial patterns (size of patches) between 1945 and 2001 that resulted from combinations of natural change and anthropogenic stress (irrigated agriculture, waste water discharge, fire, ploughing). Our approach consisted in using 1 ha cells as units of spatial resolution in principal coordinates of neighbour matrices and spatial correlograms to assess the spatial scale of interest and spatial patterns at (1) the whole wetland and (2) two areas of the wetland with locally contrasting biophysical settings and anthropogenic stress history. Results showed that vegetation spatial patterns were influenced by natural variability until the 1970s. Thereafter, anthropogenic perturbation became the main driver of vegetation change, especially in the lower part of the wetland where local impacts were stronger. Natural variability did not fragment cut-sedge patches, and the effects of biological traits were less important for its spatial pattern. By contrast, man-made change resulted in a marked cut-sedge cover decrease and patch fragmentation, and the importance of its biological traits on spatial patterns increased. The trends of reed spatial patterns were generally opposite to, but not as clear as those for, cut-sedge. The trends of spatial patterns were especially evident at very broad (3,500–10,000 m) and broad (1,000–3,400 m) spatial scales. Competition for space and abiotic factors (water quality and water depth) were not strong predictors of cover variability at the 1-ha scale, particularly in the lower area of the wetland. This suggests that other environmental variables need to be considered in spatially explicit modelling of vegetation spatial patterns in wetlands. Consideration of spatial hierarchies and species-specific ecological traits is paramount to the conservation of degraded wetlands.