Species extinctions strengthen the relationship between biodiversity and resource use efficiency

Evidence from terrestrial ecosystems indicates that biodiversity relates to ecosystem functions (BEF), but this relationship varies in its strength, in part, as a function of habitat connectivity and fragmentation. In primary producers, common proxies of ecosystem function include productivity and resource use efficiency. In aquatic primary producers, macroecological studies have observed BEF variance, where ecosystems with lower richness show stronger BEF relationships. However, aquatic ecosystems are less affected by habitat fragmentation than terrestrial systems and the mechanism underlying this BEF variance has been largely overlooked. Here, we provide a mechanistic explanation of BEF variance using a trait-based, numerical model parameterized for phytoplankton. Resource supply in our model fluctuates recurrently, similar to many coastal systems. Our findings show that following an extinction event, the BEF relationship can be driven by the species that are the most efficient resource users. Specifically, in species-rich assemblages, increased redundancy of efficient resource users minimizes the risk of losing function following an extinction event. On the other hand, in species-poor assemblages, low redundancy of efficient resource users increases the risk of losing ecosystem function following extinctions. Furthermore, we corroborate our findings with what has been observed from large-scale field studies on phytoplankton

From historical backgrounds towards the functional classification of river phytoplankton sensu Colin S. Reynolds: what future merits the approach may hold?

peer reviewedRiver phytoplankton has been studied to understand its occurrence and composition since the end of the nineteenth century. Later, pioneers addressed mechanisms that affected river phytoplankton by ‘‘origin of plankton’’, ‘‘turbulent mixing’’, ‘‘flow heterogeneity’’, ‘‘paradox of potamoplankton maintenance’’ and ‘‘dead zones’’ as keywords along the twentieth century. A major shift came with the recognition that characteristic units in phytoplankton compositions could be linked to specific set of environmental conditions, known as the ‘‘Phytoplankton Functional Group concept’’ sensu Reynolds. The FG concept could successfully be applied to river phytoplankton due to its close resemblance to shallow lakes phytoplankton. The FG approach enables one to separate the effects of ‘‘natural constraints’’ and ‘‘human impacts’’ on river phytoplankton and to evaluate the ecological status of rivers. The FG classification has mainly been advocated in the context of how the environment shaped the functional composition of phytoplankton. It may be further developed in the future by a trait-based mechanistic classification of taxa into FGs, and by the exact quantification of FGs on ecosystem functioning. These improvements will help quantify how global warming and human impacts affect river phytoplankton and corresponding alterations in ecosystem functioning

Increased contribution of parasites in microbial eukaryotic communities of different Aegean Sea coastal systems

Background-Aim: Protistan communities have a major contribution to biochemical processes and food webs in coastal ecosystems. However, related studies are scarce and usually limited in specific groups and/or sites. The present study examined the spatial structure of the entire protistan community in seven different gulfs and three different depths in a regional Mediterranean Sea, aiming to define taxa that are important for differences detected in the marine microbial network across the different gulfs studied as well as their trophic interactions. Methods: Protistan community structure analysis was based on the diversity of the V2–V3 hypervariable region of the 18S rRNA gene. Operational taxonomic units (OTUs) were identified using a 97% sequence identity threshold and were characterized based on their taxonomy, trophic role, abundance and niche specialization level. The differentially abundant, between gulfs, OTUs were considered for all depths and interactions amongst them were calculated, with statistic and network analysis. Results: It was shown that Dinophyceae, Bacillariophyta and Syndiniales were the most abundant groups, prevalent in all sites and depths. Gulfs separation was more striking at surface corroborating with changes in environmental factors, while it was less pronounced in higher depths. The study of differentially abundant, between gulfs, OTUs revealed that the strongest biotic interactions in all depths occurred between parasite species (mainly Syndiniales) and other trophic groups. Most of these species were generalists but not abundant highlighting the importance of rare species in protistan community assemblage. Conclusion: Overall this study revealed the emergence of parasites as important contributors in protistan network regulation regardless of depth

Interplay between r- and K-strategists leads to phytoplankton underyielding under pulsed resource supply

Fluctuations in nutrient ratios over seasonal scales in aquatic ecosystems can result in overyielding, a condition arising when complementary life-history traits of coexisting phytoplankton species enables more complete use of resources. However, when nutrient concentrations fluctuate under short-period pulsed resource supply, the role of complementarity is less understood. We explore this using the framework of Resource Saturation Limitation Theory (r-strategists vs. K-strategists) to interpret findings from laboratory experiments. For these experiments, we isolated dominant species from a natural assemblage, stabilized to a state of coexistence in the laboratory and determined life-history traits for each species, important to categorize its competition strategy. Then, using monocultures we determined maximum biomass density under pulsed resource supply. These same conditions of resource supply were used with polycultures comprised of combinations of the isolated species. Our focal species were consistent of either r- or K-strategies and the biomass production achieved in monocultures depended on their efficiency to convert resources to biomass. For these species, the K-strategists were less efficient resource users. This affected biomass production in polycultures, which were characteristic of underyielding. In polycultures, K-strategists sequestered more resources than the r-strategists. This likely occurred because the intermittent periods of nutrient limitation that would have occurred just prior to the next nutrient supply pulse would have favored the K-strategists, leading to overall less efficient use of resources by the polyculture. This study provides evidence that fluctuation in resource concentrations resulting from pulsed resource supplies in aquatic ecosystems can result in phytoplankton assemblages' underyielding

Everything is not everywhere: can marine compartments shape phytoplankton assemblages?

The idea that ‘everything is everywhere, but the environment selects' has been seminal in microbial biogeography, and marine phytoplankton is one of the prototypical groups used to illustrate this. The typical argument has been that phytoplankton is ubiquitous, but that distinct assemblages form under environmental selection. It is well established that phytoplankton assemblages vary considerably between coastal ecosystems. However, the relative roles of compartmentalization of regional seas and site-specific environmental conditions in shaping assemblage structures have not been specifically examined. We collected data from coastal embayments that fall within two different water compartments within the same regional sea and are characterized by highly localized environmental pressures. We used principal coordinates of neighbour matrices (PCNM) and asymmetric eigenvector maps (AEM) models to partition the effects that spatial structures, environmental conditions and their overlap had on the variation in assemblage composition. Our models explained a high percentage of variation in assemblage composition (59–65%) and showed that spatial structure consistent with marine compartmentalization played a more important role than local environmental conditions. At least during the study period, surface currents connecting sites within the two compartments failed to generate sufficient dispersal to offset the impact of differences due to compartmentalization. In other words, our findings suggest that, even for a prototypical cosmopolitan group, everything is not everywhere

Nitrogen as the main driver of benthic diatom composition and diversity in oligotrophic coastal systems

Phytoplankton is the main indicator group for eutrophication in coastal ecosystems, however its high dispersal potential does not enable the assessment of localized effects of coastal nutrient enrichment. Benthic diatoms are sessile microalgae associated with sandy substrates and have the potential to reflect more localized pollution impacts. Although benthic diatoms are widely used bioindicators in freshwater systems, they have rarely been used for assessing the eutrophication status of oligotrophic environments such as the eastern Mediterranean Sea. In the present study, we assess the efficiency of benthic diatoms as bioindicators of nutrient enrichment in oligotrophic coastal systems, by investigating the effect of different physicochemical conditions and nutrient concentrations on the assemblage composition, diversity and individual species populations. To do this, we sampled along a eutrophication gradient formed by anthropogenic nutrient inputs from a metropolitan area. The main driver of assemblage composition, diversity and biomass of diatoms was nitrogen concentration and its temporal and spatial changes. Nitrogen loadings were positively correlated with increased biomass of Cocconeis spp. and negatively correlated with Mastogloia spp. Our findings suggest that in coastal ecosystems of oligotrophic marine ecoregions, benthic diatom assemblage structure and specific taxonomic groups can be reliable predictors of coastal eutrophication offering higher spatial resolution compared to phytoplankton

Fragmentation and reconnection in aquatic metacommunities

The impacts of fragmentation and connectivity on ecosystem structure and function are central in current ecological research and management efforts on the effects of environmental change. These issues are studied within the framework of metacommunity ecology, which is directly applicable to coastal aquatic ecosystems (e.g. interconnected lagoons, rock pools) affected by pulsed nutrient inputs. As water levels vary and patches become periodically isolated and reconnected, the structure and function of an aquatic metacommunity doesn’t solely depend on connectivity among local communities but also on their previous state during fragmentation. The present study aims to assess the effect of fragmentation and connectivity on different community attributes -species composition, diversity and productivity- in aquatic metacommunities characterized by environmental heterogeneity, employing both experimental and modeling approaches. Experiments consisted of fragmented natural phytoplankton assemblages that self-organized under different nutrient pulsing frequencies. This part of the experiment, together with numerical modeling, was used to address the role of fragmentation and resource heterogeneity on community structure. These steady-state assemblages represented the local patches that were then connected at a dispersal continuum forming metacommunities, to assess the role of spatial averaging on species richness and productivity. During fragmentation, nutrient pulses of decreasing frequencies resulted in assemblages of increasing functional diversity and richness. Numerical simulations also revealed the importance of both growth rate and competitive ability in assembly processes under pulsed nutrient inflows. Moreover, these environmental conditions led to the dominance of a species at the intermediate pulsing frequency, which, during connectivity, dominated the metacommunity. Therefore, the pivotal role of spatial averaging described in the present study, is directly linked to the conditions that gave rise to the isolated local assemblages. The present thesis addressed questions that have important implications for the management of coastal zone. It revealed that optimal ecosystem function (maximal richness, functional diversity, and productivity) in heterogeneous aquatic metacommunities can be achieved when the interval between resource pulses exceeds the generation times of constituent species, and when patches are connected at intermediate connectivity levels.Η επίδραση του κατακερματισμού και της συνδεσιμότητας στη δομή και τη λειτουργία των οικοσυστημάτων είναι πρωταρχικής σημασίας στη σύγχρονη οικολογική έρευνα και στη διαχείριση υπό το πρίσμα της κλιματικής αλλαγής. Η διερεύνηση αυτή γίνεται στα πλαίσια της θεωρίας των μετακοινοτήτων, που έχει άμεση εφαρμογή στα υδατικά οικοσυστήματα, ιδιαίτερα στην παράκτια ζώνη (π.χ. συστήματα λιμνοθαλασσών) που δέχεται παλμικές εισροές θρεπτικών. Με την αλλαγή της στάθμης του νερού, τα κατατμήματα απομονώνονται και ξαναενώνονται, συνεπώς η δομή και η λειτουργία μιας υδατικής μετακοινότητας δεν εξαρτάται μόνο από τη συνδεσιμότητα των τοπικών κοινοτήτων αλλά και από τη δομή τους κατά την απομόνωση. Σκοπός της διδακτορικής διατριβής είναι η μελέτη της επίδρασης του κατακερματισμού και της συνδεσιμότητας σε διαφορετικά χαρακτηριστικά των βιοκοινοτήτων -σύνθεση ειδών, ποικιλότητα, παραγωγικότητα- σε υδατικές μετακοινότητες που χαρακτηρίζονται από περιβαλλοντική ετερογένεια, με τη χρήση πειραμάτων και μοντέλων προσομοίωσης. Τα πειράματα περιελάμβαναν μια φυσική συνάθροιση φυτοπλαγκτού που κατακερματίστηκε και αυτό-οργανώθηκε κάτω από διαφορετικές συχνότητες παλμών θρεπτικών για τη μελέτη του ρόλου του κατακερματισμού και της περιβαλλοντικής ετερογένειας στη δομή της κοινότητας. Αυτές οι αυτό-οργανωμένες συναθροίσεις έγιναν τα τοπικά κατατμήματα τα οποία επανασυνδέθηκαν σε διαφορετικά επίπεδα, δημιουργώντας μετακοινότητες, για τη μελέτη του ρόλου της ομογενοποίησης προς τις ενδιάμεσες συνθήκες στον πλούτο ειδών και την παραγωγικότητα. Κατά τον κατακερματισμό, παλμοί θρεπτικών μειούμενης συχνότητα οδήγησαν σε συναθροίσεις αυξανόμενης λειτουργικής ποικιλότητας και πλούτου ειδών. Το μοντέλο έδειξε τη σημασία του ρυθμού αύξησης και της ανταγωνιστικής ικανότητας κατά τη συνάθροιση κάτω από παλμούς θρεπτικών. Αυτές οι συνθήκες οδήγησαν στην κυριαρχία ενός είδους στην ενδιάμεση συχνότητα παλμών, που, κατά την επανασύνδεση, κυριάρχησε σε όλη τη μετακοινότητα. Συνεπώς, ο σημαντικός ρόλος της ομογενοποίησης προς τις ενδιάμεσες συνθήκες που περιγράφεται στην παρούσα μελέτη, συνδέεται άμεσα με τις συνθήκες που δημιούργησαν το απομονωμένο τοπικό κατάτμημα. Η παρούσα διατριβή έδειξε ότι βέλτιστη οικοσυστημική λειτουργία (μέγιστος πλούτος ειδών, λειτουργική ποικιλότητα και παραγωγικότητα) σε ετερογενείς υδατικές μετακοινότητες μπορεί να επιτευχθεί όταν ο χρόνος ανάμεσα στους παλμούς των θρεπτικών ξεπερνά το χρόνο γενεάς των ειδών και σε ενδιάμεσα επίπεδα συνδεσιμότητας των κατατμημάτων

Data from: Spatial averaging and disturbance lead to high productivity in aquatic metacommunities

Dispersal in heterogeneous ecosystems, such as coastal metacommunities, is a major driver of diversity and productivity. According to theory, both species richness and spatial averaging shape a unimodal relationship of productivity with dispersal. We experimentally tested the hypothesis that disturbances acting on local patches would buffer the loss of productivity at high dispersal by preventing synchronized species oscillations. To simulate these disturbances, our experimental assemblages involved species that self-organized in isolation under three inflow pulsing frequencies, where hydraulic displacement and nutrient loading affected assemblage diversity and composition. At steady-state, the emerging isolated assemblages were connected at three levels of dispersal creating three metacommunities of different connectivity. Consistent with theory, as dispersal increased, species richness in the metacommunity declined; productivity however remained high. This occurred because the most productive species in our study (which dominated the isolated patch of intermediate inflow pulsing frequency) dominated all three patches (low, intermediate and high inflow pulsing frequencies) after dispersal commenced in our metacommunities. This experimental result provides empirical support for the mechanism of spatial averaging. Furthermore, disturbances, in the form of localized pulsed inflows, prevented population oscillation synchrony caused by homogenization. Overall, our observations suggest that localized environmental fluctuations and the identity of species seem to be more influential than dispersal in shaping the diversity and composition of phytoplankton assemblages and stabilizing productivity

Geology Can Drive the Diversity–Ecosystem Functioning Relationship in River Benthic Diatoms by Selecting for Species Functional Traits

The biodiversity–ecosystem functioning (BEF) relationship has been studied extensively for the past 30 years, mainly in terrestrial plant ecosystems using experimental approaches. Field studies in aquatic systems are scarce, and considering primary producers, they mainly focus on phytoplankton assemblages, whereas benthic diatoms in rivers are considerably understudied in this regard. We performed a field study across nine rivers in Greece, and we coupled the observed field results with model simulations. We tested the hypothesis that the diversity–biomass (as a surrogate of ecosystem functioning) relationship in benthic diatoms would be affected by abiotic factors and would be time-dependent due to the highly dynamic nature of rivers. Indeed, geology played an important role in the form of the BEF relationship that was positive in siliceous and absent in calcareous substrates. Geology was responsible for nutrient concentrations, which, in turn, were responsible for the dominance of specific functional traits. Furthermore, model simulations showed the time dependence of the BEF form, as less mature assemblages tend to present a positive BEF. This was the first large-scale field study on the BEF relationship of benthic diatom assemblages, offering useful insights into the function and diversity of these overlooked ecosystems and assemblages

Spatial averaging and disturbance lead to high productivity in aquatic metacommunities

Dispersal in heterogeneous ecosystems, such as coastal metacommunities, is a major driver of diversity and productiv-ity. According to theory, both species richness and spatial averaging shape a unimodal relationship of productivity with dispersal. We experimentally tested the hypothesis that disturbances acting on local patches would buffer the loss of productivity at high dispersal by preventing synchronized species oscillations. To simulate these disturbances, our experimental assemblages involved species that self-organized in isolation under three inflow pulsing frequencies, where hydraulic displacement and nutrient loading affected assemblage diversity and composition. At steady-state, the emerging isolated assemblages were connected at three levels of dispersal creating three metacommunities of different connectivity. Consistent with theory, as dispersal increased, species richness in the metacommunity declined; productivity however remained high. is occurred because the most productive species in our study (which dominated the isolated patch of intermediate inflow pulsing frequency) dominated all three patches (low, intermediate and high inflow pulsing frequencies) after dispersal commenced in our metacommunities. is experimental result provides empirical support for the mecha-nism of spatial averaging. Furthermore, disturbances, in the form of localized pulsed inflows, prevented population oscilla-tion synchrony caused by homogenization. Overall, our observations suggest that localized environmental fluctuations and the identity of species seem to be more influential than dispersal in shaping the diversity and composition of phytoplankton assemblages and stabilizing productivity