Maintenance of Intraspecific Diversity in Response to Species Competition and Nutrient Fluctuations.

Intraspecific diversity is a substantial part of biodiversity, yet little is known about its maintenance. Understanding mechanisms of intraspecific diversity shifts provides realistic detail about how phytoplankton communities evolve to new environmental conditions, a process especially important in times of climate change. Here, we aimed to identify factors that maintain genotype diversity and link the observed diversity change to measured phytoplankton morpho-functional traits and cell size of the species and genotypes.

Grazing Induced Shifts in Phytoplankton Cell Size Explain the Community Response to Nutrient Supply.

Phytoplankton cell size is important for a multitude of functional traits such as growth rates, storage capabilities, and resistance to grazing. Because these response traits are correlated, selective effects on mean community cell size of one environmental factor should impact the ability of phytoplankton to cope with other factors. Here, we experimentally apply expectations on the functional importance of phytoplankton cell size to the community level.

Composition and Dominance of Edible and Inedible Phytoplankton Predict Responses of Baltic Sea Summer Communities to Elevated Temperature and CO.

Previous studies with Baltic Sea phytoplankton combining elevated seawater temperature with CO revealed the importance of size trait-based analyses, in particular dividing the plankton into edible (>5 and <100 µm) and inedible (<5 and >100 µm) size classes for mesozoopankton grazers. While the edible phytoplankton responded predominantly negative to warming and the inedible group stayed unaffected or increased, independent from edibility most phytoplankton groups gained from CO. Because the ratio between edible and inedible taxa changes profoundly over seasons, we investigated if community responses can be predicted according to the prevailing composition of edible and inedible groups.

Dispersal mitigates bacterial dominance over microalgal competitor in metacommunities.

Ecological theory suggests that a combination of local and regional factors regulate biodiversity and community functioning in metacommunities. The relative importance of different factors structuring communities likely changes over successional time, but to date this concept is scarcely documented. In addition, the few studies describing successional dynamics in metacommunity regulation have only focused on a single group of organisms.

Warming has stronger direct than indirect effects on benthic microalgae in a seaweed system in spring.

Using outdoor mesocosms we investigated the relative importance of the direct and indirect (here: altered grazing) effects of seawater warming on benthic microalgae in a Baltic Sea (Phaeophyceae) system during the spring season. Seawater warming had a positive main effect on microalgal total biomass accrual and growth rate and on total mesograzer abundance and biomass. Moreover, under the existing resource-replete conditions in spring the direct positive effect of warming on microalgae was stronger than its indirect negative effect through enhanced grazing.

Inter- and intraspecific phenotypic plasticity of three phytoplankton species in response to ocean acidification.

Phenotypic plasticity describes the phenotypic adjustment of the same genotype to different environmental conditions and is best described by a reaction norm. We focus on the effect of ocean acidification on inter- and intraspecific reaction norms of three globally important phytoplankton species (Emiliania huxleyi, Gephyrocapsa oceanica and Chaetoceros affinis). Despite significant differences in growth rates between the species, they all showed a high potential for phenotypic buffering (similar growth rates between ambient and high CO conditions).

Temperature effects on seaweed-sustaining top-down control vary with season.

Rising seawater temperature and CO2 concentrations (ocean acidification) represent two of the most influential factors impacting marine ecosystems in the face of global climate change. In ecological climate change research, full-factorial experiments performed across seasons in multispecies, cross-trophic-level settings are essential as they permit a more realistic estimation of direct and indirect effects as well as the relative importance of the effects of both major environmental stressors on ecosystems. In benthic mesocosm experiments, we tested the responses of coastal Baltic Sea Fucus vesiculosus communities to elevated seawater temperature and CO2 concentrations across four seasons of one year.

Effects of sea surface warming on marine plankton.

Ocean warming has been implicated in the observed decline of oceanic phytoplankton biomass. Some studies suggest a physical pathway of warming via stratification and nutrient flux, and others a biological effect on plankton metabolic rates; yet the relative strength and possible interaction of these mechanisms remains unknown. Here, we implement projections from a global circulation model in a mesocosm experiment to examine both mechanisms in a multi-trophic plankton community.

Community composition has greater impact on the functioning of marine phytoplankton communities than ocean acidification.

Ecosystem functioning is simultaneously affected by changes in community composition and environmental change such as increasing atmospheric carbon dioxide (CO2 ) and subsequent ocean acidification. However, it largely remains uncertain how the effects of these factors compare to each other. Addressing this question, we experimentally tested the hypothesis that initial community composition and elevated CO2 are equally important to the regulation of phytoplankton biomass.

Extinction debt in source-sink metacommunities.

In an increasingly modified world, understanding and predicting the consequences of landscape alteration on biodiversity is a challenge for ecologists. To this end, metacommunity theory has developed to better understand the complexity of local and regional interactions that occur across larger landscapes. While metacommunity ecology has now provided several alternative models of species coexistence at different spatial scales, predictions regarding the consequences of landscape alteration have been done exclusively for the competition-colonization trade off model (CC).

Dispersal decreases diversity in heterogeneous metacommunities by enhancing regional competition.

Experiments and models reveal that moderate dispersal rates between local communities can increase diversity by alleviating local competitive exclusion; in contrast, high dispersal rates can decrease diversity by amplifying regional competition. However, hitherto experimental tests on how dispersal affects diversity in the presence and absence of environmental heterogeneity are largely missing, although it is known that environmental heterogeneity influences diversity. For the first time we experimentally show that the interaction between dispersal rate and the presence of an environmental gradient with on-average lower resource availability than the homogeneous control treatment affects diversity.

Biodiversity in a complex world: consolidation and progress in functional biodiversity research.

The global decline of biodiversity caused by human domination of ecosystems worldwide is supposed to alter important process rates and state variables in these ecosystems. However, there is considerable debate on the prevalence and importance of biodiversity effects on ecosystem function (BDEF). Here, we argue that much of the debate stems from two major shortcomings.

Effects of grazer richness and composition on algal biomass in a closed and open marine system.

Most natural local systems exchange organisms with a regional pool of species through migration and dispersal. Such metacommunity processes of interconnected multispecies assemblages are likely to affect local dynamics of both species and processes. We present results from an artificial marine outdoor rock pool system in which we investigated the factors of (1) local grazer richness and composition, and (2) connectivity of local patches to a regional species pool, and their effects on algal biomass.

Dispersal frequency affects local biomass production by controlling local diversity.

Dispersal is a major factor regulating the number of coexisting species, but the relationship between species diversity and ecosystem processes has mainly been analysed for communities closed to dispersal. We experimentally investigated how initial local diversity and dispersal frequency affect local diversity and biomass production in open benthic microalgal metacommunities. Final local species richness and local biomass production were strongly influenced by dispersal frequency but not by initial local diversity.