“Metaecosystem dynamics of marine phytoplankton alters resource use efficiency along stoichiometric gradients”
Nils Gülzow, Yanis Wahlen, and Helmut Hillebrand (Jan 2019)
In metaecosystems the stoichiometry of resource supply affects resource use and biomass production of phytoplankton
Resources needed for autotroph growth are often distributed heterogeneously in space, which confronts the organisms with imbalanced resource ratios. The imbalance leads to abiotic flows of matter along concentrations gradients, but also interacts with the spatial mobility of organisms, as these can capture resources in different local habitats. These considerations have been well established in metaecosystem theory, where however theoretical advances occur at much larger speed than empirical assessments of the base predictions. Therefore, we conducted a metaecosystem experiment with marine phytoplankton species. Each metaecosystem consisted of five linearly connected Erlenmeyer flasks (local patches) connected by silicon tubes, which could be opened or closed in different time intervals. Thereby, we created two levels of connectivity (low and high) and crossed this with a second treatment altering resource heterogeneity in space: half of the metaecosystems had equal resource supply in each patch (“uniform”), the other received the same total amount of two nutrients (nitrogen and phosphorus, N and P) but N supplied only to patch 1 and P only supplied to patch 5 (“gradient”). Using this approach we found that nutrients were less efficiently used in the gradient metaecosystem than in the uniform ones. Moreover, at extremely low P supply in the gradient, the abundant N was not efficiently transferred into biomass production anymore. Thus, the stoichiometry of resource supply created a clear spatial imprint on algal productivity – and also composition.
Metaecosystem theory addresses the link between local (within habitats) and regional (between habitats) dynamics by simultaneously analyzing spatial community ecology and abiotic matter flow. Here, we experimentally address how spatial resource gradients and connectivity affect resource use efficiency (RUE) and stoichiometry in marine phytoplankton as well as the community composition at local and regional scales. We created gradostat metaecosystems consisting of five linearly interconnected patches, which either were arranged in countercurrent gradients of nitrogen (N) and phosphorus (P) supply or with a uniform spatial distribution of nutrients, and which had either low or high connectivity. Gradient metaecosystems were characterized by higher remaining N and P concentrations (and N:P ratios) than uniform ones, a difference reduced by higher connectivity. The position of the patch in the gradient strongly constrained elemental stoichiometry, local biovolume production and RUE. Expectedly, algal C:N, biovolume and N-specific RUE decreased towards the N-rich end of gradient metaecosystem, whereas the opposite was observed for most of the gradient for C:P, N:P and P-specific RUE. However, at highest N:P supply, unexpectedly low C:P, N:P, and P-specific RUE values were found, indicating that the low availability of P inhibited efficient use of N and biovolume production. Consequently, gradient metaecosystems had lower overall biovolume at the regional scale. Whereas treatment effects on local richness were weak, gradients were characterized by higher dissimilarity in species composition. Thus, the stoichiometry of resource supply and spatial connectivity between patches appeared as decisive elements constraining phytoplankton composition and functioning in metaecosystems.