Indices of trophic and competitive relations in a planktonic network of a shallow, temperate lagoon. A graph and structural equation modeling approach

The aim of the study was to investigate the biocenotic structure of relations among zooplankton and phytoplankton taxa using the innovative approach combining graph theory and structural equation modeling (SEM). The analysis focused on the biocenosis of the Vistula Lagoon (southern Baltic) that included 32 zooplankton species and eight phytoplankton associations. It was presented as a graph model in which negative interactions were designated that indicate feeding and predatory trophic relations and also competition among species and taxonomic groups. Based on the analysis of the graph attributes, the phytoplankton groups (picophytoplankton and diatoms), among which communication in the biocenosis proceeds mainly through the zooplankton species that feed on them, had the highest closeness centrality indicators. High values of this indicator were also noted for the Copepoda species Eurytemora affinis, because of its numerous relations with predation and competition. The relation between the Cladocera genus Ceriodaphnia sp. and the phytoplankton groups of Dinoflagellata and Cryptophytes, as measured with the edge betweenness indicator, had the greatest impact on biocenotic network cohesion, and this revealed the role of rare taxa in the biocenosis. Data from the SEM graph model that was obtained based on regression analysis indicated the domination of co-occurring taxa when the algal food base was abundant and the negative dependence of zooplankton species feeding on the main groups of phytoplankton. The dominating crustacean in the model was Calanoida nauplii, which initiated a few relational sequences in this trophic system. Among the Rotifera, the dominating species in this instance was Keratella c. cochlearis. The innovative complementary use of modeling based on graph theory and SEM to characterize the ecological systems appeared to be an effective tool with significant research and practical potential.