Network characteristics of the Cape Fear River estuary food web

Background/Question/Methods

The Cape Fear River Estuary (CFRE) is a black water system in southeastern North Carolina with a direct connection to the Atlantic Ocean. The estuary itself serves as a nursery area for important commercial fisheries, such as the blue crab, as well as providing recreation and an important shipping route. Because the estuary has a high concentration of colored dissolved organic matter, making it “black,” primary production in the system is often limited. We expect that this limitation will impact the entire food web and ecosystem functioning. To assess this impact, we constructed an initial summer food web for the polyhaline region of the estuary based on previously reported empirical work and ecological knowledge. We then applied network analyses to characterize the model using the igraph R package. This analysis includes a suite of graph-level indices as well as node degree and betweenness centrality. We used the coefficient of variation to describe the degree of centralization (concentration) in the centrality measures. 

Results/Conclusions

The food web consists of 18 different nodes, each representing a functional group. Three nodes, DOC, Suspended POC, and Sediment POC, represent non-living detritus compartments. Three nodes, Macrophytes, Microphytobenthos, and Phytoplankton, represent primary producers. Two nodes, Benthic Feeding Fishes and Carnivorous Birds, represent top carnivores. The connectance value is 0.21, and the link density value is 3.83. We compared the degree and betweenness centrality of the nodes to highlight the importance of different groups in the estuary. Degree centrality indicated that DOC is the most important compartment, followed by Sediment POC, Benthic Feeding Fishes, and Suspended POC. In contract, betweenness centrality indicated that Sediment POC is the most important, followed by Suspended POC and Free-Living Bacteria. Betweenness centrality was much more centralized (CV=1.84) than the degree centrality (CV=0.48). This suggests that from the perspective of betweenness, the power in the web connectivity is concentrated into a smaller number of nodes. Both centrality measures highlighted the importance of detrital compartments, while neither measure indicated a high relative importance of primary producers. This result matches our expectation for this black water estuary. Future work will focus on refining the model using ecosystem network analysis to further characterize carbon fluxes in the system.