Confluence network dynamics can create a spatial mosaic of predator interactions

Background/Question/Methods

Geomorphology or large scale land forms can create variation in habitat that can differentially attract and retain predators and consequently influence consumers within food webs. Specifically, tributary junctures, also known as confluences, can create ecological hotspots through heterogeneity in shape, size, substrate, water velocity, and other aspects of complexity. These hotspots can aggregate consumers and alter food webs. The network dynamics hypothesis predicts that the presence and position of confluences can alter productivity, ecosystem function, and community structure. This hypothesis has been very influential in geomorphology and ecosystem ecology, but less tested in community ecology especially related to food web interactions. Striped Bass (Morone saxatilis) are an important predator that can influence prey populations, communities, and ecosystems. Ecologically, Striped Bass provide goods and services for recreational and commercial anglers. Plum Island Estuary (PIE) in northeastern Massachusetts supports a summer resident population of migratory Striped Bass that move up the east coast from natal estuaries (Hudson, Delaware, Chesapeake) in the summer to feed.  PIE also has a complex river network that provides a model system for examining how confluences of different size, structure, complexity, and location are used by mobile predators. Previous research on Striped Bass in PIE showed that these predators were not evenly distributed throughout the estuary, but instead are aggregated by geomorphological discontinuities in the seascape. Here, we test if these mobile consumers use specific types of tributary confluences differentially by quantifying variation in confluence structure, then linking those patterns to striped bass presence, abundance, and residence at 40 sites throughout PIE.

Results/Conclusions

Using GIS, ten metrics that describe number, location, size, and variation in confluences were calculated from aerial images. The complex river network in PIE yielded a variety of confluence conditions.  Specifically, the forty sample sites in PIE grouped into five clusters based on confluence presence, size, variability, and complexity. Confluence variety was associated with higher predator density. In summary, because the structure of confluences can influence consumers in general, understanding how river network morphology affects top predator aggregations specifically has substantial basic and applied significance for both ecology and conservation. For example, this research can benefit the many Massachusetts anglers that collectively spend millions of dollars on this popular recreational sport fishery and can also help conserve ecosystem goods and services provided by this important predator.