Detritus-based food webs fueled by dead organic matter from terrestrial, estuarine and coastal ecosystems support many consumer organisms (e.g., zooplankton, crabs, clams, fish) in the coastal zone. But whether these food webs are uniformly available (i.e., highly connected) to estuarine consumers or occur discretely (compartmentalized in space and time) is not entirely clear. Although recent studies have shown that the spatial scale, strength and persistence of food web links to specific detritus sources can rapidly change over narrow transition zones, these studies were located in microtidal estuaries with little or no adjacent riverine input. This study will evaluate food web compartmentalization among five estuaries in northern Puget Sound to quantify the scale, strength and mechanisms (water flow) of food web connections along intertidal gradients of estuarine marsh, mudflat, and seagrass ecosystems under differing freshwater influences. The scale of our experiments will range from meters to kilometers and from seasonal to interannual.
Quantifying scales, strengths and persistence of food web connectivity to different living and detrital sources will help refine ecosystem-based approaches to managing estuarine and coastal restoration, protection, and resource use. Results will shed light on how estuarine restoration actions such as removing levees to reintroduce tidal processes, or establishing estuarine reserves, may expand the scale of food web connectivity beyond location of the management action. By examining a variety of Pacific Northwest estuaries influenced by rivers, this study will also address the effect of river flow and watershed alteration on ecosystem connectivity across different coastal settings.
Natural connectivity across the abrupt transition from land to ocean can provide important benefits for human societies. This is especially true of estuaries, where rivers meet the sea in tidal swamps, salt marshes, seagrass beds and mud flats. However, benefits such as storm surge protection, fish habitat, and improved water quality diminish significantly when these landscapes become more fragmented through urbanization: each patch shrinks and becomes more detached, like one bead from another on a loosening strand of pearls. Human-engineered levees and armored shorelines disconnect ecosystems that once seamlessly intermeshed. Such large-scale changes can alter the natural biological, chemical and physical processes occurring within and across ecosystems, disrupting the flow of organic energy—the food web—from plants to animals. Recently, scientists have found that estuarine food webs are particularly vulnerable to changes in the ability of materials, organisms, and energy to flow across natural ecosystem boundaries. This phenomenon occurs when organic matter produced in one part of the landscape crosses ecosystem boundaries to support organisms in adjacent parts, thereby connecting a food web across space. Such spatial food web subsidies have been reported worldwide and are now understood to be critically reliant on the exchange of organic matter across the land margin. While scientists recognize the importance of spatial food web subsidies in estuaries, the impacts of decreased ecosystem connectivity are less well understood. Our new research shows that both landscape characteristics and organism biology play important roles in determining the scale and strength of food web connectivity in Pacific Northwest estuaries. Specifically, river discharge into the estuary, the way organisms feed, and the extent to which they move about the estuary, can influence food web connections. Seasonal changes in river flow and differences in estuary landscapes, such as marsh size, play secondary but still significant roles. Hypothesizing that different food web sources would reflect varying levels of ecosystem connectivity, we used natural isotopes of carbon, nitrogen and sulfur to track fish, clam and mussel food web sources in five estuaries with different levels of river inputs. Tracing these "biomarkers," we found that mussels that filter particles of organic material from water and resident fish with limited mobility show increasing connectivity to energy produced in both land and marsh environments as river flow increases. From this, we conclude that river flow plays an important role in transporting organic matter across the estuarine landscape, and that alterations to the natural flow of rivers and tides have the potential to disrupt food web connectivity. However, in estuaries where weak river flow results in less transport of organic matter among adjacent ecosystems, mobile species like small flounders can enhance connectivity by moving across landscapes to obtain food, while resident organisms like clams cannot. Clams do not show strong connections with river-transported food sources in estuaries with the largest river flow, perhaps because high flow speeds prevent food particles from settling onto sediments where clams feed. High flow speeds can be one consequence of upstream levees that confine the river to a few distributary channels. These channels focus river flow into a fire hose plume that causes organic matter to overshoot the estuarine delta. As a result, organisms living in the delta’s sediments lose access to sources of food normally delivered by the river. Our study of clam diets suggests that such unnatural river structures can disrupt food web connectivity by altering the manner and speed by which organic materials are delivered to the habitats of estuarine organisms. We also seasonally tracked food web connections to mussels that inhabit restored marshes of different ages and design. Of significance to future restoration planners, we found mussels inhabiting restoration sites where levees were fully removed had greater food web connectivity and diet equivalence with natural marsh ecosystems than mussels in sites where only a gap was opened in the levee. This suggests that restoration designs promoting connectivity may return altered estuaries to natural conditions more rapidly than those providing limited connectivity to the surrounding landscape. We also found that reliance on marsh-produced detritus by mussels during winter suggests that restoring shallow-water wetlands (emergent marshes) can bolster food web subsidies during periods when marine sources of food, such as phytoplankton, are scarce. This study demonstrates how physical and biological factors interact to affect food web connectivity across divergent estuarine ecosystems. Managers interested in conserving and/or restoring food webs that link estuarine ecosystems can use our results to develop strategies for sustaining or increasing food web connectivity in different estuarine landscapes. In some landscapes it may be advantageous to restore natural flow, in others, the capacity for organic matter production. Conservation and restoration actions directly addressing ecosystem capacity and connectivity offer the opportunity to improve the altered state of the world’s land margins and, ultimately, to promote both natural and societal ecosystem benefits. Emily Howe and Charles Simenstad Wetland Ecosystem Team, School of Aquatic and Fishery Sciences, University of Washington
