Graduate Research Fellow - Emily Howe
NOAA/Padilla Bay NERR Graduate Research Fellow
Differences in Food Web Connectivity
Emily Howe
PhD candidate, University of Washington
School of Aquatic & Fishery Sciences
Seattle, Washington
Differences in Food Web Connectivity across Intertidal Embayment and Fluvial Dominated Estuaries
BACKGROUND
Food web connectivity, or links between primary producers and consumers within and across habitat boundaries, plays a large role in ecosystem health, stability and function. For example, benthic communities depend on a number of different primary producer assemblages to supply them with organic matter. Little is known, however, about differences in the contribution of organic matter produced by these distinct assemblages to the benthic community. It has been suggested that in well mixed estuarine ecosystems such as Padilla Bay, organic matter is widely spread and evenly assimilated by benthic consumers. However, recent evidence suggests strong gradients in the sources of organic matter supplied to benthic communities; different assemblages of primary producers contribute different amounts of organic matter to organisms within and across habitat boundaries. One important source of this organic matter is estuarine marshes.
Estuarine marshes play a key role in coastal ecosystems of the Pacific Northwest and are home to a large diversity of life including birds, mammals, invertebrates and fish. Over the last century, the area of estuarine marshlands in Puget Sound has greatly decreased as a result of human alteration. Despite this loss, recent evidence has found estuarine marshes are still contributing significant amounts of organic matter to benthic organisms in habitats outside the marsh. These findings suggest that primary productivity in emergent marshes can contribute significantly to nearby habitats such as mudflats and eelgrass beds despite the high productivity of these areas. Emily Howe, a PhD candidate from the University of Washington and recipient of a Padilla Bay Graduate Research Fellowship, is studying the food web connectivity of marshes, mudflats, and eelgrass beds in Skagit and Padilla Bays.
Fig. 1. Mud flats at Padilla Bay, Washington
The study addresses two main questions:
-Does organic matter produced by specific assemblages of primary producers contribute significantly to consumers outside the boundary of that assemblage?
-Does river-forcing affect the extent of organic matter transportation and assimilation by organisms living in nearby habitats?
Methods
Skagit Bay and Padilla Bay where selected for comparison. Both bays are shallow, well mixed embayments with extensive intertidal habitats. However, the supply of freshwater to the system differs significantly between the bays with Skagit Bay receiving a far greater freshwater input when compared to Padilla Bay. This comparison allows for an assessment of organic matter transport from marshes to intertidal habitats under two different flow regimes.
Stable isotopes where used to trace the sources and relative contributions of organic matter to the benthic food web. Five main groups of primary producers were sampled and isotopic signatures were determined for each. The sources included river-borne particulate organic matter, phytoplankton, benthic microalgae, vascular plants, and macroaglae. Bay mussels (filter-feeders) and bent-nosed clams (benthic-deposit feeders) were used as bioindicators.
Three transects were placed at each bay. Transects were drawn across three distinct habitats; marsh, mudflat, and eelgrass. At each transition point between habitats (e.g. marsh to mudflat) mussels and clams were placed at positions 2m, 10m, and 50m away from the vegetative transition point and left from March 2007 to August 2007. A second group of mussels and clams was left from August 2007 to March 2008. This process was repeated again for a total of two winter and two summer replicates. After each time period, the mussels and clams were collected and their tissue was analyzed to determine their isotopic ratios. This method, combined with a complex multiple source mixing model, enabled Howe to determine the sources and their relative contributions of organic matter eaten by benthic organisms.
Fig. 2. Containment colanders placed along a mudflat-eelgrass transition zone.
Preliminary Results
The results of this study provide evidence that marshes are contributing significantly to organisms living in the mudflats and eelgrass beds despite the low area of marshes when compared to mudflats and eelgrass. This means that organisms living in mudflats and eelgrass beds in the center of Padilla Bay rely more heavily on organic matter produced by marsh plants then previously predicted. These findings illustrate the importance of food web connectivity in Pacific Northwest estuaries.
In addition, the comparison between Skagit and Padilla Bay suggest that river forcing (as in Skagit Bay) results in a greater compartmentalization of food webs when compared to estuaries with less freshwater input (Padilla Bay). These findings imply that river forcing in Skagit Bay is actually transporting nutrients beyond the estuarine area and out of range of the benthic communities, thus isolating habitats within close proximity to one another. The channelization of the Skagit River and the resulting increase in flow velocities could explain this trend.
Project Significance
As we begin to better understand the patterns and processes of food web connectivity, more effective restoration strategies can be developed. In light of this study, it is clear that future restoration should take into consideration the restoration of emergent marshlands. Strategies should include the nearshore removal of levees and hardened shorelines that prevent the establishment of emergent marsh habitats. Thanks to the work of Emily Howe, we can begin to implement further strategies for coastal restoration projects that better protect the system as a whole.
Biographical Information
Emily Howe is currently a PhD candidate at the School of Aquatic and Fishery Sciences, University of Washington. Her graduate advisor is Dr. Charles Simenstad. Emily is a recipient of a Padilla Bay NERR Graduate Research Fellowship. She received a BA in Environmental Studies and Biology in 2001 from Middlebury College, Vermont. In 2006 she received her Master's degree from the School of Aquatic and Fishery Sciences, University of Washington.
Fig. 3. Emily Howe collecting specimens from a transect in Skagit bay.