Graduate Research Fellow - Michael Hannam
NOAA/Padilla Bay NERR Graduate Research Fellow
Environmental controls on Japanese Eelgrass
Michael Hannam
Ph.D. Candidate, University of Washington
College of Forest Resources
Seattle, WA
Inferring Limitation on the Invasive Seagrass Zostera japonica from Environmental Characteristics and LiDAR derived Topography
BACKGROUND
In recent years, coastal ecosystems of the Pacific Northwest have undergone significant alterations due to the introduction of a non-native eelgrass. Native to the sandy and muddy shores from sub-tropical Vietnam to East Asia, Z. japonica has invaded previously unvegetated mudflat habitats in the Pacific Northwest. First documented in Willapa Bay in 1957, Z. japonica was presumably introduced along with Japanese oysters. Soon after, Z. japonica was introduced to other areas including Padilla Bay, Samish Bay, and other shellfish growing areas around the region. Z. japonica has since extensively colonized formally un-vegetated tidal flats and has spread as far south as northern California and as far north as British Columbia. In Boundary Bay, B.C., about 40 miles north of Padilla Bay, there was a 17-fold increase in Z. japonica abundance between 1970 and 1991. This rapid expansion has greatly altered the habitat structure of estuarine ecosystems by replacing unvegetated intertidal mudflat ecosystems with an intertidal eelgrass ecosystem.
However, Z. japonica is unusual among invasive species in that it may be beneficial to some intertidal organisms. Z. japonica provides valuable habitat and food for invertebrates, fish, and waterfowl. Some studies have shown that Z. japonica actually increases overall macroinvertebrate diversity and abundance in areas it invades. Z. japonica is the principal food of migrating and wintering waterfowl, which prefer Z. japonica to Z. marina due to longer accessibility (higher up in the intertidal), higher energy content, and smaller, easier to handle size. In fact some 19 species of birds in the PNW consume some portion of Z. japonica making it a rare example of an exotic species being generally beneficial to major components of an ecosystem. For these reasons, Z. japonica is protected in Washington State. More research is needed, however, to fully understand the impact Z. japonica has on local aquatic organisms, in particular, the native eelgrass Z. marina.
Z. japonica typically grows in the upper intertidal zone, and thus does not overlap in distribution with Z. marina in most areas of the Pacific Northwest. However, the two species do occur in mixed patches in areas such as Padilla Bay. In these areas, Z. japonica may have adverse affects on the native eelgrass. The interaction between Z. japonica and Z. marina has therefore been given a high priority for research at the Padilla Bay National Estuarine Research Reserve. Michael Hannam, a PhD candidate at the University of Washington, College of Forest Resources and a recipient of the National Estuarine Research Reserve Graduate Research Fellowship, is currently studying the interactions between Z. japonica and Z. marina south of Samish Island in Padilla Bay. In his study, Hannam is hoping to elucidate roles that environment constraints and interspecies interactions play in the distribution of these species.
Figure 1. Hamman uses Light Detection and Ranging (LiDAR) to monitor topography in very flat conditions.
Methods
The north end of Padilla bay offers a unique opportunity to study the conditions that limit the spread of Z. japonica. The tidal flats in this area are defined by a series of small, closely situated pools and hummocks. Here, a distinctive vegetation pattern forms in which discreet assemblages of Z. japonica are found only on hummocks, while Z. marina is observed only in pools. The two species are located within close proximity but with very little intermixing between them, suggesting that there are conditions in the pools that prevent colonization by Z. japonica.
To test this idea, Hannam first analyzed the environmental conditions found in the pools and hummocks, including sediment temperature, redox potential, porewater sulfide, sediment organic matter, and topographic setting. However, the environmental differences didn’t appear dramatic enough to explain the growth pattern at this site. As a result of these finding, Hannam designed an alternative experiment to assess the importance of interactions between the two plants. Plants of each species were planted onto mounds and into pools, either with or without members of the other species, in order compare the importance of plant-to-plant interactions with the importance of environmental conditions.
Concurrently, Hannam is monitoring the topography that forms these pools and hummocks using ground-based Light Detection and Ranging, or Lidar. Conceptually similar to radar, LiDAR uses a laser (instead of radio waves) to create a three-dimensional map of the terrain. Using this tool, he hopes to determine how stable this topography is, and how these plants respond to naturally occurring shifts in the terrain.
Preliminary Results
Data from the transplantation experiment suggest that both environmental factors and interspecies interactions influence the distribution of these species. Z. marina appears to suffer when transplanted onto mounds. Z. japonica, on the other hand, grows well on the mounds and in the pools, as long as Z. marina is not present. When growing together with Z. marina in either environment, it spreads much more slowly.
Project significance
The work being done by Michael Hannam in Padilla Bay may be the first use of ground-based LiDAR in the study of seagrass and among the first uses in intertidal environments. Knowledge gained will contribute to our understanding of physical processes and vegetative trends within Padilla Bay, and provide detailed elevation data in this part of the reserve. Understanding these factors is critical for implementing effective management strategies.
Biographical Information
Michael Hannam is currently a PhD candidate at the University of Washington, College of Forest Resources. Working with University of Washington faculty, Dr. Sandy Wyllie-Echeverria and Dr. L. Monika Moskal, Michael is a recipient of the National Estuarine Research Reserve Graduate Research Fellowship. He received a Bachelor of Science from the University of Notre Dame in 1999. In 2008 he received his Master’s degree from the University of Washington, College of Forest Resources.