By Andrea Watts
From shore at low tide in Skagit County’s Padilla Bay, the limp piles of eelgrass might appear the same, but there are two species of eelgrass growing in the mud: the native Zostera marina and the introduced dwarf Zostera japonica. Though you wouldn’t think that the eelgrass is on the move, they are and researchers want to know how fast and where these two species are spreading throughout the bay.
Eelgrasses are considered ecosystem engineers by slowing the flow of water, settling sediment to form mounds, and reducing erosion. Elsewhere in the world, eelgrass coverage is decreasing while “overall in the Puget Sound, the area of native eelgrass is stable,” yet in individual locations, declines have been noted, says Michael Hannam, a recent PhD graduate from the University of Washington. He was a National Oceanic Atmospheric Agency (NOAA) Graduate Research Fellow studying the interaction of native and dwarf eelgrasses. His research results will be of interest because as Doug Bulthuis, research coordinator at Padilla Bay admits, “frankly we do not know much” on how these species of eelgrass are interacting and Michael’s research “helps with our understanding of it.” It’s important to understand this interaction because as the dwarf eelgrass spreads throughout Padilla Bay, it could be at the expense of our native eelgrass.
Research elsewhere along the Puget Sound found that the larger, native eelgrass occupies the lower, wetter areas of an intertidal zone while the dwarf eelgrass inhabits the higher, drier mounds. Michael set out to study whether this holds true at Padilla Bay but also to quantify the interaction. His research site, just off the southern area of Samish Island, contains pools and mounds that is a suitable area to test that question. In 2010 he transplanted over 800 native and dwarf plants to 35 plots throughout the intertidal zone. Tied to a transplant frame, these plants were placed in pools and mounds, sometimes alongside only plants of their species or mixed with the other species. This was to test how each species responded to the growing conditions and then to test the interaction between the two species. Michael recalls tying eelgrass at midnight in preparation for fieldwork the following day.
Throughout the experiment from June 2010 to August 2011, Michael’s “Iife was ruled by tides.” At every low tide he was at Padilla Bay, crouched on the mud flats counting eelgrass blades. The number of blades served as an indicator of the plant’s growth; the healthier the plant, the more blades of grass. As nearly all the plants survived transplant the loss of a few plants didn’t impact the counts. Michael recalls it was pretty stressful working with only a four hour window until the tide returned.
Though on some days, Michael was out in the intertidal zone counting the eelgrass blades by himself, other days found him with students who could provide moral support. And local neighbor, Frank Smith was “extraordinary helpful,” Michael says by allowing quick and easy access to his research site through Frank’s backyard. When Frank noticed Michael taking a long route to reach his research area, Frank didn’t hesitate to invite Michael onto his property. Frank recalls they got to trading crab and salmon for Michael’s homemade bread.
Another portion of Michael’s research involved one of the first known uses of ground-based LiDAR in an intertidal environment. LiDAR stands for Light Detection and Ranging and involves shooting lasers at an object and recording the returned intensity rate and return time. These data, when displayed on a computer screen, show a 3-D image of the object or landscape. Through the use of LiDAR, Michael captured a 3-D image of his research site, which covers roughly one hectare (one to two football fields). Having taken scans from 2009 through 2011, Michael can study the subtle differences in topography that have occurred and see if that has affected the growth of the eelgrass.Yet using LiDAR technology came at a price; with only a three to four hour window during low tides, Michael had to hustle to get two full scans done. If the tide returned sooner than expected, Michael was faced with the choice of which field gear to haul out first. The LiDAR equipment always won because the cooler carrying additional equipment could float.
As a result of his one year transplant experiment, Michael determined that in his portion of Padilla Bay the two species of eelgrass followed a similar growth pattern observed elsewhere: the native eelgrass grows better in pools while the dwarf eelgrass can grow in pools or on higher mounds. However, what Michael discovered is that when these two species grow alongside each other in the pool environment, the native eelgrass inhibits the growth of the dwarf eelgrass; an unexpected case of a native species being able to dominate an introduced species.
Now that the fieldwork portion of his research has ended, Michael is writing a final report for Padilla Bay’s reserve director Terry Stevens, since his research was supported by the National Oceanic Atmospheric Agency (NOAA) Graduate Research Fellowship Program of the National Estuarine Research Reserve System. There are management implications for his research, Michael says; his findings provide short-term answers on the growth patterns of the eelgrass at Padilla Bay and more importantly, show there isn’t an immediate need to control the introduced dwarf eelgrass in areas where the topography is varied.
Yet what if the topography doesn’t contain pools or mounds? How is the native and introduced eelgrass interacting there? A long-term monitoring study started in 2011 to answer that question, Doug Bulthuis says. A transect, extending over four kilometers into Padilla Bay, contains permanent plots where the cover of Z. marina and Z. japonica will be measured to study the long-term interactions over a topography that is much more gradual. And this research is high-priority because Z. japonica will continue to spread across Padilla Bay.