Eelgrass beds are both ecologically valuable and potentially threatened. They fall within the “critical” category of DFO’s habitat rating system, and DFO has concluded that eelgrass has characteristics which meet the criteria of an Ecologically Significant Species. The United Nations recently estimated a 15% loss in seagrass habitat globally over the last decade.
The Lucy Islands lie in the middle of Chatham Sound, approximately 21 km west of the city of Prince Rupert. They are a nationally listed important bird area (IBA), supporting a globally significant population of rhinoceros auklets. The Lucy Islands Provincial Conservancy is a new conservancy that was established pursuant to government’s land use decision for the North Coast planning area.
Purpose and Objectives of the Lucy Islands Eelgrass Study
Although the North Coast 2000 Aerial Video Imaging Survey showed no eelgrass visible around the Lucy Islands, eelgrass beds had been observed in the lower intertidal and subtidal regions during boat trips to the islands. Thus, in 2010, Ocean Ecology and WWF jointly applied for, and received, a research grant from MEC to carry out a study of the eelgrass beds at the Lucy Islands.
Shown below is the North Coast 2000 Aerial Video Imaging Survey of the Lucy Islands.
The purpose of the Lucy Islands Eelgrass Study was to investigate the productivity and ecological roles of, as well as the impacts of climate change and human activities on, eelgrass in northern B.C.
The objectives of the Lucy Islands Eelgrass Study were the following:
- To assess the productivity and ecological roles of eelgrass on the north coast. Eelgrass beds in northern B.C. are frequently smaller, deeper, and are found over a wider range of bottom types than eelgrass beds in southern B.C. Their ecological roles and their relationships to closely associated marsh grass habitats in northern B.C. are poorly understood.
- To compare the productivity of the Lucy Islands’ eelgrass bed with other eelgrass beds, such as Flora Bank, in the Skeena River estuary region in order to better understand the relationships between global climate changes, changes in river flow and sedimentation patterns, and changes in eelgrass productivity.
- To assess the damage caused to the Lucy Islands’ eelgrass bed by the use of anchors during the course of recreational boater activities in the area. Possible solutions to this problem were evaluated.
- To provide a clearer understanding of the ecological relationship of the Lucy Islands’ eelgrass bed to the Lucy Islands’ role as an established IBA for rhinoceros auklets, pigeon guillemots, glaucous winged gulls, and black oystercatchers.
- To assess the use of side scan sonar as a tool to quantify subtidal eelgrass beds.
Lucy Islands Eelgrass Study Design
Three surveys were carried out at different seasons:
- Summer – July 17th to July 19th, 2010
- Fall – October 27th, 2010
- Spring – April 20th, 2011
During these surveys, the following information was collected:
- Towed benthic video camera footage
- High definition video drop camera photos
- Side scan sonar imagery
- Multibeam and single-beam bathymetry
- Identification and enumeration of fish and mobile invertebrates from beach seining
- Bird, mammal, and human activity observations
Conclusions from the Lucy Islands Eelgrass Study
The following conclusions were made from the Lucy Islands Eelgrass Study:
- Eelgrass in northern B.C. is subject to large tidal ranges, strong currents, and heavy winter storm activity. At the Lucy Islands study site, strong currents and storm waves produced seasonal sand migration, with the sand moving offshore to sand bars in the winter and back to the intertidal beach area in the summer. Eelgrass was most abundant at the site in areas of mixed pebble, cobble, and sand substrate. The presence of pebbles and cobbles probably reduced the sand migration, thus increasing substrate stability and preventing up-rooting and loss of eelgrass plants. This may explain why eelgrass in northern B.C. is found more commonly in mixed substrate than in southern B.C.
- Eelgrass abundance and distribution was greatest in the summer. During the fall, in response to decreased light, both eelgrass and macroalgae growth at the site decreased. Heavy storm activity resulted in the erosion and removal of much of the flora biomass at the site.
- Macroalge reached its maximum biomass in April, whereas eelgrass did not reach its maximum biomass until sometime after July.
- During the spring, summer, and fall surveys of the Lucy Islands study site, a total of 14 species of macroalgae, 49 species of marine fauna, and 17 species of birds were observed. Species richness was greatest in spring and summer, and least during the fall. During the fall survey, juvenile salmon, most likely chinook, were present in schools at the site. Dungeness crabs were also present in both the summer and fall surveys.
- High species richness during the spring and summer was correlated with high eelgrass density. The eelgrass is clearly providing suitable niches for a number of organisms. Eelgrass beds probably function to stabilize the mobile sand substrate, thus providing areas where other organisms can become anchored, such as algae and sessile fauna. Mobile fauna find food and refuges from predators in the eelgrass blades. Birds are more common on the islands during the times when marine fauna are most abundant, and are probably feeding on these organisms.
- While the Lucy Islands eelgrass bed receives terrestrial nutrients from the Skeena River, it does not experience the full impact of the plume turbidity, and thus eelgrass may be found growing to depths of 2 – 3 m. In contrast, very little subtidal eelgrass was seen growing at Flora Bank, which is located within the turbid region of the plume year round and is severely light limited. The presence of river-derived nutrients at the Lucy Islands allows lush eelgrass growth to occur.
- Global warming may lead to reduced snow packs as a result of increasing average temperatures, but may also increase the intensity of fall storms. Precipitation falling as heavy rain rather than snow has the potential to increase the volume of sediment entering the river. Increased riverine sediment may cause eelgrass beds closest to the mouth of the river to experience increased turbidity, which could lead to reduced growth. However, eelgrass beds at the edge of the Skeena plume may avoid this sedimentation effect, either partially or totally, and continue to function as healthy ecosystems. Mobile organisms, such a juvenile fish, may end up using the “outer” eelgrass beds in preference to “inner” beds which have become silted over.
- Evidence of anthropogenic damage to the eelgrass bed was rapidly obliterated by moving sand at the Lucy Islands site. However, constant up-rooting of eelgrass plants will eventually decrease the productivity of the eelgrass bed, increase the mobility of the substrate, and create a habitat which is less rich and diverse than the one which presently exists at the site. Since the site is located at the trailhead of the Lucy Islands trail, which will encourage anchoring at the site, it is recommended that some type of fixed anchor system be put in place at the site. Two potential systems have been recommended: (1) a helical screw anchor system; and (2) a steel piling with a float collar.
- More species of birds were present, and the overall number of individual birds was higher, when the eelgrass was most abundant. A likely hypothesis is that when eelgrass is abundant, it provides both a source of food and a protective habitat for many species of marine fauna. These organisms, in turn, serve as prey items for marine birds. This may be of particular importance to nesting birds whose foraging range may be reduced by the requirements to incubate eggs and guard the nest and hatchlings.
- The image quality of the side scan data produced by the Humminbird 997c SI unit was comparable with that of images produced by more expensive systems. In light of the significant difference in set-up costs between the Humminbird system and other systems used in scientific research, this is a significant outcome. It may make it possible for small organizations with limited funding to be able to collect high quality side scan data.
- Two side scan processing techniques were used to assist in the identification and measurement of eelgrass areal coverage in side scan images: (1) texture analysis using grey level co-occurrence matrices (GLCM); and (2) false coloring of back scatter intensity. Both techniques have significant potential usefulness for future eelgrass surveys, but will need a bit more “fine-tuning”.
- Measurement of eelgrass height using the downward-looking sonar was very successful. While this technique does not differentiate between eelgrass and macroalgae, once an eelgrass bed has been located, this method could be used to track changes in eelgrass height, and thus productivity, throughout the year or over the course of several years.
See the link below for the full Lucy Islands Eelgrass Study report.
Presentation of Lucy Islands Results at the BCPARF Conference
After a very successful season of research, we were asked by BC Parks to present the results from the Lucy Islands Eelgrass Study at the British Columbia Protected Areas’ Research Forum Conference in December, 2011 at UBC. This was a great opportunity for us to share our results, network with other groups doing protected areas research, and start working on new ideas for next season’s work. See the link for our BCPARF presentation.