Inter-Annual Variation in Eelgrass (Zostera Marina) Distribution and Productivity on Roberts Bank and in Boundary Bay in Southern British Columbia

The effects of climate change and other anthropogenic stressors on eelgrass can not be accurately assessed without an understanding of the natural variability within these habitats. Natural and anthropogenic factors have greatly modified the habitat within the Roberts Bank eelgrass meadow over the past 56 years. A causeway, three kilometres in length, was constructed in 1959 that bisected the eelgrass meadow. A second causeway (five kilometres in length) was constructed in 1970 three kilometres north of the first in 1970 which further divided the meadow. Zostera japonica was discovered landward of the Z. marina meadow in the mid 1970s. A review of historical air photographs, orthophotographs, and satellite imagery estimated that the area colonized by Z. marina increased from 449 ha. in 1967 to 964 ha. in 2003. The section of the meadow that is located between two causeways was filmed (digital orthophotos) and ground truthed in 2003 and annually between 2007 and 2014 to assess inter-annual changes in distribution and productivity. Productivity data was analysed from four reference stations in between the causeways and compared with data from two stations west of the causeways, and with two stations in a meadow approximately 26 kilometres to the southeast. Large variations in inter-annual productivity were detected at all sites and trends were generally consistent between locations. Research has shown that eelgrass productivity may be influenced by many large scale environmental factors and near shore oceanic conditions. The inter-annual variation in productivity at the study sites was compared with the Pacific Decadal Oscillation (PDO), sea surface temperature, inter-annual tide height variation, and the onset of daytime spring low tides. The relationship between eelgrass productivity and distribution resulting from variation with these large scale physical influences will be discussed