Remote sensing illustrates patterns of change and permanence in eelgrass meadows, False Bay Washington

Abstract

Seagrasses are an important global resource which serve as ecosystem engineers, providing a wide range of ecosystem functions and services. Unfortunately, many seagrasses such as eelgrass (Zostera marina) are declining in abundance worldwide due to multiple local and global stressors. Declines in eelgrass abundance cause concern for the habitats it exists in, due to the important biological roles seagrasses fill. Increasing ocean temperatures predicted to intensify under current climate change models may be contributing to this global decline of seagrasses. In order to investigate this relationship, historic changes in eelgrass extent were examined within False Bay located between San Juan Island and Haro Strait in the Salish Sea. Using remote sensing techniques and spatial analysis, changes in eelgrass extent were able to be quantified in ten different years spanning 1990 - 2021. The total area of observed eelgrass extent ranged from 8316𝑚2 to 16120𝑚2 with the greatest change occurring from 2015 to 2017 (−7069𝑚2). We investigated potential relationships between variability of eelgrass within the shallow subtidal zone, recorded air temperature and the Pacific decadal oscillation index (PDO). No consistent trend of increase or decrease in eelgrass extent was found over the entire study period. However, we found a positive correlation between eelgrass growth and both negative mean PDO and lower monthly average temperatures. A strong correlation was found between a period of dramatic decline in False Bay eelgrass (2015 to 2017), and both higher average monthly temperatures and an atypical PDO positive phase. Analysis of historic eelgrass cover also revealed persistent patches of eelgrass consistent since 1990. These persistent patches deemed ‘elder patches’, show eelgrass beds within False Bay that have remained unchanged in expanse for over thirty years.