Predicted eelgrass response to sea level rise and its availability to foraging Black Brant in Pacific coast estuaries

Abstract

Managers need to predict how animals will respond to habitat redistributions caused by climate change. Our objective was to model the effects of sea level rise on total eelgrass (Zostera marina) habitat area and on the amount of that area that is accessible to Brant geese (Branta bernicla), specialist grazers of eelgrass. Digital elevation models were developed for seven estuaries from Alaska, Washington, California (USA), and Mexico. Scenarios of future total eelgrass area were derived from combinations of estuarine specific sediment and tectonic rates (i.e., bottom change rate) with three rates of eustatic sea level rise (ESLR). Percentages of total eelgrass areas that were accessible to foraging Brant were determined for December when the birds overwinter at more southerly sites and in April as they move north to sites where they build body stores on their way to nesting areas in Alaska. The modeling showed that accessible eelgrass area could be lower than total area due to how daytime low-tide height, eelgrass shoot length, and the upper elevation of eelgrass determined Brant-reaching depth. Projections of future eelgrass area indicated that present-day ESLR (2.8 mm/yr) and bottom change rates should sustain the current pattern of estuarine use by Brant except in Morro Bay, where use should decrease because eelgrass is being ejected from this estuary by a positive bottom change rate. Higher ESLR rates (6.3 and 12.7 mm/yr) should result in less Brant use of estuaries at the northern and southern ends of the flyway, particularly during the winter, but more use of mid-latitude estuaries. The capacity of midlatitude estuaries to function as Brant feeding refugia, or for these estuaries and Izembek Lagoon to provide drift rather than attached leaves, is eventually limited by the decrease in total eelgrass area, which is a result of a light extinction affect on the eelgrass, or the habitat being pushed out of the estuary by positive tectonic rates. Management responses are limited to the increase or decrease of sediment supply and the relocation of levees to allow for upslope migration of eelgrass habitat.

Climate-linked Mechanisms Driving Spatial and Temporal Variation in Eelgrass (Zostera marina L.) Growth and Assemblage Structure in Pacific Northwest Estuaries, U.S.A.

“…With the prediction that extreme events will become more frequent and that average MSL will increase, we are concerned that eelgrass will not have the ability to shift its distribution upward or downward rapidly enough to survive, and that eelgrass may be eliminated especially in tidal flat-dominated systems such as Willapa Bay. Modeling by Shaughnessy et al (2012) showed that eelgrass should expand considerably (i.e. Recent data also indicate that the tidal amplitude is increasing in the eastern Pacific Ocean, which complicates assessment of sealevel effect on eelgrass growth and distribution (Jay, 2009).…”

Coastal retreat and improved water quality mitigate losses of seagrass from sea level rise

“…However, some key recent modelling studies based on Zostera spp. in North America (Kairis & Rybczyk, ; Carr et al ., ; Shaughnessy et al ., ) have found the effects of SLR on seagrass to be variable and context dependent: both predicted increases or decreases in extent of seagrass have been reported (Kairis & Rybczyk, ; Shaughnessy et al ., ).…”

The black brant population is declining based on mark recapture

“…A significant proportion of the decline in annual survival of adults can be attributed to increased harvest rate, which has increased 2−4‐fold since 2000 (Sedinger et al , Leach et al ). Increased retrieved harvest can only account for about 48% of the decline in annual survival of adults (Leach et al ), however, suggesting other factors like quality of migration or winter habitats are also playing a role (Ward et al , Shaughnessy et al ).…”