Effect of grazing by Canada geese Branta canadensis on an intertidal eelgrass Zostera marina meadow

Abstract

Fishing Island, in Portsmouth Harbor on the Maine-New Hampshire border (USA), is the site of an intertidal eelgrass (Zostera marina L.) bed that is part of SeagrassNet, an international program for long-term seagrass monitoring. Eelgrass bed parameters of canopy height, percent cover, and aboveground biomass have been monitored quarterly since October 2001 using the SeagrassNet protocol. A flock of nearly 100 Canada geese Branta canadensis L. over-wintered at Fishing Island and grazed on eelgrass from January to April 2003, an event that had not been seen at this meadow in 2 decades of observation. Before Canada geese were present, eelgrass parameters demonstrated seasonal fluctuations typical of the region. During the grazing event, eelgrass parameters declined drastically, and biomass losses reached 680 g m -2 in parts of the meadow. SeagrassNet data demonstrated that eelgrass did not recover after the geese departed. Additional fieldwork conducted from February to July 2003 showed that eelgrass recruitment via sexual reproduction at Fishing Island was minimal, and vegetative recovery was impeded by Canada goose consumption of the plant meristems. Unlike studies in other locations, which show seagrass quickly rebounding from annual grazing events, eelgrass at Fishing Island showed little recovery from Canada goose grazing through July 2003.

SeagrassNet monitoring across the Americas: case studies of seagrass decline

“…During the first year of SeagrassNet monitoring (October 2001 through July 2002), data for the eelgrass parameters of percent cover, canopy height, and aboveground biomass showed the typical seasonal changes of eelgrass abundance at Fishing Island (Rivers & Short 2006). Eelgrass canopy height and aboveground biomass showed similar seasonal fluctuations at all three cross transects, with low values occurring in January and peak values occurring in July; percent cover and shoot density data followed a similar trend (Fig.…”

“…In conclusion, monitoring showed that 96% of the seagrass (eelgrass, Zostera marina L.) at the SeagrassNet monitoring site in New Hampshire (USA) was lost in the first quarter of 2003, a consequence of intensive grazing by Canada geese, Branta canadensis L. (Rivers & Short 2006), birds which historically migrated south during the winter. With a warmer climate, these birds now reside in New Hampshire throughout the year, increasing the grazing pressure on seagrasses.…”

Assessing the role of large herbivores in the structuring and functioning of freshwater and marine angiosperm ecosystems

“…Several other large herbivore species occasionally consume submerged angiosperms, including other cervids, mainly based on availability and comprising only a minor fraction of their diets, as far as data are available. Sources: data on habitat, diet and range for mammals from MacDonald (2001) Table 2); it is not uncommon for turtles and swans to consume ca 100% of primary production in a given season (Rivers and Short 2007, Hidding et al 2009, Kelkar et al 2013a, Christianen et al 2014. A lot of the variation in herbivory rates observed among studies is likely due to variation in herbivore densities (Wood et al 2012a).…”

Long-term Patterns of Eelgrass (Zostera marina) Occurrence and Associated Herbivorous Waterbirds in a Danish Coastal Inlet

“…Several other studies have documented that grazing by herbivorous waterbirds affect the submerged vegetation in terms of leaf length, and below ground biomass (Bortolus et al, 1998), and above ground biomass (Rivers and Short, 2007;Dos Santos et al, 2015) but temporal removal of plant material does not necessarily have a permanent impact on the plants. Some aquatic plants such as Potamogeton pectinatus are in fact extremely "tolerant" to intense and annual grazing by swans, and may overcompensate and thus produce better in the presence of grazing (Nolet, 2004).…”