Northeast USA SeagrassNet Zm percent cover

Sea surface temperature (SST) has increased worldwide since the beginning
of the 20th century, a trend which is expected to continue. Changes in SST
can have significant impacts on marine biota, including population-level
shifts and alterations in community structure and diversity, and changes
in the timing of ecosystem events. Seagrasses are a group of foundation
species that grow in shallow coastal and estuarine systems, where they
provide many ecosystem services. Eelgrass, Zostera marina L., is the
dominant seagrass species in the Northeast United States of America (USA).
Multiple factors have been cited for losses in this region, including
light reduction, eutrophication, and physical disturbance. Warming has the
potential to exacerbate seagrass loss. Here, we investigate regional
changes in eelgrass presence and abundance in relation to local water
temperature using monitoring data from eight sites in the Northeastern USA
(New Hampshire to Maryland) where a consistent monitoring protocol,
SeagrassNet, has been applied. We use a hurdle model consisting of a
generalized additive mixed model (GAMM) with binomial and beta response
distributions for modeling eelgrass presence and abundance, respectively,
in relation to the local summer average water temperature. We show that
summer water temperature one year prior to monitoring is a significant
predictor of eelgrass presence, but not abundance, on a regional scale.
Above average summer temperatures correspond to a decrease in probability
of eelgrass presence (and increased probability of eelgrass absence) the
following year. Cooler than average temperatures in the preceding year,
down to approximately 0.5°C below the site average, are associated with
the highest predicted probability of eelgrass presence. Our findings
suggest vulnerability in eelgrass meadows of the Northeast USA and
emphasize the value of unified approaches to seagrass monitoring,
conservation and management at the seascape scale.