Abstract
Seagrass meadows provide valuable ecosystem benefits but are at risk from disease. Eelgrass (Zostera marina) is a temperate species threatened by seagrass wasting disease (SWD), caused by the protist Labyrinthula zosterae. The pathogen is sensitive to warming ocean temperatures, prompting a need for greater understanding of the impacts on host health under climate change. Previous work demonstrates pathogen cultures grow faster under warmer laboratory conditions and documents positive correlations between warmer ocean temperatures and disease levels in nature. However, the consequences of disease outbreaks on eelgrass growth remain poorly understood. Here, we examined the effect of disease on eelgrass productivity in the field. We coupled in situ shoot marking with high-resolution imagery of eelgrass blades and used an artificial intelligence application to determine disease prevalence and severity from digital images. Comparisons of eelgrass growth and disease metrics showed that SWD impaired eelgrass growth and accumulation of non-structural carbon in the field. Blades with more severe disease had reduced growth rates, indicating that disease severity can limit plant growth. Disease severity and rhizome sugar content were also inversely related, suggesting that disease reduced belowground carbon accumulation. Finally, repeated measurements of diseased blades indicated that lesions can grow faster than healthy tissue in situ. This is the first study to demonstrate the negative impact of wasting disease on eelgrass health in a natural meadow. These results emphasize the importance of considering disease alongside other stressors to better predict the health and functioning of seagrass meadows in the Anthropocene.
Highlights
Rapid environmental changes significantly impact and reshape our oceans
This paper examines the impacts of seagrass wasting disease (SWD) on eelgrass (Zostera marina) health and productivity
To sustain and conserve these crucial habitats and their ecosystem services, we must determine the impacts of disease on eelgrass health
Summary
Rapid environmental changes significantly impact and reshape our oceans. Elevated temperatures can alter marine host-pathogen dynamics by increasing host stress and pathogen virulence, expanding pathogen ranges, and altering host ranges, triggering increased occurrence and severity of disease outbreaks (Harvell et al, 2002; Burge et al, 2014; Cohen et al, 2018; Burge and Hershberger, 2020). In the 1930s, SWD outbreaks decimated up to 90% of eelgrass meadows throughout the North Atlantic (Renn, 1936; Short et al, 1987; Muehlstein, 1989), reducing waterfowl, shrimp, scallop, and fish populations (Renn, 1936; Stauffer, 1937; Moffitt and Cottam, 1941; Milne and Milne, 1951) and compromising eelgrass ecosystem services (Orth et al, 2006) These and subsequent die-offs were traced to Labyrinthula zosterae (Muehlstein et al, 1988), which is recognized as a virulent pathogen in eelgrass (Groner et al, 2014, 2016; Martin et al, 2016) and other seagrasses worldwide (reviewed in Sullivan et al, 2018). Given that eelgrass creates nursery and feeding grounds (Orth et al, 1984), filters and oxygenates seawater (Costanza et al, 1997; Hasegawa et al, 2008), stabilizes sediments (Fonseca et al, 1983), efficiently stores carbon (Duarte et al, 2005; Röhr et al, 2018), and plays important roles in nutrient cycling in nearshore communities (reviewed in Moore and Short, 2006), SWD can compromise the health of eelgrass individuals, populations, and entire coastal communities
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