Warming sea surface temperatures fuel summer epidemics of eelgrass wasting disease

Abstract

Seawater temperatures are increasing, with many unquantified impacts on marine diseases. While prolonged temperature stress can accelerate host-pathogen interactions, the outcomes in nature are poorly quantified. We monitored eelgrass wasting disease (EWD) from 2013-2017 and correlated mid-summer prevalence of EWD with remotely sensed seawater temperature metrics before, during, and after the 2015-2016 marine heatwave in the northeast Pacific, the longest marine heatwave in recent history. Eelgrass shoot density declined by 60% between 2013 and 2015 and did not recover. EWD prevalence ranged from 5-70% in 2013 and increased to 60-90% by 2017. EWD severity approximately doubled each year between 2015 and 2017. EWD prevalence was positively correlated with warmer temperature for the month prior to sampling while EWD severity was negatively correlated with warming prior to sampling. This complex result may be mediated by leaf growth; bigger leaves may be more likely to be diseased, but may also grow faster than lesions, resulting in lower severity. Regional stressors leading to population declines prior to or early in the heatwave may have exacerbated the effects of warming on eelgrass disease susceptibility and reduced the resilience of this critical species.

Disease surveillance by artificial intelligence links eelgrass wasting disease to ocean warming across latitudes

“…Below an optimal threshold, warming increases eelgrass growth (Lee et al 2007); because disease severity standardizes lesion area by leaf area, enhanced plant growth may outpace lesion growth and minimize increases in our measure of severity, despite increased lesion area. Eelgrass also exhibits consistent relative growth rates of 1-2% per day globally, which means that longer leaves grow faster in absolute terms (Ruesink et al 2018), and the negative correlation between disease severity and blade area shown here and in prior work (Groner et al 2016(Groner et al , 2021 suggests that eelgrass leaves can outgrow lesions. However, in some cases, possibly when plants are stressed, lesion growth rate can outpace leaf growth (Graham et al 2021).…”

“…We trained this version of EeLISA using the positive feedback loop described above and a dataset of 1036 eelgrass scans from an earlier study of eelgrass wasting disease in the San Juan Islands, WA (Groner et al 2021). We divided the dataset into sets of 789 and 247 scans for training and testing sets respectively, and a human processor manually labeled each scan by classifying every pixel as healthy tissue, lesion tissue, or background.…”

Pathogen Filtration: An Untapped Ecosystem Service

“…Allelopathic activity in seagrasses, the defensive release of chemicals, and biological interactions of the micro and macro epibionts may also play a role in pathogen filtration services in this important ecosystem (Jacobs-Palmer et al, 2020). The outbreaks of the unicellular protist L. zosterae (Sullivan et al, 2018) in eelgrass beds (M. Bockelmann et al, 2013;Groner et al, 2021) suggest that seagrass pathogens may also accumulate within the beds and that biological filtration may not only apply to pathogens that are non-infectious to seagrass. It is important to note that the decline of seagrass beds worldwide is attributed to both pathogens and anthropogenic factors such as ocean warming and eutrophication.…”

“…Eelgrass wasting disease, likely caused by the etiological agent Labyrinthula zosterae, created a large-scale blight in the Eelgrass (Zostera marina) population along the Atlantic coast of the US in the 1930s that has still not fully recovered (Figure 1A) (Rasmussen, 1977). L. zosterae is causing outbreaks of disease in eelgrass from Europe to western North America (Bockelmann et al, 2013;Groner et al, 2021).…”

The evolutionary past and the uncertain future of foundational species

“…It is imperative that we understand the underlying constraints for foundational species in changing ecosystems and the role evolutionary history may play in these constraints. Eelgrass fringes some of the most anthropogenically influenced shorelines ( 12 ) and occupies shallow marine coastal embayments which are areas of rapid warming ( 13 ). The stressors that exist across the two oceans can differ ( 14 ).…”