Wasting disease regulates long-term population dynamics in a threatened seagrass

Host traits and temperature predict biogeographical variation in seagrass disease prevalence

Diseases are ubiquitous in natural systems, with broad effects across populations, communities and ecosystems. However, the drivers of many diseases remain poorly understood, particularly in marine environments, inhibiting effective conservation and management measures. We examined biogeographical patterns of infection in the foundational seagrass Zostera marina by the parasitic protist Labyrinthula zosterae, the causative agent of seagrass wasting disease, across >20° of latitude in two ocean basins. We then identified and characterized relationships among wasting disease prevalence and a suite of host traits and environmental variables. Host characteristics and transmission dynamics explained most of the variance in prevalence across our survey, yet the particular host traits underlying these relationships varied between oceans, with host size and nitrogen content important in the Pacific and host size and density most important in the Atlantic. Temperature was also a key predictor of prevalence, particularly in the Pacific Ocean. The strength and shape of the relationships between prevalence and some predictors differed in our large-scale survey versus previous experimental and site-specific work. These results show that both host characteristics and environment influence host-parasite interactions, and that some such effects scale up predictably, whereas others appear to depend on regional or local context.

Pathogenicity and phylogeny of Labyrinthula spp. isolated in Washington and Oregon, USA

The class Labyrinthulomycetes constitutes a multitude of species found ubiquitously in the environment, and includes pathogens of corals, hard clams, turfgrasses, and seagrasses. Labyrinthula zosterae, the causative agent of seagrass wasting disease, has been associated with declines in seagrass coverage since the 1930s. However, pathogenic and nonpathogenic Labyrinthula spp. have been isolated from seagrass tissue. These isolates are difficult to distinguish morphologically, and the diversity of isolates where seagrass wasting disease is present is often unknown. This study aimed to increase knowledge on the pathogenicity and phylogeny of Labyrinthula spp. in Washington and Oregon, USA where a high prevalence of seagrass wasting disease has been associated with eelgrass, Zostera marina, declines. We tested the pathogenicity of 14 Labyrinthula isolates and compared partial 18S rRNA gene sequences of 12 isolates to sequences from around the world through the NCBI database. We found that pathogenic isolates could be identified as Labyrinthula zosterae, while nonpathogenic isolates did not form a clade with any previously identified SSU ribotypes. These results add to the growing data on Labyrinthula and seagrass wasting disease and can improve our understanding of pathogen evolution and spread in the future.

Diverse RNA viruses discovered in multiple seagrass species

Seagrasses are marine angiosperms that form highly productive and diverse ecosystems. These ecosystems, however, are declining worldwide. Plant-associated microbes affect critical functions like nutrient uptake and pathogen resistance, which has led to an interest in the seagrass microbiome. However, despite their significant role in plant ecology, viruses have only recently garnered attention in seagrass species. In this study, we produced original data and mined publicly available transcriptomes to advance our understanding of RNA viral diversity in Zostera marina, Zostera muelleri, Zostera japonica, and Cymodocea nodosa. In Z. marina, we present evidence for additional Zostera marina amalgavirus 1 and 2 genotypes, and a complete genome for an alphaendornavirus previously evidenced by an RNA-dependent RNA polymerase gene fragment. In Z. muelleri, we present evidence for a second complete alphaendornavirus and near complete furovirus. Both are novel, and, to the best of our knowledge, this marks the first report of a furovirus infection naturally occurring outside of cereal grasses. In Z. japonica, we discovered genome fragments that belong to a novel strain of cucumber mosaic virus, a prolific pathogen that depends largely on aphid vectoring for host-to-host transmission. Lastly, in C. nodosa, we discovered two contigs that belong to a novel virus in the family Betaflexiviridae. These findings expand our knowledge of viral diversity in seagrasses and provide insight into seagrass viral ecology.

High infectivity and waterborne transmission of seagrass wasting disease

Pathogen transmission pathways are fundamental to understanding the epidemiology of infectious diseases yet are challenging to estimate in nature, particularly in the ocean. Seagrass wasting disease (SWD), caused by Labyrinthula zosterae, impacts seagrass beds worldwide and is thought to be a contributing factor to declines; however, little is known about natural transmission of SWD. In this study, we used field and laboratory experiments to test SWD transmission pathways and temperature sensitivity. To test transmission modes in nature, we conducted three field experiments out-planting sentinel Zostera marina shoots within and adjacent to natural Z. marina beds (20 ± 5 and 110 ± 5 m from bed edge). Infection rates and severity did not differ among outplant locations, implicating waterborne transmission. The infectious dose of L. zosterae through waterborne exposure was assessed in a controlled laboratory experiment. The dose to 50% disease was 6 cells ml-1 and did not differ with the temperatures tested (7.5°C and 15°C). Our results show L. zosterae is transmissible through water without direct contact with infected plants. Understanding the transmission dynamics of this disease in the context of changing ocean conditions will improve Z. marina protection and restoration in critical coastal habitats worldwide.

Deeper habitats and cooler temperatures moderate a climate-driven seagrass disease

Eelgrass creates critical coastal habitats worldwide and fulfills essential ecosystem functions as a foundation seagrass. Climate warming and disease threaten eelgrass, causing mass mortalities and cascading ecological impacts. Subtidal meadows are deeper than intertidal and may also provide refuge from the temperature-sensitive seagrass wasting disease. From cross-boundary surveys of 5761 eelgrass leaves from Alaska to Washington and assisted with a machine-language algorithm, we measured outbreak conditions. Across summers 2017 and 2018, disease prevalence was 16% lower for subtidal than intertidal leaves; in both tidal zones, disease risk was lower for plants in cooler conditions. Even in subtidal meadows, which are more environmentally stable and sheltered from temperature and other stressors common for intertidal eelgrass, we observed high disease levels, with half of the sites exceeding 50% prevalence. Models predicted reduced disease prevalence and severity under cooler conditions, confirming a strong interaction between disease and temperature. At both tidal zones, prevalence was lower in more dense eelgrass meadows, suggesting disease is suppressed in healthy, higher density meadows. These results underscore the value of subtidal eelgrass and meadows in cooler locations as refugia, indicate that cooling can suppress disease, and have implications for eelgrass conservation and management under future climate change scenarios. This article is part of the theme issue 'Infectious disease ecology and evolution in a changing world'.

Environment predicts seagrass genotype, phenotype, and associated biodiversity in a temperate ecosystem

Coastal vegetative ecosystems are among the most threatened in the world, facing multiple anthropogenic stressors. A good example of this is seagrass, which supports carbon capture, coastal stabilization, and biodiversity, but is declining globally at an alarming rate. To understand the causes and consequences of changes to these ecosystems, we need to determine the linkages between different biotic and abiotic components. We used data on the seagrass, Zostera marina, collected by citizen scientists across 300 km of the south coast of the United Kingdom as a case study. We assembled data on seagrass genotype, phenotype, infauna, and associated bathymetry, light, sea surface temperature, and wave and current energy to test hypotheses on the distribution and diversity of this temperate sub-tidal ecosystem. We found spatial structure in population genetics, evident through local assortment of genotypes and isolation by distance across a broader geographic scale. By integrating our molecular data with information on seagrass phenotype and infauna, we demonstrate that these ecosystem components are primarily linked indirectly through the effects of shared environmental factors. It is unusual to examine genotypic, phenotypic, and environmental data in a single study, but this approach can inform both conservation and restoration of seagrass, as well as giving new insights into a widespread and important ecosystem.

Spatial Patterns of Thalassia testudinum Immune Status and Labyrinthula spp. Load Implicate Environmental Quality and History as Modulators of Defense Strategies and Wasting Disease in Florida Bay, United States

Seagrass wasting disease, caused by protists of the genus Labyrinthula, is an important stressor of the dominant macrophyte in Florida Bay (FB), United States, Thalassia testudinum. FB exhibits countervailing gradients in plant morphology and resource availability. A synoptic picture of the Thalassia-Labyrinthula relationship was obtained by assessing the activity of four immune biomarkers in conjunction with pathogen prevalence and load [via quantitative PCR (qPCR)] at 15 sites across FB. We found downregulated immune status paired with moderate pathogen load among larger-bodied host phenotypes in western FB and upregulated immunity for smaller-bodied phenotypes in eastern FB. Among the highest immune response sites, a distinct inshore-offshore loading pattern was observed, where coastal basins exposed to freshwater runoff and riverine inputs had the highest pathogen loads, while adjacent offshore locations had the lowest. To explain this, we propose a simple, conceptual model that defines a framework for testable hypotheses based on recent advances in resistance-tolerance theory. We suggest that resource availability has the potential to drive not only plant size, but also tolerance to pathogen load by reducing investment in immunity. Where resources are more scarce, plants may adopt a resistance strategy, upregulating immunity; however, when physiologically challenged, this strategy appears to fail, resulting in high pathogen load. While evidence remains correlative, we argue that hyposalinity stress, at one or more temporal scales, may represent one of many potential drivers of disease dynamics in FB. Together, these data highlight the complexity of the wasting disease pathosystem and raise questions about how climate change and ongoing Everglades restoration might impact this foundational seagrass species.

Low genotypic diversity and long-term ecological decline in a spatially structured seagrass population

In isolated or declining populations, viability may be compromised further by loss of genetic diversity. Therefore, it is important to understand the relationship between long-term ecological trajectories and population genetic structure. However, opportunities to combine these types of data are rare, especially in natural systems. Using an existing panel of 15 microsatellites, we estimated allelic diversity in seagrass, Zostera marina, at five sites around the Isles of Scilly Special Area of Conservation, UK, in 2010 and compared this to 23 years of annual ecological monitoring (1996–2018). We found low diversity and long-term declines in abundance in this relatively pristine but isolated location. Inclusion of the snapshot of genotypic, but less-so genetic, diversity improved prediction of abundance trajectories; however, this was spatial scale-dependent. Selection of the appropriate level of genetic organization and spatial scale for monitoring is, therefore, important to identify drivers of eco-evolutionary dynamics. This has implications for the use of population genetic information in conservation, management, and spatial planning.

Eutrophication assessment in the transit area German Bight (North Sea) 2006-2014 - Stagnation and limitations

The eutrophication status of the German Bight (North Sea) has been assessed the third time since 1998 according to the OSPAR-Comprehensive Procedure between 2006 and 2014. Since the 1980s nutrient discharges and atmospheric nitrogen deposition had declined significantly but chlorophyll a and nutrient concentrations remained above assessment levels inshore and in inner coastal waters, reflecting continuing eutrophication. Recently local river discharges stagnated or increased again and total nitrogen remained above a reduction target of 200 μM. Most nutrients and conversion products were imported by a coastal current, passing the German Bight. Organic matter was trapped in offshore bottom waters in the ancient Elbe valley, causing repeated annual oxygen minima (<6 mg/L) and a classification as Problem Area. Effects of national reduction measures are limited in the transit area German Bight because improvements in open coastal waters require international efforts, based on comprehensive analyses.

Seagrass wasting disease: Nitrate enrichment and exposure to a herbicide (Diuron) increases susceptibility of Zostera marina to infection

Seagrass meadows suffered large-scale declines in the past century. The 'wasting disease', pathognomonically associated with Labyrinthula zosterae, reduced populations of Zostera marina on both sides of the North Atlantic in, and since, the 1930s, coinciding with intensive agricultural use of artificial fertilizers and herbicides. This study tests the long-standing hypothesis that nutrient enrichment and a herbicide increases vulnerability to pathogens. Z. marina shoots from the Thames Estuary grown in elevated nitrate concentrations had significantly higher rates of infection by L. zosterae than controls, but not by Aplanochytrium sp., another slime-mould like protist. Z. marina shoots grown in 2μg·l^-1 Diuron solutions and infected separately by L. zosterae and Aplanochytrium sp. had significantly higher wasting indices than controls. The results identified Aplanochytrium sp. as another opportunistic pathogen causing a seagrass wasting-type disease and support the hypothesis that pollution by herbicides and nitrate increases the susceptibility of Z. marina to infections.

Impacts on the seagrass, Zostera nigricaulis, from the herbicide Fusilade Forte® used in the management of Spartina anglica infestations

The herbicide Fusilade Forte^® (FF) is widely applied in agricultural weed management and in the management of the invasive saltmarsh grass, Spartina anglica (ricegrass or cordgrass). FF (active ingredient fluazifop-P acid, FPA) is selective for poaceous grasses. Its primary mode of action is inhibition of the acetyl coenzyme-A carboxylase (ACCase) specific to this taxonomic group, and its secondary mode is by promotion of oxidative stress. FF is applied to S. anglica infestations in the intertidal zone, in proximity to seagrass meadows. Despite the potential for vital seagrass ecosystems to be exposed to FF, there is limited knowledge of any potential impacts. We investigated impacts of FPA on the endemic Australian seagrass, Zostera nigricaulis, measuring ACCase activity and parameters that reflect oxidative stress: photosynthetic performance, lipid peroxidation and photosynthetic pigment content. Seagrass was exposed to FF (0.01-10mgL^-1 FPA and a control) for 7d, followed by a 7-d recovery in uncontaminated seawater. An enzyme assay demonstrated that FPA ≤10mgL^-1 did not inhibit the activity of ACCase isolated from Z. nigricaulis, demonstrating that this seagrass is resistant to FF's primary mode of action. However, physiological impacts occurred following 7 days exposure to ≥0.1mgL^-1 FPA, including up to a 72% reduction in photosynthetic pigment concentration. After 7-d recovery, photosynthetic pigment content improved in treatment plants; however, treated plants exhibited higher levels of lipid peroxidation. This study demonstrates that while Z. nigricaulis is resistant to FF's primary mode of action, significant physiological impacts occur following 7 days exposure to ≥0.1mgL^-1 FPA. This study provides valuable information on the effects of FF on a non-target species that can better inform approaches to Spartina management in coastal seagrass ecosystems.

Disease transmission promotes evolution of host spatial patterns

Ecological dynamics can produce a variety of striking patterns. On ecological time scales, pattern formation has been hypothesized to be due to the interaction between a species and its local environment. On longer time scales, evolutionary factors must be taken into account. To examine the evolutionary robustness of spatial pattern formation, we construct a spatially explicit model of vegetation in the presence of a pathogen. Initially, we compare the dynamics for vegetation parameters that lead to competition induced spatial patterns and those that do not. Over ecological time scales, banded spatial patterns dramatically reduced the ability of the pathogen to spread, lowered its endemic density and hence increased the persistence of the vegetation. To gain an evolutionary understanding, each plant was given a heritable trait defining its resilience to competition; greater competition leads to lower vegetation density but stronger spatial patterns. When a disease is introduced, the selective pressure on the plant's resilience to the competition parameter is determined by the transmission of the disease. For high transmission, vegetation that has low resilience to competition and hence strong spatial patterning is an evolutionarily stable strategy. This demonstrates a novel mechanism by which striking spatial patterns can be maintained by disease-driven selection.

Fractal measures of spatial pattern as a heuristic for return rate in vegetative systems

Measurement of population persistence is a long-standing problem in ecology; in particular, whether it is possible to gain insights into persistence without long time-series. Fractal measurements of spatial patterns, such as the Korcak exponent or boundary dimension, have been proposed as indicators of the persistence of underlying dynamics. Here we explore under what conditions a predictive relationship between fractal measures and persistence exists. We combine theoretical arguments with an aerial snapshot and time series from a long-term study of seagrass. For this form of vegetative growth, we find that the expected relationship between the Korcak exponent and persistence is evident at survey sites where the population return rate can be measured. This highlights a limitation of the use of power-law patch-size distributions and other indicators based on spatial snapshots. Moreover, our numeric simulations show that for a single species and a range of environmental conditions that the Korcak-persistence relationship provides a link between temporal dynamics and spatial pattern; however, this relationship is specific to demographic factors, so we cannot use this methodology to compare between species.

Plant characteristics associated with widespread variation in eelgrass wasting disease

Seagrasses are ecosystem engineers of essential marine habitat. Their populations are rapidly declining worldwide. One potential cause of seagrass population declines is wasting disease, which is caused by opportunistic pathogens in the genus Labyrinthula. While infection with these pathogens is common in seagrasses, theory suggests that disease only occurs when environmental stressors cause immunosuppression of the host. Recent evidence suggests that host factors may also contribute to disease caused by opportunistic pathogens. In order to quantify patterns of disease, identify risk factors, and investigate responses to infection, we surveyed shoot density, shoot length, epiphyte load, production of plant defenses (phenols), and wasting disease prevalence in eelgrass Zostera marina across 11 sites in the central Salish Sea (Washington state, USA), a region where both wasting disease and eelgrass declines have been documented. Wasting disease was diagnosed by the presence of necrotic lesions, and Labyrinthula cells were identified with histology. Disease prevalence among sites varied from 6 to 79%. The probability of a shoot being diseased was higher in longer shoots, in patches of higher shoot density, and in shoots with higher levels of biofouling from epiphytes. Phenolic concentration was higher in diseased leaves. We hypothesize that this results from the induction of phenols during infection. Additional research is needed to evaluate whether phenols are an adaptive defense against Labyrinthula infection. The high site-level variation in disease prevalence emphasizes the potential for wasting disease to be causing some of the observed decline in eelgrass beds.

The roles of flowering, overwinter survival and sea surface temperature in the long-term population dynamics of Zostera marina around the Isles of Scilly, UK

Interaction between biotic and abiotic drivers of dynamics is an important topic in ecology. Despite numerous short-term studies, there is a paucity of evidence about how environmental structure modifies dynamics in marine systems. We quantified Zostera marina flowering and non-flowering shoot density annually from 1996 to 2012 around the Isles of Scilly, UK, parameterizing a population dynamic model. Flowering is structured in time and space, with temperature and flowering positively associated at some locations only. We found no evidence that flower production contributes to seagrass density but 'patchiness' was positively associated with flowering in the previous year. With evidence of substantial overwinter survival, findings support the hypothesis that local populations are maintained largely through vegetative reproduction but sexual reproduction may contribute to colonisation of vacant habitat. This long-term study (1) tests validity of shorter-term investigations, (2) quantifies interaction between biotic and abiotic factors and (3) promotes seagrass as a model ecosystem.

Host demography influences the prevalence and severity of eelgrass wasting disease

Many marine pathogens are opportunists, present in the environment, but causing disease only under certain conditions such as immunosuppression due to environmental stress or host factors such as age. In the temperate eelgrass Zostera marina, the opportunistic labyrinthulomycete pathogen Labyrinthula zosterae is present in many populations and occasionally causes severe epidemics of wasting disease; however, risk factors associated with these epidemics are unknown. We conducted both field surveys and experimental manipulations to examine the effect of leaf age (inferred from leaf size) on wasting disease prevalence and severity in Z. marina across sites in the San Juan Archipelago, Washington, USA. We confirmed that lesions observed in the field were caused by active Labyrinthula infections both by identifying the etiologic agent through histology and by performing inoculations with cultures of Labyrinthula spp. isolated from observed lesions. We found that disease prevalence increased at shallower depths and with greater leaf size at all sites, and this effect was more pronounced at declining sites. Experimental inoculations with 2 strains of L. zosterae confirmed an increased susceptibility of older leaves to infection. Overall, this pattern suggests that mature beds and shallow beds of eelgrass may be especially susceptible to outbreaks of wasting disease. The study highlights the importance of considering host and environmental factors when evaluating risk of disease from opportunistic pathogens.

Local competition and metapopulation processes drive long-term seagrass-epiphyte population dynamics

It is well known that ecological processes such as population regulation and natural enemy interactions potentially occur over a range of spatial scales, and there is a substantial body of literature developing theoretical understanding of the interplay between these processes. However, there are comparatively few studies quantifying the long-term effects of spatial scaling in natural ecosystems. A key challenge is that trophic complexity in real-world biological communities quickly obscures the signal from a focal process. Seagrass meadows provide an excellent opportunity in this respect: in many instances, seagrasses effectively form extensive natural monocultures, in which hypotheses about endogenous dynamics can be formulated and tested. We present amongst the longest unbroken, spatially explict time series of seagrass abundance published to date. Data include annual measures of shoot density, total above-ground abundance, and associated epiphyte cover from five Zostera marina meadows distributed around the Isles of Scilly, UK, from 1996 to 2011. We explore empirical patterns at the local and metapopulation scale using standard time series analysis and develop a simple population dynamic model, testing the hypothesis that both local and metapopulation scale feedback processes are important. We find little evidence of an interaction between scales in seagrass dynamics but that both scales contribute approximately equally to observed local epiphyte abundance. By quantifying the long-term dynamics of seagrass-epiphyte interactions we show how measures of density and extent are both important in establishing baseline information relevant to predicting responses to environmental change and developing management plans. We hope that this study complements existing mechanistic studies of physiology, genetics and productivity in seagrass, whilst highlighting the potential of seagrass as a model ecosystem. More generally, this study provides a rare opportunity to test some of the predictions of ecological theory in a natural ecosystem of global conservation and economic value.

An integrative method for the evaluation, monitoring, and comparison of seagrass habitat structure

Assessing environmental condition is essential for the management of coasts and their resources, but better management decisions occur when large databases are simplified into more manageable units of information. Here we present the habitat structure index (HSI), which enables rapid assessment and direct comparison of seagrass habitat structure using scores of 0 (poor) to 100 (excellent) based on integrating five habitat variables: area, continuity, proximity, percentage cover, and species identity. Acquiring data to calculate the HSI can be done in situ or from video recordings, and requires relatively simple methodology of belt transects, estimating percentage cover, and basic taxonomy. Spatiotemporal comparisons can usefully identify locations and periods of seagrass habitat change, potentially providing an early warning indicator of habitat damage and decline in environmental quality. Overall, the integrative approach of the HSI represents a step toward simplifying the exchange of environmental information among researchers, coastal managers, and governing bodies.