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RESEARCH ARTICLE (Open Access)

Decadal monitoring shows seagrass decline and community shifts following environmental disturbance in Moreton Bay, south-eastern Queensland, Australia

Joanna N. Smart https://orcid.org/0000-0002-0717-0390 A B * , Nicholas M. Hammerman A , Kirsten M. Golding A , Kathryn Markey B , Eva Kovacs A and Chris Roelfsema A
+ Author Affiliations
- Author Affiliations

A Marine Ecosystem Monitoring Lab, School of the Environment, University of Queensland, Brisbane, Qld 4072, Australia.

B Earth Observation Research Centre, School of the Environment, University of Queensland, Brisbane, Qld 4072, Australia.

* Correspondence to: joanna.smart@uq.edu.au

Handling Editor: Michelle Casanova

Marine and Freshwater Research 76, MF25049 https://doi.org/10.1071/MF25049
Submitted: 6 March 2025  Accepted: 20 June 2025  Published: 10 July 2025

© 2025 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

Abstract

Context

Seagrass meadows are declining globally. Most seagrass research focuses on small-scale dynamics, with less known about change over larger spatial and temporal scales. The Eastern Banks in Moreton Bay, Australia (142 km2), provide a case study for investigating large-scale seagrass dynamics.

Aims

Determine whether seagrass species community composition and cover vary spatially and temporally across the Eastern Banks.

Methods

Georeferenced photoquadrats (n = 48,629) were collected across four banks between 2011 and 2024. Seagrass cover was classified using machine learning platform ReefCloud. Beta regression models and PERMANOVA analysis assessed trends in seagrass composition across years and among banks. Seagrass cover was assessed against water quality data to identify potential environmental drivers of change.

Key results

Oceana serrulata increased at Amity and Wanga-Wallen Banks, replacing Zostera muelleri as the dominant species. Total seagrass cover declined across three banks, with changes in cover and community composition being potentially associated with fluctuations in water quality.

Conclusion

Seagrass cover has declined significantly, with shifts in community composition occurring across the Eastern Banks during the study period.

Implications

These findings have highlighted the vulnerability of seagrass meadows to environmental disturbances, and the need for monitoring to identify resilience mechanisms and support ecosystem management.

Keywords: community composition, long-term monitoring, Moreton Bay, Oceana serrulata, seagrass decline, seagrass dynamics, seagrass meadows, Zostera muelleri.

Introduction

Seagrass meadows are critical ecosystems in coastal marine and estuarine environments worldwide (Orth et al. 2006; Nordlund et al. 2018). Healthy seagrass meadows provide essential ecosystem services such as nutrient cycling (Human et al. 2015), carbon sequestration (Macreadie et al. 2014; Duarte and Krause-Jensen 2017), coastal protection (Ondiviela et al. 2014), habitat creation (Heck et al. 2008), food generation (Nordlund et al. 2018) and oxygen production (Felisberto et al. 2015). However, anthropogenic pressures on marine habitats are causing extensive changes to coastal environments (Barbier et al. 2011; Priya et al. 2023; Venegas et al. 2023). In particular, seagrass meadows are experiencing a global decline (Waycott et al. 2009; Unsworth et al. 2019; Dunic et al. 2021). Factors such as marine heatwaves, extreme weather events (e.g. floods), water pollution (e.g. agricultural and urban runoff, wastewater discharge) and physical damage (e.g. dredging, land reclamation, bottom trawling) impact the physiology, composition, extent and density of coastal seagrass meadows (McKenzie et al. 2020; Strydom et al. 2020; Grech et al. 2011; Bass and Falkenberg 2023, Clemente et al. 2023, Moir et al. 2025).

Despite the extent of degradation to seagrasses worldwide, there is limited information on how seagrass meadows respond to disturbance events at ecologically relevant spatial scales and over extended timeframes (i.e. decadal) (Roelfsema et al. 2014; Kilminster et al. 2015). For instance, while many studies have focused on small scale seagrass dynamics, from individual plants to local patches of between 1 and 100 m2 (Orth and Heck 2023), there is a distinct scarcity of research investigating seagrass communities at broader spatial extents exceeding 100 km2 and spanning more than a few years (Orth et al. 2010; Lyons et al. 2012, 2013; Roelfsema et al. 2014; Kilminster et al. 2015; Kovacs et al. 2022). This gap in research hinders our ability to understand how seagrass communities respond to environmental and anthropogenic stressors at ecologically relevant scales. Comprehensive, long-term datasets are necessary to evaluate the resilience of seagrass meadows and to inform conservation and management strategies effectively (York et al. 2017; Unsworth et al. 2019; Orth and Heck 2023).

The shallow seagrass meadows of the Eastern Banks, Moreton Bay (Queensland, Australia), have been utilised as a study site for exploring seagrass dynamics including through remote sensing, in situ field surveys and manipulative experiments (Abal and Dennison 1996; Udy and Dennison 1997; Lyons et al. 2010, 2012; Kovacs et al. 2022). Covering ~142 km2, the Eastern Banks support many endangered and vulnerable species such as dugongs (Dugong dugon) (Department of Climate Change, Energy, the Environment and Water 2000; Lanyon 2003), green sea turtles (Chelonia mydas) (Arthur et al. 2007; Department of the Environment and Energy 2017) and commercially important species, such as snapper, prawns and crabs (Lanyon 2003; Arthur et al. 2007; Tibbetts et al. 2019). The Eastern Banks are comprised of four sub-banks, each separated by a deep channel (Amity, Wanga-Wallen, Moreton and Maroom), which are home to the following six main seagrass species: Halophila spinulosa, Halophila ovalis, Halodule uninervis, Zostera muelleri, Oceana serrulata and Syringodium isoetifolium (Young and Kirkman 1975), with a total of seven species occurring in the broader Moreton Bay region (Tibbetts et al. 2019). The most dominant species across the Eastern Banks by percentage cover include O. serrulata, Z. muelleri and H. uninervis (Phinn et al. 2008; Lyons et al. 2010, 2011). The region has experienced significant environmental disturbances that could affect seagrass dynamics, including flooding and storms in 2011, 2013 and 2022 (Yu et al. 2011; Hanington et al. 2015; Grinham et al. 2024). Additionally, anthropogenic activities in the bay, such as dredging, wastewater discharge and land reclamation, may influence the spatial extent and coverage of seagrass meadows in the region (Gibbes et al. 2014; Grinham et al. 2024).

To understand how seagrass communities have changed over time, we aimed to (1) quantify changes in community composition of the seagrass species both spatially (between banks) and temporally (between years) across shallow estuarine seagrass habitat, and (2) determine whether those changes are associated with acute disturbances or environmental change occurring during the study period. We used ~50,000 photoquadrats to assess the spatial and temporal patterns of seagrass coverage and species composition across 13 sampling events between June 2011 and December 2024. These findings highlight the vulnerability of seagrass meadows to environmental disturbances, emphasising the need for continued large-scale spatial monitoring and research to identify resilience mechanisms and support ecosystem management.

Materials and methods

Study site

The Eastern Banks are located within Moreton Bay (south-eastern Queensland, Australia), ~14 km east of Brisbane City. Specifically, the banks are found in the eastern part of Moreton Bay, south of Moreton Island and north-west of North Stradbroke Island (see Fig. 1). The Eastern Banks have been the focus of satellite remote sensing-based habitat mapping research since 2000 (Phinn et al. 2005). The study included the collection and analysis of an extensive in situ georeferenced photoquadrat data sets (Roelfsema et al. 2015a) used for verification for 13 sampling events up to 2024. The consistent methods for collection and analysis across time and space with high thematic classification of the photoquadrats (i.e. to species level) provide an opportunity to assess long-term changes in seagrass community composition, which has not yet been fully leveraged for ecological assessments.

Fig. 1.

Map of the survey area within the Eastern Banks, Moreton Bay (Map data ©2025 Google). (a) The location of Moreton Bay, denoted by a yellow dot, on the south-eastern coast of Queensland, Australia, (b) the location of the Eastern Banks within Moreton Bay, and (c) a fine-scale map with red dots showing the location of all survey transects within the Eastern Banks surveyed at any point between 2011 and 2024. Green lines delineate the four sub-banks within the survey area. Blue points indicate the location of water quality monitoring, from which environmental data have been annually measured (e.g. temperature, turbidity, salinity, dissolved oxygen).


MF25049_F1.gif

The area is dynamic, experiencing strong currents and a tidal range of ~1.5–2 m (Maritime Safety Queensland, Department of Transport and Main Roads 2024). Covering an area of ~142 km2, the Eastern Banks generally have a good water clarity and are shallow in depth, making them ideal for seagrass growth (Udy et al. 2024). However, the banks are also influenced by runoff from the surrounding rivers (e.g. Caboolture, Pine, Brisbane and Logan Rivers) and catchments, which can periodically affect water quality in the region (Tibbetts et al. 2019). The Eastern Banks can be divided into four major individual banks, each demarked by deep channels or land masses. These banks include Amity Bank, Moreton Bank, Maroom Bank and Wanga-Wallen Bank (Fig. 1) on which we focus this study because of the availability of consistent field data.

Field sampling

Georeferenced photoquadrats were collected for each of the 13 sampling events between June 2011 and December 2024, by using a repeatable and fine spatial-scale survey method that has been previously described (Roelfsema et al. 2015b). Snorkellers captured photoquadrats of the benthos while towing a float equipped with a handheld GPS, which was logging positions so photographs could be assigned coordinates through time synchronisation. Photoquadrats of the seafloor representing an ~1-m2 quadrat were captured at 2–4-m intervals with a waterproof digital camera along a transect, with transects ranging in length from 100 to 800 m. Transect locations were distributed throughout the different banks to capture the variation in seagrass habitat (Fig. 1). The total dataset consisted of a total of 48,629 photoquadrats across 13 sampling events (Table 1). In some years (i.e. 2012, 2013, 2014, 2024), two sampling events occurred, whereas in all other years, one sampling event occurred (Table 1). Not all transects were surveyed each year because of environmental conditions and logistics.

Table 1.Summary of field survey events between 2011 and 2024.

YearMonthSeasonNumber of transectsNumber of photographs
2011JuneWinter343759
2012JulyWinter313069
2012FebruarySummer312352
2013JanuarySummer302782
2013MayAutumn333946
2014AprilAutumn141181
2014JulyWinter242267
2015JuneWinter383562
2021JulyWinter284451
2022JuneWinter314026
2023JulyWinter304427
2024JulyWinter335392
2024DecemberSummer315729

Water quality data

Water quality data for the Eastern Banks was compiled to assess whether variability in water quality could be a potential driver of seagrass compositional change. Data were collected between 2011 and 2023 as part of the Ecosystem Health Monitoring Program (EHMP). The EHMP is an ongoing ambient monitoring program established in 1999 and is currently coordinated by Healthy Land and Water (HLW) (Healthy Land and Water 2023). Water samples were collected at three locations across the Eastern Banks (Fig. 1) during 8 months of the year (February, March, May, August, September, October, November and December). Depth profiles of temperature, salinity, pH, turbidity and dissolved oxygen were collected in situ with a multiparameter water quality sonde. Surface-water samples were collected and tested for total nitrogen, total phosphorus, ammonia, nitrogen oxides, filterable reactive phosphorus and chlorophyll-a.

Analysis

Seagrass community composition and coverage

Seagrass species composition and percentage cover were derived from the photoquadrats by utilising the machine learning platform, ReefCloud (Australian Institute of Marine Science, see https://reefcloud.ai/). Although ReefCloud was originally developed to analyse coral reef photoquadrats, its underlying architecture can be adapted for seagrass habitat classification. The flexible architecture, point-based classification interface, and capacity for user-defined training datasets made it possible to apply ReefCloud successfully to seagrass habitats, with performance suitable for ecological analysis. ReefCloud utilises a point count method where images as annotated by assigning classification categories to points randomly overlaid on the image. Point count methods are commonly used to assess benthic cover across tropical, subtropical and temperate ecosystems (Pante and Dustan 2012; Perkins et al. 2016). A proportion of the randomly overlaid points is assigned to a class by the user, which trains the algorithm for classification of the remaining points.

For this study, fifty points were randomly overlaid onto each photoquadrat in ReefCloud (see https://reefcloud.ai/). This resulted in a total of 2,427,350 points labelled across 48,629 photographs. In total, 19,105 (3.9%) points were manually classified by an expert for training data to support the machine learning model. Model training was undertaken until further training no longer resulted in increased model F1 and accuracy scores, ensuring a reasonable balance between classification accuracy and the need to process and analyse the full dataset. A seagrass habitat classification scheme was used for this study, which was previously determined to capture the variation of the main habitat types present in the study area (Roelfsema et al. 2015b).

To ensure all points could be correctly assigned a category, the categories used for classification included 28 categories within a hierarchical design, which included seven functional categories (Supplementary Table S1). Category accuracy was assessed by calculating the class F1 score. The F1 score is a performance metric that combines precision (correct positive predictions of all predicted positives) and recall (correct positive predictions of all actual positives) (Naidu et al. 2023; Diallo et al. 2025). In this study, only classes with an F1 score greater than 0.5 were used to ensure that model predictions were reasonably reliable, with a balanced trade-off between precision and recall, and to exclude poorly predicted or highly uncertain classes. The percentage cover of each category in each photoquadrat was determined by ReefCloud, by summing the total count for each category per quadrat and dividing by the number of points surveyed in the quadrat.

Spatial and temporal trends for seagrass cover and composition

To analyse the changes in seagrass cover within each bank over time, we calculated the mean percentage cover and associated confidence intervals for each seagrass species at each bank for each sampling event by using the statistical environment R (ver. 4.4.3, R Foundation for Statistical Computing, Vienna, Austria, see https://www.r-project.org/). This aggregation step was used to address sample imbalance, as the number of photoquadrats differed among sampling events (Table S3). By using the mean percentage cover as the response variable, we ensured that each time point contributed equally to the model, regardless of sample size.

To determine how the percentage cover of each seagrass species across the four different banks had changed over time, we fit beta regression models for each seagrass species within each bank by using the R package betareg (ver. 3.2-1, see https://cran.r-project.org/package=betareg; Cribari-Neto and Zeileis 2010). Beta regression is an extension of the generalised linear model, using a logistic link function to ensure logical predictions and assuming that errors are beta-distributed. Beta regression is well-suited for analysing percentage cover data because it models proportional outcomes (ranging between 0 and 1), while accounting for the skewness and heteroscedasticity often present in ecological data (Douma and Weedon 2019). Prior to analysis, percentage cover data were scaled to between 0 and 1, as required by beta regression. Any values of 0 or 100% were converted to 0.001 and 0.999 respectively, as the beta regression structure cannot utilise exact 0 and 1 values. The mean percentage cover per sampling event was used as the response variable, and sampling year was included as the predictor variable.

Given the time-series nature of the data, residuals from each model were assessed for temporal autocorrelation by using autocorrelation function (ACF) plots. Although some residual autocorrelation was detected, especially at short lags, explicit autocorrelation structures were not included because of the limited number of time points (n = 13) and irregular sampling intervals, which restrict the implementation and reliability of time-series models. Further model diagnostics were performed using the R packages performance (ver. 0.13.0, see https://cran.r-project.org/package=performance; Lüdecke et al. 2021), car (ver. 3.1-3, see https://CRAN.R-project.org/package=car; Fox and Weisberg 2019) and lmtest (ver. 0.9-40, see https://cran.r-project.org/package=lmtest; Zeileis and Hothorn 2002). Residual plots and McFadden’s pseudo-R2 were generated to assess model fit and evaluate the adequacy of the chosen distribution. Likelihood ratio tests were used to compare models and assess the significance of predictor variables. Residuals were examined for normality and homoscedasticity to validate beta regression assumptions.

To assess whether seagrass community composition and cover varied spatially or temporally, a permutational analysis of variance (PERMANOVA) was conducted using the R package vegan (ver. 2.6-8, J. Oksanen et al., see https://cran.r-project.org/package=vegan/). Percentage cover data were averaged for each transect within each bank. The explanatory variables included year of sampling, bank (Amity, Wanga-Wallen, Moreton and Maroom), and transect. Season was not included as a factor, because seasonal sampling was highly unbalanced across years, with most surveys occurring in either winter or summer, and limited representation from other seasons (Table 1).

To evaluate the assumption of homogeneity of multivariate dispersion, the function betadisper() from the vegan package was used. Significant differences in dispersion were found across both banks and years, suggesting that some variation detected by PERMANOVA could be influenced by unequal within-group variability. However, interpretation of PERMANOVA results was supported by complementary analyses. Pairwise PERMANOVA comparisons were conducted to identify which groups differed significantly, and non-metric multidimensional scaling (NMDS) plots were used to visualise compositional patterns across banks and years. These visualisations aided in distinguishing whether observed differences were due to shifts in group centroids (i.e. true compositional differences) or simply due to dispersion.

The final PERMANOVA model included ‘year,’ ‘bank,’ and ‘transect’ as explanatory variables. An interaction term between year and bank was included to account for spatially structured temporal changes and transect was nested within bank to account for spatial autocorrelation, because transects within the same bank are likely to share similar environmental and ecological conditions. A Bray–Curtis dissimilarity matrix was used as the resemblance measure, and 9999 permutations were applied to generate the reference distribution.

Patterns in community composition and seagrass cover were visualised using NMDS, implemented using vegan (ver. 2.6-8). Percentage cover data were averaged by transect within each bank, and Bray–Curtis dissimilarities were calculated on square-root transformed species abundance data to down-weight dominant species. A three-dimensional solution (k = 3) was selected to better capture the complex variation in the data, while maintaining acceptable stress values. Model fit was assessed using Shepard plots and stress values. Environmental vectors were fitted to the ordination, and variables with a Spearman rank correlation of >0.5 were overlaid on the NMDS plots to highlight associations with species and community structure.

Water quality

To assess when water quality deviated significantly from normal conditions, we calculated the 5th and 95th percentiles for indicators of water quality, including total nitrogen (mg L−1), ammonia (mg L−1), salinity (recorded by a multi-parameter water quality sonde, g L−1) and turbidity (NTU) in R (R Foundation for Statistical Computing). These percentiles were used as thresholds to flag extreme deviations in the dataset (Australian Government Initiative 2023) and identify when water quality conditions on the Eastern Banks had deviated from normal, potentially affecting seagrass species. Quantitative comparisons between water quality data and seagrass cover could not be conducted because of temporal and spatial disparity between water quality sampling and seagrass surveys.

Results

Point classification model accuracy

The point classifier correctly identified 83% of the training points, with a corresponding F1 score of 0.83 (Table S1). Of the seagrass species, the classification of Oceana serrulata was most accurate, with an F1 score of 0.92. The F1 scores for the classification of H. uninervis, H. spinulosa, S. isoetifolium and Z. muelleri all exceeded 0.70. Classification accuracy for H. ovalis was comparatively low, with an F1 score of 0.53. The model performed well when classifying other non-seagrass benthic habitats.

Trends in seagrass percentage cover

Analysis of seagrass cover trends across banks over time showed species-specific responses during the study period (Tables S2, S4). Oceana serrulata showed significant increases in cover at both Amity (Fig. 2, slope = 0.1976 ± 0.034, P < 0.001) and Wanga-Wallen banks (slope = 0.1281 ± 0.0193, P < 0.001), with particularly strong growth at Wanga-Wallen resulting in a 33.90% increase in cover. However, this species showed a significant decline at Maroom Bank (slope = −0.1289 ± 0.0007, P < 0.001).

Fig. 2.

Seagrass cover trends from the beta regression model for each seagrass species at each bank over the 13-year study period, with 95% confidence intervals. Solid lines represent significant model predictions, with dashed lines showing non-significant relationships. Dots represent mean cover from photoquadrats with ±95% confidence intervals.


MF25049_F2.gif

Conversely, Zostera muelleri displayed significant decreases across all banks, with particularly severe declines at Wanga-Wallen (slope = −0.2105 ± 0.0322, P < 0.0001) and Amity (slope = −0.1734 ± 0.0347, P < 0.0001), leading to cover reductions of 40.45 and 34.13% respectively. The trends in seagrass cover suggested that declines in Z. muelleri were replaced by O. serrulata at both Amity and Wanga-Wallen banks, with O. serrulata becoming the new dominant species.

Halophila ovalis exhibited consistent significant declines across all banks (all P < 0.001), with the strongest decrease observed at Maroom Bank (slope = −0.1688 ± 0.0464) and Wanga-Wallen Bank (slope = −0.1523 ± 0.0277). However, it should be noted that accuracy and F1 scores were lower for this species than for other seagrasses. Similarly, Halophila spinulosa demonstrated significant decreases across all banks, with the most pronounced decline at Maroom Bank (slope = −0.171 ± 0.403, P < 0.001), resulting in a 20.33% reduction in cover. Syringodium isoetifolium showed significant declines at three of the four banks, with Moreton Bank experiencing the most substantial decrease (slope = −0.232 ± 0.0537, P < 0.001), equating to a 17.40% reduction in cover. The only seagrass species that did not experience a significant decline across the banks was Halodule uninervis (all P > 0.05).

Despite increases in O. serrulata across two of four banks, total seagrass cover declined significantly across Maroom, Moreton and Wanga-Wallen banks (Fig. 3, all P < 0.001). The most substantial reduction was observed at Wanga-Wallen Bank (slope = −0.2105 ± 0.0322, P < 0.001), resulting in a 56.20% decrease in total cover. Amity bank was the only bank that did not record significant seagrass decline, which was likely because of the increase in O. serrulata replacing the declines observed in Z. muelleri.

Fig. 3.

Predicted seagrass cover trends from the beta regression model for total seagrass cover at each bank over the 13-year study period, with 95% confidence intervals. Solid lines represent significant model predictions, with dashed lines showing non-significant relationships. Dots represent mean cover from photoquadrats with ±95% confidence intervals.


MF25049_F3.gif

Trends in species composition

For the shallow waters of the Eastern Banks, significant differences in seagrass community composition across multiple factors were found (Table 2). Temporal variation (year) explained 16.47% of the total variation (F = 22.81, P = 0.001). This suggested that seagrass community composition has changed significantly over time. Differences among banks accounted for 22.08% of the variation (F = 48.89, P = 0.001), indicating that seagrass community composition varies among these spatially distinct locations. The interaction between year and bank was also significant (F = 3.20, P = 0.001), explaining 8.9% of the variation. The significance of interaction suggests that the effect of time on community composition differs among banks. The strongest source of variation was attributed to the nesting of transect within bank, which explained 33.24% of the total variation (F = 8.76, P = 0.001). This result suggested that there are meaningful differences in seagrass community composition among transects within the same bank.

Table 2.PERMANOVA results assessing the effects of bank, year and their interaction on seagrass community composition over the 13-year study period, with transect nested within bank.

ItemDegrees of freedomSum of squaresR2FPr (>F)
Year96.6530.1647722.80650.001***
Bank58.9150.2207848.89520.001***
Year:bank343.6200.089663.20270.001***
Bank/transect4613.4230.332438.76430.001***
Residual2207.7670.19236
Total31440.3781.00000

The table includes pseudo-F values, degrees of freedom and significance levels for each factor. Probabilities are significant at: ***, P ≤ 0.001.

Distinct temporal and spatial patterns in seagrass community composition were found within the NMDS ordination (stress = 0.06, Fig. 4, 5). The first axis (MDS1) represented a gradient from areas dominated by Oceana serrulata and Zostera muelleri to more sparse communities. The second axis (MDS2) separated communities characterised by Halophila ovalis and brown macroalgae from those dominated by Lyngbya majuscula. Temporal trajectories showed a general shift in community composition from 2011 to 2024, with the most pronounced changes occurring at Maroom bank, which displayed the greatest dispersion along MDS1.

Fig. 4.

Non-metric multidimensional scaling (NMDS) ordination of seagrass community composition coloured by year of monitoring. Points represent each bank at each survey year, with closer points indicating greater similarity in species composition.


MF25049_F4.gif
Fig. 5.

Non-metric multidimensional scaling (NMDS) ordination of seagrass community composition coloured by bank. Points represent each bank at each survey year, with closer points indicating greater similarity in species composition.


MF25049_F5.gif

The spatial separation among banks was evident, with Maroom bank showing the most distinct composition and greatest temporal variation. Amity, Moreton and Wanga-Wallen banks displayed more clustered patterns, suggesting more stable community composition over time, although still exhibiting significant temporal changes as indicated by the PERMANOVA results. These patterns suggested that although all banks underwent significant temporal changes in seagrass community composition, the magnitude and direction of these changes varied among locations.

Water quality dynamics

Water quality parameters showed notable deviations beyond the 95th percentile during the study period, particularly following flood events (Fig. 6). Ammonia concentrations exhibited several distinct peaks above the 95th percentile at location EHMP-506, with pronounced spikes occurring after flooding events. Total nitrogen similarly showed episodic elevations beyond the baseline conditions, with the most notable exceedances occurring between 2011 and 2012. Salinity demonstrated characteristic flood-related drops, particularly in 2011 and 2022, when values fell substantially below typical marine conditions. Turbidity was generally more variable throughout the monitoring period, with location EHMP-506 showing an increased frequency of elevated turbidity readings after 2016. These periodic departures from baseline water quality conditions, especially following flood events, suggest potential environmental stressors that may have influenced the observed changes in seagrass community composition.

Fig. 6.

Temporal trends in water quality variables (ammonia, salinity, total nitrogen and turbidity) and total seagrass cover across all banks over the study period. Red horizontal lines indicate the 95th percentile for each variable, whereas blue vertical lines mark the dates of significant flooding events in Brisbane. Dot colour represents EHMP monitoring sites EHMP 506 (green), EHMP 507 (orange) and EHMP 531 (purple).


MF25049_F6.gif

Discussion

We documented overall declines in seagrass percentage cover across the Eastern Banks, along with clear shifts in seagrass community composition. This analysis was based on 13 years of monitoring data for seagrass species and percentage cover, encompassing a wide spatial area within the Eastern Banks. Our results align with global findings indicating that seagrass meadows are declining, leading to shifts in species composition and reductions in overall spatial cover (Amone-Mabuto et al. 2018; Arias-Ortiz et al. 2018; Buckee et al. 2021; Orth and Heck 2023). Such reductions in seagrass functioning have profound ecological and economic implications (Roca et al. 2016; Unsworth et al. 2019). In the discussion below we outline these ecological trajectories, considering both acute disturbance and environmental variability. We then frame this insight in terms of its implications for the continued functioning of seagrasses within the Bay and how this spatio-temporal understanding of seagrass dynamics can inform conservation and restoration efforts.

Past studies have documented seagrass decline across the banks between 1988 and 2010 through remote sensing (Lyons et al. 2013). Previous studies in Moreton Bay have documented substantial losses of Syringodium isoetifolium following major flood events (Hanington et al. 2015) and seagrass extent within the broader Moreton Bay has been shown to decline following flood events (Udy et al. 2024). Additionally, extreme weather events such as cyclones have led to seagrass uprooting and light deprivation in Hervey Bay, located ~240 km north of the study area (Preen et al. 1995). These results contribute to the growing body of evidence showing a widespread reduction in seagrass extent in Moreton Bay, more regionally (i.e. south-eastern Queensland) and worldwide (Short et al. 2006, 2014; Buckee et al. 2021; Green et al. 2021; Leblanc et al. 2023).

In addition to overall declines in seagrass cover, shifts in community composition have also been observed, with notable differences among the four banks. For instance, Amity and Wanga-Wallen banks share a similar species composition over time, whereas Moreton and Maroom exhibit greater divergence. This variation appears to be primarily driven by the presence of Oceana serrulata, which has replaced Zostera muelleri as the dominant species at Amity and Wanga-Wallen during the monitoring period. By contrast, Moreton and Maroom have lost most of their seagrass cover. In particular, O. serrulata is known for its ability to rapidly colonise disturbed areas (Collier et al. 2012). Seagrass species with colonising life-history traits (i.e. fast growth, rapid reproduction and wide dispersal) are generally expected to recover quickly from disturbance, whereas opportunistic species, such as Z. muelleri, recover more slowly, resulting in community composition change after disturbance events (Turschwell et al. 2021). Similar shifts have been observed elsewhere, including in Shark Bay, Western Australia, where opportunistic species replaced more persistent species following a heatwave, leading to a decline in ecosystem services (Nowicki et al. 2017). The ecological drivers behind the apparent shift from Z. muelleri to O. serrulata remain unclear. It is uncertain whether O. serrulata is outcompeting Z. muelleri because of faster growth or whether Z. muelleri is more sensitive to specific environmental pressures that do not affect O. serrulata as strongly. Further research is needed to clarify these dynamics; however, the pattern in changing species composition observed at the Eastern Banks is consistent with the broad environmental disturbances experienced within Moreton Bay.

Changes in seagrass percentage cover and species composition have important ecological consequences for Moreton Bay, affecting primary production, carbon storage and trophic interactions (Walker and McComb 1992; Hughes et al. 2009; Do et al. 2013). Different seagrass species contribute varying amounts of biomass for the same percentage cover, influencing carbon cycling and habitat structure (Duarte and Chiscano 1999; Roelfsema et al. 2014; Stankovic et al. 2018). Oceana serrulata has a particularly high below-ground biomass and greater physiological resilience than do the other species present on the banks (Kilminster et al. 2015). Faunal grazing dynamics may also shift in response to these changes. Halophila ovalis is a key food source for dugongs and green turtles and is resilient to grazing (Kuiper-Linley et al. 2007). A decline in H. ovalis could therefore have cascading effects on herbivore populations, as seen in studies linking seagrass loss to declining green turtle populations (Meylan et al. 2022). Furthermore, shifts in seagrass composition can alter benthic infaunal communities (Mills and Berkenbusch 2009), potentially disrupting ecosystem services such as sediment stabilisation, carbon sequestration and nutrient cycling.

The observed changes in seagrass composition and cover show a critical vulnerability of coastal marine ecosystems to environmental fluctuations. Climate-change models predict increased frequency of extreme weather events, including more intense flooding and prolonged periods of environmental stress (Pörtner et al. 2022). Our findings indicated that seagrass communities might not simply recover linearly after disturbances, but could undergo fundamental restructuring. The replacement of Z. muelleri by O. serrulata at Amity and Wanga-Wallen banks between 2011 and 2024 demonstrates this adaptive response in the face of changing environmental conditions.

Water quality may be a key driver of these changes, particularly in response to extreme weather events. Seagrass meadows are highly sensitive to fluctuations in sediment composition and light availability (Dennison 1987; de Boer 2007; Cabaço et al. 2008; Carr et al. 2016). The timing of the observed decline coincides with major climatic disturbances. Salinity, turbidity and nutrient inputs have been previously identified as significant predictors of seagrass cover and composition, underscoring the role of water quality in shaping these ecosystems (Longstaff and Dennison 1999; Udy et al. 1999; Lirman and Cropper 2003; Saunders et al. 2017; Bell et al. 2019). Previous studies have shown that when exposed to prolonged light stress, seagrasses shed leaves and shoots as a survival response (York et al. 2013). In Moreton Bay, declining water quality has already been linked to losses in rock oyster populations (Diggles 2013), reduced coral cover and condition (Lybolt et al. 2011; Hammerman et al. 2022) and shifts in benthic diatom communities (Grinham et al. 2011). Projected sea level rise and sediment accretion could further exacerbate this decline, with models predicting a 17% reduction in seagrass extent across Moreton Bay by 2100 (Saunders et al. 2013). In addition, major flood events (e.g. 2022) have elevated sediment resuspension (i.e. turbidity) and nutrient loads (e.g. ammonium, nitrates, total nitrogen) to levels ~10 times above background concentrations and such fine sediment deposition has affected 98% of the Bay’s seafloor (Grinham et al. 2024). These habitats are increasingly under threat from acute disturbances such as flooding, coastal development, dredging, urbanisation and ocean warming. Increased urbanisation can lead to greater coastal development and the expansion of paved surfaces, which in turn alters natural water flow and increases stress on coastal ecosystems. These changes could contribute to higher freshwater runoff, sedimentation and nutrient loads, posing significant risks to the ecological health of Moreton Bay.

There are several limitations to this study that should be considered. The classification accuracy for H. ovalis was lower than for other species, introducing some uncertainty in abundance estimates. This is perhaps due to the cryptic nature of H. ovalis, forming small leaves, which often occur as mixed communities with other larger species of seagrass. However, we elected to keep H. ovalis in our dataset because of its ecological importance as a food source for dugongs and green sea turtles. Regardless of this inclusion, we do acknowledge the lower accuracy score of H. ovalis from our ReefCloud model, and this warrants careful consideration of assessing its ecological trajectory.

Additionally, the photoquadrats were originally intended as validation and calibration data points for remote-sensing work throughout the Eastern Banks (Roelfsema et al. 2013, 2015a, 2015b). Hence, sites were selected to represent variation in habitat and were not permanent quadrats. As a result, variation in location could introduce some spatial variability. However, the strength of these data is that they cover a large spatial area over each bank and were designed to encompass all the possible types of substrate and seagrass communities expected. Water quality data were also coarser in resolution and not directly aligned with seagrass monitoring events, which may have obscured potential correlations. This temporal mismatch means that water quality measurements were not taken at the exact timing of disturbance events and these measurements were not taken directly overtop our study area (i.e. the HLW monitoring stations were adjacent to our banks rather than directly over them and led us to utilise these data qualitatively).

Through this study, we have provided novel insights into seagrass dynamics over extended spatial and temporal scales. Our results have highlighted a significant decline in seagrass percentage cover since 2011, alongside a shift in community composition, with O. serrulata emerging as the dominant species in two of the four banks. These findings suggest that acute disturbance and a broader trend in declining water quality across Moreton Bay may have potentially played a major role in shaping the seagrass assemblages observed today.

Although changes in water quality appear to coincide with seagrass decline, further research is needed to quantify these relationships more precisely. Water quality programs that more closely match seagrass monitoring surveys spatially and temporally may help untangle such relationships. The Eastern Banks may also provide a valuable site for future seagrass restoration efforts, particularly for Z. muelleri, which has been the focus of recent restoration trials in Australia (Wendländer et al. 2020). However, such work often focuses on restoration following near complete loss of seagrass, rather than when there are shifts in community composition, as was observed at Amity and Wanga-Wallen banks. As such, traditional restoration approaches may need refinement. Instead of attempting to recreate historical ecosystem states, management strategies should focus on enhancing adaptive capacity and promoting resilience. This could involve identifying and protecting areas with more stable seagrass communities, developing targeted restoration protocols that consider species-specific responses to environmental stress, and implementing comprehensive water quality management strategies that mitigate the impact of flood events and urban runoff. Continued monitoring of these seagrass communities is critical for understanding long-term environmental drivers and detecting shifts in spatial and temporal patterns. Future research should focus on the resilience mechanisms of seagrass in Moreton Bay, as well as the potential for restoration to mitigate ongoing declines and sustain the critical ecosystem services these meadows provide.

Supplementary material

Supplementary material is available online.

Data availability

Data used in this study are accessible through the GitHub repository (see https://github.com/joannasmart/seagrass_photoquadrat_analysis). Code used in the analyses and further supplementary information is provided within the repository.

Conflicts of interest

The authors declare they have no conflicts of interest.

Declaration of funding

Funding was provided by the: University of Queensland; CSIRO; Cooperative Research Centre Coastal Zone, Estuaries and Waterways Management; ARC Linkage Grant awarded to Prof. J. Marshall and Prof. S. Phinn; UQ-UWA Collaborative Research Grant awarded to Prof. van Niel; Goodman Foundation; SmartSat CRC and The Lott Corporation.

Author contributions

C. Roelfsema and J. Smart conceived the idea. J. Smart, C. Roelfsema and N. Hammerman designed the methodology. J. Smart, C. Roelfsema, N. Hammerman, K. Golding, E. Kovacs, K. Markey and C. Roelfsema collected the data. J. Smart, N. Hammerman, K. Golding, E. Kovacs and K. Markey contributed to data training and validation. J. Smart conducted the formal analysis, investigation and visualisation. J. Smart and N. Hammerman contributed to writing of the original draft. C. Roelfsema supervised the research. All authors contributed critically to the drafts and gave final approval for publication.

Acknowledgements

We acknowledge the Quandamooka People as the Traditional Owners and their custodianship of the lands on which Moreton Bay Research Station operates and the Sea Country within which this research was conducted. We pay our respects to their ancestors and their descendants, who continue cultural and spiritual connections to Country. We recognise their valuable contributions to Australian and global society. We thank the Healthy Land and Water Environmental Health Monitoring Program for providing water quality data. We thank Moreton Bay Research Station, and all staff and volunteers involved in collecting the data.

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