Seagrass Shoot Density Research Articles

Seagrass blue carbon stocks and air-sea CO2 fluxes in seagrass meadows Jepara, Indonesia

Abstract This research was conducted on August 2022 (east season) and February 2023 (west season) in Jepara waters. There are four types of seagrasses found: Ea (Enhalus acoroides), Cr (Cymodocea rotundata), Os (Oceana serrulata), and Th (Thalassia hemprichii). The analysis results suggest that the Os species has the largest organic carbon stock within seagrass (OCS-Sg), whereas Ea has the lowest. Aside from morphological measures, in this study seagrass shoot density also determined OCS-Sg for each type of seagrass. This study found that OCS-Sg is higher during the east season (26.2±2.81 gCorg m−2) than the west season (11.2±1.78 gCorg m−2). Whereas, organic carbon stock within sediment (OCS-S) is lower during the east season (41.5±0.9 gCorg m−2) and higher during the west season (56.3±0.9 gCorg m−2). During the east and west seasons, Jepara waters act as a CO2 source. However, during the east and west seasons vegetated OCS-S had higher carbon storage (41.9 and 57.2 gCorg m−2) than unvegetated (39.2 and 53.7 gCorg m−2). This is also indicated by the CO2 flux in vegetated which is lower (13.1±4.8 mmol CO2 m−2 d−1) than unvegetated (16.5±4.7 mmol CO2 m−2 d−1) during the east season. On the other hand, vegetated waters have higher CO2 flux (22.7±2.7 mmol CO2 m−2 d−1) in the west season than unvegetated (18.0±2.5 mmol CO2 m−2 d−1), this is related to seagrass shoot density during the west season which is decreasing. These findings lead to the conclusion that seagrass plays an important role for storing and absorbing carbon and storing carbon and store it in seagrass biomass and sediment even though the role of waters returns CO2 back to the atmosphere.

Edge effects of a fragmented seagrass habitat on flow, bivalve recruitment, and sediment dynamics

In both continuous and fragmented seagrass ecosystems, the vegetation edge can be a location of abrupt hydrodynamic change, with impacts to both ecological and physical processes. We address how flow and wave activity change across seagrass meadow edges and the effects of vegetation on sediment dynamics and bivalve recruitment. Two Zostera marina seagrass meadow sites were monitored: a high-density site with >500 shoots m-2 and a low-density site with <250 shoots m-2. Mean flow velocities were significantly reduced in seagrass vegetation adjacent to edges, with reductions compared to unvegetated areas ranging from 30-75%. Recruitment of juvenile bivalves was significantly elevated within vegetation. No significant differences in wave activity or sediment suspension and/or deposition were found spatially across a 10 m distance from a seagrass edge, but significant temporal variability was observed, caused by periodic storms. Wave height was a major predictor for sediment movement along seagrass edges, with an observed 10-fold increase in sediment collection within benthic traps following severe storms. These results were found across various heterogeneous edge configurations and reveal abrupt hydrodynamic responses of both mean flow and turbulence to occur at short spatial scales (1-10 m), with changes to wave and sediment deposition and/or suspension conditions only occurring over larger spatial distances (~100 m). Changes to the hydrodynamic regime were therefore found to be driven by meteorological conditions (e.g. winds, storms) on daily timescales and by changes in seagrass shoot density, altering both bivalve recruitment and sediment dynamics on longer temporal and/or spatial timescales.

A successful method to restore seagrass habitats in coastal areas affected by consecutive natural events.

Seagrass meadows, known for providing essential ecosystem services like supporting fishing, coastline protection from erosion, and acting as carbon sinks to mitigate climate change effects, are facing severe degradation. The current deteriorating state can be attributed to the combination of anthropogenic activities, biological factors (i.e., invasive species), and natural forces (i.e., hurricanes). Indeed, the global seagrass cover is diminishing at an alarming mean rate of 7% annually, jeopardizing the health of these vital ecosystems. However, in the Island Municipality of Culebra, Puerto Rico, losses are occurring at a faster pace. For instance, hurricanes have caused over 10% of cover seagrass losses, and the natural recovery of seagrasses across Culebra's coast has been slow due to the low growth rates of native seagrasses (Thalassia testudinum and Syringodium filiforme) and the invasion of the invasive species Halophila stipulacea. Restoration programs are, thus, necessary to revitalize the native seagrass communities and associated fauna while limiting the spread of the invasive species. Here, we present the results of a seagrass meadow restoration project carried out in Punta Melones (PTM), Culebra, Puerto Rico, in response to the impact of Hurricanes Irma and María during 2017. The restoration technique used was planting propagation units (PUs), each with an area of 900 cm2 of native seagrasses Thalassia testudinum and Syringodium filiforme, planted at a depth between 3.5 and 4.5 m. A total of 688 PUs were planted between August 2021 and August 2023, and a sub-sample of 88 PUs was monitored between August 2021 and April 2023. PUs showed over 95% of the seagrass survived, with Hurricane Fiona causing most of the mortalities potentially due to PUs burial by sediment movement and uplifting by wave energy. The surface area of the planting units increased by approximately 200% (i.e., 2,459 cm2), while seagrass shoot density increased by 168% (i.e., 126 shoots by PU). Additionally, flowering and fruiting were observed in multiple planting units, indicating 1) that the action taken did not adversely affect the PUs units and 2) that the project was successful in revitalizing seagrass populations. The seagrass restoration project achieved remarkable success, primarily attributed to the substantial volume of each PUs. Likely this high volume played a crucial role in facilitating the connection among roots, shoots, and microfauna while providing a higher number of undamaged and active rhizome meristems and short shoots. These factors collectively contributed to the enhanced growth and survivorship of the PUs, ultimately leading to the favorable outcome observed in the seagrass restoration project.

Stressor fluctuations alter mechanisms of seagrass community responses relative to static stressors

Ecosystems are increasingly affected by multiple anthropogenic stressors that contribute to habitat degradation and loss. Natural ecosystems are highly dynamic, yet multiple stressor experiments often ignore variability in stressor intensity and do not consider how effects could be mediated across trophic levels, with implications for models that underpin stressor management. Here, we investigated the in situ effects of changes in stressor intensity (i.e., fluctuations) and synchronicity (i.e., timing of fluctuations) on a seagrass community, applying the stressors reduced light and physical disturbance to the sediment. We used structural equation models (SEMs) to identify causal effects of dynamic multiple stressors on seagrass shoot density and leaf surface area, and abundance of associated crustaceans. Responses depended on whether stressor intensities fluctuated or remained static. Relative to static stressor exposure at the end of the experiment, shoot density, leaf surface area, and crustacean abundance all declined under in-phase (synchronous; 17, 33, and 30 % less, respectively) and out-of-phase (asynchronous; 11, 28, and 39 % less, respectively) fluctuating treatments. Static treatment increased seagrass leaf surface area and crustacean abundance relative to the control group. We hypothesised that crustacean responses are mediated by changes in seagrass; however, causal analysis found only weak evidence for a mediation effect via leaf surface area. Changes in crustacean abundance, therefore, were primarily a direct response to stressors. Our results suggest that the mechanisms underpinning stress responses change when stressors fluctuate. For instance, increased leaf surface area under static stress could be caused by seagrass acclimating to low light, whereas no response under fluctuating stressors suggests an acclimation response was not triggered. The SEMs also revealed that community responses to the stressors can be independent of one another. Therefore, models based on static experiments may be representing ecological mechanisms not observed in natural ecosystems, and underestimating the impacts of stressors on ecosystems.

Megaherbivore exclusion led to more complex seagrass canopies and increased biomass and sediment Corg pools in a tropical meadow

In some regions of the Caribbean Sea, seagrasses have been negatively affected by sea turtle overgrazing. Seagrass canopy complexity has declined at a long-term monitoring site in Costa Rica. We deployed megaherbivore exclosures for 13 months and found an increase over time in seagrass cover and maximum canopy height to ~ 75% and 20 cm respectively in the exclosures; while they remained steady in controls at < 25% and ~ 5 cm. Following exclusion, above ground biomass was higher in exclosures (320 ± 58 g DW m-2) compared to controls (171 ± 60 g DW m-2). Leaves were longer and wider in the exclosures (8 ± 5 cm and 0.8 ± 0.2 cm) compared to controls (2 ± 2 cm and 0.5 ± 0.1 cm). Above ground biomass Corg pools in exclosures (1.2 ± 0.2 Mg ha-1) were two-times higher than in controls (0.6 ± 0.2 Mg ha-1). Meanwhile, there was no variation between treatments in seagrass shoot density (1,692 ± 803 shoots m-2), below ground biomass (246 ± 103 g DW m-2) and its Corg pool (0.8 ± 0.4 Mg ha-1). Relative sediment level increased up to 4.4 cm within exclosures revealing a net increase in sediment Corg, while surficial sediment Corg percentage was similar between exclosures and controls. Releasing these meadows from megaherbivore grazing therefore led to a clear increase within exclosures of seagrass cover, canopy complexity, above ground biomass, and Corg pools in above ground biomass and sediment. Our study reveals that the decline in canopy complexity over time at this meadow is linked to megaherbivore grazing and has most likely led to a decrease in blue carbon pools. Excessive megaherbivore grazing at this site could lead to a continued decline or potential loss of the meadow, and seagrass conservation and restoration initiatives should include consideration of trophic dynamics.

Seasonal growth and senescence of seagrass alters sediment accumulation rates and carbon burial in a coastal lagoon

AbstractSeagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively trapped sediment at meadow edges during spring–summer growth seasons, during winter senescence low‐density meadows (< 160 shoots m−2) were erosional with rates sensitive to density. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change.

Regional variation in seagrass complexity drives blue crab Callinectes sapidus mortality and growth across the northern Gulf of Mexico

Seagrass meadows provide greater predator refuge and resource availability than unvegetated habitats and generally improve the survival and growth rates of associated animals. Few studies, however, have examined how these relationships might vary at a region-wide spatial scale. The blue crabCallinectes sapidusis a commercially important species that uses turtlegrassThalassia testudinumhabitats, but it is unclear if blue crab use of seagrass habitats varies across the northern Gulf of Mexico (GOM), USA. We conducted synoptic predation and growth experiments at 6 turtlegrass-dominated estuaries in the northern GOM to evaluate the role of seagrass structural complexity on juvenile (9.7-44 mm carapace width) blue crab mortality due to predation and growth. Relationships of blue crab predation and growth rate with seagrass shoot density, canopy height, temperature, and seagrass leaf area index (LAI) were evaluated using linear and generalized linear mixed effects models. Mortality rates due to predation (50 ± 9.6% [mean ± SD] crabs eaten, n = 286) showed negative relationships with LAI across the northern GOM. Conversely, mean crab growth rate (0.513 ± 0.317 mm d-1) varied across the northern GOM but was independent of seagrass shoot density or canopy height. We found that: (1) turtlegrass-dominated beds with a greater seagrass LAI provided more effective cover for juvenile blue crabs across the northern GOM, (2) blue crabs across multiple GOM estuaries responded similarly to changes in LAI, and (3) blue crab growth varied across the northern GOM and was likely dependent on regional factors unrelated to structural complexity. This study illustrates the value of conducting synchronous field-based experiments across broad spatial scales to identify regional patterns and the influence of regional versus local drivers.

Seaweed farming and land-use impacts on seagrass meadows in the region of Rote Island, Indonesia

The international demand for macroalgae products has increased driving the expansion of seaweed farming to many coastal rural areas worldwide. Yet it is not fully understood what the impacts of this practice are on local habitats, including seagrass meadows, one of the most productive coastal ecosystems. Additionally, high nutrient inputs to coastal systems via direct freshwater run-off or groundwater can cause algal blooms with negative consequences to seagrasses and seaweed production. This study aims to detect the short term ecological impacts of seaweed farming on seagrass meadows, as well as the influence of nutrient inputs via groundwater on seaweed productivity in the near-shore aquatic system of Rote Island, Indonesia. Seaweed farming plots were set up over seagrass meadows in two locations on the island with assumed higher and lower anthropogenic pressure in Nemberala village and Tunggaoen beach, respectively. Seaweed production rates, seagrass indices, and environmental indicators were measured within the seaweed farm and in plots directly outside of the farm over two seaweed growth cycles. Seaweed production was positively affected by environmental changes during the rainy season in the region, including lower temperatures, higher water motion and higher nutrient concentrations. On the other hand, results show 15% reduction in seagrass shoot density, as well as reduced leaf length, plant biomass, and percentage cover under the seaweed farms. Nuisance algae were found attached or entangled in the seaweed fronds, especially in Nemberala, causing further light reduction under the farms. The impacts on seagrasses also led to diminution of Corg (22-23%) and nitrogen (22-29%) in surface sediment in the plots compared to unfarmed areas. δ15N of the suspended particulate matter of sampled groundwater from the beach seepages in both sites were high, 13.4‰ in Nemberala and 17.8‰ in Tunggaoen, indicating heavier sources of N either from sewage contamination or isotopic discrimination due to microbial processing of groundwater. During the rainy season, the N- and P-enriched water entering the system led to an increase in nutrient concentrations in Nemberala bay seawater, causing eutrophication.

Factors modulating herbivory patterns in Cymodocea nodosa meadows

AbstractIn coastal marine food webs, seagrass‐grazer interactions play a fundamental role in ecological processes by regulating the structure and functioning of plant communities. Therefore, assessing the strength of these seagrass‐grazer links and identifying the mechanisms that regulate these relationships are crucial to increasing our understanding of community and ecosystem structure and dynamics. Herbivory on the temperate seagrass Cymodocea nodosa was evaluated in four locations with contrasting abiotic and biotic factors (i.e., depth; meadow size; seagrass shoot density, productivity, and leaf traits; and herbivore presence) using cross‐transplantation experiments carried out once per season to account for seasonal variation. Patterns of feeding rates on C. nodosa meadows showed high temporal variability, with peaks occurring in spring and summer. Results indicate that between 46.59% and 74.08% of the annual leaf production is lost to herbivory at the locations sampled in this study. Herbivory rates increased with higher nitrogen content and shoot density for all types of consumers (mesograzers, fishes, and sea urchins). This work highlights the need to integrate the abiotic and biotic factors modulating herbivory patterns, particularly in a foundation species like C. nodosa, for which changes in plant traits and grazer abundance may trigger trophic cascades with far‐reaching consequences for associated species.

Modeling seagrass bed dynamics under environmental impacts of intensive mariculture activities in Bolinao and Anda, the Philippines

This study assessed the estuarine-scale dynamics of mixed seagrass beds in Bolinao and Anda coastal areas in the Philippines, where seagrass beds have declined primarily due to eutrophication-induced degradation of light environment caused by unregulated mariculture activities in the adjacent areas. A seagrass model, driven by a coupled hydrodynamic-biogeochemical model, was developed and applied to the two-dominant species in the area – Thalassia hemprichii and Enhalus acoroides, and validated using observed spatial seagrass shoot density. The results showed the degradation of light environment in the seagrass beds, especially near mariculture areas, due to advective fluxes of water with high phytoplankton biomass and high nutrients from fish farms. The seagrass model driven by the reproduced water quality showed high reproducibility of seagrass above-ground biomass distribution of the two co-existing species, with R2 values of 0.78 and 0.60 for T. hemprichii and E. acoroides, respectively. Scenario analysis was conducted to examine the effectiveness of individual- and inter-municipality mariculture management (e.g., regulating fish feed input) in improving the light environment and facilitating the recovery of seagrasses. Results showed that the light environment could be significantly enhanced by an inter-municipality scheme compared to individual efforts of Bolinao and Anda municipalities, since their waters are interconnected through hydrodynamics. In conjunction with the improvement of light conditions, the recovery in seagrass abundance is most significant under the cooperative management scheme, thus highlighting its effectiveness.

In situ Responses of the Eelgrass Zostera marina L. to Water Depth and Light Availability in the Context of Increasing Coastal Water Turbidity: Implications for Conservation and Restoration.

Accelerating losses of seagrass meadows has led to efforts to restore these highly productive and beneficial ecosystems globally. Depth and light availability are critical determinants of seagrass restoration success. Eelgrass (Zostera marina L.) is the dominant seagrass species in the temperate northern hemisphere, but its global distribution has reduced dramatically. The main aims of this study were to determine: (1) the depth limit for Z. marina survival in Ailian Bay, north China, and (2) how light availability affects the growth and recruitment of Z. marina as a basis for identifying a suitable depth range for successful restoration. To achieve these aims, Z. marina shoots were transplanted from a nearby donor site, Swan Lake, to an experimental site, Ailian Bay, and the temporal responses of Z. marina shoots to light availability at water depths ranging from 1 to 8 m were investigated using in situ suspended cultures. Four suspended shoot transplantation experiments were conducted in 4 years. The results showed that the transplanted Z. marina shoots could survive and branch during an annual growth cycle, permanently underwater, at a depth ≤3 m. Due to the local turbidity of the waters in Ailian Bay, a depth of 4 m led to sufficient light deprivation (reduced to 6.48–10.08% of surface irradiance) to negatively affect seagrass shoot density and clonal reproduction. In addition, reproductive shoot density also tended to decline with water depth and light deprivation. Our results indicated that Z. marina population recruitment, through sexual and asexual (clonal growth) reproduction, were negatively affected by increasing water depth and light deprivation. These findings may provide a suitable depth range for the successful restoration of Z. marina in local coastal waters. They may also be applied to the management and restoration of Z. marina globally.

Seagrass of Vasiliko Bay, Eastern Mediterranean: Lost Cause or Priority Conservation Habitat?

Mediterranean coasts are affected by multiple mounting pressures. In Cyprus, marine fish farming has grown rapidly in the past decade and is concentrated in the west side of Vasiliko Bay. The east coast of this bay has ports, a power station, a desalination unit, a cement factory, a major new oil terminal, and gas storage facilities. The bay is earmarked to create the largest hydrocarbon processing, storing, and transport facility in the region. Here, we assess the status of Posidonia oceanica habitat in an understudied region at the upper thermal, and eastern limit, of this Mediterranean endemic seagrass. An extensive ancient seagrass meadow was revealed, covering about 200 ha across 10 km of coastline, over soft substrata at ca 10–30 m depth, and over hard substrata at ca 0–6 m depth. Seagrass shoot density and leaf surface area decreased, both with increasing depth and with proximity to industrial developments; part of the meadow had been destroyed by dredging to build a jetty. Close to fish farms the seagrass had higher epiphytic biomass as well as lower leaf number, mass, and surface area, all of which indicate adverse effects of eutrophication and increased turbidity. Despite these multiple stressors, most of the meadow was in good ecological status, with some of the highest shoot densities ever reported. Furthermore, iconic species like sea turtles, monk seals, and dolphins were seen during sampling. Posidonia oceanica meadows off Cyprus are among the most valuable in the Mediterranean due to their tolerance of high seawater temperatures. Managers of future coastal developments in the region will need to adhere to European legislation and international conventions designed to secure the socioeconomic benefits of seagrass beds.

Temporal changes of the fish community in a seagrass bed after disappearance of vegetation caused by disturbance of the sea bottom and sediment deposition

AbstractTo investigate the response of the fish community structure to a natural disturbance in their habitat, fish abundance, biomass and species composition were analysed in relation to temporal variability of environmental conditions in a seagrass Zostera marina bed. A total of 3024 fishes belonging to 46 taxa (22 families) were collected by quantitative sampling for 10 years from 2007 to 2016 in the Seto Inland Sea, south-western Japan. Seagrass shoot density decreased to less than 1/20 of its original density after disappearance of vegetation caused by heavy rain in the autumn of 2011 and the area did not recover for the next five years. In order to analyse temporal changes of fish community, the fishes were divided into three groups depending on their habitats or lifestyles: pelagic or migratory species (PM), sand or mud bottom-associated species (SM) and seagrass (Z. marina) – or substrate (rocky bottom including macrophytes) – associated species (ZS). Multiple regression analysis showed seagrass shoot density had the most significant effect on biomass of ZS among the three groups, with higher fish biomass under higher seagrass shoot density. Fish community composition changed after the disappearance of the seagrass vegetation coverage with an increase in abundance of SM during the five years of the post-disturbance period. Seagrass vegetation was concluded to affect temporal change of fish community structure through a stronger influence on fish species that are more dependent on seagrass beds as habitat.

Influence of the seagrass Thalassia hemprichii on coral reef mesocosms exposed to ocean acidification and experimentally elevated temperatures

Ocean acidification (OA) and warming currently threaten coastal ecosystems across the globe. However, it is possible that the former process could actually benefit marine plants, such as seagrasses. The purpose of this study was to examine whether the effects of the seagrass Thalassia hemprichii can increase the resilience of OA-challenged coral reef mesocosms whose temperatures were gradually elevated. It was found that seagrass shoot density, photosynthetic efficiency, and leaf growth rate actually increased with rising temperatures under OA. Macroalgal growth rates were higher in the seagrass-free mesocosms, but the calcification rate of the model reef coral Pocillopora damicornis was higher in coral reef mesocosms featuring seagrasses under OA at 25 and 28 °C. Both the macroalgal growth rate and the coral calcification rate decreased in all mesocosms when the temperature was raised to 31 °C under OA. However, the variation in gross primary production, ecosystem respiration, and net ecosystem production in the seagrass mesocosms was lower than in seagrass-free controls, suggesting that the presence of seagrass in the mesocosms helped to stabilize the metabolism of the system in response to simulated climate change.

Spatiotemporal variability in the structure of seagrass meadows and associated macrofaunal assemblages in southwest England (UK): Using citizen science to benchmark ecological pattern.

Seagrass meadows underpin a variety of ecosystem services and are recognized as globally important habitats and a conservation priority. However, seagrass populations are currently impacted by a range of biotic and abiotic stressors, and many are in decline globally. As such, improved understanding of seagrass populations and their associated faunal assemblages is needed to better detect and predict changes in the structure and functioning of these key habitats. Here, we analyzed a large dataset—collected by recreational scuba divers volunteering on a citizen science project—to examine spatiotemporal patterns in ecological structure and to provide a robust and reliable baseline against which to detect future change. Seagrass (Zostera marina) shoot density and the abundance of associated faunal groups were quantified across 2 years at 19 sites nested within three locations in southwest UK, by collecting in situ quadrat samples (2,518 in total) during 328 dives. Seagrass shoot density and meadow fragmentation was comparable across locations but was highly variable among sites. Faunal abundance and assemblage structure varied between areas with or without seagrass shoots; this pattern was largely consistent between locations and years. Overall, increased seagrass density was related to increased faunal abundance and explained shifts in faunal assemblage structure, although individual faunal groups were affected differently. More broadly, our study shows that well‐funded and orchestrated citizen science projects can, to some extent, gather fundamental information needed to benchmark ecological structure in poorly studied nearshore marine habitats.

The role of flow velocity combined with habitat complexity as a top–down regulator in seagrass meadows

Large‐scale losses of seagrass areas have been associated with eutrophication events, which have led to an overproduction of photosynthetic organisms including epiphytes. Grazers that feed on epiphytes can exert a significant top–down control in the system, but the effects of physical factors on grazing activity and feeding behaviour have been rarely examined. We addressed the combination of hydrodynamic regime and seagrass shoot density can alter the feeding and foraging behaviours of mesograzers. A full factorial experiment, with flow velocity (high, medium and low) and shoot density (high versus low) as main factors, was conducted in a racetrack flume using artificial seagrass plots. The results showed that when high flow velocity conditions were combined with low shoot density, consumption of epiphytes by mesograzers was strongly reduced. In contrast, when flow velocity was low or shoot density was high, mesograzers exhibited high feeding rates and vigorous swimming behaviour. These results clearly indicate that hydrodynamic stress reduces the time that mesograzers can spend feeding, since it inhibits their swimming behaviour, and thus indirectly affecting to the density of epiphytes. Therefore, the triggering of trophic cascade effects in seagrass communities under these experimental conditions depended on the interrelationship and feedbacks among shoot density, abiotic (flow velocity) and biotic (epiphytes and mesograzers) compartments, with flow velocity exerting a top–down control on seagrass ecosystems.

Distribution and relative predation risk of nekton reveal complex edge effects within temperate seagrass habitat

Edge effects may be important drivers of community dynamics across marine habitat mosaics. Past research has consistently suggested that within temperate and sub-tropical seagrass habitats, organisms inhabiting meadow edges experience lower survivorship, presumably correlated with increased predation. However, these survivorship trends have not consistently translated to differences in faunal densities between edge and interior regions of seagrass meadows. We evaluated the evidence of edge effects on predation upon two dominant mesopredators within temperate eelgrass (Zostera marina) communities of the U.S. East Coast: blue crabs (Callinectes sapidus) and pinfish (Lagodon rhomboides). We considered three lines of data to arbitrate the null hypothesis that edge has no impact on distribution or predation on blue crabs or pinfish: (1) relative density as measured by catch-per-unit-effort (CPUE) of blue crabs and pinfish within edge (<1 m from unstructured sandflat boundaries) versus interior (>3 m from unstructured sandflat boundaries) regions of eelgrass meadows; (2) distribution of acoustically tagged red drum (Sciaenops ocellatus), a recognized predator of both blue crabs and pinfish, within eelgrass meadows (at 1-m bin resolution); and (3) relative mortality of tethered blue crabs and pinfish within edge versus interior regions of eelgrass meadows. Additionally, we manipulated seagrass shoot density to evaluate potential interactions between local habitat complexity and edge effects. We found no statistically detectable difference in catch rates of blue crabs or pinfish in edge versus interior habitats, and red drum detection frequencies were statistically indistinguishable moving from the seagrass-sandflat boundary toward the meadow interior. Despite these findings, we did record statistically and ecologically significant edge effects on predation rates of tethered blue crabs and pinfish. Counter to previous work, we found that blue crabs survived >2.5× longer, and pinfish survived >2× longer, along the meadow edge relative to interior. Furthermore, the strength of these predation-related edge effects was most notable for blue crabs within plots with higher shoot density, while the opposite pattern was true for pinfish. These findings are, in part, consistent with the dichotomy apparent in the seagrass literature with respect to edge effects on faunal density and survivorship. Additionally, our work provides new detail on how habitat edges may affect the population ecology of larger bodied, more mobile mesopredator species that have not received as much attention in previous studies (i.e., higher survivorship possible along edges).

Thresholds for morphological response to light reduction for four tropical seagrass species

Seagrasses worldwide are highly vulnerable to, and at increasing risk from reduced light availability, and robust light thresholds are required for evaluating future impacts of changing light conditions. We tested the morphological response (shoot density and growth) of four Indo-West Pacific seagrass species (Cymodocea serrulata, Halodule uninervis, Halophila ovalis and Zostera muelleri) to six daily light levels ranging from 0 to 23molm−2d−1 (0–70% surface irradiance) in cool (∼23°C) and warm temperatures (∼28°C) over 14 weeks. The impact of light limitation on shoot densities and growth rates was higher at warm than at cool temperatures, and for Z. muelleri and H. ovalis than for C. serrulata and H. uninervis, in terms of both the time taken for the low light treatment to take effect and the predicted time to shoot loss (e.g. 17–143 days at 0molm−2d−1). Using fitted curves we estimated temperature-dependent thresholds (with estimates of uncertainty) for 50% and 80% protection of growth and shoot density, defined here as “potential light thresholds” in recognition that they were derived under experimental conditions. Potential light thresholds that maintained 50% and 80% of seagrass shoot density fell within the ranges 1.1–5.7molm−2d−1 and 3.8–10.4molm−2d−1, respectively, depending on temperature and species. Light thresholds calculated in separate in situ studies for two of the same species produced comparable results. We propose that the upper (rounded) values of 6molm−2d−1 and 10molm−2d−1 can be used as potential light thresholds for protecting 50% and 80% of shoot density for these four species over 14 weeks. As management guidelines should always be more conservative than thresholds for biological declines, we used error estimates to provide a quantitative method for converting potential light thresholds into guidelines that satisfy this criterion. The present study demonstrates a new approach to deriving potential light thresholds for acute impacts, describes how they can be applied in management guidelines and quantifies the timescales of seagrass decline in response to light limitation. This method can be used to further quantify cumulative impacts on potential light thresholds.

Effect of Chronic Versus Pulse Perturbations on a Marine Ecosystem: Integration of Functional Responses Across Organization Levels

Human impacts accelerate the intensity and frequency of perturbations on ecosystems; approaches that integrate responses across organization levels are, however, lacking, particularly in the ocean. We experimentally simulated the frequency of fertilization (‘chronic’ versus ‘pulse’ events) in orthogonal combinations of two intensities (‘large’ versus ‘moderate’ fertilization) to determine physiological and biological responses by the seagrass Cymodocea nodosa and associated flora (epiphytes and green seaweeds, specifically Caulerpa prolifera), as well as functional changes (community primary and secondary productivity) at the ecosystem level. We predicted that the absence of recovery time from chronic perturbation would more severely affect responses at population and ecosystem levels relative to discrete events (pulses). Nutrient enrichment increased the biomass of C. prolifera irrespective of its frequency, whereas seagrass biomass and shoot density particularly decreased under a chronic scenario. These demographic responses were connected with varying photo-physiological performance of both C. nodosa and C. prolifera. Fertilization, regardless of its intensity and frequency, decreased the maximum photosynthetic rate of C. nodosa, concomitant with increased pigments, particularly under chronic fertilization, and decreased photoprotective (phenols) compounds. In contrast, fertilization boosted the maximum photochemical yield of C. prolifera, in addition to an increase in pigments and photoprotective compounds. Community primary and secondary productivity, however, did not vary under fertilization of varying intensity and frequency. In summary, fertilization precipitated population-level changes in physiological and biological attributes of vegetation. However, fertilization effects did not entirely cascade into ecosystem-level processes, that is, ecosystem productivity, which suggests a functional compensation (that is, increased algal performance to offset losses of seagrass production) during the initial stages of fertilization.

Changes in fish community in seagrass beds in Mangoku-ura Bay from 2009 to 2014, the period before and after the tsunami following the 2011 off the Pacific coast of Tohoku earthquake

In order to evaluate the impact of the tsunami following the 2011 off the Pacific coast of Tohoku earthquake on habitat condition and fish community structure in seagrass Zostera marina beds, seagrass vegetation and structure of fish communities were investigated in Mangoku-ura Bay, on the Pacific coast of northern Japan, from 2009 to 2014. Underwater observation revealed a decrease in the vegetation coverage in the seagrass bed after the tsunami. A total of 1764 fishes belonging to 37 species (18 families) were collected. Effects of year on mean fish abundance (no. fish 100 m−2) and biomass (fish weigh in g 100 m−2) in the seagrass bed were significant, while there was no significant effect of year on number of fish species. Seagrass-associated or substrate-associated species such as Sebastes cheni and Ditrema viride were dominant by weight in the fish community in 2009, and were replaced with sand or mud bottom-associated fish species such as Acanthogobius flavimanus in 2011. An increase in abundance of sand or mud bottom-associated fish species such as Acentrogobius virgatulus and Takifugu niphobles contributed to significant increase in the mean fish abundance in 2011, when seagrass shoot density decreased due to the disturbance of the sea bottom and sediment deposition by the tsunami. Changes in sea bottom and vegetation conditions are concluded to have been factors causing the change in dominant fish species.