Metals sequestered in coastal sediments are normally considered to be stable, but this investigation shows – somewhat surprisingly – that mercury concentrations in a previously contaminated area, Harboøre Tange, Denmark, have decreased since the 1980s. Mercury concentrations were determined in sediment and benthic biota and present values were compared to values in the 1980s and values from areas without known; history of mercury contamination. Concentrations in both the upper 20 cm of the sediments and; biota are considerably lower now compared to latest monitoring (1980s). Sediment.concentrations at most locations have decreased from the 100–300 ng Hg g⁻¹ dry weight (dw) level to levels below the Background Concentration (BC) of 50 ng Hg g⁻¹ dw defined by Oslo-Paris Convention for the Protection of the Marine Environment of the North-East Atlantic; some stations are at the 2–10 ng Hg g⁻¹ dw level characteristic of Danish coastal sediments with no known history of mercury contamination. Concentrations of mercury in the benthic biota along Harboøre Tange have also decreased since the 1980s but despite the lowered mercury concentrations in the sediments, concentrations in most samples of benthic invertebrate fauna still exceed those in uncontaminated coastal areas and also the Environmental Quality Standard (EQS) of 20 ng Hg g⁻¹ wet weight (≈100 ng Hg g⁻¹ dry weight) defined by the European Union’s Water Framework Directive. Concentration ranges in selected organisms are: (Harboøre Tange l980s/Harboøre Tange now/uncontaminated areas - given in ng Hg g⁻¹ dw): Periwinkles Littorina littorea 9000/150–450/55-77, blue mussels Mytilus edulis up to 9000/300–500/40–170, cockles Cerastoderma edule up to 8000/400–1200/200, brown shrimp Crangon crangon 700–2200/150-450/47, eelgrass Zostera marina up to 330/25–70/12. The present results - together with a literature review - show that a simple and straight forward relationship between the concentrations of mercury in sediment and benthic organisms does not necessarily exist.

Marine canopies formed by seagrass and other coastal vegetated ecosystems could act as sinks of microplastics for being efficient particle traps. Here we investigated for the first time the occurrence of microplastic retention by marine canopies in a hydraulic flume under unidirectional flow velocities from 2 to 30 cm s⁻¹. We used as model canopy-forming species the seagrass Zostera marina with four canopy shoot density (0, 50, 100, 200 shoots m⁻²), and we used as microplastic particles industrial pristine pellets with specific densities from 0.90 to 1.34 g cm⁻³ (polypropylene PP; polystyrene PS; polyamide 6 PA; and polyethylene terephthalate PET). Overall, microplastics particles transported with the flow were retained in the seagrass canopies but not in bare sand. While seagrass canopies retained floating microplastics (PP) only at low velocities (<12 cm s⁻¹) due to a barrier created by the canopy touching the water surface, the retention of sinking particles (PS, PA, PET) occurred across a wider range of flow velocities. Our simulations revealed that less dense sinking particles (PS) might escape from the canopy at high velocities, while denser sinking particles can be trapped in scouring areas created by erosive processes around the eelgrass shoots. Our results show that marine canopies might act as potential barriers or sinks for microplastics at certain bio-physical conditions, with the probability of retention generally increasing with the seagrass shoot density and polymer specific density and decreasing with the flow velocity. We conclude that seagrass meadows, and other aquatic canopy-forming ecosystems, should be prioritized habitats in assessment of microplastic exposure and impact on coastal areas since they may accumulate high concentration of microplastic particles that could affect associated fauna.

This study firstly quantified the responses of seeds of Zostera marina to different planting times (22 September, 5 October, 23 October, 7 November and 20 November in 2015) through a field seed-planting experiment over a two year period. The suitable seed planting time required by the seeds of Z. marina was evaluated. The seedling establishment rate of Z. marina subjected to different planting times ranged from 7% to 55%, with the higher values attained on the treatments of 22 September and 5 October. New plant patches from seed were fully developed and well maintained on the planting time of 22 September, 5 October and 23 October after 2 years following planting. The shoot density under the three treatments ranged from 62 shoots per replicate to 72 shoots per replicate with an average of 67 shoots per replicate in September 2017. According to the propagation assessment and growth analysis, we found that the planting time from mid-September to mid-October may be the optimal time to plant seeds of Z. marina in our experimental site. Our results demonstrate that seed planting time has an important effect on the effectiveness of eelgrass restoration and provide data that could prove helpful in the development of successful eelgrass restoration.

Microplastic (MPL) contamination in the marine environment is extensively studied yet little is known about the extent of MPL abundance in seagrass beds. The aim of this study was to evaluate MPL accumulation in coastal seagrass (Zostera marina) beds in the Baltic Sea, Estonia.Surface water was sampled by pumping using 40 μm plankton net, and sediments by trowel. MPL was extracted with NaCl, identified by microscopy and ATR-FTIR on selected samples.Surface water in the seagrass beds had 0.04–1.2 (median 0.14) MPL/L, similar to other areas of the Baltic Sea. Sediments had 0–1817 (median 208) MPL/kg (dwt), much higher than previously recorded from adjacent unvegetated and offshore sediments, thereby suggesting a strong ability of the sediments in seagrass beds to retain MPL. Of identified MPL, blue fibres were dominant in both the sampled media. Sediment characterization showed a correlation between MPL counts with poorly sorted sediments.

We extracted and analyzed microplastics (MP) in archived sediment cores from Great Bay Estuary (GBE) in the Gulf of Maine region of North America. Results indicated that MP are distributed in GBE sediments, 0–30 cm, at an average occurrence of 116 ± 21 particles g⁻¹ and that morphology varies by site and depth. Analysis by sediment depth and age class indicated that MP accumulation increased over several decades but recently (5–10 years) has likely begun to decrease. Hydrodynamic and particle transport modeling indicated that bed characteristics are a more controlling factor in MP distribution than typical MP properties and that the highest accumulation likely occurs in regions with weaker hydrodynamic flows and lower bed shear stress, e.g., eelgrass meadows and along fringes of the Bay. These results provide a baseline and predictive understanding of the occurrence, morphology, and sedimentation of MP in the estuary.

Given a large quantity of epiphytes and other material attached on eelgrass leaf blades, we explored the relationship between eelgrass sheaths and different-aged leaf blades (1st, 2nd, 3rd, and 4th leaf blade) on nutrient content and their ratios (C, N, P, C/N, C/P, and N/P) to identify whether eelgrass sheaths could be used to instead of leaf blades in terms of nutrient content. In addition, we explored the relationship between eelgrass sheath length and shoot length. Results showed that there were significant relationships between the sheath and leaf blades in terms of N and P content and their ratios. For length analysis, there was a significant relationship between sheath length and shoot length, and shoot length was approximately four to five times (mean 4.4659) longer than sheath length, such that shoot length can be estimated by sheath length. These significant relationships suggest that eelgrass sheath could be used as a suitable predictor of leaf blade in length and nutrient stoichiometry, thus eelgrass sheath could be used as an indicator for further eelgrass nutrient monitoring and research.

Seagrass beds are recognized as pivotal and among the most vulnerable coastal marine ecosystems globally. The eelgrass Zostera marina L. is the most widely distributed seagrass species and dominates the temperate northern hemisphere. However, an alarming decline in seagrass has been occurring worldwide due to multiple stressors. Seagrass meadow degradation is particularly serious in the Bohai Sea, in temperate China; however, large areas (> 500 ha) of seagrass meadows and population recruitment have rarely been reported in this area. In the present study, we report on a large eelgrass bed in a eutrophic bay of the Bohai Sea. Sonar and field survey methods were used to investigate the distribution of seagrass and its population recruitment. We also analyzed the major threats to this large seagrass bed. Results showed that a large Z. marina bed with an area of 694.36 ha occurred in this area of the Bohai Sea, with a peripheral area of ~25 km². Seagrass canopy height and plant coverage had a significant correlation with water depth. Asexual reproduction principally occurred in autumn and played a dominant role in population recruitment in vegetated areas, where no seedlings successfully colonized. In contrast, a considerable number of seedlings survived in the seagrass meadow gaps, and thus played a critical role in the recruitment in these areas. The maximum reproductive shoot densities were about 100 and 70 shoots m⁻² at sampling site (S)-1 and S-2 in 2018, respectively, which was about two times more than in 2019 (50 and 20 reproductive shoots m⁻² at S-1 and S-2, respectively). The potential seed output per unit area in 2019 was about 1020 seeds m⁻² at S-1 and 830 seeds m⁻² at S-2, and the seed output in the study area was at a low level compared with global values. Overall, high spring and summer water temperature appeared to induce sexual reproduction of Z. marina in the study area, including reproductive effort, reproductive investment, and seedling development. Furthermore, eelgrass height, aboveground biomass, and density were significantly related to water temperature. Among the potential threatening factors to seagrass in this area, the activities of clam harvesting were intense with daily clam catches >2000 kg, leading to patchy seagrass meadows, especially in the fringe areas. The seagrass bed was also threatened by marine pollution (nutrient loading) and land reclamation. Therefore, the protection and restoration of this seagrass bed are strongly recommended. Our study will provide fundamental information for the conservation and management strategies of large eelgrass beds in the Bohai Sea.

Iron is recognized as an efficient method to alleviate sulfide stress. This study tested the response of Zostera marina plants to different levels of sedimentary sulfides (100.0–818.7 μmol L⁻¹) and iron inputs (590.0–825.3 μg L⁻¹) in a field experiment performed over an eighty-day period. We measured plant responses in terms of shoot density and plant morphology and productivity. The relationship between the propagation effort (PE, in %) and sulfide content (S, in μmol L⁻¹) was expressed as: PE = −14.01 × ln (S) + 86.86 (R² = 0.99, p < .01), which indicates that the toxic limit of the pore-water sulfide concentration for the survival of eelgrass is 493 μmol L⁻¹. The addition of iron can reduce the toxicity of sulfides to eelgrass beds, resulting in an increase in plant density and productivity, and can even reverse the decline of eelgrass beds exposed to high sulfide concentrations.

The interaction between bottom-up and top-down processes in coastal ecosystems has been scarcely studied so far. Temporal changes in trophic interactions of Zostera marina along the Swedish west coast are relatively well studied, with the exception of epifaunal communities. Epifauna was used as a model study to explore resource (bottom-up) or predator (top-down) regulated in a vegetated ecosystem. We conducted a 21-year comparative study (1997 and 2018) using epifauna of 19 Zostera marina meadows along the Swedish Skagerrak coast. Large changes were observed in the composition of small (0.2–1 mm) and large (>1 mm) epifauna. In the small-sized epifauna, the nematode Southernia zosterae and harpacticoids showed an increase of 90% and a decrease of 50% of their abundances, respectively. In the large-sized epifauna, the polychaete Platynereis dumerilii and chironomid larvae were absent in 1997 but thrived in 2018 (>2000 ind. m⁻²). Mesoherbivores (Idoteids and gammarids) were locally very abundant in 1997 but disappeared in 2018. An 83% decline of mytilids settling in Zostera marina leaves was observed. Our results showed that epifauna is predominantly top-down regulated. An integrative framework of the study area is outlined to shed light on the causes and consequences of the environmental shifts reported in Zostera meadows from the northern Skagerrak area throughout the last three decades.

Seagrass meadows are declining at alarming rates globally due to both anthropogenic activities and natural threats. Seagrasses play key ecological roles in coastal ecosystems as primary producers and providers of habitat and environmental structure. Therefore, mapping seagrass beds is indispensable for the effective monitoring and management of coastal vegetated habitats. In contrast to direct sampling techniques and optical remote sensing, active hydroacoustic techniques are relatively inexpensive and efficient for the detection of seagrass. We used a single beam echosounder to detect the spatial and temporal distribution characteristics of the eelgrass Zostera marina L. in an important overwintering habitat for the whooper swan Cygnus cygnus (Swan-Lake lagoon), northern China. We also distinguished echograms of the macroalgae Chaetomorpha linum K. and outlined its threat to seagrass. We also propose a method for calculating the accuracy of interpolation analyses. Results showed that: (1) The distribution of seagrass in Swan Lake varies with seasons, with maximum distribution area in summer. The maximum distribution area of seagrass beds in Swan Lake was 199.09 ha–231.67 ha, accounting for 41.48%–48.26% of the area of Swan Lake; (2) C. linum is a growing threat for seagrass beds of Swan-lake, with distribution area as high as 129.28 ha in May 2018. The invasion and competition by C. linum against seagrass beds could be one of the reasons for the decline in seagrass beds in Swan-Lake; (3) Topo to Raster has the highest interpolation accuracy and is the most conservative among three interpolation methods. Topo to Raster was the most suitable interpolation method for the sonar detection of seagrass beds. The findings may facilitate the application of sonar technology in seagrass monitoring and provide data for the formulation of appropriate seagrass bed management and restoration strategies and policies.