3.1. The Barrier Role of Macrophytes
3.1.1. The influence of aquatic plant ecology, their morphological features and plant density on the barrier role
Among the many mechanisms influencing the behavior and retention of plastic microparticles by macrophytes, their morphological features and growth density are of primary importance. Table 2 shows typical values of density indices for some macrophyte species in Central Russia. In general, the relationships between macrophyte density indices are nonlinear, since when thickets become denser, the upper parts of individual plants overlap.
For floating microparticles made of high-density polyethylene with a size of 4.5 mm at a concentration of 500 particles/m2, the rate of retention by macrophytes ranges from 22% (Potamogeton perfoliatus L.) to 76% (Ceratophýllum demérsum L.) with a projective cover of 50% of the water area by vegetation and from 31% (Glyceria spectabilis Mert. & W.D.J. Koch) to 100% (Nymphaea candida J. Presl & C. Presl, Nuphar lutea (L.) Smith, Trapa natans L.) with a projective cover of 100% (Table 3).
Table 2
ǀ Relationships between the projective cover and the number of plants per unit area for some species of higher aquatic plants
| Species of macrophytes | The projective cover (%) | The number of plants (pcs/m2) |
| Phragmites australis (Cav.) Trin. ex Steud. | 30 40 100 | 25 33 100–196 |
| Glyceria spectabilis Mert. & W.D.J. Koch | 20 80 | 18 28–36 |
| Carex acuta L. | 100 | 625 |
| Potamogeton natans L. | 70 | 35 |
| Nymphaea candida J. Presl & C. Presl | 15 | 11 |
| Nuphar lutea (L.) Smith | 50 90 | 18 36–38 |
The ecology of aquatic plants is no less important factor. For instance, although Potamogeton perfoliatus L. has a spatial structure close to Ceratophýllum demérsum L. and both species are submerged, with the same projective cover, the tops of Potamogeton perfoliatus L. plants reach the surface of the water significantly less often than Ceratophýllum demérsum L. plants do. Furthermore, Potamogeton perfoliatus L. plants and their leaves almost never intertwine, which is not the case for Ceratophýllum demérsum L., whose leaves are dichotomously branched and form a dense mesh structure. As a result, Ceratophyllum demersum L. retains 32–54% more polyethylene microfilms than Potamogeton spp. Do at a projective cover of 50% and 12–32% more polyethylene microfilms at a projective cover of 100%. An additional factor is that Ceratophýllum demérsum L. usually grows in low-flow stagnant zones, where weak water exchange also contributes to the retention of plastic microparticles. Close in morphology and growing conditions to Ceratophyllum demersum L. are the canadian waterweed (Elodea canadensis Michx.) and brittle naiad (Najas minor All.). In contrast, Potamogeton perfoliatus L. plants can withstand significant mechanical loads; this species is found in areas with significant exposure to wind and ship waves. Other examples of the influence of aquatic plant ecology, morphological features and density of growth on the degree of retention of plastic microparticles are given below.
Table 3
ǀ The average value of the retention rate (%) of high-density polyethylene microparticles (films) 4.5 mm in size at a concentration of 500 particles/m− 2 by 1 m2 thickets of different species of higher aquatic plants
| | The projective cover (%) |
| Species of macrophytes | 50 | 100 |
| Phragmites australis (Cav.) Trin. ex Steud. | 36 | 44 |
| Typha angustifolia L. | 53 | 33 |
| Glyceria spectabilis Mert. & W.D.J. Koch | 31 | 31 |
| Acorus calamus L. | 31 | 36 |
| Iris pseudacorus L. | 50 | 58 |
| Carex acuta L. | 49 | 62 |
| Ceratophýllum demérsum L. | 76 | 76 |
| Potamogeton perfoliatus L. | 22 | 51 |
| Potamogeton lucens L. | 44 | 64 |
| Potamogeton natans L. | 31 | 47 |
| P. heterophyllus × P. Perfoliatus | 27 | 44 |
| Nymphaea candida J. Presl & C. Presl | 60 | 100 |
| Nuphar lutea (L.) Smith | 58 | 100 |
| Trapa natans L. | 42 | 100 |
3.1.2. The barrier role of submerged macrophytes
The studied submerged water plants with a projective cover of 50% demonstrated a similar level of retention of plastic microparticles (27–44%), and the morphological and ecological features of the species easily explained the observed differences. For example, P. heterophyllus × P. perfoliatus has a sparse arrangement of thin leaves submerged in water, and Potamogeton lucens L. is a long (up to 7 m) plant with many wide and long leaves (up to 20 cm), a significant part of which float on the surface of the water.
With a projective cover of 100%, submerged vegetation generally occupies an intermediate position in terms of the degree of retention of plastic microparticles between emergent macrophytes and floating plants with roots. With the exception of Ceratophýllum demérsum L., which, owing to its morphological and ecological features, had the highest rate of microparticle retention among all the studied submerged aquatic plant species at a projective cover of 50%, with increasing density, the degree of microparticle retention increased by 45–131%, which is several times greater than that for emergent macrophytes. Ceratophýllum demérsum L. equally retained microparticles in dense overgrowth and in more sparse thickets, while remaining the most effective microparticle trapper among submerged macrophytes. Potamogeton natans L. and Potamogeton lucens L. have similar spatial structures; with increasing density, their retention rates of plastic microparticles are increased by 48% and 45%, respectively. Moreover, Potamogeton lucens L. approached Ceratophýllum demérsum L. (76%) in terms the efficiency of trapping microparticles (64%). Among the submerged macrophytes, the efficiency of plastic microparticle retention increased the most with increasing thicket density for Potamogeton perfoliatus L. (2.3 times greater). This is apparently due to the emergence of more plants on the surface of the water. Moreover, although P. heterophyllus × P. perfoliatus increased the rate of microparticle retention by 63% in denser stands, it remained the least effective barrier for plastic microparticle retention among submerged macrophytes at the level of Phragmites australis (Cav.) Trin. ex Steud. with the same projective cover. Both mentioned species (Potamogeton perfoliatus L. and P. heterophyllus × P. perfoliatus) with a projective cover of 100%, have approximately the same distance between plants – approximately 5 cm.
3.1.3. The barrier role of emergent macrophytes
Emergent vegetation with a projective cover of 50% thickets generally has a higher rate of retention of plastic microparticles (31–53%) than submerged vegetation does, with three studied species tending toward the lower limit (Glyceria spectabilis Mert. & W.D.J. Koch, Acorus calamus L., Phragmites australis (Cav.) Trin. ex Steud.), and three others approach the upper limit (Carex acuta L. Iris pseudacorus L., Typha angustifolia L.). The first group has a similar stem structure and similar thickness: a round cross-section with a diameter of 4–8 mm for Phragmites australis (Cav.) Trin. ex Steud. and elliptical cross-sections of 4×8 and 6×15 mm, respectively, for Glyceria spectabilis Mert. & W.D.J. Koch and Acorus calamus L. Plants of the second group either have thick elliptical cross-section stems with average sizes of 20×35 and 20×40 mm, respectively, for Typha angustifolia L. and Iris pseudacorus L., or many individual thin leaves, such as Carex acuta L. (cross-section 2×5 mm), which create similar conditions for the retention of plastic microparticles. An additional factor in the retention of microparticles by Carex acuta L. is the specific surface of plants with many sharp protruding spines, on which microparticles are caught and retained. Notably, emergent macrophytes intensively dampen the movement of air masses and their impact on the surface of water, as a result of which polystyrene foam microspheres do not roll over the surface but have neutral buoyancy, such as floating microparticles of other shapes.
We did not find a direct relationship between the density of plants of the same species and the rate of retention of plastic microparticles. For example, with a projective cover of 50% vegetation, the greatest variation in the rate of retention of microplastics was characteristic of submerged vegetation, but with a projective cover of 100%, the minimum values of the rate of retention were for many species of emergent vegetation: Glyceria spectabilis Mert. & W.D.J. Koch (31%), Typha angustifolia L. (33%), and Acorus calamus L. (36%). For these species, with a twofold increase in projective cover, only Acorus calamus L. increased the rate of retention of plastic microparticles by 16%, whereas this indicator did not change for Glyceria spectabilis Mert. & W.D.J. Koch, and even decreased for Typha angustifolia L. In the latter case, with an increase in the density of Typha angustifolia L. thickets, plants with thick stems grow in separate small groups without changing the overall projective cover but create a wider free space in the lower part of the stems for the flow of water with greater water exchange, where plastic microparticles move freely. For other species of emerging macrophytes, although the absolute values of the plastic microparticle retention rate are significantly higher (44–62%), the relative values of this indicator have increased only slightly (16–26%). The effect of increasing vegetation density on the proportional increase in the retention rate of floating microparticles is most clearly observed in thickets of Carex acuta L., which has thin leaves that evenly dissect the flow of water or air carrying microparticles.
3.1.4. The barrier role of floating plants with roots
Floating plants with roots retain more floating plastic microparticles (42–60% with a projective cover of 50%), which is also associated with their morphology and environmental characteristics, as they grow in places with poor water exchange. In the Ivankovskoye Reservoir, the leaves of Nuphar lutea (L.) Smith have an average diameter of approximately 10 cm or an elliptical shape of 8×11 cm. The lower rate of microfilm retention by Trapa natans L. can be explained by the structure of the rosette of this plant floating on the surface of the water, which does not have a solid structure like the leaves of Nymphaea candida J. Presl & C. Presl and Nuphar lutea (L.) Smith but is similar to swollen petioles on which diamond-shaped leaves 2–3 cm long are located. Moreover, the diameter of the rosette and the spaces between the leaves (the space of the free water surface) can be significantly larger than those of the solid floating leaves of Nymphaea candida J. Presl & C. Presl and Nuphar lutea (L.) Smith. Under the conditions of the Ivankovskoye Reservoir, the diameter of the Trapa natans L. rosette varies within 12–24 cm, and according to the author’s earlier studies, on the Lower Volga it can reach 45 cm. Notably, according to the results of our experiments on the Donkhovka River, one of the tributaries of the Ivankovskoye Reservoir, the retention of light and mobile microparticles of polystyrene foam with a diameter of 4.5 mm on the leaves of Nymphaea candida J. Presl & C. Presl occurs at an average flow rate of up to 6 cm/s, although the retention rate of microparticles in this case is only 10%, with the same projective plant cover of approximately 10%. Apparently, such a constant water velocity rate is the upper limit for the growth of this species. With a projective cover of 100%, floating plants with roots retain all microparticles floating within their growth zone.
3.1.5. Influence of microplastic size on the barrier role
Reducing the average size of microfilms made of the same material (high-density polyethylene) from 4.5 to 1.0 mm resulted in a decrease in the retention of plastic microparticles by Ceratophýllum demérsum L. by 42% and 12%, respectively, for projective coverages of 50% and 100%. Most likely, smaller particles pass more freely through the lattice structure of the stems and leaves of Ceratophýllum demérsum L. Experiments on the effect of the size of plastic microparticles on the rate of their retention by macrophytes were conducted only for this species.
3.1.6. Influence of microplastic concentration on the barrier role
The barrier role of macrophytes in trapping floating plastic microparticles is also influenced by their concentration on the water surface within the thickets. Detailed studies were carried out for Phragmites australis (Cav.) Trin. ex Steud. The concentration of microfilms with an average size of 4.5 mm varied within the range of 50–6000 particles/m2. For both studied projective covers of macrophytes, the graph of the change in the rate of retention of microparticles by Phragmites australis (Cav.) Trin. ex Steud. from their concentration has an S-shape. First, with an increase in the concentration of plastic microparticles, the rate of their retention by Phragmites australis (Cav.) Trin. ex Steud. also increases, but at a concentration of approximately 700–800 particles/m2, the direction of the process changes – a relatively larger number of plastic microparticles begin to pass freely through the thickets of macrophytes. At a concentration of 3000 particles/m2 of plastic microparticles, only 17 and 26% are retained, respectively, for 50 and 100% of the projective cover of Phragmites australis (Cav.) Trin. ex Steud. thickets. With a further increase in the concentration of plastic microparticles, they stick together, damming up the narrow spaces between plants and creating chains of microfilms attached to each other. This leads to a repeated increase in the rate of retention of microparticles by Phragmites australis (Cav.) Trin. ex Steud. Moreover, at a concentration of 6000 particles/m2, it is 41% and 57%, respectively, with a projective cover of 50% and 100%, which is 14% and 29% higher than the retention rate at the basic studied concentration of 500 particles/m2. However, not for all macrophytes, an increase in the concentration of microparticles by an order of magnitude leads to an increase in the rate of their retention. For Glyceria spectabilis Mert. & W.D.J. Koch, this value remained at the same level, and for Carex acuta L., a relative decrease was even observed.
3.1.7. The influence of the behaviour of microplastics is related to their material of origin, shape and degree of rigidity on the barrier role
The behavior of plastic microparticles in the aquatic environment, associated with the material of their origin (not necessarily the material of the polymer matrix), shape, and degree of rigidity, also affects their ability to be retained by macrophyte thickets. The physicochemical properties of polymers can vary significantly depending on the history of the material. There is so-called “polymer memory”, which is the ability of polymers to respond and adapt to changing environmental conditions (Khanna et al. 1988; Reiter 2020). The presence of polymer memory remains a complex phenomenon that is not fully understood (Kim et al. 2017). Recent studies have shown that no two polymers are fundamentally identical or have exact physicochemical characteristics, even if they have the same chemical structure or are synthesized via the same procedure (Charles E. & Carraher J. 2018; Seiffert 2020; Koltzenburg et al. 2023). The structure of plastics is even more complex, where in addition to the base polymer, there are additives, initiators, plasticizers, diluents, hardeners, thickeners, stabilizers, dyes, flame retardants, and lubricants.
Our experiments with high-density polyethylene microfragments showed different retention rates, both higher and lower, than those of microfilms made of the same polymer material and the same size. For the studied macrophyte species (Iris pseudacorus L., P. heterophyllus × P. perfoliatus, Ceratophýllum demérsum L.) with a projective cover of 50%, the retention rates of the microfragments were similar (41–51%). For microfilms, the weakest barrier is P. heterophyllus × P. perfoliatus, which can be explained by its morphological features, although it retains microfragments one and a half times more intensively than microfilms do. Plastic microfragments are retained somewhat better (49%) by Ceratophýllum demérsum L., which is similar in morphology to P. heterophyllus × P. perfoliatus, but this is significantly less than for microfilms (76%). Iris pseudacorus L., with a projective cover of 50%, demonstrated the same retention rate of microparticles in the form of fragments and films, which cannot be said with a density of 100%. At this density, the effect of the formation of microholes with increased water exchange appears, where floating microparticles rush, as was the case in dense thickets of Typha angustifolia L. As a result, the retention rate of microfragments in dense thickets of Iris pseudacorus L. is lower than that in sparser ones with a projective cover of 50%. This effect was not observed in a study of microfilm retention using Iris pseudacorus L. Apparently, the different degrees of adhesion of the surfaces of the microfragments and microfilms to the surface of Iris pseudacorus L. leaves had an effect. The other two studied macrophyte species, with increasing density, proportionally increased the retention rate of plastic microfragments by 29% and 18%, respectively, for P. heterophyllus × P. perfoliatus and Ceratophýllum demérsum L.
When the average size of plastic microfragments is reduced from 4.5 to 1.0 mm, as with microfilms, there is a proportional decrease in the retention rate of microparticles by the studied Ceratophýllum demérsum L. thickets, which is equal to 5% in absolute value with a projective cover of 50% and 100%. For the studied P. heterophyllus × P. Perfoliatus, this value remained virtually unchanged, although it decreased somewhat.
3.1.8. Influence of the seasonal state of macrophytes on their barrier role
Since macrophytes have seasonal dynamics, the efficiency of their retention of floating plastic microparticles also changes. Among the studied species, the tops of submerged macrophytes are the first to disappear from the water surface. In reservoirs of the central zone of Russia, this occurs in late August – early September. Thus, after dying, the integrity of the leaves and the upper part of the stems of Potamogeton perfoliatus L. is lost within 5–6 days. In the same state, P. heterophyllus × P. perfoliatus plants were immersed in water from the 10th-11th day and gradually sank to the bottom. This time is enough for the high-density polyethylene microfilms attached to the plants to also sink to the bottom together with the plants or independently. In laboratory experiments with microfilms with an average size of 4.5 mm, after 65 hours, more than 10% of the microparticles became attached to emergent macrophytes in the water column, which was approximately half of the submerged microparticles. For Ceratophýllum demérsum L., with an average microfilm size of approximately 1 mm, after 15–16 hours, 2/3 of the microparticles are already attached to plants in the water column.
Emergent vegetation performs a direct barrier function for a longer period of time. When emergent macrophytes die, the barrier role may even increase for some time, since the leaves of most species of these plants (except Phragmites australis (Cav.) Trin. ex Steud.) falls to the surface of the water, increasing the area of water covered by them. In addition, the degree of adhesion of microparticles to leaves and stems subjected to degradation increases. Thickets of Phragmites australis (Cav.) Trin. ex Steud. in one form or another are preserved in the water body throughout the year. In places of contact of plants with variable water levels, under the dynamic impact of wind and ship waves, the leaves of Phragmites australis (Cav.) Trin. ex Steud. degrade faster and disappear completely, even during the vegetation period. However, some decrease in the density of thickets as a result of leaf loss and a possible decrease in the rate of retention of plastic microparticles is compensated for by an increase in the roughness of stems, which have aged by this time; thus, the end of the vegetation period of Phragmites australis (Cav.) Trin. ex Steud. does not noticeably affect its barrier role.
3.2. Mechanisms of Microplastic Retention by Macrophytes
3.2.1. The appearance of additional hydraulic resistances
A characteristic feature of plastic microparticles that determines their behavior in the environment is their density. Among all the microparticles found in water bodies, only microplastics can float on the surface of the water, be in the water column, be concentrated at different depths, lie on the bottom, mix with other mineral, organic and anthropogenic particles, move in the vertical plane, sometimes sink to the bottom, and sometimes float to the surface of the water. This behavior is associated, first, with the density of polymeric materials, which is close to the density of water; as a result, the threshold values of buoyancy characteristics under the influence of multiple hydroecological processes can be easily overcome in one direction or another. These patterns are fundamentally different from the patterns of dynamics of suspended mineral particles of the same size, which, under stationary conditions, are concentrated at the bottom of a water body.
The density of particles affects the rate of their sedimentation in the aquatic environment. Real microplastic particles that settle in natural water are polydisperse suspensions with a wide range of sizes and shapes. In addition, the sedimentation rate of particles depends on the speed of movement of water masses, which is affected by a complex set of external conditions. Macrophytes occupying a certain part of the water column or the entire water column create special conditions for water movement in this area and its environments. The characteristics of vegetation, such as shape, flexibility and height, significantly affect flow structures. Vegetation creates additional hydraulic resistance, thereby enhancing the damping of the kinetic energy of the flow, helping to reduce the speed of water movement and its transport capacity, and reducing the intensity of turbulent mixing, transformation and attenuation of wind waves. Water exchange in overgrown areas decreases by 1–2 orders of magnitude. The hydraulic roughness coefficients of such areas are 4–6 times greater than those of nonovergrown water areas (Kazmiruk 1990).
The hydraulic resistance at the bottom of water bodies is always greater than the resistance of different layers of liquid. In overgrown zones, this resistance is increased by the root parts of plants, roots, rhizomes and turions, as well as the irregularities in the surface of the sediments created by them. In addition, plant litter of both autochthonous origin and from the catchment area often accumulates at the bottom of coastal zones. All this, together with the minimal speeds of water movement, creates significant obstacles for particles moving in the bottom region and rolling over the surface of the bottom.
3.2.2. The emergence of eddy zones in the rear of the plants
The flow around plants has a complex kinematic structure. The flow regime is affected by the flexibility of plants. Flow around submerged and emergent vegetation occurs differently. In front of plants, there is backwater on the water surface, which can be both general and local in nature. In the rear part of the plants, in relation to the oncoming flow, vortex shedding occurs, where captured microparticles accumulate. The same zones are formed behind a group of plants growing separately. The width of these zones is approximately equal to the width of the obstacle (a plant or a group of plants), and the length is approximately 10–12 times greater than the width. The resulting velocity in the vortex zones is significantly less than the background velocity and can be directed against the general water flow.
3.2.3. Decrease in the wind speed near the water surface
A decrease in water exchange and, in general, the hydrodynamic activity of water masses also occurs due to a decrease in wind speed at the water surface in areas overgrown with emergent rigid vegetation. In such areas, the wind speed is 6–10 times lower than that in similar open areas. The development of turbulent mixing of water from wind action is also prevented by submerged vegetation, the tops of which are above the water, as are floating plants with roots.
3.2.4. The suppression of wind waves leading to the resuspension of deposited particles
There is a close relationship between wave hydrodynamic characteristics in the coastal zones of water bodies and the structure of macrophyte cenoses. Intensive damping of the energy of incoming waves, both wind and tidal, by emergent rigid vegetation has been noted in many studies (Leonard & Luther 1995; Chen et al. 2007; Quartel et al. 2007). Thus, indirectly, through a decrease in the level of hydrodynamic activity of water masses, thickets of all groups of macrophytes retain plastic microparticles through their sedimentation in overgrown zones and prevent resuspension of particles located on the bottom.
3.2.5. Decreasing the kinetic energy of raindrops and reducing the likelihood of their direct impact on floating and already trapped microparticles
A canopy of emergent plants, as well as floating-leaved plants, helps to dampen the kinetic energy of raindrops and reduce the likelihood of their direct impact on floating and already retained microparticles. Submerged plants, floating–leaved plants, and free-floating plants retain heavy particles (for example, polyethylene terephthalate particles) on their surface or support them on the surface of the water, preventing them from sinking into the water and falling to the bottom. Phytoplankton perform the same function, especially when they form surface films. In addition, all these groups of plants, which are on the surface of the water, “reinforce” the surface microfilm of the water in their presence, preventing it from being destroyed by the interaction of air masses and the surface of the water.
3.2.6. Mechanical retention of particles by irregularities in the structure of plants
Depending on the morphological features of different types of aquatic plants, their spatial structure, and the presence or absence of irregularities on stems and leaves, the rate of retention of plastic microparticles can vary significantly even with the same density of thickets. Unsurprisingly, the greatest number of floating microparticles are retained by plants whose leaves float on the surface of the water. Many species of emergent vegetation have massive stems and leaves and similar morphologies. A significant portion of natural and artificial microparticles are retained in these plants at the nodes where leaves separate from stems.
3.2.7. Retention of particles by sieve-like structures from interlaced stems and leaves
The overlap and intertwining of plant leaves and stems form a sieve-like structure of thickets, where plastic microparticles are retained. This phenomenon is especially pronounced in emergent plants. In continuous thickets with a projective cover of approximately 100%, the number of plants per unit area and the diameter of their stems can vary significantly (see Table 2 for Phragmites australis (Cav.) Trin. ex Steud.). Significant intertwining of different parts of plants is observed in Ceratophýllum demérsum L. and species similar in morphology.
3.2.8. Creation by macrophytes at the bottom, surface and water column of a bulk of plant litter
Particles floating on the water surface are mainly retained by the above-water parts of emergent plants, the tops of some species of submerged plants that reach the water surface (e.g., Ceratophýllum demérsum L.), or leaves and stems floating on the water surface. Free-floating plants, such as floating fern (Salvinia natans (L.) All.), greater bladderwort (Utricularia vulgaris L.), or star duckweed (Lemna trisulca L.), as well as floating plant litter from fragments of dead macrophytes and leaf litter from trees and shrubs, can create porous dams on the water surface that trap virtually all floating particles. The rigid framework of such dams is made up of thickets of emergent vegetation. These dams often form at the mouths of small streams flowing into reservoirs, which is facilitated by a general decrease in water velocity, as well as in thickets of riparian emergent vegetation under the influence of surge winds. In spring, floating mats of plant litter, especially Phragmites australis (Cav.) Trin. ex Steud., can form on the surface of the water, and in areas with weak water exchange, floating islands, which have existed for many years and are sources of swamping water areas.
The retention of solid material carried by the water flow occurs not only on its surface but also throughout the water column and at the bottom. Particles, plant fragments and objects of anthropogenic origin, the density of which is close to the density of water, move throughout its entire thickness and, having reached the thickets of macrophytes, are retained by the stems and leaves of the submerged vegetation and the underwater parts of emergent plants. In the water column, it is also possible to form volumetric dams both from the material moving under water and from the material accumulated on its surface and squeezed into the depths of the water column under the pressure of the accumulated upper layers and the forming water pressure. The main materials of such dams, which can reach the bottom, are large and small wood fragments that make up their framework and are supplemented by hard fragments of emergent plants and leaves of both macrophytes and woody and shrubby vegetation. The gaps and pores in this structure eventually become clogged with detritus in the water. The porous structure of such dams and alongshore formations of growing emergent vegetation, undecomposed plant litter and detritus, is an effective natural filter that prevents solid particles of various natures from entering the water body from the catchment area.
3.2.9. Retention of microparticles as a result of the adhesion of the surfaces of macrophytes and particles
There is no clear relationship between the degree of leaf roughness and the number of microparticles attached to plants since most species have smooth or slightly rough leaves (Phragmites australis (Cav.) Trin. ex Steud., Glyceria spectabilis Mert. & W.D.J. Koch). However, as already noted, Carex acuta L., which has very rough leaves, generally demonstrates approximately 20% greater efficiency in retaining plastic microparticles in all studied projective covers. In all types of macrophytes, adhesion to leaves and stems is most often demonstrated by microparticles originating from flexible material (microfibers, microfilms) and microparticles that have been in the environment for a long time, on which a sticky bacterial biofilm has formed.
3.2.10. Attachment of particles to the sticky surfaces of periphyton covering the leaves and stems of macrophytes
Changes in the adhesive properties of macrophyte surfaces can occur as a result of the development of diverse and extensive microbial communities, as well as phytoperiphytic algae, which form biofilms with sticky surfaces on plants. Biofilms are common adaptations of natural bacteria and other microorganisms. The presence of an adhesive surface that allows quick and reliable attachment to the substrate is a fundamental property of benthic algae, ensuring their survival (Tarakhovskaya 2014; Kerrison et al. 2019; Karimi et al. 2021). In addition, the matrix of extracellular polymeric secretions forms adhesive coatings that act as a stabilizing anchor for buffer cells and their extracellular processes during frequent physical changes (Decho 2000). Notably, research on the water‒particle‒biofilm triad is still in its infancy (Gerbersdorf et al. 2021).
3.2.11. The adhesion of particles to plants and to each other as a result of the interaction of electric fields
Plastic microparticles made of materials similar to expanded polystyrene and, to a lesser extent, of polyethylene-based materials are typically attracted to other objects as a result of the interaction of electric fields (Fig. 3). Under laboratory conditions, the author reported that the distance at which the forces attracting microfragments to the stems and leaves of macrophytes begin to manifest themselves depends on the internal structure of the plant stems or leaves, the degree of their wetting with water and the size of the particles. Owing to the tubular and highly porous structure of the stems, the forces of attraction are weaker and act at a shorter distance. A decrease in the particle size also leads to a decrease in the interaction distance between the plant and the particle, although not always proportional to the same plant species. For example, a decrease in the size of experimental expanded polystyrene particles from 3 to 1 mm led to a decrease in the level of particle attraction by Acorus calamus L. plants to the level of Phragmites australis (Cav.) Trin. ex Steud., although for larger particles, it was at the level of Glyceria spectabilis Mert. & W.D.J. Koch and Carex acuta L. The stem diameter of plants of the same species does not significantly affect the distance of particle attraction by plants. When the stem diameter of Phragmites australis (Cav.) Trin. ex Steud. was reduced from 8 to 4 mm, the distance from the beginning of attraction decreased from 1.5 to 1.4 cm, i.e., by only 1 mm.
For widespread species of emerged vegetation, the distance at which polystyrene foam particles are attracted in the studied size range varies within 1.0–1.9 cm. Carex acuta L., whose leaves are approximately 4 mm wide, attracts polystyrene foam particles the most strongly. The distance at which the attraction of polystyrene foam particles begins is apparently influenced by the internal anatomical structure of the stems. The lower degree of attraction of microparticles by Typha angustifolia L. and Phragmites australis (Cav.) Trin. ex Steud. can be explained by the presence of large spongy tissues in these species, creating spaces or air channels (aerenchyma), which are significantly smaller in the other studied species.
3.2.12. Aggregation of free-floating particles with already attached to plants
At high concentrations of plastic microparticles on the water surface within macrophyte thickets of approximately 1000 particles/m2 or more, aggregation of free-floating particles with those already attached to plants is possible. Thus, in our experiments with Phragmites australis (Cav.) Trin. ex Steud. plants at different concentrations of microfilms with an average size of 4.5 mm and a concentration of other microparticles of 6000 particles/m2, they stuck together, and chains of microfilms attached to each other emerged. These chains can contain up to 25 linked particles. The probability of aggregation and the resistance of aggregates to destruction increase with the interaction of plastic microparticles in the form of microfilms and microfibers, the interaction of aged microplastics on which a sticky biofilm has formed, as well as in water with a high content of natural organic matter. This phenomenon was observed with both studied projective covers of macrophytes.
3.3. Factors of Indirect Influence of Macrophyte Thickets on their Barrier Role
3.3.1. Influences of particle origin and the state of the aquatic environment
The role and intensity of action of certain mechanisms in each specific case depend on the species composition of macrophytes, growth density, season of the year, growing conditions, hydrological and hydrodynamic characteristics of the water body, and size and origin of the particles that affect their behavior. Whether a particle floats on the surface of water, moves in the water column or sinks to the bottom depends on the density of the original material, the particle mass and shape, and the characteristics of the aquatic environment, such as density, surface tension, speed of movement, turbulence and viscosity of water, which depend on temperature. If for terrestrial ecosystems the density of microparticles is not significant, then for particles that have entered the aquatic environment, this is a threshold characteristic that determines their buoyancy, ability to migrate and further fate. In addition, with a long duration in the natural environment, the original physical characteristics of synthetic polymeric materials are modified under the influence of mechanical destruction, biogeochemical processes, photochemical degradation, biological fouling, sorption and coagulation.
Before entering a water body and actually becoming fragments of some plastic products, plastic fragments must be present in wastewater or on contaminated land where other pollutants are present for a sufficiently long period of time. Our field experiments revealed that even after the formation of microparticles during the destruction of objects made of synthetic polymeric materials located in coastal zones, several months to several years are needed for these microparticles to be transported from land into water bodies. Municipal and spontaneous landfills always contain a significant amount of organic matter from household waste. Soil humus and organic matter from waste and clay particles, the density of which is 2.5–2.7 times greater, have a high degree of adhesion to hydrophobic materials made of plastic. Therefore, it is unlikely that plastic microparticles, being in wastewater or moving on land under the influence of water or air currents, will have the density of the original polymers or polymer-based composites. Since the density of the absolute majority of organic substances and minerals is significantly greater than the density of water and microplastics, even particles based on polyethylene and polypropylene that have formed heteroaggregates, have already sorbed mineral particles or are simply contaminated with organic matter, when entering a water body, will most likely sink into the water.
If the polyethylene particles are not contaminated on land during their formation from larger objects, they float on the surface of the water for some time, since the density of this polymer is lower than that of water. In addition, the particles are supported on the surface of the water by the force of surface tension, the value of which for water is quite high compared with that of other liquids and among common liquids is second only to mercury. Our field and laboratory experiments revealed that even previously uncontaminated particles of high-density polyethylene films begin to sink into the water quite quickly. The reason for this is the presence and adhesion of humus and suspended mineral particles in the water and not biofouling.
In the presence of higher aquatic vegetation in a laboratory vessel, transferred from their natural environment of growth and is a source of natural microparticles and conditions of water mixing, after 24–47 hours more than 10% of high-density polyethylene microfilms with an average size of 4.5 mm are immersed in the water column. In addition, approximately the same number of microfilms sink to the bottom. Smaller particles of the same material (average size of approximately 1 mm) begin to sink into the water even faster. After 15–16 hours, approximately 10% of microparticles of this size are already in the water column, and in the presence of Ceratophyllum demersum L. this value increases to 33%. Over time, the process of microfilms sinking into the water increases. Notably, that the adhesion of natural microparticles to microplastic particles begins and occurs more intensively in places where there are surface irregularities, such as bends, tears, splits, scratches, abrasions.
We studied the effects of the presence of Phragmites australis (Cav.) Trin. ex Steud., Glyceria spectabilis Mert. & W.D.J. Koch, Acorus calamus L., Carex acuta L., Potamogeton natans L. and Ceratophyllum demersum L. on the immersion of polyethylene microfilms into the water column. Under laboratory conditions, the onset and intensity of microfilm immersion into water depend on the surface area of the plants and their structure, which facilitates the retention of mineral and humus particles. In our experiments, the sedimentation of high-density polyethylene microfilms began most quickly in the presence of Potamogeton natans L. plants, which have a large area of smooth leaves floating on the surface of the water; these plants retain suspended matter well and easily release it into the surface layer of water, where the microfilms are located. The same can be said of Ceratophyllum demersum L., whose leaves, although having a different structure, also effectively retain fine particles, both on the leaves themselves and in the space around them.
The importance of mixing the upper layer of water and microparticles on plants during the transition of high-density polyethylene microfilms to a submerged state is demonstrated by the results of an experiment conducted by the author, where Carex acuta L. plants and microfilms were carefully placed in a vessel with clean tap water. The Carex acuta L. plants were prerinsed in tap water. The microfilms remained on the surface of the water for 170 hours. In the following 24 hours, 6% of the microparticles sank into the water, and the same amount settled to the bottom. The lack of mixing prevented the washing of microparticles from the surface of the plants and their dispersion in the water and adhesion to the microfilms, slowed the exchange processes between the plants and the water, and increased the effect of surface tension forces. After the conditions for mixing the water were artificially created in the experiment, the amount of microparticles in its thickness increased to 10%, and after another 24 hours it was already 18%. At the same time, the amount of microfilms that settled to the bottom increased to 38%.
How long polyethylene microfragments with rigid structures should spend on the water surface before they change their density and sink into the water column requires additional experimental studies. Our field and laboratory experiments revealed that high-density polyethylene fragments with an average size of 5 mm have neutral buoyancy, react weakly to intensive mixing of water (more than 90% of the particles remain on the surface), are not affected by wind and are less mobile on the water surface than microfilms of the same material are. They do not sink after a sufficiently long time, measured in weeks.
3.3.2. Influence of surface tension forces
Among the microparticles based on mass-produced polymer materials, the density of which is greater than the density of water and which, under normal conditions, should sink into water, are often polyethylene terephthalate particles. As a result of experimental studies, we found that despite the high density of the base polymer, microparticles of polyethylene terephthalate in the form of flakes can remain on the surface of water for a long time, which is supported by surface tension forces. In addition, the hydrophobicity of the polymer contributes to maintaining microparticles of polyethylene terephthalate on the surface of water. The calculations revealed that surface tension forces can hold particles whose mass is measured in units of grams. This value is significantly greater than the mass of plastic microparticles, which are usually found on the surface of water and whose mass is expressed in units of milligrams.
The presence of polyethylene terephthalate microparticles on the water surface is weakly affected by the speed of water flow and wind-wave mixing, and buoyancy is maintained until the surface film of water is destroyed during waves or rain and when the particles are immersed in water. This can explain of detection of polyethylene terephthalate particles on the surface of water bodies. During heavy rain, the probability of a direct droplet hit the microparticles increases significantly. As a result, mass disappearance of plastic microparticles from the water surface is possible. When immersed under the water surface, polyethylene terephthalate flakes quickly sink, in accordance with the sedimentation rate inherent in these particles, and reach the bottom.
Approximately the same behavior is typical for polystyrene microparticles and polyester microfibers. Although polyester is one and a half times heavier than polyethylene terephthalate is, owing to its size, microfibers made from it are among the lightest among the large microparticles found in the environment. Therefore, microfibers stay on the surface of the water longer, even in windy weather, remain in the water column for a long time and sink to the bottom more slowly, which affects the speed and level of microplastic pollution of various components of the aquatic ecosystem.
3.3.3. Influence of gas exchange in the water column
Maintaining heavy microparticles in the water column and even reaching the water surface is possible not only because of the turbulent mixing of water masses but also because of gas exchange in the aquatic ecosystem. It was previously suggested that plastic microparticles can float due to the rise of gas bubbles exiting of bottom sediments (Kukulka et al. 2016). The fact that such a mechanism is possible is evidenced by the operating principle of elutriation units for separating different particles, including microplastics, by their size, shape and density (Claessens et al. 2013).
Under laboratory conditions, we observed cases in which polyethylene terephthalate microflakes that had previously settled to the bottom floated back to the surface of the water. This occurred when there were plants in the vessel, on the surface of which gas bubbles formed, or after intensive mixing. The same gas bubbles, especially during daylight hours, formed on the edges of the microflakes and, in significantly smaller quantities, on their upper surface. The average diameter of the bubbles is approximately 0.5 mm. With a nonuniform distribution of bubbles on different edges of the particles, the edge with more bubbles rises above the bottom, and the particle remains in a vertical plane for some time and then floats to the surface of the water. Floating is also facilitated by minimal hydraulic resistance when the particle is located vertically and a decrease in hydrostatic pressure. When the particle is torn off the bottom, its floating speed is approximately 0.03 cm/s. After reaching the water surface, the gas bubbles partially collapse, but the microparticle remains on the water surface, which is supported by the Archimedes force, the force of surface tension and, for some time, the remaining attached gas bubbles. Large particles with edges of 5 mm or more, having a large weight, float up less often and later than smaller particles.
In addition to weight, adhesion forces also help retain particles on the bottom. On polyethylene terephthalate microflakes that fit tightly to the bottom, gas bubbles do not form or form in significantly smaller quantities and sizes. Some microflakes with attached gas bubbles do not float to the surface, but unlike particles without bubbles, which are in a vertical position, they easily move along the bottom by shear and saltation during increased hydrodynamic activity in the bottom region. In addition, increased hydrodynamic activity leads to the detachment of gas bubbles from the surfaces of solid objects in water and their floating, creating conditions similar to elutriation. We also observed isolated cases of floating polyethylene terephthalate microflakes without visible gas bubbles on their surface. Apparently, in this case, the change in the density of microparticles occurred as a result of the adsorption of gases by their surfaces.
3.3.4. Some features of the behavior of polystyrene expanded microparticles
The dynamics and migration of plastic microparticles in the form of polystyrene foam microspheres that have entered a water body are no less complex. In addition to the previously mentioned diversity of buoyancy, several factors influence the behavior and fate of microparticles in each specific case. A characteristic feature of polystyrene foam microparticles is their high sail area, which can significantly affect their speed of migration. These extremely light spherical particles, which have a high sail area, do not float but roll along the water surface and are capable of covering a distance of approximately 250 km per day (Chubarenko et al. 2016).
Polystyrene foam microparticles are easily blown off land in windy weather, and at first glance, it is logical to assume that the stronger the wind is, the more microparticles will end up in a water body. In some cases, this is what happens. For example, when blown particles end up in a bay closed to wind waves or when the wind speed is sufficient to carry light polystyrene foam particles but insufficient to develop wind waves, as well as when these particles are attached to larger objects floating or stationary on the water surface. This attachment and retention of polystyrene foam microparticles is possible as a result of the interaction of electric fields. Otherwise, approximately 80% of the particles are thrown back onto the land by wind waves at their maximum vertical splash, where these particles remain, mixed with other litter thrown onto the shore and covered by it. In this state, polystyrene foam particles will enter the water body much later if they are not buried under a layer of mineral particles. Only approximately 20% of polystyrene foam microparticles remain in the water body under conditions of developed wind waves in the coastal zone and the coastal source of their inflow. The same spatial differentiation of polystyrene foam particles also occurs in the presence of a strip of Phragmites australis (Cav.) Trin. ex Steud. thickets located at a distance of approximately 5 m from the water edge.
If the source of polystyrene foam microparticles is located in a water body at some distance from the shore, the particles quickly disperse across the water area. This situation occurs when a large polystyrene foam object enters a water body and is destroyed there. Under the conditions of Glyceria spectabilis Mert. & W.D.J. Koch thickets with a projective cover of 80% at a surge wind of approximately 7 m/s, a wave height of 6 cm near the shore and a water flow velocity of 1–2 cm/s, polystyrene foam particles with an average diameter of 4.5 mm, which simultaneously separate from a point source, disperse over an area of 1 m2 within 1 min, and occupy an area of 6 m2 within 5 min. The movement of polystyrene foam particles on the water surface is determined by a complex interaction of factors in the contact zone at the boundary between water and air. Since microparticles are very light and practically do not sink into the water, supported, among other things, by the forces of surface tension, their movement is determined not only by the movement of water in the upper millimeter layer but also by the translational movement and turbulent microvortices of the air.
