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Article

Use of Drone Remote Sensing to Identify Increased Marine Macro-Litter Contamination following the Reopening of Salgar Beach (Colombian Caribbean) during Pandemic Restrictions

by
Rogério Portantiolo Manzolli
* and
Luana Portz
*
Department of Geology and Geochemistry, Universidad Autónoma de Madrid, 28049 Madrid, Spain
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(13), 5399; https://doi.org/10.3390/su16135399
Submission received: 28 May 2024 / Revised: 20 June 2024 / Accepted: 21 June 2024 / Published: 25 June 2024
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Abstract

:
This study involves an integrated and innovative approach employing high-frequency monitoring, which is rare in studies focusing on solid waste on beaches. Eight drone flights were performed over a tourist beach in the Colombian Caribbean to achieve two main objectives: (i) to quantify the changes in marine macro-litter (>2.5 cm) density, focusing on the differences between the period when the beach was closed due to the COVID-19 pandemic and the subsequent reopening period; and (ii) to map changes in the abundance of marine macro-litter on the coast, with an emphasis on single-use waste. The number of items of litter on the beach increased 9-fold between the closed and reopening periods, and the main items found were crisp/sweet packets (n = 304, 13% of the total waste), plastic cups (n = 248, 11%), and expanded polystyrene (food containers) (n = 227, 10%). The factors contributing to the presence and distribution of the marine macro-litter were tourists, the use of the beach, and offshore wind direction. The results revealed that Salgar Beach can be considered a marine macro-litter exporter since waste is incorporated into the longshore current and redistributed either to nearby beaches or the ocean. This study emphasizes the potential for using drone images in an integrated approach to monitoring the presence of marine macro-litter as well as the efficiency of programs for combatting litter at sea.

1. Introduction

Several scientific articles have provided an important picture of the timeline of plastic pollution in coastal and marine environments [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. Plastic production has increased exponentially since its use became widespread in the 1950s [18] and plastic items have become dominant in the global waste stream. The increase in plastic production and its poor management have led to a rise in plastic waste, especially along densely populated coasts [19].
With increasing evidence of the impacts marine litter has on ecosystems and organisms [20,21,22,23,24,25], numerous campaigns to increase conscientious consumption and reduce the use of single-use plastics have achieved important results (see General Assembly of the United Nations; [26]). In addition, many nations have employed various legal and political tools to limit and/or control the use of disposable plastics. In the Caribbean, approximately 27 countries and territories have put laws in place or proposed some form of policy control to reduce the use of plastics over the last decade [27].
However, the COVID-19 pandemic severely disrupted plastic reduction governmental policies and generated significant waste management changes, with potential adverse impacts on the environment and human health [28]. To combat the spread of the virus, the world population employed masks and gloves as personal protective equipment (PPE) [29]. Nevertheless, poor solid waste management practices have proven to be an important factor in ecosystem pollution [30]. Items such as masks and gloves have been found and recorded on several natural and tourist beaches [30,31].
In addition to the increased use and disposal of PPE, there has also been an increase in the use of disposable plastic [32]. On beaches, the packaging waste from consumer goods (such as foam meal boxes, plates, cups, and cutlery) has increased due to safety recommendations related to the reopening of beaches. In addition, most governments mandate the use of PPE in public places. On beaches, climate conditions, such as the frequent presence of wind, facilitate the loss and dispersion of these items. In addition to the use of PPE, during the main COVID-19 contagion period, the spread of the virus led to intensive measures being implemented to prevent transmission, including the closure of public areas and borders. Indeed, the complete closure of the beaches in the Atlántico Department in Colombia facilitated an evaluation of litter contributions from other solid-waste sources.
In recent years, several studies have noted the importance of the Magdalena River as a source of the marine macro-litter found on the beaches of the Atlántico Department [33,34,35]. The Magdalena drainage basin contains 734 municipalities that account for 83% of the total population of the country; 334 of these municipalities (45%) have no solid waste disposal system, and a large part of the municipal, agricultural, industrial, and even hazardous solid waste is dumped within the basin [35].
With beach access restrictions and the risks of potentially contaminated waste, the use of drone technology and artificial machine learning provide an approach for rapidly evaluating the distribution of waste. The use of drones for analyzing the distribution, composition and weight estimation of marine litter on beaches has become frequent [36,37,38,39,40,41]. Gonçalves et al. [42] carried out a review on the use of drones for beach litter surveys, discussing the potential standardization of the methodologies employed. This work highlighted the need for standardized protocols to improve comparability between studies and the accuracy of the data collected. In this sense, the high-resolution images that are obtained are very useful for evaluating marine macro-litter items over large areas, consequently reducing the time spent in the field.
In addition to monitoring the surface of beaches, it is crucial to expand the analysis to submerged areas, where marine litter also poses a significant threat. Complementing this approach, Escobar-Sánchez et al. [43] explored the combined use of aerial and underwater drones to monitor marine litter in shallow coastal waters. The integration of these two technologies allows for more comprehensive and detailed coverage of affected areas, significantly improving the ability to identify and quantify waste in different marine environments. In addition, drones have proven effective in inaccessible locations, such as coral reefs. For example, Kabiri et al. [44] demonstrated the effectiveness of drone-based methods for mapping coral reefs in shallow coastal waters [45].
Although there is substantial scientific literature on the abundance, causes, and impacts of marine macro-litter in locations where various sources of waste interact, this unique opportunity to evaluate beaches at a time when one of the main sources is excluded (in this case, both local and regional beach users) makes it possible to determine the main contributors more clearly to the waste, and hence more efficiently focus on the main causes.
This study establishes the baseline for understanding the spatial variability and main sources of marine macro-litter on Salgar Beach in the Colombian Caribbean during the interval of complete beach closure and the subsequent period when both the beaches and the economy reopened. In addition, we discuss the impact the increase in disposable plastic consumption during exceptional circumstances, such as a pandemic, had on the beach due to health and safety measures as well as the environmental variables that impact this sector. Although the beaches of the Colombian Caribbean are the focus of this study, the results presented have broader implications. This study used a highly useful method involving high-frequency monitoring, which remains significant and valuable in studies of solid waste on beaches.

2. Materials and Methods

The coast of the Atlántico Department (northern Colombian Caribbean) has a wide variety of ecosystems and coastal landscapes [46] that are influenced by the interaction of the Magdalena River and the Caribbean Sea. The Magdalena River is the largest fluvial system in Colombia, extending for more than 1600 km, with an annual sediment load of 144 × 106 t·year−1 [47,48] (Figure 1B).
Salgar Beach has a humid tropical climate with predominantly N and NE winds (Figure 1D). Oceanic undulation is closely linked to the seasons, especially in terms of wind action. In comparison to other seasons, the dry season has higher waves and greater wind energy, predominantly towards the NE, with significant wave heights of up to 3.6 m [45]. The region experiences micro-tides with amplitudes ranging from 0.2 to 0.5 m [49].
The dates selected for the drone flights were chosen to capture a range of conditions before, during, and after the reopening of Salgar Beach. This approach provides a comprehensive view of the changes in marine macro-litter contamination over time. The dates in April (8 and 29) and early May (5) reflect conditions during the period of restricted beach use, establishing a baseline. The selected dates in mid-May (14 and 16) include the day before the reopening (Friday, May 14) and the day of the reopening (Sunday, May 16), allowing for an immediate post-reopening assessment. Subsequent flights at the end of May (30), June (30), and July (15) were conducted to assess long-term impacts as beach usage normalized. These dates represent typical beach use and conditions, covering both weekdays and weekends to account for potential variations in visitor numbers and activities.

Marine Macro-Litter Mapping

A Mavic 2 PRO (DJI) drone equipped with a 24 MP RGB camera was used to capture high-resolution aerial images during the interval the beach was closed during the COVID-19 pandemic and the period when the beach reopened to tourism. The flights were performed after 4 pm, once the beach had been closed to the public.
The flight altitude was 10 m above sea level, and the camera gimbal was adjusted to 75°, with a longitudinal overlap of 80% and a lateral overlap of 76%. The flight plan was drawn parallel to the shoreline, aiming to survey the area between the swash zone and the area used by beachgoers (kiosks and cabanas). Due to the positioning of the groin, the supply of sediments available for maintaining the equilibrium of the beach profile may vary with the seasonal direction and intensity of the winds, meaning the area of sand available may vary weekly.
The RGB orthomosaics were produced using the Pix4Dmapper PRO Version 4.2.15 by Pix4D software, applying the Photogrammetric Processing Structure from Motion-MultiView Stereo (SfM-MVS). The nominal spatial resolution of the final image, expressed in ground sample distance (GSD), was 0.71 cm·px−1.
Each of the orthophotos was analyzed individually against the RGB orthomosaic to identify marine macro-litter on the beach area. The resolution of the orthophotos and orthomosaics allowed for the identification of items >2.5 cm in size. Each orthophoto was visually screened in a GIS environment (ArcGIS Pro) by two operators. Once the aerial photographic surveys had been completed, to validate the results obtained by quantification using orthomosaics, an in situ evaluation of three 10 m wide transects, perpendicular to the shoreline, was implemented with the help of a GPS device and a camera, in line with [9,10,11,12]. Each item was manually marked on the image, and its category was entered into the attribute table according to the Convention for the Protection of the Marine Environment of the North-East Atlantic [50]. Any items that could not be associated with a specific category were labeled “Undetermined”.
The statistical tests were performed using Past® 4.06b software [51,52,53]. The data were first checked for heterogeneity and normality. Levene’s test for homogeneity of variances resulted in a p-value of 0.085, indicating that the variances of the two groups are homogeneous. The Shapiro–Wilk normality test resulted in p-values of 0.136 for the beach closed (lockdowns) data and 0.855 for the beaches reopened data, suggesting that both sets of data follow a normal distribution. One-way analysis of variance (ANOVA) was then conducted to determinate significant differences in marine litter density (items·m−2) between the open and closed beach periods. The one-way ANOVA revealed significant differences in the density of marine litter between the closed and open beach periods (p < 0.05).

3. Results

The aerophotogrammetric surveys using drones varied in the coverage area of the flights, ranging from 2490 m2 (May 14) to 5392 m2 (June 30). The visual screening of the 8 orthomosaic images showed a total of 2113 waste items, resulting in a density of between 0.004 and 0.25 items·m−2 (Table 1). Concurrently, there was a total of 1396 items of large wood debris (LWD) on the beach, resulting in a density of between 0.02 and 0.251 items·m−2 (Figure 2).
The calibration of the visual observation resulted in a comparative analysis of the three 10 m shoreline segments. A total of 45.8% (11 of 24 segments) received the same density rating during calibration, all prior to beach reopening. One of the segments, (May 14—prior to opening) had a higher manual in situ item count than the orthophoto count, with a difference of less than 2% (standard error of 0.98). In the remaining segments, after the reopening of the beaches, there was a variation in the actual number of items identified between the original analysis and the calibration, in which the mean difference in item count was +5.5, with a standard error of 0.98. Therefore, the magnitude of inter-observer error increases with higher debris counts.
The sampling area (distance between the beach line and the kiosks/cabanas) varied according to the exact positions. The kiosks/cabanas were periodically moved closer to the shoreline, which resulted in different surface areas in each collection period, so the specific area for each period was used to calculate the density. These structures affect the results because they block the drone’s full view, resulting in an underestimation of the quantity of waste. The beach macro-litter concentrations showed highly variable values. During the beach closure period (April 08 and May 14), very low marine macro-litter densities were observed (0.003–0.037 items·m−2). In contrast, in the post-opening period, after the COVID-19 related restrictions were lifted, greater marine macro-litter densities were found (0.121–0.208 items·m−2), with some critical accumulation points, especially near the water line and the kiosks/cabanas (Table 1).
The distribution of LWD on the beach showed less density variation between the evaluation dates. The presence of LWD was mainly linked to discharge from the Magdalena River and the lack of beach cleaning. With the closure of the beach to tourism, the number of wood pieces increased between 8 and 29 April as the beach was not cleaned during this period. Based on the orthomosaics obtained in May, beach cleaning resumed on the eve of the reopening, with a drastic reduction in the amount of LWD (Figure 2 and Figure 3).
The most common marine macro-litter categories were those consisting mainly of plastic polymers (86% of the total waste). In this category, crisp/sweet packets (n = 284, 13.5% of the total waste) were the most abundant, followed by plastic cups (n = 228, 11% of the total waste) and expanded polystyrene (foam meal box) (n = 208, 10% of the total waste). The macro-litter categorized as “undetermined” represented 4% of the total waste. The presence of a substantial number of surgical masks (n = 72, 3.5% of the total waste) occurred mainly after the beach reopened to tourism (on/after May 16). Only 235 items (10% of the total) were non-plastic waste; this included glass bottles (n = 12, 1% of the total waste) and beverage cans (n = 102, 5% of the total waste) (Table 1/Figure 4).
The origin of the marine macro-litter on this beach is associated with tourist activities. Because it is a recreational beach and constantly cleaned, the items that arrive from the ocean are removed in the early hours of the day. As a result, the marine macro-litter mapped in this study represents daily accumulation (Figure 5). The high number of single-use plastic items, such as cups, plates (plastic and expanded polystyrene), and plastic cutlery in the surveys of the beach after reopening (from May 16 to July 15) is notable.
There were significant temporal trends in the abundance of marine macro-litter, with a highly significant increasing trend after the post-COVID-19 reopening of the beach (ANOVA: F-21.4; p (same) −0.00241). The abundance of marine macro-litter was spatially heterogeneous but showed considerable temporal variation. Between April 08 and May 14, there were 210 items (May 14: 107; May 05: 17; April 29: 69; April 08: 17) (Figure 6). The total number of marine macro-litter items found during these four surveys together (in a period during which the beach was not being cleaned) did not exceed the number of items identified on May 16 (n = 372), the first Sunday when the beach was reopened. For the later dates, the residual densities remained higher than the average residual densities for the entire beach closure period.
The marine macro-litter was distributed along the whole length of the beach, with higher volumes being observed close to the beach kiosks/cabanas, and the water line, reflecting the concentration of beach users and their consumption of food and beverage/other items, as well as the beach dynamics (wave swash). With the predominant offshore wind direction (Figure 1D), low-density waste (including plastics) was distributed along the entire length between the beach kiosks/cabanas and the water line (Figure 7).

4. Discussion

This study defines the baseline distribution of marine macro-litter during a period when the beaches were closed due to the COVID-19 health emergency and, subsequently, the beach reopening period (Figure 8A,B, respectively). The beaches of Puerto Colombia are predominantly used by a mixture of day trippers (locals, inlanders, and regional tourists), with higher occupancy rates at the weekends, making these beaches a useful case study for determining the accumulation and dispersion of marine debris during the pandemic closure and the reopening of the beaches for recreational use. The mean annual temperature in this area is 28 ± 2 °C, meaning its beaches are used throughout the year.
Remote sensing techniques allow for intensive monitoring of the area, making this repetitive measurement much more feasible. Indeed, performing field surveys with drones is 39 times faster than the standard approach involving surveying a beach by walking [54]); in addition, only one person is needed to perform a field survey using a drone. In the context of the pandemic, requiring fewer people in the field just to check the consistency of the evaluations between the different survey dates was an important consideration. However, one advantage of ground transect surveys over drone passes are that litter items can be collected for further study, and a hands-on examination of specific items may yield information on specific sources (e.g., logos, brand names, and lists of contents).
The densities of the marine macro-litter items found on Salgar Beach (maximum of 0.208 items·m−2) were lower than the densities determined by Rangel-Buitrago et al. [52] in a nearby area (plastic 0.74 items·m−2; other waste 0.42 items·m−2). This difference can be explained by the fact that in the reopening period of this study, the amount of marine macro-litter corresponded only to that accumulated over a single day, since the beach was cleaned each morning (06:00 and 06:30). The amount of litter in this study is like that found on a tropical recreational beach, Mkomani Beach, in Kenya (0.383 ± 0.260 items·m−2) [53].
Despite the high resolution of the images (GSD < 1 cm·px−1), the underestimation of the marine macro-litter density using the images can be expected to be similar to that obtained from in situ observations. When litter items are smaller, it is difficult to determine the type. In addition, other factors, such as shadows, the presence of vegetation, discontinuous surfaces, the absence of a third dimension, and the presence of shells, may also hinder the determination of larger items [54]. Additionally, is assumed that the number of marine macro-litter items may be underestimated due to the presence of beach kiosks, where some of the litter may not be observable in the images. During the field surveys, a large concentration of marine macro-litter was observed below the kiosks/cabanas and this could not be quantified using orthomosaics (Figure 9). Considering these limitations, future drone monitoring designs could include manual quantification of litter in areas obstructed by structures such as kiosks. This approach would help to ensure a more accurate estimation of litter density and provide a more comprehensive understanding of litter distribution in coastal environments.
Through image processing, it could be determined that the marine macro-litter primarily consisted of plastic items such as cups, plates, and cutlery, as well as items used by beach visitors during the day. Due to the almost daily cleaning of the beaches by tourism support services (in the morning, Figure 9), the number of items and their type (determined at the end of the day) were primarily based on the daily accumulation of waste from tourist activities. Thus, this scenario resulted in a positive identification of the marine macro-litter items in the images because the waste items were neither degraded nor very fragmented.
Face masks were found in considerable numbers after the reopening of the beaches, and because these are made of low-density material and have a low purchase value, they are easily carried away by the wind and not recovered by their users. In contrast, fabric masks were rarely identified on the beach. The most common masks are composed mainly of polypropylene (PP), making them a source of fossil fuel-based plastic and microplastic pollution [54,55]. This high number of masks in a small beach area evidences the negligent handling of face masks. As these masks are easily transported by the predominant offshore wind, they readily enter the marine environment. Masks have become an important new category of both cloth and plastic items found in the natural environment, as evidenced in several beach locations around the world [37,53,56,57,58]. In natural environments, masks can increase contamination with microfibers and hazardous chemicals and have the potential to induce serious effects on fauna, from invertebrates to vertebrates, and at different biological system levels [28].
In addition to this new category (masks), since 2020, the pandemic has led to a dramatic increase in the use of single-use items, not only billions of masks but also gloves and foam meal boxes. At Salgar Beach, in this study, the increase in these types of plastic waste can be observed in the difference in waste density between the beach closure period and the subsequent reopening period, during which large amounts of plastic and expanded polystyrene packaging (Styrofoam), in addition to many other products, such as the indispensable face masks, were found.
According to several studies undertaken during the pandemic, the increased use of plastics and medical waste is a reality worldwide [59], and existing recycling systems have collapsed in some places. Food services on beaches have been forced to use single-use plastics, e.g., plastic cups, bags, Styrofoam containers, straws, bottles (glass and plastic), and cans, to reduce the likelihood of contamination. In the study area, all marine macro-litter items were used in a typically windy environment and there was a lack of collection protocols, resulting in a high concentration of items. The zone immediately inland (backshore) includes areas that service tourists, meaning the predominantly offshore winds transport waste from both the service and leisure areas to the beach line. This combination of factors must be added to the highly dynamic environment, where a combination of oceanographic (tides, waves, and currents) and meteorological (wind and rain) processes influence the distribution, abundance, and types of marine macro-litter that occur, as well as their variability in space and time. In addition, the macro-litter that reaches the water line may be dispersed to nearby beaches that do not have intensive tourism, exacerbating the contamination of those areas. The predominant wind direction at this beach, which affects fragmentation and abrasion processes, can also have a potential impact as the marine macro-litter is transported directly from the remaining beach area into the waves.
Disposable plastics were already the most found marine macro-litter on Colombian tourist beaches [10,11,35,60,61]. On Santa Marta beaches, glass items (fifth category in terms of the number of items) are also found extensively [60]. On Salgar Beach, low quantities of glass items are observed because these are not transported by the wind; thus, they remain below the kiosks/cabanas and were not identified in the images. Kiosk and cabana litter items are easily collected during cleaning the next morning.
The presence of LWD on the beaches is characteristic of the study area, and the current patterns influencing LWD abundance and accumulation are related to the characteristics of the beach as well as the degree of exposure. In addition to these factors, the presence of LWD reflects transport by coastal currents that carry the wood from the mouth of the Magdalena River (this is the most important drainage basin in the country). The number of wood items in the study area did not vary greatly, with the largest number of items being observed on April 29. As wood is periodically removed from the beach in this area, with the closure of the beach in the first days of April and less frequent beach cleaning, a high natural accumulation of wood was evident on April 29. A study conducted by Rangel-Buitrago et al. [62] indicated a density of 0.8 items·m−2 at a nearby tourist beach, while the wood on non-tourist beaches (where the timber is not removed periodically) was as high as 3.8 items·m−2. In this Caribbean region, wood plays an important role in maintaining the beach by decreasing erosion [63]. However, most people ignore the importance of wood in these processes because of its unattractiveness on the beach. Due to the expectations of urban beach users, LWD is increasingly removed as beach waste. Currently, LWD removal processes are developed without any regulation and are based on the extraction, transport, and abandonment of LWD in adjacent areas (e.g., backshore or dunes) for subsequent removal by burning [62]. In the study area, both small wood pieces (during daily cleaning) and the larger logs (removed to areas adjacent to the groins and subsequently transported away by the local authorities with the aid of backhoes and trucks) were removed from the beach.

4.1. Temporal and Spatial Variability

Determining and classifying the spatial and temporal patterns of marine macro-litter are key to identifying its possible sources and helping plan focused management actions, both preventive and restorative [54]. The accumulation of coastal waste is highly variable and can occur at both regional and local scales (for example, [9,64]). This accumulation is influenced by several factors, including oceanographic parameters [11,65], the geomorphological characteristics of the beach [38,66], and beach use and services [4,9,10].
The proximity of a beach to land-based waste sources, as well as the activities that take place there and surrounding services, can result in a greater volume of marine litter [64]. However, in practice, inaccessible beaches often accumulate considerable amounts of marine litter [11,34,67], while tourist beaches that are constantly cleaned experience less waste contamination [10].
The accumulation of marine litter on Salgar Beach, particularly influenced by tourist activities and the prevalent use of single-use plastics, poses significant challenges. The beach experiences increased waste accumulation near the shoreline due to the extensive use of disposable items by beach visitors. This includes plastic bottles, food packaging, and other single-use plastics commonly discarded in beachside recreational areas. The proximity of the beach to land-based waste sources and the continual use of beach services contribute to the higher volume of marine litter observed, despite periodic cleaning efforts.
The variability in coastal marine litter accumulation also has a direct relationship with the degree and characteristics of ocean transport [65]. This sector of the Colombian Caribbean coast is strongly influenced by longshore currents, which transport part of the waste that reaches the ocean through the Magdalena River [34]. Another important parameter in the marine macro-litter distribution on this beach is the predominant wind direction, which can be considered the main factor in the litter distribution along the entire beach strip. With the predominant offshore wind direction and the intensive use of the backshore, the waste remaining on the beach is transported across and along the beach to the wave swash line (Figure 7). Thus, tourist-derived marine litter is added to the marine litter transported by the currents. On most beaches, onshore winds are predominant, and marine litter tends to accumulate in the dune system or back-beach vegetation [9,12,36]. Thus, as identified in the waste dispersion model developed by Critchell et al. [65], the physical characteristics of the source location have the greatest effect on the destination of the litter.
In this sense, our study highlights the impact of beach geomorphology, local oceanographic conditions, and anthropogenic activities on marine litter accumulation patterns. For example, the Colombian Caribbean coast is influenced by longshore currents that carry substantial amounts of litter and wood from the Magdalena River [63] to Salgar Beach. In addition, the prevailing offshore winds play a crucial role in the distribution of marine litter along the beach, exacerbating contamination levels close to the strand line. By understanding these dynamics, coastal management authorities can implement specific interventions.

4.2. Litter Management in Exceptional Circumstances, Such as a Pandemic

The management of marine litter is complex and involves an analysis of various parameters. The situation of beach closures due to the COVID-19 pandemic in many countries caused the reduced movement of people in and around coastal areas, resulting in cleaner beaches and clearer waters in the surrounding areas [68]. However, as evidenced by our study, the reopening of these beaches may have increased pollution due to the increase in the use of single-use plastics implemented as a measure to reduce the transmission of the virus [69]. Since the beaches of the Colombian Caribbean reopened, the number of visitors to Salgar Beach has increased considerably, mainly because this beach is close to the state capital and is easily accessible, facilitating the use of the beach without necessitating an overnight stay (Figure 8).
Beachgoers generally value water quality highly and have supported initiatives to protect the coastal zone; they may also stop visiting beaches if they have low environmental quality conditions [70]. However, a study conducted by Enriquez-Acevedo et al. [71] showed that visitors to Salgar Beach care little about its environmental quality in comparison to other beaches in the surrounding area, which underscores our findings. They show a willingness to engage in recreational activities on the beach, regardless of environmental quality. This lack of environmental culture correlates with observed poor waste management practices, influencing litter accumulation patterns, and it is confirmed by comparison with the results during the pandemic period. Inadequate management practices have also been identified as an important factor in PPE pollution at Cox’s Bazar (Bangladesh) [30]. A culture of caring for others as well as the environment was expected during the pandemic [72]; however, we did not find this phenomenon reflected on Salgar Beach.
A reduction in the influx of waste from tourists on beaches is essential to increasing beach health, and this becomes especially pronounced in exceptional waste-generation circumstances, such as a pandemic. To achieve healthy beaches, the first step is to understand where the waste is located, how it gets there, and its characteristics, as well as the effectiveness of cleaning efforts. These are the main questions defined by the results of this study.
Initiatives to reduce solid waste and minimize environmental impact are being developed by national and local authorities. The aim is to reduce marine litter before it reaches the sea and to encourage an economic model that gains a benefit from this waste. At the national level, the Colombian government approved a law, in June 2022, to eliminate single-use plastics. This law is a historic step forward in favor of water resources and the sea. The law bans the use of 14 categories of plastics, including bags, plates, trays, knives, forks, spoons, and cups, these being the types seen in the highest percentages in this study.
At the local level, actions should focus on the appropriate disposal of waste by beach users and service providers. Public incentives should be introduced for companies specializing in the separation and economic use of this waste. Efforts implemented near waste sources will be more cost-effective and will have a greater impact than actions performed in the open ocean. In addition, increasing the public perception of the risk of plastic pollution is essential for reducing plastic waste generation. Advances in research into the adverse effects of plastic waste may influence the general public’s view of plastic pollution, boosting the global governance of marine plastic [32].
Some practical recommendations include fining people for the irregular disposal of waste; installing informative signs and waste collection bins; prioritizing informative and educational actions related to the impact of marine litter; and establishing continuous waste monitoring to constantly adapt the implemented actions.
Managing marine litter during exceptional circumstances such as a pandemic requires not only understanding marine litter dynamics and their origins but also implementing effective waste reduction strategies at local and national levels. Our data underscore the urgent need for targeted actions to enhance waste disposal practices and public awareness, ensuring sustainable beach environments for future generations.

5. Conclusions

This study presents a method for mapping marine macro-litter in a tourist beach system using drones; this approach requires few people in the field and can detect marine macro-litter on the coastline. The approach presented in this article overcomes the limitations imposed by the COVID-19 pandemic and, therefore, is suitable for integration into international strategies aimed at reducing marine macro-litter and its impacts.
  • There was a substantial increase in macro-litter during the beach reopening period compared to the lower litter values observed during the beach closure period. This result indicates that at Salgar Beach, the macro-litter sources are the beach use services provided on site, and this source surpasses the waste originating from the Magdalena River, an important source of waste in the central Colombian Caribbean sector.
  • The main driving force behind the litter accumulation is the environmental culture established in this sector. Inadequate waste disposal facilities in the service sector (kiosks) result in marine macro-litter and subsequent offshore transport.
  • In this study, Salgar Beach can be considered a waste exporter as macro-litter can be incorporated into the longshore current and redistributed either to nearby beaches or the ocean.
  • The method described in this article may contribute to a best practice protocol for monitoring both marine macro-litter and the efficiency of waste reduction practices in coastal areas.
  • It should be noted that it was easy to identify the marine macro-litter items in the images, with a very good correlation with the visual observation calibration, mainly because the waste items were neither degraded nor very fragmented.
  • Actions to reduce marine macro-litter in this sector should be focused on environmental education and the waste disposal processes used in tourist services. In addition, actions are needed to improve the behaviors of beachgoers in terms of their waste disposal practices while at the beach.

Author Contributions

Conceptualization, R.P.M. and L.P.; methodology, R.P.M. and L.P.; software, R.P.M. and L.P.; validation, R.P.M. and L.P.; formal analysis, R.P.M. and L.P.; investigation, R.P.M. and L.P.; resources, R.P.M. and L.P.; data curation, R.P.M. and L.P.; writing—original draft preparation, R.P.M. and L.P.; writing—review and editing, R.P.M. and L.P.; visualization, R.P.M. and L.P.; supervision, R.P.M. and L.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Study area. (A) National context. (B) Regional context. (C) Area corresponding to drone flights (ArcGIS PRO 3.3.0 image base). (D) Wind rose showing a predominance of NE winds (source [45]).
Figure 1. Study area. (A) National context. (B) Regional context. (C) Area corresponding to drone flights (ArcGIS PRO 3.3.0 image base). (D) Wind rose showing a predominance of NE winds (source [45]).
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Figure 2. Size and density (items·m−2) of the LWD observed on the beach on the different sampling dates. Note the significant increase in the number of LWD just after the beaches closed and a drastic reduction on the day they reopened, due to beach cleaning.
Figure 2. Size and density (items·m−2) of the LWD observed on the beach on the different sampling dates. Note the significant increase in the number of LWD just after the beaches closed and a drastic reduction on the day they reopened, due to beach cleaning.
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Figure 3. (A) Evidence of timber removal from the beach. The LWD is gathered and then removed by the urban cleaning service. (B) Image of the different LWD sizes found. Note that the size of the LWD varies and may be very large.
Figure 3. (A) Evidence of timber removal from the beach. The LWD is gathered and then removed by the urban cleaning service. (B) Image of the different LWD sizes found. Note that the size of the LWD varies and may be very large.
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Figure 4. Density (items·m−2) and types of marine macro-litter found per sampling day. Note the sharp increase in density of all items, but especially plastics, on the reopening date, May 16th, and subsequently.
Figure 4. Density (items·m−2) and types of marine macro-litter found per sampling day. Note the sharp increase in density of all items, but especially plastics, on the reopening date, May 16th, and subsequently.
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Figure 5. Examples of the more common marine macro-litter items. Specific items identified include (a) a plastic cup, (b) expanded polystyrene plate, (c) surgical mask, (d) crisp/sweet packet, (e) water bottle, (f) water bottle, (g) beer can, (h) plastic spoon, and (i) plastic bag.
Figure 5. Examples of the more common marine macro-litter items. Specific items identified include (a) a plastic cup, (b) expanded polystyrene plate, (c) surgical mask, (d) crisp/sweet packet, (e) water bottle, (f) water bottle, (g) beer can, (h) plastic spoon, and (i) plastic bag.
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Figure 6. Time-series data of items distributed on the beach (between the kiosks/cabanas and the waterline). (a) April 08; (b) April 29; (c) May 05; (d) May 14; (e) May 16; (f) May 30; (g) June 30; and (h) July 15. Note the increase in the number of items and their distribution as of May 16.
Figure 6. Time-series data of items distributed on the beach (between the kiosks/cabanas and the waterline). (a) April 08; (b) April 29; (c) May 05; (d) May 14; (e) May 16; (f) May 30; (g) June 30; and (h) July 15. Note the increase in the number of items and their distribution as of May 16.
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Figure 7. Visualization of the distribution of marine macro-litter along the beach line on the day the beaches reopened (May 16) (A) as a function of waste disposal along the line of beach kiosks/cabanas and its subsequent transport by the dominant offshore wind (B).
Figure 7. Visualization of the distribution of marine macro-litter along the beach line on the day the beaches reopened (May 16) (A) as a function of waste disposal along the line of beach kiosks/cabanas and its subsequent transport by the dominant offshore wind (B).
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Figure 8. Image of Salgar Beach, Puerto Colombia. (A) 4 April 2021; (B) 23 May 2021. Note the increased number of cars and the density of beach structures in image B.
Figure 8. Image of Salgar Beach, Puerto Colombia. (A) 4 April 2021; (B) 23 May 2021. Note the increased number of cars and the density of beach structures in image B.
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Figure 9. (A) Example of the presence of waste under the kiosks/cabanas (photo: personal archive—June 15). (B) Example of daily cleaning of the beach area (photo: personal archive—June 15). Note that the presence of kiosks/cabanas is one of the limitations of drone use since they hide the waste.
Figure 9. (A) Example of the presence of waste under the kiosks/cabanas (photo: personal archive—June 15). (B) Example of daily cleaning of the beach area (photo: personal archive—June 15). Note that the presence of kiosks/cabanas is one of the limitations of drone use since they hide the waste.
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Table 1. Number of items and density on each data collection date. The residual counts are for all identified marine macro-litter. Note the sharp increase in the total number of plastic items between May 14 and the reopening of the beach on May 16, as well as the similar sharp rise in metal cans and face masks. The high number of “undetermined” items reflects the limits of the drone imagery surveys.
Table 1. Number of items and density on each data collection date. The residual counts are for all identified marine macro-litter. Note the sharp increase in the total number of plastic items between May 14 and the reopening of the beach on May 16, as well as the similar sharp rise in metal cans and face masks. The high number of “undetermined” items reflects the limits of the drone imagery surveys.
Beach ClosedOpen
April 08 April 29 May 05 May 14 May 16 May 30 June 30 July 15
Plastic fragment21243130263516
Plastic cutlery/trays220237387041
Plastic lolly sticks/straw101338414932
Plastic drinks (bottles, containers, drums)044425326525
Plastic bags21601532293935
Plastic caps103625114522
Plastic crisp/sweet packets0301142518988
Plastic cups 140860476840
Plastic pots 05036584
Expanded polystyrene (frag)060220303523
Expanded polystyrene330833585548
Rubber (shoes)00040232
Metal (drink cans)020115224220
Other000000010
Glass 01000605
Surgical masks00449221518
Undetermined5111510221517
Density (items·m−2)0.0040.0130.0030.0370.1210.1780.2080.142
Area (m2)41135392523728863074249030453143
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Manzolli, R.P.; Portz, L. Use of Drone Remote Sensing to Identify Increased Marine Macro-Litter Contamination following the Reopening of Salgar Beach (Colombian Caribbean) during Pandemic Restrictions. Sustainability 2024, 16, 5399. https://doi.org/10.3390/su16135399

AMA Style

Manzolli RP, Portz L. Use of Drone Remote Sensing to Identify Increased Marine Macro-Litter Contamination following the Reopening of Salgar Beach (Colombian Caribbean) during Pandemic Restrictions. Sustainability. 2024; 16(13):5399. https://doi.org/10.3390/su16135399

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Manzolli, Rogério Portantiolo, and Luana Portz. 2024. "Use of Drone Remote Sensing to Identify Increased Marine Macro-Litter Contamination following the Reopening of Salgar Beach (Colombian Caribbean) during Pandemic Restrictions" Sustainability 16, no. 13: 5399. https://doi.org/10.3390/su16135399

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