Water quality in coastal environments has decreased in many areas around the world due to anthropogenic impacts (Mallin et al., 2000; Bordalo, 2003; Ahn et al., 2005; Dwight et al., 2011; Wiegner et al., 2021). Some of these human activities include watershed alteration for agriculture, irrigation practices, domestic and recreational activities, land development, and littering (Khatri and Tyagi, 2015; Sidabutar et al., 2017; Tavakoly Sany et al., 2019). Furthermore, according to a recent investigation conducted by Lin and colleagues (2022), over 80% of sewage produced by human actions is released into rivers and oceans untreated, leading to environmental contamination and over 50 different diseases, affecting valuable coastal ecosystems and the quality of people’s lives (Halliday and Gast, 2011; Freeman et al., 2019; Nalley et al., 2021; Lin et al., 2022). As a result, efforts to reverse the effects of poor planning of land alterations have been made in some regions, one example being the creation of the Guánica Bay Watershed Management Plan (CWP, 2008).

The Guánica Bay Watershed is a 151 square miles system located in southwest Puerto Rico, encompassing Guánica’s municipality, a portion of Yauco, Sabana Grande, San Germán, Maricao, Adjuntas, and Lares municipalities, and a segment of the Lajas Valley Agricultural region, all of which drain to the bay through the Loco River (CWP, 2008; Viqueira and Rios, 2018). Watersheds in southern Puerto Rico have been highly developed to sustain domestic activities, irrigation, and hydroelectric power infrastructures (CWP, 2008; Gómez-Gómez et al., 2014). Specifically, in Guánica, the Guánica Lagoon, which covered an area of approximately 1,200 acres, was drained in the 1950s as part of the Southwest Puerto Rico Project for agricultural and hydroelectric power purposes (Viqueira Rios et al., 2012). These developments come with a cost to marine coastal ecosystems, as impacts such as marine sedimentary deposition, eutrophication, and shifts on coastal species composition have been reported in some countries (Morais, 2008; Sanger et al., 2013).

One of the most impacted ecosystems of watershed development are adjacent coral reefs that face challenges like the introduction of pollutants causing nutrient enrichment beyond tolerance of reefs, along with marine sedimentary deposition, pathogens, and toxic contaminants (Richmond et al., 2007; CWP, 2008). In Puerto Rico, stressed and degraded coral reefs have been reported along the insular shelf, particularly in the Guánica to Guayanilla region in the southwest (Warne et al., 2005). As a result, the Guánica Bay Watershed was selected as a priority area for management by the U.S. Coral Reef Task Force (Whitall et al., 2013; Takesue et al., 2021). The Guánica Bay Watershed Management Plan was developed in 2008 to reduce the non-point sources of pollution that arrived at the bay and protect adjacent coral reefs (CWP, 2008). Some of the recommendations in the plan included the restoration of the Guánica Lagoon, agricultural outreach, the creation of treatment wetlands, among others (CWP, 2008). Reports created by Viqueira Ríos, 2018, Viqueira Ríos, 2021) highlight the management actions implemented from 2012 to 2022, which include the application of hydroseeding (2013–2017), training farmers on Best Management Practices (BMP) and shade coffee farming, and the creation of infiltration systems in farms using native trees and vetiver grass (2010–2016), to name a few. However, some of these management actions, i.e., the restoration of the Guánica Lagoon and the creation of the treatment wetlands, which were the first two most important recommendations from the management plan, had not yet been implemented by the end of this work.

Assessment of watershed management requires integrating a broad range of spatial and temporal data (Wang et al., 2016). Although no reports assessing the effectiveness of the management actions in Guánica were found, refinements and suggestions to improve the management plan considering stakeholder concerns have been made (Carriger et al., 2013; Smith et al., 2017). Therefore, the goal of this work was to evaluate the effectiveness of the management actions implemented from the Guánica Bay Watershed Management Plan created to protect adjacent coral reefs using remote sensing techniques. We aimed to describe temporal trends of the diffuse attenuation coefficient at 490 nm (Kd₄₉₀) in one managed (Guánica Bay) and three non-managed (reference) estuaries in Southwest Puerto Rico before and after the implementation of the Guánica Bay Watershed Management Plan. It is noteworthy to mention that during the after period of this study the following major events affected our area of study: Hurricanes Irma and María in 2017, and Fiona in 2022, with significant impacts to the area. Also, the Covid-19 pandemic outbreak and associated lockdowns, and a magnitude 6.4 earthquake near Guánica, all of which could result in decreased human impact in the watershed and coastal waters.

The diffuse attenuation coefficient (Kd) of downward irradiance is an apparent optical property of water widely used in oceanographic studies, a good indicator of turbidity, and one of the parameters for which ocean color algorithms have been developed using satellite sensors (Lee et al., 2005; Wang et al., 2009; Tomlinson et al., 2019; Y; Wang et al., 2022). For this purpose, the European Space Agency (ESA) launched in 2002 the Medium Resolution Imaging Spectrometer (MERIS) on board the Envisat satellite to provide ocean color observations. The Ocean Land Colour Instrument (OLCI) was launched in 2016 and 2018 on Sentinel-3 A and B, respectively, as successors of MERIS to ensure long term monitoring with improvements such as additional spectral bands, mitigation of sun-glint by tilting the sensors, and improved signal to noise ratio (Nieke et al., 2015). In Puerto Rico, satellite estimates have been used to study long term trends and disturbance impacts (e.g., hurricanes) in coastal water quality using chlorophyll-a, Kd at 490 nm (Kd₄₉₀), and absorption of colored dissolved organic matter (Gilbes et al., 2001; Amirrezvani, 2016; García-Sais et al., 2017; Hernández et al., 2020; Ortiz-Rosa et al., 2020). Furthermore, an inverse relationship between Kd and percent coral cover was observed in Ponce, Guayanilla, La Parguera, Mayagüez Bay, and other areas, which suggests that Kd can be used for indirect assessment of the percent living coral cover in coral reefs (Cardona-Maldonado, 2008; García-Sais et al., 2017; Freitas et al., 2019). While Puerto Rico has no coastal water quality standards of light attenuation, Hernández et al. (2020) suggest using 0.1 m^-1 to reference degraded water quality adopted from Hawai’i standards.

If management actions implemented in the Guánica Bay watershed have been effective, it was expected that: i) values of Kd₄₉₀ decreased in Guánica Bay after the implementation of management actions, and ii) this temporal change would be observed only in Guánica Bay as compared to reference estuaries. It is important to note that the matter in question was not to compare estuaries as they are part of different watersheds and might change through time but to detect whether trends in Guánica Bay were unique when compared to reference estuaries which could be attributable to the implementation of management actions to this watershed.

To evaluate the effectiveness of the Guánica management actions, it was hypothesized that Kd₄₉₀ values would decrease in Guánica after the implementation of the management actions. The temporal distribution patterns observed were not consistent with an improvement, i.e., a decrease of Kd₄₉₀, following the management actions, thus, the hypothesis is rejected. The results of the analysis indicated that there were differences between the before and after periods, but they were unique to each location within the estuary. This means that variability within the estuaries was high, and each location followed different patterns of temporal variability of Kd₄₉₀ driven by conditions unique to the location. Even in the managed estuary, Guánica Bay, variability of turbidity between the locations (Close, East, West, and Far) was high. The oceanic continuity analysis indicated that there was not a significant difference between sensor estimates, which allows for the integration of the sensors estimates for long-term trend monitoring and time series analysis. Any differences between sensor estimates can be explained by the improvements applied in the OLCI sensor (mitigation of sun-glint, improved signal-to-noise ratio) and improved revisit times, thus being able to detect more variability and accurate estimates than the MERIS sensor (See Supplementary Figure S2 showing the increase in frequency sampling) (Nieke et al., 2015).

Waters in the estuaries can be considered non-turbid ocean waters (Kd₄₉₀ < 0.3 m^-1) (Wang et al., 2009). Generalized patterns were observed in the southern estuaries which showed the highest Kd₄₉₀ values in the Close location and the lowest values in the Far location as expected. The Guanajibo River estuary, located in southwest Puerto Rico, followed different patterns which can be attributed by the difference in prevailing trade winds in this area causing higher resuspension rates near the shore (Miller and Cruise, 1995; Gómez-Gómez et al., 2014). The 2018 peak observed in the southern estuaries coincides with the fact that the impact of Hurricane Maria in September 2017 has been reported to have lasted up to 4 months after its passage through the island driven by mass wasting, sediment runoff, coastal turbidity, and bottom sediment resuspension (Takesue Sany et al., 2019). Keellings and Hernández Ayala, (2019) reported the extreme rainfall event associated with the hurricane while Bessette-Kirton et al. (2019) reported a low landside density (1–25 landslides/Km²) in the upland region of the Guánica Bay watershed. In terms of water quality, Ortiz-Rosa and colleagues (2020) reported that absorption of colored dissolved organic matter or detrital gelbstoff (aCDOM/ADG) was high (>0.13 m^-1) in Guánica 4 months after the event as well as the coastal total suspended sediment (TSM) (∼5–6 mg/L) values as reported by Miller et al. (2019). Additionally, Hernández et al. (2020) reported high Kd₄₉₀ (>0.1 m^-1) and Chlorophyll-a concentration (>0.45 μg/L) values and Cheriton et al. (2019) reported increased concentrations of suspended particulate material (SPM) and polychlorinated biphenyls (PCBs), a highly toxic industrial contaminant, after the hurricane. This peak was not observed in the Guanajibo River which could be explained due to the low impact of the hurricane on the west side of the island and possibly to its difference in sediment type, i.e., mixed terrigenous and carbonate clastic. The COVID-19 lockdowns as well as a major earthquake offshore of southwest Puerto Rico in 2020 caused less anthropogenic disturbance to the coastal environment in this area, and therefore improved water quality, which is why a trough can be observed in 2020–2021 in some of the estuaries, a phenomenon also observed in other areas of the world (Supplementary Figure S2) (Callejas et al., 2021; Ormaza-González et al., 2021; Vijay Prakash et al., 2021).

This study is limited to changes observed in light attenuation and does not account for other coral reef threats such as increased seawater temperature and disease outbreaks (Rogers and Ramos-Scharrón, 2022). Events of resuspension, changes in precipitation, and tropical cyclone frequency and intensity cannot be eliminated as sources of change in Kd₄₉₀ values. Furthermore, Guánica’s water discharge is managed and not direct and the primary discharge occurs in Guánica Bay and then flushes out to the south where our stations were located. Additionally, a recent study found that watersheds located east of Guánica are a predominant source of sediments to Guánica’s reefs, suggesting that management efforts should also be implemented in these watersheds (Takesue et al., 2021). In addition, the impact of the hurricanes on water clarity probably outweighed the efforts made in the watershed to improve water quality. It has also been of discussion that ecosystems follow complicated responses after restoration efforts because there can be a lag time between efforts and evidence of recovery as well as other environmental stressors, e.g., climate change, affecting water quality, resulting in shifting baselines (Jeppesen et al., 2005; Duarte et al., 2009).

This work is a preliminary assessment of the management actions implemented in the Guánica Bay Watershed by 2022, in which the two most important recommendations by the management plan, i.e., restoration of the Guánica Lagoon and construction of treatment wetlands, had not been completed (Viqueira Rios, 2021). Future assessments should be done after the implementation of these two management actions and a more comprehensive analysis that includes other remotely sensed water quality parameters and in situ measurements is recommended.

This study is the first attempt, to our knowledge, to evaluate the Guánica Bay Watershed Management Plan in terms of water quality. Results from this work could potentially contribute to determining whether current strategies in the Guánica Bay watershed should be continued, stopped, or modified. Other contributions could include the implementation of management actions in other watersheds with anthropogenic impacts like those of the Guánica Bay Watershed or the refinement of future watershed management plans developed to improve water quality in Guánica Bay and other areas around the world.