Assessing temporal dynamics on pelagic Sargassum influx and its relationship with water quality parameters in the Mexican Caribbean

Highlights

• Higher Sargassum coverage and turbidity during winter months.

• High correlation between Sargassum coverage and turbidity in near shore waters.

• Satelital estimation of turbidity is a way to detect Sargassum in coastal zones.

• Sargassum accumulated near-shore waters affects the water quality of this zone.

Abstract

Since 2011, massive inflows of pelagic Sargassum have been recorded in the Caribbean region, which has affected coastal ecosystems, mainly water quality and the flora and fauna that inhabit these areas. The aim of this study is to evaluate the temporal dynamics of Sargassum arrival and its relationship on water quality in three sites of the Mexican Caribbean (Mahahual, Xahuayxol and Xcalak). We used remote sensing tools to estimate Sargassum coverage and turbidity to know if these parameters vary throughout the year and are related. We measured in situ water quality parameters to identify variations, depending on the Sargassum quantity and leachate concentration. Our results of 26 Landsat 8-OLI images show higher Sargassum coverage and turbidity during the winter months and a high relationship between Sargassum coverage and turbidity in areas near the coast. Additionally, turbidity calculated from multispectral bands complemented satellite detection of Sargassum, as high turbidity values were identified in near-shore areas where satellite images could not detect the alga. Finally, water quality parameters showed an increase in ammonium ${}_{}$ concentration and decreased pH and oxygen levels in the areas with higher Sargassum accumulation and leachate concentration. This study provides a better understanding about temporal dynamics of Sargassum influx in the southern Mexican Caribbean and its relationship with water turbidity in order to develop strategies, management plans and harvesting policies that could prevent huge masses of Sargassum reaching the coast.

Introduction

Pelagic Sargassum has a significant ecological importance, in the “Sargassum Sea” located in the North Atlantic (Gower and King, 2011, Guiry and Guiry, 2011, Laffoley et al., 2011), functioning as habitat for endemic species, supporting local fisheries and providing refuge for various species of vertebrates and invertebrates (Butler et al., 1983, Coston-Clements et al., 1991, Pendleton et al., 2014, Hu et al., 2016). However, since 2011 the Caribbean coast has experienced massive influxes of Sargassum (Smetacek and Zingone, 2013, Franks et al., 2016) that comes from the North Equatorial Recirculation Region (NERR, located off the coasts of Brazil and Africa) through winds, currents, and tides (Johns et al., 2020). In this region, Sargassum blooms due to increased surface temperature, the contribution of nutrients from the Amazon, Orinoco, and Congo rivers, the upwellings of the west coast of Africa and the Sahara dust (Johnson et al., 2013, Westrich et al., 2016, Wang et al., 2019, Chávez et al., 2020, Johns et al., 2020, Lapointe et al., 2021).

Sargassum that accumulates in near-shore areas has negative effects on the water quality due to its decomposition, causing a depletion of dissolved oxygen, a reduction of light and pH, an increase of turbidity and temperature and the production of compounds with toxic potential such as ammonium (NH₄) and hydrogen sulfide (H${}_2$S) (van Tussenbroek et al., 2017, Cabanillas-Terán et al., 2019, Rodríguez-Martínez et al., 2019, Antonio-Martínez et al., 2020, Chávez et al., 2020). Likewise, it has been reported that pelagic Sargassum contains in its tissue heavy metals such as: As, Ca, Cl, K, Mn, P, Rb, S, Si, Sr, Th, and U, with arsenic (which can be toxic) in high concentrations (24–172 ppm DW) (Rodríguez-Martínez et al., 2020). Those damages to the water quality caused mortality on seagrasses, coral reefs and the fauna that inhabit mainly in reef lagoons (van Tussenbroek et al., 2017, Rodríguez-Martínez et al., 2019). Also, the presence of this alga generated an alteration on the trophic ecology of the sea urchin Diadema antillarum (Cabanillas-Terán et al., 2019) and the swimming behavior of the larvae of the coral Acropora palmata (Antonio-Martínez et al., 2020).

Remote sensing tools applied to satellite images have been useful to quantify the biomass and cover of Sargassum that reaches the Caribbean, Central Atlantic, Gulf of Mexico and the Mexican Caribbean (Wang and Hu, 2016, Cuevas et al., 2018, Gower and King, 2019, Wang et al., 2019, Chávez et al., 2020, Trinanes et al., 2021). When analyzing large extensions MODIS (Moderate Resolution Imaging Spectroradiometer) and OLCI (Ocean and Land Color Imager) images are used due to their low spatial resolution covering areas of kilometers (Wang and Hu, 2016, Gower and King, 2019, Wang et al., 2019). These kinds of images detected that the amount of Sargassum arriving to the Caribbean and Central Atlantic have a year-to-year increase, reporting a coverage of 199.5 km² in 2011 with a five-fold increase in 2015 (956.2 km²) (Wang and Hu, 2016). In the Caribbean and Equatorial Atlantic Sargassum arrivals of 5 million tons in 2011 and 15 million tons in 2017 were estimated (Gower and King, 2019). However, during 2018, one of the largest influxes of this alga was recorded, from Africa to the Caribbean; with a biomass of 20 million tons covering an area of 6000 km² (Wang et al., 2019). On a regional scale Landsat 8 images with high spatial resolution and MODIS images have been used (Cuevas et al., 2018, Chávez et al., 2020, Trinanes et al., 2021). For example, in the northeast of the Yucatan peninsula, temporary variations in Sargassum cover were detected, with the highest peaks observed during December 2014 (4000 ha) and summer 2015 (6000 ha) (Cuevas et al., 2018). In the Mexican Caribbean, the most important peaks were reported during September 2018 (22,900 ha) and April 2019 (19,000 ha) (Chávez et al., 2020). Similarly, during 2019–2020, Cancun, Mexico was exposed to a high inundation of Sargassum (for 8 weeks) mainly in the summer months (Trinanes et al., 2021). Nevertheless, the use of satellite imagery and crowdsourcing in conjunction with deep learning tools has been effective in detecting Sargassum off the coasts of Quintana Roo and Yucatan with high accuracy (90%) (Arellano-Verdejo et al., 2019, Arellano-Verdejo and Lazcano-Hernández, 2021).

Interestingly, no previous studies have evaluated the effect of Sargassum on turbidity, even though it has been observed that its decomposition generates a loss in the transparency of the water (van Tussenbroek et al., 2017). Turbidity can be calculated from the combination of multispectral bands in satellite images, without the need of in situ measurements. In Cam Rahn Bay and the Black Sea, reflectance data from the red and infrared bands were used to calculate semi-quantitative indexes of turbidity and was supported by in situ measurements. In both sites the turbidity was observed to increase, in the first one due to the rainy season, while in the second because of the discharge and flow from delta of Danube River (Constantin et al., 2017, Quang et al., 2017). These studies suggest that the turbidity data calculated from reflectance values are reliable, since when making a comparison between these and the in situ measurements, there was a correlation of 0.84–0.96.

Although there are studies related to Sargassum detection and its interannual​ changes in almost the entire Mexican Caribbean (Chávez et al., 2020), what is lacking is focus on Sargassum temporal dynamics and changes in water turbidity in the southern part of the Mexican Caribbean.

In this work we hypothesized that Sargassum arrival and turbidity in the southern Mexican Caribbean vary throughout the year and that these parameters have a positive relationship, likewise turbidity calculated through multispectral bands is useful to indicate the presence of Sargassum, in near-shore areas. In addition, we hypothesized that water quality parameters have a variation depending on the Sargassum quantity and leachate concentration in near-shore areas. The aim of this study was to evaluate the seasonal dynamics of Sargassum arrival and its effect on water quality in three sites of the Mexican Caribbean (Mahahual, Xahuayxol and Xcalak) during an event with a high influx of the alga that occurred in 2018–2019 (Chávez et al., 2020). The temporal changes in the Sargassum cover and turbidity were detected and accompanied by in situ measurements of water quality parameters.