Impact of Siltation on Water Quality and Mangrove Diversity in Mining-Affected Hinatuan Island, Philippine
Abstract
Mangroves are important ecosystems that provide protection of the coastal areas by trapping sediments from inland activities, particularly mining. However, they are facing various challenges from anthropogenic activities resulting in sedimentation and siltation. This study aimed to demonstrate the extent of siltation through visual satellite imageries, compare the water physico-chemical characteristics, and diversity of two mangrove stands situated in Hinatuan Island (adjacent to a mining site) and Barangay Cagutsan (distant from the mining site) of Surigao del Norte, respectively. Satellite images were generated to show the extent of siltation and inventory of mangrove species was done using a strip split-plot design. The established landward and seaward zones of each site showed no significant difference (p>0.05) in the mean water physico-chemical measurements, abundance and the diversity indices. However, significant differences were observed in the mean water physico-chemical parameters (temperature, surface water pH, dissolved oxygen and salinity) between sites (p<0.05) except for total suspended solids and total dissolved solids (p>0.05). A total of six (6) true mangrove species namely, Avicennia rumphiana, Sonneratia alba, Aegiceras corniculatum, Bruguiera cylindrica, Rhizophora apiculata and Rhizophora mucronata were found, with Rhizophora apiculata as the most abundant and Avicennia rumphiana categorized as vulnerable. The diversity indexes of the two mangrove stands, which are namely; H’=0.83 and H’=0.62 in Barangay Cagutsan and HInatuan Island, respectively are not significantly different from each other (p>0.05). In this study, mangrove stand density, and the length of mangrove forest cover needs to be increased to further serve its purpose to trapped sediments from inland sources.
Introduction
The Philippines is considered as one of the top 15 most mangrove-rich countries in the world holding more than 50% of the world’s mangrove species (Primavera et al., 2004) due to its diverse habitats and high rates of endemism (Long and Giri, 2011). Mangrove forest contributes to the diversity, richness, and productivity of the coastal ecosystems. It is found along the interface between land and sea and can exists under a wide range of harsh environmental conditions (Abino et al., 2014). Mangroves provide various ecosystem services such as habitats for coastal fauna (Li et al., 2017), natural water breaks from waves and tsunamis (Alongi, 2002), coastal erosion (Naylor et al., 2002), and function as a trap for sediments from the inland sources (Noor et al., 2015).
Despite the significant role of mangroves, a sharp decline in the mangrove forest was observed (Primavera, 2000; Chapman, 1976). This decline was attributed to overexploitation by coastal dwellers, conversion to agriculture, salt ponds, industry, and settlements (Primavera, 2000). Mining is also one of the industries that contributed to these disturbances (Sari and Rosalina, 2016). Two of the most abundant mineral resources in the Philippines are nickel and iron. These deposits occur in the form of nickeliferous laterite which are mostly found along the eastern portion of the country and northeastern part of Mindanao, specifically, the Surigao provinces. Since the 1970’s the Philippines have the biggest nickel reserves in the world and continues to be one of the biggest suppliers of nickel and iron. Nickel lateritic soils contain very fine particles. Mining operations induce nickel laterite run-off, contribute to the buildup of nickel laterite particles (Apodaca et al., 2018), and cause siltation in the coastal areas (Modassir, 1994; Al-Usmani, 2011). Extraction of these resources results in massive erosion and the disturbed soils are being carried and transported by run-off water and deposited to the coastlines of the mining areas (Wickham et al., 2013; Baay and Tan, 2015).
Mangrove ecosystems serve as a trap for the sediments from landward sources (Lee et al., 2014) adding to the organic-rich load in the coastlines ranging from 10 cm to over 3 m in depth (Pendleton et al., 2012). Its ability to trap sediments slows down the decay process resulting in much massive amounts of stored carbon (Alongi and Mukhopadhyay, 2015). However, excessive sediment accumulation in mangrove areas may lead to stunted growth (Ellison, 1999), seedling mortality, and even death of mature tree or of an entire stand (Ochieng et al., 2013).
Another form of Mangrove responses to sediments can also result in zonal distribution (Nguyen et al., 2020). The structural and distribution patterns of mangroves have been linked with the genetic structure (Geng et al., 2021) and physiological tolerance such as water logging, salinity, nutrients, siltation, and sediment characteristics (Sherman et al., 2000; Clarke and Allaway, 1993). This tolerance may also include the interactions within and between species which may provide internal and complementary protection when various natural stressors are present (Whittaker, 1974). Knowing the species composition and structure is important to assess the quality of the mangrove forest prior to any conservation and management initiatives.
The strategies developed to trap these sediments and lessen the damage in the coastal ecosystem of Hinatuan Island is the use of siltation ponds, sandbags, boom traps, and mangrove reforestation. Reforestation efforts provide an important buffer from inland pollution like toxic waste and trap silt. Likewise, prevent coastal erosion and filters solid wastes from the sea.
The purpose of the study was to compare the extent of siltation using satellite imageries, physico-chemical water properties, and mangrove structure and composition in the sites near (Hinatuan Island) and far from mining operations (Brgy. Cagutsan).
Source : Effects of siltation on water quality, community structure and diversity of mangrove species of a mining operation in Hinatuan Island, Taganaan, Surigao del Norte, Philippines
