Sains Malaysiana 44(4)(2015): 545–558 Water Quality and Aquatic Insects Study at the Lower Kinabatangan River Catchment, Sabah: In Response to Weak La Niña Event (Kajian Kualiti Air dan Serangga Akuatik di Hilir Sungai Kinabatangan, Sabah: Respons kepada Fenomena La Niña Berskala Lemah) SAHANA HARUN*, SALMAN ABDO AL-SHAMI, RAMZAH DAMBUL, MARYATI MOHAMED & MOHD. HARUN ABDULLAH ABSTRACT A study on water quality and aquatic insects has been conducted at the Lower Kinabatangan River Catchment, Sabah, Malaysia. The quality of water surface and aquatic insects’ composition were studied in streams near to oil palm plantation (OP), secondary forest (SF) and oxbow lake (OB). The study also aims to identify the seasonal variation in the aquatic insects’ composition during the weak La Niña event. A total of 135 water samples and 1678 aquatic insect individuals (four orders and 14 families) were collected during ieldwork campaign that spanned over the inter-monsoonal period, wet and dry seasons between October 2004 and June 2005. OP has the highest abundance of aquatic insects particularly during the dry season. Biological indices showed that all stations were in moderate water category. The water quality index (WQI) was calculated and indicated that the quality of the river categorised into Class II. Discriminant analysis (DA) was employed to classify the independent variables into mutually-exclusive groups. Suspended sediment (SS) and chemical oxygen demand (COD) were high during the wet season. Suspended sediment was high in oxbow lake and could be a strong reason behind low abundance of aquatic insects. Precipitation anomalies were found to affect seasonal variations of water quality and aquatic insects at the Lower Kinabatangan River. Keywords: Aquatic insects; average score per taxa (ASPT); biological monitoring work party (BMWP); Lower Kinabatangan River catchment; water quality index (WQI) ABSTRAK Sebuah kajian kualiti air dan serangga akuatik telah dilakukan di Hilir Sg. Kinabatangan, Sabah, Malaysia. Kajian ini bertujuan untuk mengkaji kualiti air serta serangga akuatik dari tiga jenis guna tanah: ladang kelapa sawit (OP), hutan sekunder (SF) dan tasik ladam (OB); serta untuk mengenal pasti variasi musim sewaktu fenomena La Niña berskala lemah. Sebanyak 135 sampel air telah dianalisis dan 1678 individu serangga akuatik (empat order dan 14 famili) telah berjaya dikumpul sewaktu kerja lapangan dijalankan. Kajian dilaksanakan sewaktu musim perantara-monsun, musim hujan dan kering dari Oktober 2004 sehingga Jun 2005. OP didapati mempunyai taburan serangga akuatik yang tertinggi khususnya sewaktu musim kering. Indeks biologi menunjukkan semua stesen persampelan dikategorikan sebagai sederhana bersih. Pengiraan Indeks Kualiti Air juga turut dilakukan dan kualiti Hilir Sg Kinabatangan berada di dalam kelas II. Analisis diskriminan (DA) telah dilaksanakan bagi mengelaskan pemboleh ubah tak bersandar berdasarkan kumpulan yang sama. Sedimen terampai (SS) dan permintaan oksigen kimia (COD) mempunyai nilai yang tinggi sewaktu musim hujan. Nilai SS yang tinggi di kawasan tasik ladam berkemungkinan menjadi faktor penyumbang jumlah taburan serangga akuatik yang rendah di situ. Anomali hujan didapati mempengaruhi kualiti air dan taburan serangga akuatik mengikut variasi musim di Hilir Sg. Kinabatangan. Kata kunci: Hilir Sungai Kinabatangan; indeks kualiti air (WQI); purata skor setiap takson (ASPT); serangga akuatik; unit kerja kawalan biologi (BMWP) INTRODUCTION Tropical loodplains play a signiicant role in providing highly productive ecosystem services (Pettit et al., 2011), vital to a range of ecosystem processes (Hamilton 2002). They are mainly characterised by seasonal inundation (Hamilton 2002; Mayora et al. 2013), received high solar radiation (Saigusa et al. 2008) and rich in biodiversity (Bruford et al. 2010). However, deforestation, active land development, land conversion for mechanized agriculture such as oil palm plantations (Mercer et al. 2013), industrial and domestic waste discharge (Azrina et al. 2006) often contribute to river water quality deterioration in the tropical regions. In Malaysia, rainforest conversion to cocoa and oil palm plantations in Bukit Tekam, Pahang circa 1977 and 1986 (Douglas 1999) and Sabah (Jakobsen et al. 2007) was showed to increase surface run-off, suspended sediment loads and nutrients (i.e. nitrogen and phosphorus). Furthermore, Azrina et al. (2006) reported that oil palm plantations and sewage wastes are among the main contributors to pollutants in Langat River, while