Pemodelan Dispersi Air Bahang (Thermal Plume) dan Respons Struktur Komunitas Plankton di Perairan Pesisir Lombok

Abstract

Pembuangan air pendingin dari pembangkit listrik pesisir dapat membentuk thermal plume yang mengubah kondisi termal perairan penerima. Perubahan suhu, meskipun bersifat lokal, berpotensi memengaruhi kualitas perairan dan struktur komunitas plankton sebagai komponen dasar jejaring makanan pelagik. Penelitian ini bertujuan menganalisis pola dispersi thermal plume, mengevaluasi struktur komunitas plankton, serta menilai keterkaitan eksploratif antara sebaran panas, kualitas perairan, dan komunitas plankton di perairan pesisir Lombok. Penelitian menggunakan data sekunder hasil pemantauan lingkungan Mei 2025 dan pemodelan numerik dua dimensi menggunakan MIKE 21. Data meliputi batimetri, pasang surut, arus, angin, kualitas perairan, dan komposisi plankton pada empat titik pantau. Simulasi dilakukan pada 1–15 Mei 2025 dengan debit outlet 4,32 m³/s, suhu ambien 30 °C, dan suhu outlet 36 °C. Struktur komunitas plankton dianalisis menggunakan indeks kelimpahan, keanekaragaman Shannon-Wiener, keseragaman, dan dominansi, sedangkan hubungan antarparameter dianalisis menggunakan korelasi Spearman dan Analisis Korespondensi. Hasil penelitian menunjukkan bahwa pasang surut bertipe campuran condong harian ganda (Formzahl 0,327) dengan tunggang pasut sekitar 2,28 m. Arus permukaan berkisar 0,1–0,3 m/s dan menjadi mekanisme utama adveksi panas. Thermal plume memanjang sejajar garis pantai, dengan suhu tertinggi terkonsentrasi di sekitar outfall dan menurun ke arah perairan terbuka. Kelas suhu 30–30,5 °C mendominasi luasan model, sedangkan area bersuhu >31,5 °C terbatas (<1 km²) di sekitar sumber buangan, menunjukkan bahwa zona panas tinggi bersifat lokal. Kualitas perairan menunjukkan suhu 30,1–31,6 °C, salinitas 30,1–30,2‰, pH 7,62–8,27, DO 4–5 mg/L, BOD5 3–6 mg/L, TSS 2,5–6 mg/L, amonia 0,02–0,20 mg/L, nitrat 0,02–0,38 mg/L, dan ortofosfat 0,01–0,04 mg/L. Struktur komunitas plankton relatif stabil (H' 1,93–2,21; E 0,86–0,96; D 0,12–0,17) dan didominasi Bacillariophyceae, terutama Chaetoceros sp., sedangkan TP3 berasosiasi dengan diatom yang responsif terhadap nutrien. Analisis Spearman menunjukkan bahwa suhu tidak berkorelasi langsung dengan keanekaragaman, tetapi memiliki kecenderungan hubungan kuat dengan keseragaman. Mengingat jumlah titik pantau terbatas, hasil statistik diperlakukan sebagai indikasi ekologis awal. Secara keseluruhan, hasil penelitian menunjukkan bahwa thermal plume masih berdampak lokal dan belum menjadi faktor dominan yang mengubah struktur komunitas plankton. Variasi komunitas plankton lebih dipengaruhi oleh interaksi kondisi termal, nutrien, beban organik, dan dinamika hidrodinamika dibandingkan oleh peningkatan suhu semata.

Cooling-water discharge from coastal power plants may generate thermal plumes that alter the thermal condition of receiving waters. Although spatially localized, these temperature anomalies may influence water quality and plankton communities as the foundation of the pelagic food web. This study aimed to analyze the dispersion pattern of a thermal plume, evaluate plankton community structure, and assess the exploratory relationship among heat dispersion, water quality, and plankton communities in the coastal waters of Lombok. The study used secondary environmental monitoring data collected in May 2025 and two-dimensional numerical modeling using MIKE 21. Simulations were conducted from 1 to 15 May 2025 using a discharge rate of 4,32 m³/s, an ambient seawater temperature of 30 °C, and an outlet temperature of 36 °C. Plankton community structure was evaluated using abundance, Shannon–Wiener diversity, evenness, and dominance indices, while relationships among variables were explored using Spearman rank correlation and Correspondence Analysis. The results showed that the study area was characterized by mixed mainly semidiurnal tides, with a Formzahl value of 0,327 and a tidal range of approximately 2,28 m. Surface current velocity ranged from 0,1 to 0,3 m/s and served as the primary mechanism for heat advection. The thermal plume extended parallel to the coastline, with the highest temperatures concentrated near the outfall and decreasing toward offshore waters. The 30–30,5 °C temperature class dominated the modeled area, whereas areas warmer than 31,5 °C were generally <1 km² and confined to the vicinity of the discharge point, indicating that elevated temperature zones remained localized. Water quality ranged from 30,1–31,6 °C for temperature, 30,1–30,2‰ for salinity, 7,62–8,27 for pH, 4–5 mg/L for dissolved oxygen, 3–6 mg/L for BOD5, 2,5–6 mg/L for TSS, 0,02–0,20 mg/L for ammonia, 0,02–0,38 mg/L for nitrate, and 0,01–0,04 mg/L for orthophosphate. The plankton community remained relatively stable (Shannon–Wiener diversity 1,93–2,21, evenness 0,86–0,96, and dominance 0,12–0,17) and was dominated by Bacillariophyceae, particularly Chaetoceros sp., while TP3 was associated with nutrient-responsive diatom taxa. Spearman analysis indicated no direct relationship between temperature and diversity but suggested a strong tendency toward an association with evenness. Given the limited number of monitoring stations, these statistical results should be interpreted as preliminary ecological evidence. Overall, the findings indicate that the thermal plume remained localized and was not the dominant factor shaping plankton community structure during the study period. Instead, plankton community variation was more strongly influenced by the combined effects of thermal conditions, nutrient availability, organic loading, and local hydrodynamic processes than by temperature alone.