Sungai Kromong merupakan aliran sungai yang berada di kawasan wisata Pacet Mojokerto, tepatnya di Desa Pacet, Kecamatan Pacet, Kabupaten Mojokerto. Sungai kromong dimanfaatkan penduduk untuk keperluan mandi, mencuci, irigasi pertanian, obyek wisata rafting atau arum jeram, dan penambangan pasir. Berbagai aktivitas manusia dapat memungkinkan terjadinya perubahan faktor fisika kimia perairan yang secara langsung mempengaruhi keanekaragaman biota air terutama plankton. Untuk mengetahui kualitas air dapat menggunakan indeks keanekaragaman plankton. Tujuan penelitian ini yaitu untuk menentukan kualitas air di sungai Kromong Pacet, Mojokerto berdasarkan indeks keanekaragaman plankton. Jenis penelitian ini yaitu deskriptif observasi. Penelitian ini dilakukan di 3 stasiun yang berbeda, identifikasi plankton dilakukan di Laboratorium Ekologi, Universitas Negeri Surabaya. Perhitungan indeks keanekaragaman plankton menggunakan rumus indeks diversitas Shannon-weaner. Hubungan indeks keanekaragaman plankton dengan kualitas air dianalisis menggunakan uji korelasi Bivariate Pearson dengan SPSS. Hasil penelitian menunjukkan bahwa keanekaragaman plankton di sungai Kromong Pacet, Mojokerto terdiri dari 11 divisi, 22 famili, 24 genus, indeks keanekaragaman plankton sebesar 3,02479, termasuk kondisi belum tercemar. Kualitas air menunjukkan bahwa semua memenuhi standar baku mutu kecuali CO₂ dan BOD dengan rata-rata 36,44 mg/L dan 3,10 ppm. Suhu, kecepatan arus, pH, DO, dan BOD berkorelasi negatif, sedangkan parameter kecerahan, kedalaman, dan CO₂ berkorelasi positif dengan indeks keanekaragaman plankton.

Kata kunci: sungai kromong, kualitas air, indeks keanekaragaman plankton.

The kromong river was a river in the pacet mojokerto tourist area, precisely in pacet village, pacet district, mojokerto regency.  The kromong river was used by residents for bathing, washing, agricultural irrigation, rafting or rafting as a tourist attraction, and sand mining.  Various human activities could enable changes in the physical and chemical factors of waters which directly affect the diversity of aquatic biota, especially plankton.  To determine watered quality, you could used the plankton diversity index.  The aimed of this researched was to determine the watered quality in the kromong pacet river, mojokerto based on the plankton diversity index.  This typed of researched was descriptive observation.  This researched was carried out at 3 different stations, plankton identification was carried out at the ecology laboratory, surabaya state university.  Calculation of the plankton diversity index used the shannon-weaner diversity index formula.  The relationship between plankton diversity index and watered quality was analyzed used the bivariate pearson correlation test with spss.  The researched results showed that plankton diversity in the kromong pacet river, mojokerto consists of 11 divisions, 22 families, 24 genera, the plankton diversity index was 3. 02479, including unpolluted conditions.  Watered quality shows that everything met quality standards except co₂ and bod with an average of 36. 44 mg/l and 3. 10 ppm.  Temperature, current speed, ph, did, and bod were negatively correlated, while brightness, depth, and co₂ parameters were positively correlated with the plankton diversity index

Key word: kromong river, water quality, plankton diversity indeks.