Monitoring of Water Quality and Microalgae Species Composition of Penaeus monodon Ponds in Pulau Pinang, Malaysia

Abstract

Many reports have revealed that the abundance of microalgae in shrimp ponds vary with changes in environmental factors such as light, temperature, pH, salinity and nutrient level throughout a shrimp culture period. In this study, shrimp cultivation period was divided into three stages (initial = week 0–5, mid = week 6–10 and final = week 11–15). Physical and chemical parameters throughout the cultivation period were studied and species composition of microalgae was monitored. Physical parameters were found to fluctuate widely with light intensity ranging between 182.23–1278 ?mol photon m^–2s^–1, temperature between 29.56ºC –31.59ºC, dissolved oxygen (DO) between 4.56–8.21 mg/l, pH between 7.65–8.49 and salinity between 20‰–30‰. Ammonium (NH₄⁺-N), nitrite (NO₂^–-N), nitrate (NO₃^–-N), and orthophosphate (PO₄^3–-P) concentrations in the pond at all cultivation stages ranged from 0.017 to 0.38 mg/l, 0.24 to 2.12 mg/l, 0.06 to 0.98 mg/l and 0.16 to 1.93 mg/l respectively. Statistical test (ANOVA) showed that there were no significant difference (p<0.05) in nutrients concentrations among the cultivation stages. All nutrients concentrations however were still in the tolerable level and safe for shrimp culture. The chlorophyll a contents were found to range from 5.03±2.17 to 32.61±0.35 ?g/l throughout the cultivation period. A total of 19 microalgae species were found in the shrimp pond, with diatoms contributing up to 72% of the species followed by Chlorophyta (11%) and Cyanophyta (11%). However, weekly species abundance varied through the study period. At the initial stage, when there were no shrimps in the pond, Anabaena spp. and Oscillatoria spp. (Cyanophyta) were the dominant species, followed by Chlorella sp. and Dunaliella sp. (Chlorophyta). When shrimps were introduced into the pond, Amphora sp., Navicula sp. Gyrosigma sp. and Nitzschia sp. (diatoms) started to exist. At the middle and towards the final stage of the shrimp culture period diatoms were the dominant species. The Chlorophyta (Chlorella sp.) domination took place only twice, which was at week 2 and 13. The absence of some of the coastal water microalgae species in the shrimp pond was most likely due to the fact that they could not tolerate the physicochemical factors of harsh environment. In this study, Cylindrotheca closterium was regarded as the most tolerant species among the microalgae due to its ability to exist for 6 weeks out of the 15 weeks of cultivation.

Laporan terdahulu telah mendedahkan bahawa kelimpahan mikroalga di dalam kolam udang adalah berbeza-beza dengan perubahan faktor persekitaran seperti cahaya, suhu, pH dan status nutrien sepanjang tempoh penternakan udang. Dalam kajian ini, tempoh penternakan udang dibahagikan kepada tiga fasa (awal = minggu 0–5, pertengahan = minggu 6–10 dan akhir = minggu 11–15). Faktor fizikal dan kimia sepanjang tempoh penternakan ini dikaji dan komposisi mikroalga dipantau. Faktor fizikal didapati berubah-ubah dengan nilai julat keamatan cahaya antara 182.23–1278 ?mol foton m^–2s^–1, suhu antara 29.56ºC–31.59ºC, oksigen terlarut antara 4.56–8.21 mg/l, pH antara 7.65–8.49 dan saliniti antara 20‰–30‰. Kepekatan amonium (NH₄⁺-N), nitrit (NO₂^–-N), nitrat (NO₃^–-N), dan ortofosfat (PO₄^3–-P) di kolam udang pada semua fasa penternakan setiap satunya mempunyai julat bacaan antara 0.017–0.38 mg/l, 0.24–2.12 mg/l, 0.06–0.98 mg/l dan 0.16–1.93 mg/l masing-masing. Ujian statistik (ANOVA) menunjukkan bahawa tidak terdapat perbezaan yang signifikan (p<0.05) terhadap kepekatan nutrien antara fasa-fasa penternakan tersebut. Walau bagaimanapun, semua kepekatan nutrien tersebut masih berada dalam julat bacaan yang selamat untuk perternakan udang. Kepekatan klorofil a menunjukkan julat bacaan antara 5.03±2.17 hingga 32.61±0.35 ?g/l sepanjang tempoh penternakan. Sebanyak 19 spesies mikroalga ditemui di kolam udang. Daripada jumlah itu, 72% adalah diatom, diikuti oleh Chlorophyta (11%) dan Cyanophyta (11%). Namun, kelimpahan spesies mikroalga adalah berbeza-beza pada setiap minggu sepanjang tempoh kajian. Pada fasa awal, ketika udang belum dimasukkan ke dalam kolam, Anabaena spp. dan Oscillatoria spp. (Cyanophyta) merupakan spesies yang dominan di dalam kolam, diikuti oleh Chlorella sp. dan Dunaliella sp. (Chlorophyta). Apabila udang dimasukkan ke dalam kolam, spesies diatom iaitu Amphora sp., Navicula sp., Gyrosigma sp. dan Nitzschia sp. didapati mula wujud di dalam kolam. Pada fasa pertengahan dan menuju fasa akhir perternakan udang, diatom didapati mendominasi kolam udang. Spesies Chlorophyta (Chlorella sp.) mendominasi sebanyak 2 kali, iaitu pada minggu ke 2 dan 13. Ketidakhadiran beberapa spesies mikroalga air masin di dalam kolam udang kemungkinan besar disebabkan tidak boleh bertoleransi terhadap perubahan fizikal dan kimia persekitaran kolam yang berubah secara mendadak. Dalam kajian ini, Cylindrotheca closterium merupakan spesies yang paling toleran di antara mikroalga yang terdapat di kolam udang kerana kemampuannya mendominasi selama 6 minggu daripada 15 minggu tempoh penternakan.