<![CDATA[In order to determine the waters fertility and Phytoplankton abundance of Lake Toba in the Ajibata District, and to compare them with other sub-districts in Lake Toba, this study was conducted. The research was conducted by in-situ observation and sampling for laboratory analysis according to ISO 5667-6:2014 and ISO 5667-3:2012 procedures. This study revealed that the waters of Lake Toba in the District of Ajibata contain total dissolved inorganic nitrogen (nitrite-N, nitrate-N and NH4-N) dissolved 0.158–0.290 mg/l; and total phosphate 0.040–0.060 mg/l. This indicates that the waters are already included in fertile waters. These waters are inhabited by at least 24 types of phytoplankton which are included in 4 classes, namely Bacillariophyceae (11 sp.), Chlorophyceae (8 sp.), Cyanophyceae (3 sp.) and Dinophyceae (2 sp.). Even though they are fertile waters, when compared to the waters of Lake Toba in other sub-districts, the concentration of inorganic nitrogen and total phosphate is still much lower. This study also revealed that the presence of floating net cage (KJA) greatly affected the concentration of dissolved nitrogen and phosphate as well as the abundance of phytoplankton in the vicinity. This study did not show a positive correlation between nutrient concentration and abundance of phytoplankton where waters with higher nutrient concentrations did not always have a higher abundance of phytoplankton. Keywords: Toba, Water ferlitity, Phytoplankton, Abundance ABSTRAK Dalam rangka mengetahui kesuburan perairan dan komunitas fitoplankton Danau Toba di wilayah Kecamatan Ajibata, dan perbandingannya dengan wilayah kecamatan lain di Danau Toba penelitian ini dilakukan. Penelitian dilakukan dengan pengamatan in-situ dan pengambilan sampel untuk analisis laboratorium sesuai prosedur ISO 5667-6:2014 dan ISO 5667-3:2012. Penelitian ini mengungkapkan bahwa perairan Danau Toba di Wilayah Kecamatan Ajibata mengandung total nitrogen anorganik (nitrit-N, nitrat-N dan NH4-N) terlarut 0,158–0,290 mg/l; dan total fosfat 0,040–0,060 mg/l. Hal ini mengisyaratkan bahwa perairan tersebut sudah terasuk perairan yang subur. Perairan tersebut dihuni oleh paling sedikit 24 jenis fitoplankton yang termasuk dalam 4 kelas yakni Bacillariophyceae (11 sp.), Chlorophyceae (8 sp.), Cyanophyceae (3 sp.) dan Dinophyceae (2 sp.). Meskipun termasuk perairan yang subur namun jika dibandingkan dengan perairan Danau Toba di wilayah kecamatan lain konsentrasi nitrogen anorganik dan total fosfat tersebut masih jauh lebih kecil. Penelitian ini juga mengungkapkan bahwa keberadaan keramba jaring apung (KJA) sangat berpengaruh terhadap konsentrasi nitrogen dan fosfat terlarut serta kelimpahan fitoplankton disekitarnya. Penelitian ini tidak menunjukkan adanya korelasi positif antara konsentrasi hara dan kelimpahan fitoplankton dimana perairan dengan konsentrasi hara yang lebih tinggi tidak selalu memiliki kelimpahan fitoplankton yang lebih tinggi. Kata kunci: Toba, Kesuburan, Fitoplankton, Kelimpahan]]>
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