Phytoplankton Community Structure and Ecological Status of Lake Limboto, Gorontalo

Abstract

Anthropogenic activities around Lake Limboto have triggered a decline in water quality and a critical lake status. This study aimed to identify the plankton community structure as a bioindicator of the current ecological conditions of Lake Limboto. Water samples were collected using purposive sampling through horizontal towing with a plankton net at a single observation station (coordinates 0.597094 N and 123.005491 E). Quantitative identification applied a zigzag counting technique inside a Sedgwick-Rafter Counting Chamber (SRC) under a digital microscope at 100× magnification. Scanning results on 96 clearly identified grid squares showed the presence of 43 phytoplankton taxa (genera) from 3 classes, with a total abundance of 111 individuals. The class Bacillariophyceae (diatoms) harbored the highest taxonomic richness with 16 genera, dominated by Aulacoseira sp., reflecting the dynamic, shallow, and actively circulating hydrological characteristics of the lake. Conversely, in terms of quantity, the largest number of individuals was collected by the class Cyanophyceae (blue-green algae), reaching 38% of the total sample (42 individuals). The dominance of blue-green algae genera such as Oscillatoria sp. and Lyngbya africana serves as a strong biological indicator of acute nutrient enrichment (eutrophication) caused by domestic and agricultural waste. The study concludes that the phytoplankton community structure of Lake Limboto is currently in a critically disturbed status with a high risk of Harmful Algal Blooms (HABs). Further research is recommended to measure water volume precisely to determine absolute abundance values and diversity indices