Nitrogen limitation due to mariculture-derived phosphorus and the recurring incidence of algal blooms in Bolinao and Anda, Pangasinan, Philippines

posted 3 Apr 2017, 00:35 by Abigail Melendres
Charissa Ferrera, PhD
Tokyo Institute of Technology

The mariculture of milkfish (Chanos chanos) has been a major source of income in the shared semi-enclosed coastal waters of Bolinao and Anda, Pangasinan. After nearly two decades of operation, degradation of water quality has been observed from long-term monitoring of Bolinao waters. Although the number of fish farming structures in Bolinao has been regulated to within the allowable limit after the massive fish kill that occurred in 2002, conditions have remained eutrophic.  Results of seasonal surveys indicate that mariculture areas are nitrogen-limited as shown by nitrogen to phosphorus ratios (N/P) in the water that are consistently lower (~6.6) than the Redfield ratio (16).  Decomposition of wasted fish feeds containing more P than the nutritional requirement of fish, fish feces and excretions are the major sources of nutrients in the area.  At the start of the wet season, N-limitation is relieved by N-supply from the watershed, oftentimes resulting to algal blooms that lead to fish kills.  The increase in fish structures in the adjacent waters of Anda, as seen through satellite images, contributes to higher amounts of mariculture-derived nutrients brought in to Bolinao by residual current during the dry season.  Correspondingly, recurring blooms of dinoflagellates and diatoms that often proliferate in low N/P ratio conditions have been reported for Bolinao and Anda.  Understanding the environmental-socio-economic aspect is important in the recovery of a coastal area from the harmful effects of algal blooms arising from cultural eutrophication.