WATER QUALITY MONITORING OF

MAHAJAMBA BAY, MADAGASCAR

Aaron Anthony McNevin
Doctorate of Philosophy, December 17, 2004
(M.S., Auburn University, 2003)
(B.S., Mansfield University, 2000)

136 Typed Pages

Directed by Claude E. Boyd

Shrimp farming has been one of the most profitable and controversial forms of aquaculture in the world. One of the major issues of the controversy has been the eutrophication of coastal systems receiving shrimp farm effluent. Situated on the northwest coast of Madagascar is a 750-ha semi-intensive black tiger shrimp farm. This farm is located between the Masokoenja and Marovoaikely Rivers. The farm pumps water from the Marovoaikely River for the culture activity, and discharges 30 % of its effluents back into the Marovoaikely River. The main discharge river is the Masokoenja and it receives 70 % of farm effluent. Suggestions arose eluding to concerns about eutrophication of the estuary system and the prospect of an unsustainable shrimp production activity led to this study. A coastal water quality monitoring program was implemented to determine the trophic status of Mahajamba Bay, the impact of the shrimp farming on the bay, and a general understanding of hydrodynamics of the estuary. Water quality analyses included: total phosphorus, total nitrogen, nitrate-nitrogen, total ammonia-nitrogen, total suspended solids, 5-day biochemical oxygen demand (BOD5), and chlorophyll a. Samples were collected each month and alternated between spring and neap tidal cycles. In addition to water quality analyses, vertical profiles of salinity were made monthly at both high and low tides. Results showed relatively high total nitrogen, nitrate-nitrogen, and total phosphorus, but low chlorophyll a and BOD5 concentrations. Evaluation of the monitoring data by statistical procedures revealed the shrimp operation had little impact on the water quality in the estuary. Total suspended solids concentrations were high in the estuary as a result of intense erosion on watersheds that is a widespread phenomenon in Madagascar. Salinity profiles of the estuary showed the bay exchanging water with the Mozambique Channel at a relatively high rate. A southward long-shore current exists outside the mouth of Mahajamba Bay, and there is evidence to suggest once water is expelled from the bay, it is moved southward through the Mozambique Channel. Madagascar is subjected to intense precipitation in the wet season. The drainage basin of Mahajamba Bay was delineated on a map and its area determined by planimetry. Freshwater runoff into the bay (estimated as rainfall – potential evapotranspiration) was found to be 36.1 billion cubic meters each year, which equates to approximately 6.8 times the volume of the bay. The evidence found thus far suggests that the bay is showing signs of eutrophication with respect to nitrogen and phosphorus. However, there is no evidence of high primary production or high organic matter loading in the bay suggesting that the estuary is light-limited. The bay appears to be exchanging water with the Mozambique Channel at a rapid rate. Land-use patterns are not well documented in the bay drainage basin and monitoring should continue so as to provide long-term water quality data.