Scope for growth of Mar Menor flat oysters (Ostrea edulis) under eutrophic conditions: Implications for future bioremediation actions.
Supervisor: Marina Albentosa (IEO)
The Mar Menor lagoon and its adjacent watershed are located in the Murcia region (SE Spain). It is one of the largest coastal lagoons in Europe, and one of the most important Spanish wetlands. Tourism and agricultural intensification released large amounts of nitrogen and phosphorus nutrients into the lagoon. This situation triggered a phytoplankton bloom in 2015 that turned the clear and transparent water turbid and greenish, and about 85% of benthic macrophytes was completely lost. Since then, there have been a series of catastrophic events that have led to the eutrophication of the lagoon, one of them being a severe stormy meteorological phenomenon (DANA) that transformed the depths into an anoxic layer causing the death of all organisms in the deepest areas of the lagoon. Nutrient bioextraction by flat oysters has been proposed as a nature-based solution for remediation of these eutrophication events and recover water quality. The flat oyster colonized the lagoon from the adjacent Mediterranean and reached a population of 135 million of individuals during the 80s and 90s as a consequence of the reduction of the lagoon salinity due to the increase of water exchange with the Mediterranean. This research aims to quantify the clearance potential of Mar Menor oysters and their scope for growth under the environmental scenarios observed during the phytoplankton blooms. For that, three particulate matter conditions (Low=1.3 mg L-1, Medium= 3.0 mg L-1 and High= 5.5 mg L-1) were replicated in the laboratory and the oyster feeding behaviour (clearance, ingestion rates and absorption efficiency) was studied using a flow-through system. Scope for growth (SFG) was estimated after integration of absorption and respiration rates into the energy balance equation. Results showed that there are two groups of oysters in the lagoon according to their clearance rate, the high feeders (HF) and low feeders (LF). In both cases, oyster food ingestion increased with the increase of food concentration although in different order of magnitude. Moreover, different physiological responses to diet quantity between both feeding behaviour were observed. Whereas, absorption efficiency remains constant for all food conditions in both feeding behaviour. As a consequence, SFG for the high feeding oysters (22.4, 73.2 and 81.1 J g-1 h-1) was higher compared to the low feeding ones (-10.1, 8.7 and 28.0 J g-1 h-1) and estimated growth rate increased with increase in food concentrations. Thus, the use of oysters for removal of phytoplankton during blooms in Mar Menor seems promising, especially with the high feeding oysters as they have the potential for converting excess nutrients in the lagoon into energy which are used for growth.