Do macrophytes provide adequate refuge for zooplankton against fish in Mediterranean brackish lakes?

BRUCET, S., BALAYLA, D., BOIX, D., JENSEN, E., GASCÓN, S., QUINTANA, X., TROCHINE, C. & JEPPESEN, E.

Punta del Este

Conferencia; Shallow lakes Conference 2008; 2008

Macrophytes play an important role in temperate freshwater shallow lakes. They act as daytime refuge for zooplankton against fish predators, thus promoting cascading effects leading to increased water transparency (Burks et al., 2002). However, recent experiments have shown that in tropical lakes and in brackish waters, the role of macrophytes on maintaining water clarity is significantly reduced (Meerhoff et al., 2007, Jeppesen et al., 2007). Both warm and brackish lakes hold a fish community characterized by the dominance of small sizes and a high degree of omnivorous feeding. In warm freshwater lakes, small fish aggregate in high numbers among vegetation and exert a high predation pressure on zooplankton, which can lead to a decrease in water transparency. Whether the same holds true for brackish lakes is not clear.The main objective of our study was to assess whether macrophytes provide proper refuge to zooplankton against fish in Mediterranean brackish lakes. We also examined the effects of salinity on the zooplankton community structure. Artificial plants mimicking submerged plants were introduced in four Mediterranean brackish lakes located in Catalonia (NE Spain) with salinities ranging from approx. freshwater (0.4‰) to oligohaline water (2.2‰). Zooplankton, fish and macroinvertebrates were sampled during the day and night periods within the artificial plants and in open water. Results show that the fish community in Mediterranean brackish lakes is similar to that in tropical lakes, being composed mainly of small sizes (Gambussia holbrooki, Atherina boyeri).  Fish aggregated in high numbers among the plants, especially during night, and were then significantly less abundant in the open water. Rotifers were the dominant zooplankton group in all lakes while copepods populations were mainly composed of nauplii, probably as a response to high fish predation. Large-bodied pelagic cladocerans were absent. Diel migration patterns of zooplankton in these lakes are discussed related to changes in fish predation pressure. References Burks, R.L. et al., 2002. Diel horizontal migration of zooplankton: costs and benefits of inhabiting the littoral. FWB 47: 343-365Jeppesen, E.  et al., 2007. Salinity Induced Regime Shift in Shallow Brackish Lagoons. Ecosystems 10: 47-57Meerhoff, M. et al., 2007. Effects of habitat complexity on community structure and predator avoidance behaviour of littoral zooplankton in temperate versus subtropical shallow lakes. FWB 52: 1009-1021