The role of macroinvertebrates and fish in regulating the provision by macrophytes of refugia for zooplankton in a warm temperate shallow lake

GONZALEZ SAGRARIO M. A. AND E. BALSEIRO

FRESHWATER BIOLOGY (PRINT)

WILEY-BLACKWELL PUBLISHING, INC

Año: 2010 vol. 55 p. 2153 - 2166

0046-5070

1. The zooplankton often undergoes diel horizontal migration (DHM) from the open waterto the littoral of shallow lakes, thus avoiding predators in the former. This behaviour hasfunctional impacts within the lake, as it enhances zooplankton survival, increases theircontrol of phytoplankton and tends to stabilise the clear water state. However, most of theevidence supporting this migration pattern comes from cold north temperate lakes, andmore evidence from tropical and subtropical areas, as well as from southern temperateareas, is needed.2. We conducted a field study of the diel horizontal and vertical migration of zooplankton,and the horizontal distribution of potential predatory macroinvertebrates and fish, overtwo consecutive days in the summer in a temperate lake in the southern hemisphere. Wetook zooplankton samples at two depths, at three sampling stations (inside beds of aquaticmacrophytes, at their edge and in open water) along three transects running from thecentre of a bed of Ceratophyllum demersum to open water. At each sampling station, we alsotook samples of macroinvertebrates and fish and measured physical and chemicalenvironmental variables.3. Zooplankton (pelagic cladocerans, calanoid copepods and rotifers) avoided theshore, probably because of the greater risk from predators there. Larger and morevulnerable cladocerans, such as Diaphanosoma brachyurum and Moina micrura, were two tofour times more abundant in open water than at the edge of or inside beds of macrophytes,respectively, by both day and night. Less vulnerable zooplankton [i.e. of medium body size(Ceriodaphnia dubia) or with the ability to swim fast (calanoid copepods)] were distributedevenly between open water and the edge of the plant beds. Small zooplankton, Bosminahuaronensis and pelagic rotifers, showed an even distribution among the three samplingstations. Accordingly, no DHM of zooplankton occurred, although larger organismsmigrated vertically inside C. demersum stands.4. Macrophytes contained high densities of predatory macroinvertebrates and fish. Thepredator assemblage, composed of large-bodied macroinvertebrates (including odonatesand shrimps) and small littoral fish, was permanently associated with submergedmacrophytes. None of these groups moved outside the plant beds or changed theirpopulation structure (fish) over the diel cycle.5. Submerged macrophyte beds do not represent a refuge for zooplankton in lakeswhere predators are numerous among the plants, implying a weaker top-down control of phytoplankton biomass by zooplankton and, consequently, a more turbid lake. Theeffectiveness of macrophytes as a refuge for zooplankton depends on the associatedassemblage of predatory macroinvertebrates and fish among the plants.