Abstract
Climate warming may lead to changes in the trophic structure and diversity of shallow lakes as a combined effect of increased temperature and salinity and likely increased strength of trophic interactions. We investigated the potential effects of temperature, salinity and fish on the plant-associated macroinvertebrate community by introducing artificial plants in eight comparable shallow brackish lakes located in two climatic regions of contrasting temperature: cold-temperate and Mediterranean. In both regions, lakes covered a salinity gradient from freshwater to oligohaline waters. We undertook day and night-time sampling of macroinvertebrates associated with the artificial plants and fish and free-swimming macroinvertebrate predators within artificial plants and in pelagic areas. Our results showed marked differences in the trophic structure between cold and warm shallow lakes. Plant-associated macroinvertebrates and free-swimming macroinvertebrate predators were more abundant and the communities richer in species in the cold compared to the warm climate, most probably as a result of differences in fish predation pressure. Submerged plants in warm brackish lakes did not seem to counteract the effect of fish predation on macroinvertebrates to the same extent as in temperate freshwater lakes, since small fish were abundant and tended to aggregate within the macrophytes. The richness and abundance of most plant-associated macroinvertebrate taxa decreased with salinity. Despite the lower densities of plant-associated macroinvertebrates in the Mediterranean lakes, periphyton biomass was lower than in cold temperate systems, a fact that was mainly attributed to grazing and disturbance by fish. Our results suggest that, if the current process of warming entails higher chances of shallow lakes becoming warmer and more saline, climatic change may result in a decrease in macroinvertebrate species richness and abundance in shallow lakes.
Highlights
Recent studies have shown cold and warm shallow freshwater and brackish lakes have substantially different trophic structure and dynamics [1,2,3]
We investigated the potential effects of contrasting temperature, salinity and fish on the plant-associated macroinvertebrate community structure by comparing similar shallow lakes located in two regions of contrasting temperature regime with salinities corresponding to ca. freshwater (0.3 salinity) to oligohaline waters (3.8 salinity)
Significant differences were found in the density and richness of the plant-associated macroinvertebrate communities in shallow brackish lakes between the cold temperate and the Mediterranean climatic regions
Summary
Recent studies have shown cold and warm shallow freshwater and brackish lakes have substantially different trophic structure and dynamics [1,2,3]. The higher densities of small fish, which tend to aggregate among the macrophytes [5], often exert a high predation pressure on the zooplankton and reduce grazing on phytoplankton, with implications for the clear water state of warm shallow lakes [7,8,9]. Fish may consume plant-associated macroinvertebrate grazers and indirectly enhance periphyton growth with cascading effects on plant growth and on water clarity, at least in temperate lakes [10]. Studies in subtropical shallow lakes [1] have shown lower taxon richness and significantly lower densities of plant-associated macroinvertebrates compared to similar temperate lakes, presumably as a result of the high densities of fish occurring within the macrophytes.
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