Mediterranean Lakes

The Mediterranean climate is usually characterized by dry arid summers and wet winters. Climate and hydrological regimes are likely to affect lake communities differently in warmer regions. Mediterranean

Temperate

Subtropical

Piscivor. fish Other fish Shrimps

Periphyton

250 200 150

150 200 250

Piscivor. fish

Piscivor. fish

Periphyton

Piscivor. fish

Clad. herb P

Omnivorous

Carn. fish shrimps

Clad. herb P

Inv. herb L

150 200 250

Piscivor. fish

Omnivorous

Carn. fish shrimps

Inv. herb L

Periphyton

Figure 8 Comparison of the structure of the main communities associated to aquatic plants in temperate (left) and subtropical (right) shallow lakes. Above and from top to bottom: density of potentially piscivorous fish, all other fish, shrimps, littoral (L) carnivorous (carn) invertebrates (inv), pelagic (P) carnivorous invertebrates, littoral herbivorous (herb) macroinvertebrates, pelagic herbivorous cladocerans (clad), and biomass of periphyton. Below: simplified scheme of trophic interactions among the same trophic groups. The densities in the subtropics are expressed in relation to those in the temperate lakes, and except fish the same taxa share the same trophic classification in both climate regions. Shrimps were absent in the temperate lakes. Data are sample means (±1 SE) of five lakes varying in size and nutrient state, but paired between climate regions in terms of phytoplankton biomass and physico-chemical and morphometric characteristics. Modified from Meerhoff etal. (2007). Global Change Biology 13: 1888-1897.

shallow water bodies have significantly higher annual water temperatures and seasonal fluctuations in water level, but smaller seasonal changes in light and temperature than temperate lakes. Many Mediterranean small lakes and ponds are temporary and often dry out in summer, with consequent fish kills. The thresholds of nutrient loading and in-lake TP required to avoid a turbid state in the permanent lakes seem lower than those described from temperate shallow lakes, and it is suggested to be <0.05 mg TP L_1. The high annual temperatures influence fish growth and enhance predation rates on zooplankton and macroinvertebrates. Owing to mild winter temperatures and lower density of large crustacean grazers, Mediterranean shallow lakes seem suitable habitats for cyanobacteria over a wide range of nutrient concentrations and water turbidity. On the other hand, the lower water levels in summer may improve the light conditions for submerged plant development, which can afterwards persist all the year round owing to the higher temperatures.

(Sub)Tropical Lakes

(Sub)tropical shallow systems share some characteristics with Mediterranean lakes. Some of the most important differences from temperate lakes involve the fish community: the species richness and proportion of omnivores are frequently high, while many species (both juveniles and adults) are strongly associated to the submerged plant habitats. Also density, but not necessarily biomass, is substantially higher (1-2 orders of magnitude) than in comparative temperate lakes, independently of trophic state (Figure 8). Owing to high predation by fish, the zooplankton communities in tropical and subtropical lakes are frequently dominated by small cladocerans (e.g., Diaphanosoma, Ceriodaphnia and Bosmina) and rotifers, and by copepodites among the copepods. The littoral macroinvertebrate communities also seem impoverished in terms of taxon richness and densities compared with similar temperate lakes. Although submerged and free-floating plants (e.g., Eichhornia crassipes, Salvinia spp., Pistia stratiotes) may occur all year-round in these systems, their effects on water transparency seem much weaker than in temperate lakes. In particular, submerged plants offer scarce refuge towards large-bodied zooplankton against fish predation. Consequently, the predator-avoidance behavior of zooplankton seems to differ, and diel vertical migration seems more frequent than in similar temperate shallow lakes, where diel horizontal migration prevails. The trophic web appear more truncated in the (sub)tropics, likely due to the omnivorous nature of most fish species and the structure of the predatory assemblages.

Shallow lakes and ponds located in warm climate regions therefore seem much more sensitive than cold lakes to external impacts, such as temperature increases (due to climate warming), water level changes (natural or anthropogenic), and nutrient loading increases (eutrophication).

See also: Abundance and Size Distribution of Lakes, Ponds and Impoundments; Aquatic Ecosystem Services; The Benthic Boundary Layer (in Rivers, Lakes, and Reservoirs); Benthic Invertebrate Fauna, Lakes and Reservoirs; Biomanipulation of Aquatic Ecosystems; Ecological Zonation in Lakes; Effects of Climate Change on Lakes; Lakes as Ecosystems; Mixing Dynamics in Lakes Across Climatic Zones; Origins of Types of Lake Basins; Trophic Dynamics in Aquatic Ecosystems.

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