The Littoral Zone

Abstract

The littoral zone represents an interface between the land of the drainage basin and the open water of the lake, reservoir, or stream (Wetzel, 1990). The extent of the development of the littoral zone is highly variable and depends on both the geomorphology of the basin and the rate of sedimentation that has occurred since its formation.

The emergent macrophytes of littoral wetlands and the shore regions of lakes are among the most productive habitats of the biosphere [cf., Whittaker and Likens (1975)]. Productivity decreases with increasing distance from the lake, as one passes into terrestrial conditions, and decreases as one passes toward the lake through zones of floating-leaved macrophytes, submersed macrophytes, and phytoplankton. For a number of physiological reasons [cf., Wetzel (1983a, 1990)], the surface area of submersed vegetation is much greater than that for other types of littoral macrophytes. These surfaces are colonized densely by epiphytic algae which can contribute much to the total productivity of the lake. In addition, algae can grow profusely when attached to sediments or in loosely attached clumps among the littoral macrovegetation. All of this primary productivity in the littoral zone results in a large input of dissolved and particulate organic detritus to the lake system. Particulate organic detritus is deposited largely in the sediments of the littoral zone; a portion is transported to sediments of deeper water where degradation continues.

The high productivity of living organic matter and the detrital accumulations in sediments of the littoral zone provide an abundance of habitats and food resources for Zooplankton, invertebrates, and vertebrates such as fishes, amphibians, birds, and muskrats. Many of the littoral animals are specialized in their adaptation to conditions in the littoral zone (cf. many articles in Jeppesen et al., 1997). As with the littoral algae, the diversity of littoral fauna is very high.

The spatial conditions within the littoral zone are complex. Extreme heterogeneity exists in the distributions and productivities of the macro- and microflora along the gradient from the emergent wetland vegetation to submersed conditions. The heterogeneity and abundance of microhabitats make quantitative sampling difficult. As a result, knowledge of the metabolism of the flora and fauna is poor in comparison with what is known of the pelagic biota. Nonetheless, it now is known that the littoral biota are a major component of the metabolism of the whole ecosystem in most lakes of the world (Wetzel, 1979, 1990). An understanding of the physiological relationship of the littoral biota to the ecology of the lake presents an imposing challenge to ecologists when evaluating the role of this interface zone in the metabolism of the whole ecosystem.

The following exercise is designed to introduce some aspects of the structure and function of littoral biota. The limitations of the various analytical techniques will become readily apparent. The problems of extreme heterogeneity in the distribution and metabolism of organisms also will be obvious. However, the interrelationships of the littoral organisms to each other, to environmental gradients, and to the rest of the lake system can be seen in overview.