D L Strayer, Cary Institute of Ecosystem Studies, Millbrook, NY, USA
© 2009 Elsevier Inc. All rights reserved.
Benthos includes all animals that live in association with surfaces in lakes and reservoirs. This includes animals that live in and on sediments of all kinds (mud, sand, stones), as well as animals that live in, on, or around aquatic plants or debris. Animals large enough to be retained on a coarse sieve (usually 0.5mm mesh) are called macrobenthos, those that pass through a coarse sieve but are retained on a fine sieve (usually ^0.05 mm) are called meiobenthos, and those that pass through even fine sieves sometimes have been called microbenthos, although this last term is rarely used. It is customary to ignore the small benthic animals, but studies have shown that animals too small to be caught on a 0.5-mm mesh may constitute >95% of the individuals, ^25% of the biomass, ^50% of the metabolic activity, and >50% of the species in a zoobenthic community.
Composition and Biological Traits of the Lacustrine Zoobenthos
The density of zoobenthos depends strongly on the mesh size of the sieve used for the study (Figure 1). Densities of macrobenthos (i.e., animals large enough to be caught on a 0.5-mm mesh) in lakes typically are 1000-10 000 m-2, while the total density of all benthic metazoans probably usually is ^1000 000 m~2. There is a great deal of variation in densities within and among lakes, so that densities reported for a given mesh size range over ~ 100-fold within and among lakes.
Estimates of zoobenthic biomass usually include only the macrobenthos and exclude large bivalves. Because small benthic animals usually constitute a small part of zoobenthic biomass, estimates of the biomass of the macrobenthos probably are nearly equivalent to the entire zoobenthos. However, it appears that the meiobenthos may be especially important in unproductive habitats (Figure 2). Macrobenthic biomass without large bivalves ranges from ^0.2 to 100 g dry mass m~2, and dense populations of large bivalves can increase these values by >10 g DM m~2. Zoobenthic biomass rises up to a point with increasing phytoplankton production, then asymptotes or even declines with further increase in phytoplankton production. Zoobenthic biomass also tends to be highest in shallow lakes, where rooted plants are abundant, where the lake bottom is flat or gently sloping, in warm lakes or in the epilimnion of stratified lakes, and in lakes of low color.
There are relatively few data on the production rates of entire macrobenthic assemblages and almost no estimates that include the small benthic animals. One would expect to see as much variation in rates of production as in biomass, and based on the expected ratio of annual production to biomass of for animals of macrobenthic size, production of the lacustrine macrobenthos might range from to 500 g DM m~2 year-1. Production of smaller benthic animals in lakes might be about the same order of magnitude, based on the sparse data now available. Production of the macrobenthos increases with increasing primary production (Figure 3), with no hint of an asymptote or downturn at very high primary production, as has been seen for macrobenthic biomass.
The lacustrine zoobenthos is enormously diverse; a typical lake contains hundreds of species from 12 to 15 animal phyla (Table 1, Figure 4). The macro-benthos often is numerically dominated by oligo-chaetes and chironomid and chaoborid midges, although large-bodied mollusks may dominate the biomass of the community. Aquatic insects other than dipterans (such as mayflies) may be abundant, especially in shallow lakes. Nematodes usually are by far the most numerous of the meiobenthos (and of the zoobenthos as a whole), although gastrotrichs, rotifers, and microcrustaceans often are abundant as well. Many important families are ecologically important in lakes around most parts of the world, including the Chironomidae and Chaoboridae in the Diptera; the Ephemeridae in the Ephemeroptera; the Tubificidae (including the 'Naididae') in the Oligo-chaeta; the Unionidae and Sphaeriidae in the Bivalvia; the Lymnaeidae and Planorbidae in the Gastropoda; the Chydoridae, Canthocamptidae, and Cyclopidae among the microcrustaceans; and the Chaetonotidae in the Gastrotricha. In contrast, several important groups are restricted to particular biogeographic regions (mysid shrimps and gammarid amphipods chiefly in the Northern Hemisphere, hyriid bivalves in the Southern Hemisphere) or habitats (ephydrid flies or brine shrimp in fishless or saline inland waters). Human introductions have vastly increased the ranges
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