Trophic Interactions in the Pelagial

The biomass and production of fish per unit of area at a given nutrient level do not depend on lake depth. Therefore, shallow lakes have a substantially higher fish biomass per unit of volume than deep lakes. This relationship may reflect the higher nutrient recycling and availability from settled material and probably as well the availability of feeding and spawning sites for fish offered by the aquatic plants. The biomass and production of benthic invertebrates are also generally higher in shallow lakes (Figure 4), probably due to the

Figure 4 Changes in some trophic variables in temperate lakes along a mean depth gradient derived from published empirical equations. Biomass per unit area of zooplankton (a) and phytoplankton (b), zooplankton:phytoplankton biomass ratio that typically declines with increasing fish predation (c), zoobenthos biomass (d), and the zooplankton:zoobenthos ratio (e) in lakes with an epilimnion concentration of 0.1 mg TP L~1. Reproduced with kind permission from Jeppesen etal. (1997) Hydrobiologia 342/343: 151-164.

Figure 4 Changes in some trophic variables in temperate lakes along a mean depth gradient derived from published empirical equations. Biomass per unit area of zooplankton (a) and phytoplankton (b), zooplankton:phytoplankton biomass ratio that typically declines with increasing fish predation (c), zoobenthos biomass (d), and the zooplankton:zoobenthos ratio (e) in lakes with an epilimnion concentration of 0.1 mg TP L~1. Reproduced with kind permission from Jeppesen etal. (1997) Hydrobiologia 342/343: 151-164.

faster settlement of organic matter from the water column (more fresh material) and higher oxygen concentrations due to lack of stratification. The easier access to benthic invertebrates in shallow lakes also helps maintain a relatively higher fish biomass and a consequent high predation pressure on zooplankton (a process called 'benthic facilitation'), as most fish can shift between benthic and pelagic feeding. Moreover, the biomass ratio of zoobenthos:zooplankton increases with decreasing depth allowing stronger facilitated control of zooplankton in shallow lakes (Figure 4). In recent years, analysis of the stable iso-topic signals of freshwater temperate communities has shown that most fish species are actually omnivores, to a much greater extent than historically perceived. The relatively higher impact of fish likely leads to a higher predation pressure on macroinverte-brates and zooplankton with decreasing mean depth, with consequent impacts on periphyton and phyto-plankton communities (Figure 4).

However, in small lakes and ponds, fish may be scarce or even absent, which increases the likelihood of a clear-water, plant-dominated state. As a consequence of the lack of fish predation and higher habitat heterogeneity caused by the macrophytes, other groups may flourish. Although lakes and ponds behave similarly in many aspects, ponds do not necessarily respond with high phytoplankton biomass to an increase in nutrient concentrations as do larger lakes, due to other controlling factors. Also, the phy-toplankton community responds differently in shallow than in deep lakes. Green algae (Chlorophyceae) frequently dominate in shallow hypertrophic lakes, whereas in deep lakes cyanobacteria are usually dominant under such conditions. The high nutrient release from the sediments and continuous mixing conditions may favor green algae over nutrient-storing, slow-growing cyanobacteria.

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