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Pond Food Web

       A generalized diagram linking the major groups of organisms found in a small pond in this region is shown in Figure 8. Each group is described more fully in sections that follow. The arrows connecting the biological compartments indicate the direction of energy flow. Zooplankton, for example, depend in large part on energy contained in the phytoplankton they eat. Thus, management measures intended to control a particular compartment in the Figure (for example, by reducing phytoplankton abundance) inevitably affect other foodweb components (e.g, zooplankton and fish) that depend on it.

Fig. 8.Generalized food web showing major groups of primary producers (in green), invertebrate consumers (in light blue) and vertebrate predators (in red). Arrows indicate the direction of energy flow.

       Ponds provide habitat for an array of primary producers (photosynthetic organisms), all of which are influenced by water chemistry and also interact with each other. They form the base of a food web for consumers, including a variety of invertebrates, fish and waterfowl. The following groups are especially important:
       Primary producers are classified here into four general groups, distinguished by their form, location and ecological roles within the pond.
  • 1. The phytoplankton consists of microscope, free-floating algae composed of individual cells or small colonies.
  • 2. The periphyton refers to substrate-associated algae, normally forming a thin layer that covers rocks, the sediments and other surfaces in well-lit portions of the pond. These surface-dwelling “biofilms” contribute more primary production in many ponds than the phytoplankton.
  • 3. The metaphyton is a scum of filamentous algae, clearly visible at the surface or suspended in the water column of hyper-eutrophic ponds. Metaphyton “clouds” typically appear only in ponds with high nutrients. Scums of metaphyton originate as periphyton that lifts off the pond sediments or other surfaces, buoyed upward by oxygen bubbles produced in photosynthesis. The metaphyton mats often decompose at the surface, releasing their stored nutrients to the water column and potentially stimulating phytoplankton growth.
  • 4. Aquatic vascular plants, and a few large algae resembling aquatic plants, are collectively termed macrophytes. Rooted plants typically obtain most of their nutrients from the sediments. When they die and decompose, most of the nutrients taken up into the stems and leaves of the plants are released to the water column, often stimulating algal growth. Some plants are not rooted in the sediments (e.g., duckweed), and thus compete with phytoplankton and metaphyton for nutrients in the water column.

       Each group of primary producers has a unique assemblage of invertebrate consumers that depend on it (e.g., as food or shelter). Within all four invertebrate assemblages shown in Figure 8 consist of species that consume algae or plants directly, as well as predators that consume other invertebrates. They are generally found with the algae or plants on which they directly or indirectly depend:
  • 1. Zooplankton, consisting primarily of microcrustacea (cladocerans and copepods) and rotifers, is actually a community of both grazers on phytoplankton and invertebrate predators that eat other zooplankton. When larger grazers dominate the zooplankton, they can effectively control phytoplankton biomass in some ponds.
  • 2. Consumers found in the clouds of metaphyton include a variety of small-bodied microcrustacea, as well as some larger invertebrates (e.g., snails, tadpoles). Many of these animals feed on the bacteria and smaller algae attached to the much larger filaments that give the metaphyton its mat-like structure, rather than on the filaments themselves.
  • 3. Associated with the periphyton on rock surfaces or on the sediments is a diverse group of benthic invertebrates, including a wide variety of aquatic insects (larval dragonflies, beetles, midges) and crustacea (e.g., isopods, scuds, crayfish). Benthic invertebrates also colonize the surfaces of aquatic plants. Some of these invertebrates actually consume plant tissues, but most glean attached periphyton and bacteria from the plant surfaces.
       Fish and waterfowl are important predators on invertebrates, and some species also consume primary producers directly (for example, Grass Carp are often used to control aquatic plant growth, and resident swans often “overeat” plants to the point where they no longer provide sufficient habitat for invertebrates and fish). The suitability of a pond for the growth of fish or sustaining waterfowl thus depends in large part on these other components of the food web, and an excess of these “top predators” can also destabilize the food web on which they depend.