The “chain of relationships” based on nutrients


       Although food webs are often organized in terms of energy flow (Fig. 6), components of the pond ecosystem are linked in other ways as well. Figure 9 describes a “chain of relationships” (Portielje and van der Molen, 1999) among components of a typical pond that are directly or indirectly affected by nutrient supply.
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Fig. 9. Effects of nutrients on other components of a pond ecosystem. Arrows indicate cause-effect relationships among compartments rather than energy flow.


       As shown in Figure 9, increased loading of growth-promoting nutrients from the watershed (for example, through overland runoff, stream inputs, or groundwater seepage) can lead to elevated nutrient concentrations in the water column, which in turn stimulate the growth of phytoplankton and metaphyton. A portion of the incoming nutrients is precipitated to the sediments, which form a second major reservoir capable of resupplying nutrients to the water column. Increased abundances of phytoplankton and metaphyton in turn increase the rate of light depletion within the water column, which suppresses the growth of aquatic plants and algal periphyton deeper in the pond. Dizzy yet from trying to locate all the arrows? The important thing to remember is that many important elements of the pond ecosystem are influenced directly or indirectly by nutrient supply. We consider two nutrients, phosphorus and nitrogen, in Section J, and focus particularly on system responses to phosphorus.
       Nutrient supply is considered the primary determinant of pond trophic state, a concept that recurs frequently in this report. Trophic state refers to the abundance and productivity of photosynthetic algae and plants, and the nutrient supply on which they depend. Deep lakes in pristine watersheds with little nutrient inflow, low primary producer abundances and excellent light penetration are termed “oligotrophic” (poorly nourished). Most ponds in Chester County are shallow (typically 1-3 m average depth), have watersheds that supply abundant nutrients, and are periodically fertilized as well by wind-driven mixing of nutrient-rich bottom sediments into the water column. Ponds with these characteristics are termed “eutrophic” (well nourished), and typically have green water (an indication of abundant phytoplankton) and/or large quantities of rooted aquatic plants. Ponds with problem-levels of overabundant plant or algal growth (e.g., surface scums of metaphyton, or carpets of duckweed/watermeal) are often termed “hyper-eutrophic”, and may be considered “overfed” with nutrients (Fig. 10). Hyper-eutrophic ponds are common in Chester County, and their symptoms constitute the principal causes for management efforts by landowners. We will describe a quantitative method for classifying the trophic state of ponds in Section O.
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Fig. 10. A hyper-eutrophic pond in Newlin Township, showing breeze-generated patterns of watermeal at the pond surface.