Phytoplankton abundance provides a major indication of pond trophic state. More eutrophic ponds typically support higher phytoplankton biomass, usually measured by the concentration of the photopigment chlorophyll-a in the water column. The phytoplankton consists of an array of species that vary in their seasonal dominance, light and nutrient requirements, and susceptibility to consumption by zooplankton. Three major groups of species typically dominate the phytoplankton. They are described briefly below, and examples of each group are shown in Fig. 30.
        Diatoms are often particularly abundant during early spring. Unlike most other algae, diatoms require silica (SiO2) in large amounts for cell wall construction and ponds dominated by diatoms often experience sharp declines in silica concentrations during the growing season because of uptake by diatoms. Uptake by diatoms also likely caused the pronounced retention of silica within the ponds noted in Table 3. Diatoms may be present either as individual cells or as colonies of many cells, such as the star-shaped colony of Asterionella shown in the figure.
        Green algae include many species which have small, fast-growing cells, such as the Scenedesmus shown in Figure 30, that are highly palatable to zooplankton. Other species may have cells encased in gelatinous mucilage, rendering them larger and less edible. Under conditions of high nutrient loading in eutrophic ponds it is often the green algae that become particularly abundant.


Fig. 30  Three groups of algae most commonly found in the phytoplankton of ponds in Chester County.

        Blue-green algae, or cyanobacteria, usually have smaller cells than do members of the other two groups. Many species are very tolerant of warmer water, and are less preferred by zooplankton, so often grow rapidly and experience less mortality, thereby dominating ponds during summer. Their presence is often indicative of ponds with excess P and limiting N concentrations. In hyper-eutrophic ponds some species may form algal “blooms” which can be unsightly and toxic to livestock.
        Phytoplankton biomass is commonly estimated as chlorophyll-a , a photopigment used for photosynthesis and present in all groups of algae. Chlorophyll-a in the 13 Chester County ponds studied during 2002 varied from 1 to 552 µg/L, with highest values in July (Fig. 31). As is evident from the figure, light depletion in the ponds largely results from the interception of light by phytoplankton.

Fig. 31  Relationship of light depletion (LD) to phytoplankton chlorophyll-a.