Water Column Nutrients

        Two nutrients often needed by primary producers in larger amounts than are available in ponds for sustained growth are nitrogen (used to make proteins) and phosphorus (used in phospholipids, adenosine triphosphate and other biomolecules). In addition, concentrations of carbon (taken up via photosynthesis by algae and macrophytes, and present in all organic molecules) and silica (needed in large amounts by one group of algae, the diatoms, for cell wall construction) may occasionally limit the growth of particular species, but are unlikely to control overall primary producer biomass. This report focuses on seasonal changes in nitrogen and phosphorus. The forms of both nutrients are described in Table 1.
Table 1

        Nitrogen (N) may be taken up by primary producers either as ammonium (NH4+) or nitrate (NO3-). Both are available for uptake by phytoplankton and metaphyton in the water column, but sometimes occur in low enough concentrations to limit growth. Total nitrogen (TN) in the water column includes ammonium, nitrate, dissolved organic nitrogen (DON) and particulate nitrogen (PN: nitrogen incorporated into phytoplankton and other particles suspended in the water column), and is frequently used to assess the potential for nitrogen limitation of algal growth. Whereas phytoplankton, metaphyton and free-floating aquatic plants obtain nitrogen from the water column, total sediment nitrogen is a better indicator of potential limitation of the growth of rooted aquatic plants, which obtain the bulk of their nutrients from the sediments.
        Phosphorus (P) is frequently in short supply relative to the needs of primary producers and thus potentially capable of controlling their growth in many ponds. Phosphorus is taken up by primary producers as soluble reactive P (SRP, or PO43-) and incorporated internally into phosphorus-containing organic molecules. Total phosphorus (TP), including orthophosphate, dissolved organic phosphorus (DOP) and particulate phosphorus (PP), is usually used to evaluate the potential for P-limitation.
        Both nitrogen and phosphorus are essential for growth, and the growth of primary producers is limited by whichever nutrient is in least supply relative to need. If, for example, phosphorus is the “limiting nutrient” for the phytoplankton community then the growth of phytoplankton is determined solely by the availability of phosphorus, regardless of the concentrations of nitrogen. Although the needs of primary producers are known to vary according to species, an approximate ratio of need for the two nutrients is thought to be between [7.2 mg N:1 mg P] (Redfield, 1958) and [14 mg N:1 mg P] (Downing and McCauley, 1992).
        The ratio of relative availability of nitrogen and phosphorus is normally expressed as TN:TP (Dodds, 2003), although recognizing that some forms of both nitrogen and phosphorus are not directly usable by primary producers. This means that if the (mg:mg) ratio of TN:TP in the water column greatly exceeds 14:1, then nitrogen is in excess and phosphorus is considered the limiting nutrient. If the TN:TP ratio is much less than 7.2:1 then nitrogen is considered limiting. The interval between 7.2:1 and 14:1 may be taken as a zone of “joint limitation” by nitrogen and phosphorus (some species may be slightly N-limited, others P-limited, but the overall growth of primary producers will remain relatively unchanged unless both nitrogen and phosphorus levels increase). Identifying whether the limiting nutrient is nitrogen or phosphorus is often considered a critical first step in developing a management plan for controlling excessive growth by primary producers. For example, if phosphorus either limits or jointly limits growth, then reducing the supply of phosphorus (e.g., by using phosphorus-free detergents, or lowphosphorus fertilizers) can be used to reduce primary producer biomass.
        Total nitrogen levels declined between March and July in 8 of the 13 ponds studied, and total phosphorus increased in 9 of the 13 ponds (Fig. 26). Both the decline in N and the increase in P during the growing season have been observed elsewhere in shallow, highly productive ponds (Søndergaard et al., 1999; Søndergaard et al., 2003). Briefly, nitrate undergoes bacterially-mediated “denitrification” under low oxygen conditions, and is converted to nitrogen gas which is lost from the pond; phosphorus in contrast is released from binding to iron in the sediments under low oxygen and enters the water column. Both processes are facilitated later in the growing season by a combination of warmer temperatures, lowered oxygen near the bottom, and increased bacterial activity at the sediment surface. We have not measured either process directly, but both are reasonable explanations for the opposing seasonal trends of nitrogen and phosphorus in the 13 study ponds.
Figure 26

Fig. 26. Concentrations of total nitrogen and total phosphorus relative to values in March in the water column of 13 study ponds. Lines through the boxes are median values. Upper and lower limits of the boxes indicate quartiles, and whiskers indicate ranges.

        As a consequence of declines in TN but increases in TP in most ponds, ratios of TN:TP typically declined between March (when most ponds were P-limited) and July (when many ponds were jointly limited by nitrogen and phosphorus) (Fig. 27). Because P either limited or jointly limited growth, however, this report has focused on the sources and management of phosphorus as a means of controlling algal growth.
Figure 27

Fig. 27. Ratios of TN:TP for 13 ponds in Chester County, PA. “Optimal” ratios of 7.2N:1P and 14N:1P are shown as diagonal lines and demarcate approximate zones of N limitation, P limitation and joint limitation.

        As one indication of the importance of TP to primary producers, phytoplankton biomass was strongly related to TP in the water column; ponds with greater total phosphorus supported greater phytoplankton growth (Fig. 28).
Figure 28

Fig. 28. Relationship of phytoplankton biomass as chlorophyll-a to total P in surface water samples taken in March (inverted triangles and dotted trendline) May (open squares and dashed trendline) and July (solid circles and solid trendline).