09/27/1990 - Laserfiche WebLink

Home Browse Search

Zooplankton Although no quantitative examination was made of the zooplankton, it was clear from empirical review of the samples taken that the community is dominated by small - bodied copepods, rotifers, and cladocera. No large -bodied Daphnia were observed. This qualitative observation suggests that the numerous minnows and other small fish observed in the pond are visually preying on larger forms of zooplankton whereas smaller forms are escaping detection. If a complete winterkill of fish occurred, large Daahnia_ would become common and in turn would graze algae -more efficiently. This would result in a lower amount of chlorophyll per unit phosphorus, and could result in a bloom of large colonies of the nitrogen -fixing blue-green alga Aphanizomenon. C. Discussion Silver View Pond is rich in phosphorus, averaging between 65 and 123 ug/L in the different basins. The west and south basins appear to be receiving the majority of the phosphorus loading from storm -sewer runoff; however, the phosphorus concentration -is reduced substantially by the time the water makes its way to the east basin. Algae grow in proportion to the concentration of phosphorus, so algae populations are lowest in the east basin. High densities of algae reduce light significantly, so that macrophyte growth is inhibited. In addition, macrophytes have more access to light in shallow water. The eastern basin is shallow and has the least algal growth; consequently, macrophyte growth is highest in this part of the pond. The nitrogen -to -phosphorus ratio (TN/TP) indicates that Silver View Pond is slightly limited by nitrogen, so that blue-green algae are sometimes found floating on the surface. It is somewhat surprising (but desirable) that blue -greens are not more abundant in this system. It is likely that the TN/TP ratio need only be lowered slightly more to shift the algae completely to the less -desirable blue-green algae. This shift could occur if slightly more phosphorus is introduced to the system. Therefore, future watershed management should minimize additional loading of phosphorus. However, if phosphorus is greatly reduced, water transparency could increase sufficiently to greatly increase macrophyte growth. It appears that some algal growth is desirable to shade macrophytes; thus, it probably is most desirable to limit future phosphorus loading through altering land -use practices, and to not pursue large reductions in`phosphorus loading. Reasonable measures might include additional upstream ponding for any neva developments 88