Plankton to Plants—Keys to Productive Waters

Fish and Lake Management
Different plants thrive in different types of water and nutrients
Different plants thrive in different types of water and nutrients

Farmers and gardeners know certain facts about plant growth. A basic one is that to grow the kind of plants they desire, they must plant seeds, transplant seedlings or small plants from a nursery, or install turf. They also know that plants usually require nutrient applications of fertilizer for optimum growth, liming materials may be required to adjust soil pH, and each plant has a temperature range within which it grows. Farmers and gardeners also bemoan that though not intentionally planted, weeds (unwanted plants) pop up among the desirable plants originating from seed or other propagules of weeds present in the environment.

Pond owners are faced with these same facts. They need certain types of plants in their ponds and the exclusion of weeds. Aquatic plants serve as the base of the food web, which culminates in sportfish. The most desirable pond plants are certain phytoplankton species, mainly green algae and diatoms. The pond owner cannot go to a store and purchase packets of green algae or diatom propagules.

However, through natural seeding, ponds will develop a community of phytoplankton. The abundance of phytoplankton can be increased by applying nutrients in fertilizer and using liming materials in ponds with acidic water. Moreover, as with land plants, phytoplankton grow best in the warmer part of the year, and ponds have weeds.

The phytoplankton in ponds originate from natural sources. Algae occurs in moist surface soils, and they are found in the atmosphere because wind passing over the land and water suspends algae, which later are deposited on surfaces, including pond waters.

Algae in water bodies also attach to surfaces of wading birds and other waterfowl, allowing these birds to move algae from another water body to a fishpond. Seeds and pieces of other types of aquatic plants also can enter a pond on birds and other animals. Algae and other aquatic plants sometimes become attached to boats or fishing equipment used in one water body to be transferred to another.

There is an axiom in ecology that "nature abhors a vacuum," which can be translated as "build a pond, put water in it, and the aquatic plants will appear." As farmers and gardeners must contend with weeds, weeds await to offend pond owners. The significant difference is, as a general rule, ponds receive a more significant number of plant species from nature than do fields and gardens, and the good is mixed with the evil. Most plants are minute planktonic algae, and the pond owner does not know the species present, the relative abundance of each species, and which species are desirable. The best assessment is that blue-green algae are less desirable than other species, and an excessive abundance of phytoplankton is apt to lead to low dissolved oxygen concentration.

To a large extent, fishpond vegetation still must be managed as a "green box" because relatively little is known about which species are best and how to encourage the best species. The presence of larger aquatic plants (large algae and vascular plants) is generally objectionable in sportfish ponds, mainly if they infect large areas. Because many larger aquatic plants, often called aquatic weeds, grow up from the pond bottom, infestations are most common in clear water. Phytoplankton blooms shade the pond bottom, and in ponds without larger expanses of shallow water (less than 2-3 feet in depth), shade from phytoplankton blooms promoted by pond fertilization is often effective in controlling underwater weeds. Particularly pesky are the small floating duckweed species that shading cannot eliminate.

Another significant difference in phytoplankton in ponds and plants in fields, gardens, and lawns is their capacity for amazingly rapid growth. A pond with clear water can be fertilized with nitrogen and phosphorus, and a phytoplankton bloom may develop in 3 or 4 days, making the water appear green. This rapid growth results because phytoplankton cells reproduce by a process of division in which one cell splits into two cells within one to a few hours; each new cell repeats the process, which continues. Each cell can lead to hundreds or thousands of new cells in a few days.

The algal cells present when fertilizer is applied can rapidly absorb nitrogen and phosphorus from the fertilizer. As cells divide, nutrients absorbed by the previous generation become the inheritance of the present generation and are passed to their heirs. Of course, phytoplankton cells have a life span of only 1 or 2 weeks, but when they die, they generally rupture before settling to the bottom and a portion of their nutrients is rapidly recycled and used by other phytoplankton. Nevertheless, fertilizers typically should be applied at 3-4-week intervals to maintain a phytoplankton bloom dense enough to shade aquatic weeds capable of growing up from the pond bottom. However, it is crucial to understand that when a pond is already infested with larger aquatic plants, they may absorb nutrients from fertilizer to increase their growth without producing a phytoplankton bloom. Herbicide treatment or grass carp may be needed to eliminate such growth.

Developing overly abundant blue-green algae is a typical problem in sportfish ponds. There is no practical way of preventing this phenomenon, but excessive fertilization with phosphorus fertilizer encourages excessive phytoplankton.

The phytoplankton in a pond is not generally the direct food for the fish. The phytoplankton is consumed by various microscopic animals upon which sunfish feed. Dead phytoplankton settles to the bottom, becoming food for insect larvae and other tiny organisms that sunfish eat. Small sunfish are a favorite bass food, but small bass, in particular, eat zooplankton and insects.

Many of the principles that apply to terrestrial plant cultivation apply to the management of plants in sportfish ponds, especially in the south. A significant difference is the lack of an effective way of seeding ponds and maintaining them with the most desirable species. It is possible to avoid many aquatic weed infestations by deepening shallow water areas and maintaining turbidity in water through fertilization. Attempts to manage phytoplankton blooms in ponds to avoid blue-green algae and other undesirable species are often ineffective. The wisest approach seems to be a conservative fertilization program to maintain a phytoplankton bloom, sufficient to shade larger aquatic weeds but not so abundant as to result in low dissolved oxygen concentration. This effort may require frequent adjustment of the timing and amount of fertilizer applied.

Dr. Claude Boyd is a professor emeritus in the School of Fisheries, Aquaculture and Aquatic Sciences, Auburn University, Auburn, Alabama 36849. His work with water quality is internationally renowned. His most recent book, Handbook for Aquaculture Water Quality, is a must for anyone interested in learning about water chemistry and how it relates to your pond. It's technical and thorough but easy to read and understand. Buy it at in the online store.

Reprinted with permission from Pond Boss Magazine